The Design of an Osseointegrated Upper Leg Prosthesis Fixation System

Transcription

1 The Design of an Osseointegrated Upper Leg Prosthesis Fixation System A Master s Thesis Mike van Diest BSc Supervisors: Prof. dr. ir. G.J. Verkerke, Prof. dr. S.K Bulstra Co-supervisor: P.K. Tomaszewski Master thesis educational programme Biomedical Engineering, Faculty of Mathematics and Natural sciences and Faculty of Medical Sciences, University of Groningen, The Netherlands October 4,

2 The Design of an Osseointegrated Upper Leg Prosthesis Fixation System A Master s Thesis Author: Mike van Diest BSc Department of Biomedical Engineering, University Medical Centre Groningen A Deusinglaan 1, 9713 AV Groningen. Faculty of Mathematics and Natural Sciences, University of Groningen Nijenborgh 9, 9747 AG Groningen Electronic mail: Mikevandiest@gmail.com Supervisor: Prof. dr. ir. G.J. Verkerke Department of Biomedical Engineering, University Medical Centre Groningen A Deusinglaan 1, 9713 AV Groningen. Electronic mail: g.j.verkerke@med.umcg.nl Co-referent: Prof. dr. S.K. Bulstra Department of Orthopaedics, University Medical Centre Groningen, University of Groningen, A Deusinglaan 1, 9713 AV Groningen. Tutor: P.K. Tomaszewski Department of Biomedical Engineering, University Medical Centre Groningen A Deusinglaan 1, 9713 AV Groningen. October 4, 2010 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the author or the supervisor. This report is the result of an educational project. The use in this report of trade names, trademarks, service marks and similar terms, even if they are not identified as such, is not to be identified as an expression of opinion as to whether or not they are subject to proprietary rights. 2

3 Index 1 Summary... 5 Part 1 Analysis phase Problem definition Introduction Osseointegrated trans-femoral prosthetic limb fixation The Brånemark System ESKA ISP Endo/Exo Prosthesis Current problems with the OPRA and the ISP systems A new design Summary of problems Stakeholder analysis Goals Design assignment Program of Requirements and Whishes Function analysis Part 2: Synthesis I Brainstorm Applied creativity method Results Brainstorm From useful ideas to Pre-concepts Pre-concepts Combination of pre-concepts Selection of combined pre-concepts Conclusions Part 3: Synthesis II Concepts Concepts Selection of the best concept Part 4: Synthesis III Final Concept: The alternatives The alternatives Comparison of the three alternatives Shape of the torque transferring part Final concept

4 13.1 Design of the final concept Materials used Finite Element Model (FEM) Analysis Prototype Methods of production Costs Failure Mode and Effect Analysis (FMEA) Test protocol In vitro tests Conclusion Evaluation Acknowledgements References Appendix I Project Planning Appendix 2 Selection of the final concept Appendix 3 Technical drawings final concept version 1 (for the workshop) Appendix 4 Material properties Ti-6Al-4V Appendix 5 Material Properties PEEK Appendix 6 Technical drawings final concept version 2 (for the workshop) Appendix 7 Material Properties Titanium grade

5 1 Summary Each year around 600 transfemoral amputations are performed in the Netherlands [1]. After the surgery the relatively young and health patients usually receive a prosthetic limb which is attached to the remnant limb by a socket. However, this fixation method is often reported as unsatisfactory due to skin irritation, inappropriate control over the prosthetic leg, a reduced hip range of motion and stump pain[2, 3]. An alternative solution is the percutaneous attachment of the prosthetic leg to the femur bone via an osseointegrated implant. This implant, the OPRA system (Integrum AB, Göteborg Sweden), was already developed two decades ago and since then more than 100 patients have used it. The socket system problems are reported to be overcome and patients are in general satisfied with the implant [4-6]. Recently a similar concept, the ISP Endo/Exo prosthesis (ESKA Implants AG, Lübeck, Germany), was developed and tested in patients. Again the patients were reported to be satisfied [7]. However, a few serious issues with these implants have been reported. Infection may occur due to the open connection between the bone and the external environment, and there is a risk of fracture of both the bone and the implant. The risk of fracture is increased due to bone remodeling during which the distal part of the femur bone is resorbed [4, 5, 8]. In the Departments of Biomedical Engineering and Orthopaedics of the University Medical Center Groningen and the University Medical Center Radboud in Nijmegen a new osseointegrated prosthesis fixation device is being developed that has superior stress distribution properties, which should lead to a reduction of the bone remodeling process. The concept consists of a PEEK bushing, which is inserted in the medullary canal of the femur, and a titanium pin, which is inserted in the bushing. Two functions however were not yet included in the concept: Transfer of torsion from the bushing to the pin and prevention of pull-out of the pin from the bushing. In this master s thesis a solution was found for these two problems, while the superior stress distribution properties of the previous concept were preserved. The methodical design process was used to ensure optimal creativity in the search for the best solution. In the analysis phase the problem was thoroughly analyzed and made more abstract. In the Synthesis I phase a lot of ideas were generated and the best ideas were detailed and used in 11 pre-concepts. In the Synthesis II phase the three best pre-concepts were selected and further detailed, leading the three concepts. In the Synthesis III phase a final concept was selected using the program of requirements and whishes. This final concept was further detailed and several alternatives were considered. The final concept consists of a PEEK bushing, which will be inserted in the femoral medullary canal, and a titanium pin, which will be inserted in the bushing. Torque is transferred from the pin to the bushing by using a hexagonal cylinder. Pull-out is prevented by a titanium rod which runs through the pin and is tightly connected to the upper part of the bushing and the lower part of the pin. A finite element model (FEM) was built to analyze the stress distribution when torque, a pull-out force, a bending moment and a realistic load case were applied. The results showed that the implant does not fail under these loads and the stress distribution appeared to be more physiological than the OPRA and the ISP implant. A prototype of the final concept was produced. The prototype was tested in vitro to check if the loads that the prototype is supposed to withstand were withstood. These tests were successful. The concept that was developed during this master s thesis showed in the finite element model analysis that the stresses that are generated by torsional and pull-out loading are distributed uniformly and no peak stresses were observed. Moreover, the prototype passed the in vitro tests. More improvements to the concept have been suggested and in the near future more in vitro and possibly also in vivo studies can be performed, after which clinical studies might be started. It is not sure if the concept will make it that far in the process, but the first results look promising. It would be great if patients can benefit from this master s thesis within the next decade. 5

6 Part 1 Analysis phase In the analysis phase the problem is analyzed thoroughly. The phase consists of the following sub-phases: Problem definition: The problems are described in detail. Questions like: what is the problem?, Why is this a problem? etc. are answered. Stakeholder analysis: Description of the interests, expectations and capabilities of the groups of people that are involved in the project. Goals: Overview of the situation that should be reached. Design assignment: Description of the strategy to reach the goals and a description of the demarcations. Program of Requirements and Whishes: A list of requirements, to which the solution must comply, and a list of whishes, which are additional criteria which the solution does not have to fulfill but are additional, are presented. Function Analysis: The assignment is made more abstract. The basic functions that the solution should perform are shown schematically. 6

7 2 Problem definition 2.1 Introduction In the USA there are approximately 1.7 million people living with limb loss [9]. The majority of amputations in developed countries occur due to complications of the vascular system. The main reason for these complications is atherosclerosis and up to a third of these patients have diabetes [10]. These dysvascular amputations account for 82 percent of limb loss discharge and 25 percent of these amputations are at above knee level [11, 12]. Other causes of limb loss include trauma and cancer [12]. In the Netherlands each year around 600 transfemoral amputations are performed [1]. In the USA 266,465 transfemoral amputations were performed between 1988 and This is an average of 29,607 annually [12]. Upper leg amputation is a permanent disfigurement and results often in profound physical and psychological consequences [12]. It greatly limits the daily functioning of the patient [13] and has a negative effect on the quality of life [14]. An upper leg prosthesis, which usually consists of a knee joint and a lower leg prosthesis (Fig. 2.1), can be used to enable the patient to regain mobility. Two methods to attach the prosthesis to the femoral stump are currently being used. The most common fixation method is fixation by using a socket. An alternative solution is direct fixation of the prosthesis to the femur bone. The standard method to attach a prosthesis to the femoral stump is to place a socket around the stump. The socket provides load transmission, stability and efficient control of the prosthesis [15]. Although sockets have been improved a lot over the past two decades and can be considered high tech, still many problems concerning sockets are being reported. A lot of patients suffer from sores and skin irritation, sweating in the socket and the inability to walk in woods and fields. Other complaints are stump pain, phantom limb pain, back pain and pain in the contralateral leg [2]. Due to these problems the socket can only be worn for a few hours per day. The socket also reduces hip range of motion, resulting in discomfort during sitting [3]. Fig. 2.1: C-leg, Otto Bock [16] 2.2 Osseointegrated trans-femoral prosthetic limb fixation An alternative for prosthesis fixation using a socket is direct prosthesis fixation to the femur bone. A strong connection between the femur bone and the prosthesis fixation system is realized by using a process called osseointegration. Osseointegration is defined as the structural and functional connection between living bone and the surface of a load carrying implant. A fixture is osseointegrated if there is no progressive relative motion between the fixture and the surrounding bone and marrow under functional levels and types of loading [4]. At the moment two osseointegrated trans-femoral prosthetic limb fixation devices are being used. 7

8 2.2.1 The Brånemark System The Brånemark system consists of three titanium parts; an intra-osseous implant, an abutment and an abutment retention bold (Fig. 2.2). The implant is permanently situated in the medullary canal of the residual femur and provides the mechanical attachment to the cortical bone. It has a selftapping thread which engages the cortical bone of the femur over the maximum area. Perforations in the implant allow vascular communication with the marrow cavity. The abutment penetrates through the skin at the most distal part of the femoral stump. The upper end of the abutment is attached to the implant with a retention bolt. The lower end of the abutment can be attached to a prosthesis, which has a screw-tightened clamp at the upper end to grip the abutment. Two surgical procedures are necessary to install the system. In the first procedure the intro-osseous implant is inserted in the medullary canal of the residual femur. In order for osseointegration to take place in the femur, the implant should be situated there for six months. During the second surgery the scar is re-opened and the abutment is inserted into the implant, fixed by the retention bold. Also the skin penetration is made. When the patient is fully healed, a training program commences. The whole osseointegration program takes about 18 months [4]. This treatment has been performed in Sweden since 1990 and in 1999 a treatment protocol called OPRA (Osseointegrated Prostheses for the Rehabilitation of Amputees) was standardized. Today more than one hundred patients in several countries were treated with this method [6]. This group consisted for 67% of patients who lost their leg due to trauma. It should be noted that this group is not representative for the whole population of limb amputees, which consists for 82% of patients who lost their leg due to circulatory dysfunction [11]. Fig. 2.2 The Brånemark System, Integrum AB. On the left side the titanium screw, the abutment and the abutment retention bold are shown. On the right side a schematic overview of the location of the fixation device in the bone is shown. Compared to sockets, important advantages of the Brånemark System were reported. The donning and doffing of the prosthesis is easy and fast, there is always a proper fit, there are no hip range of motion restrictions and sores, sweating and discomfort that are associated with sockets are absent. Other positive findings include an improved sense of grounding with the prosthetic foot and improved prosthetic limb control [3, 5, 6, 14, 17]. Furthermore, amputees equipped with an osseointegrated fixation device have, compared to amputees using a socket, improved sensory feedback, referred to as osseoperception [18]. This was also found by Jacobs et al, who showed that the threshold level of perception of pressure stimulation and vibrations in osseointegrated fixation systems are low compared to prostheses that are attached with a socket [19]. 8

9 Disadvantages of the Brånemark System There are also negative experiences with the Brånemark System. Superficial infections are quite common complications. The site where the abutment penetrates the skin is an open connection between the bone tissue and the environment and therefore prone to infection. These infections can be treated with antibiotics. This might cure the infection, but the infection can also escalate to a deeper infection [6]. This is difficult to cure with antibiotics and can cause implant loosening, which might lead to the necessity to remove the implant. During this surgical procedure the remnant limb will get shorter, which might impede getting a new osseointegrated prosthesis or a socket. The patient will, after removal of the implant, be unable to walk for a long time. Other complications include mechanical failure. The abutment for instance can deform due to trauma. This can be repaired quite easily. A new abutment can be sterilely installed in minutes and does not require anesthetics [5]. When a patient stumbles or falls, bone fracture or splitting of the bone along its length can occur. This is a severe complication and usually requires the implant to be removed [4]. The abutment and the implant can be protected from excessive torque by a device called the ROTASAFE, which consists of two parts that can rotate relative to one another (Fig. 2.3). This device attaches the abutment to the leg prosthesis and prevents rotation of the abutment when a predetermined torque is applied by rotating its own two parts, thereby protecting the abutment from excessive torque. Fig. 2.3 the ROTASAFE [20] ESKA ISP Endo/Exo Prosthesis In 2001 Staubach et al. published a new osseointegrated percutaneous anchor for above knee implantation [21]. This ISP Endo/Exo prosthesis was developed by the firm ESKA implants GmbH & Co in Lübeck (Germany). The core of this implant is a surface-structured Chrome- Cobalt alloy (CoCrMo) pin that is implanted in the femur (no. 1 in Fig. 2.4). A metal bushing (no 2 in Fig. 2.4) with an identical surface closes off the medullary canal and provides an interface with the soft tissue. A conical adapter which is attached to a silicone cylinder is screwed to the end of the pin (no 3 and 4 in Fig. 2.4). The silicone cylinder is situated subcutaneously and ends in a titanium mesh (no 5 in Fig. 2.4). A percutaneous adapter (no 6 in Fig. 2.4) is attached to the conical pin with a screw. The device is, just like the OPRA implant, inserted in two surgical steps. There is at the moment of writing only one official publication, published in by Aschoff et al in 2009, in which promising results were reported in a group of 32 transfemoral amputees [7]. There were however infections reported (14 patients had infections) and three out of 33 prosthesis had to be explanted, of which two were reimplanted. Also Buell reported promising results of the treatment with the ISP Endo/Exo prosthesis in a dissertation [8]. 9

10 Fig. 2.4 ESKA ISP Endo-Exo Prosthesis [8]. On the left side a schematic overview of the location of the prosthesis is shown. 1: pin, 2: bushing, 3: adapter, 4: silicone cylinder, 5: mesh, 6: adapter. On the right side the skin protruding adapter is shown Current problems with the OPRA and the ISP systems Since there are little official publications concerning the results of the ISP Endo/Exo prosthesis it is difficult to compare the OPRA and the ISP devices. It is to be expected however that the ISP implant has the same problems, although perhaps in different levels of occurrence and severity compared to the OPRA, i.e. infection, mechanical failure and fracture of the bone. This is supported by the findings of Tomaszewski et al, who studied the mechanical consequences of implantation of both of the implants and the potential consequences on long-term remodeling using finite element models. In this study it was found that the ISP Endo/Exo prosthesis generated higher bone stresses, thereby increasing the bone fracture risk, but promoted physiological strain energy distribution, which favors the long term bone maintenance [22]. These results suggest that the ISP might be slightly favorable compared to the OPRA implant. An important problem with both the OPRA and the ISP implant include stress shielding, a process that occurs when the bone is subjected to stresses that are different from the natural situation. When a stiff implant, like titanium, is inserted in the bone, certain areas in the bone will endure a much lighter load than the natural load. According to Wolff s law bone resorption and bone remodeling will occur in these areas, which might lead to implant loosening [23]. Another problem with the currently available osseointegrated prostheis fixation devices is the contraindications for the patients. Both of the protocols exclude patients with diabetes and patients with diseases of the cardiovascular system [24], while these patients represent the bulk of the population of amputees. It should be noted though that fixation of a prosthesis by direct skeletal fixation implies that the patient wants to regain a lot of mobility. Elderly who suffer from vascular complications and/or diabetes might not be interested in being able to do sports and walk several hours per day. Therefore, the target group of patients consists of people who want to lead a normally active to active life. This will be further discussed in chapter 6, Requirements and whishes. 10

11 It can be concluded that there are several problems with the available osseointegrated prosthesis fixation devices. Infection, both superficial and deep, mechanical failure and bone fracture might require the implant to be removed. This results in substantial inconvenience for the patient, since the treatment takes a lot of time and requires a lot of effort from the patient. This is also a burden to their families and society. Other problems include stress shielding and the strict criteria which the patients have to fulfill. All of these problems and complications are comprehensively summarized in figure 2.5. The consequences of these complications are also presented in the figure. These include pain and reduced mobility which might lead to unemployment. Complications do not only lead to grief and disappointment but also to high healthcare costs. Therefore complications are both a burden for the patient with his/her family and the society, which result in suboptimal patient and community satisfaction with the current osseointegrated fixation devices. When a patient has an osseointegrated prosthesis fixation device, every normal day activity bears a risk; there is always a chance at complications. These complications can occur during normal activities like swimming, which increases the chance of infection, and running, jumping, heavy manual work or lifting weights in the gym, which bears the risk of mechanical failure or fracture. 11

12 Suboptimal patient and community satisfaction for the current treatments Burden to family and friends Financial burden for society Patient loses his employment Psychological damage High healthcare costs Reduced mobility, disability Pain Patient has to start the treatment all over again (two surgeries, long revalidation etc.) Implant has to be removed (surgery) and the femur bone has to heal Implant loosening might occur Risk of infection Superficial infection can escalate to deep infection Excessive loads can cause bone fracture Parts have to be replaced Bacteria can cause superficial infection Bone remodeling Excessive loads are transferred to osseointegrated fixture Mechanical failure Skin penetration enables bacteria to enter the body Stress shielding occurs A patient can experience excessive loads on prosthesis Fig. 2.5: Problem analysis 12

13 2.3 A new design In the Departments of Biomedical Engineering and Orthopaedics of the University Medical Center Groningen and the University Medical Center Radboud in Nijmegen a new osseointegrated prosthesis fixation device is being developed (Fig. 2.6). The main parts are a PEEK bushing and a titanium alloy pin. The bushing, which is relatively flexible, is inserted in the medullary canal of the femur bone and the skin penetrating pin is inserted into this bushing. This system ensures that the bending stresses are distributed over a large area of the femur bone compared to the OPRA en ISP systems. This should reduce the chance on fracture and prevent stress shielding, which causes bone remodeling[23]. The axial stresses are directly transferred to the cortex of the bone by the use of a collar. The collar is part of the pin, as can be seen in the figure, and a relatively flexible PEEK ring is put on top of the collar to prevent the bone from peak stresses. Bending stresses are transferred to the bone through the collar and through the bushing, using the spheres in the pin Fig. 2.6 Endo-Exo prosthesis as invented by the Departments of Biomedical Engineering and Orthopaedics of the University Medical Center Groningen and the University Medical Center Radboud in Nijmegen. 1: pin, 2:collar, 3:bushing. This design however has two shortcomings that need to be solved. The first shortcoming concerns torsion. Regarding the current design, the pin can revolve freely in the bushing, which is undesirable. Torque should therefore be transferred from the pin to the bushing and then to the bone. The amount of torque that has to be transferred will be calculated in chapter 6, Requirements and Whishes. The second problem is the pull-out problem. The pin should stay in the bushing, also when no ground force is acting upon the pin and when a patient stumbles. The pull-out forces that have to be transferred will also be calculated in chapter 6, Requirements and Whishes. 13

14 2.4 Summary of problems The main problems with current osseointegrated fixation systems are stress shielding, bone fracture, material failure and infection. The new osseointegrated prosthesis fixation system which is developed by the Departments of Biomedical Engineering and Orthopaedics of the University Medical Center Groningen and the University Medical Center Radboud in Nijmegen is expected to reduce stress shielding and fracture due to more advanced distribution of bending forces. The design however has two main shortcomings. These main shortcomings to face during this project are torque transfer of the pin to the bushing and prevention of pull-out of the pin. These two problems and the consequences are summarized in figure 2.7 Suboptimal patient and community satisfaction for the new design The patient is a burden for family and friends due to high need for care. Healthcare costs will be high, which is a burden for society The implant will have a high risk of getting damaged, which will lead to repair costs or even explantation, thereby decreasing the mobility of the patient The patient will experience a lot of discomfort during his activities The prosthetic leg, which is attached to the pin, is revolving during the walking cycle The patient is unable to walk and has an increased risk on infections. The pin can rotate freely in the bushing The pin will drop out of the bushing during activities Torque cannot be withstood Pull-out forces cannot be withstood Fig. 2.7: Summary of problems 14

15 3 Stakeholder analysis Stakeholder Characteristics, interests and expectations Potential, resources and capabilities Project Conclusions impact Patient Patients want to regain their mobility but differ in their expectations and demands. They want to be able to perform as much of their normal daily activities as possible and without pain. They also want as little risk of infection as possible and the fixation system should not fail or harm the patient The customer is always right. If the patient is not satisfied with the solution, then it won t be a success, resulting is less publications and little attention from companies and other research groups. The patient has to offer some of his time for tests and is capable of giving user ++ The best solution should be found in order to help the patient regain his mobility. This is for the best of the patient, the biomedical engineer, the (stress shielding, fracture etc). experience which can be used for improving the surgeon and the The costs should be as low as possible. Patients differ in the extent to which they want to regain mobility. Some only want to walk again. Others want to do sports etc. concept. department of biomedical engineering. The patient should be well studied, to get as much information Orthopedic surgeon Manufacturing companies The surgeons want to help their patients and provide the patient with the best care. Companies care about volume, profit and public image. The orthopedic surgeon is capable of installing the fixation device into the patient. When he does not believe that the solution will be in the patient s best interest, he will not install the system. The surgeon has experience and can give feedback and tips about improving the design or give feedback during the design process. Finally, the surgeon might have interesting connections with companies that can help in the design or manufacturing stage. Companies are capable of manufacturing the fixation system. The product volume will be relatively small, which makes the system of little interest for these companies. This product might however have a positive effect on the public opinion about this company. It could enhance the public image. as possible + The surgeon should be consulted during the design process. This might yield valuable information and increases the will of the surgeon to perform surgery. +/- Manufacturers must be found to produce the product. The manufacturer should be cheap, but deliver high quality. In the beginning, sponsoring might be an 15

16 Biomedical Engineers (University) Hospitals Insurance companies Relatives patient Society of Want to design a solution that enhances the quality of life of the patient the most. Give low priority to profit. Want to publish results in prominent journals and care about the image of the university and especially the biomedical engineering department. They expect to find a solution. Want to provide the patient with good care and have to make profit. Care about their image. Want to reduce healthcare costs, make profit and care about their image. Want to have the best solution for the patient. Want the patient to be as independent as possible Want the common goods to be allocated efficient and fair. Want the patient to participate in society and pay taxes as soon as possible, yet sustainable. Are capable of designing a product that solves the problem and thus helps the patient. More engineers increase creativity. Are capable of taking care of the patient and providing the environment for optimal revalidation. Hospitals have resources, but the costs have to be covered by the insurance companies. Houses the departments of orthopedics, and thus the surgeons. Have control over funding. Are capable of paying for the surgeries, the hospital and the product. Can provide free care and mental support to the patient. Can provide feedback on the design. Might be able to put pressure on surgeons and insurance companies and can alert the media. The society has an opinion about hospitals, insurance companies, manufacturers etc. They are capable of making or breaking these companies. interesting option. + Multiple biomedical engineers should be consulted during the design process to increase the amount of possible solutions. +/- Hospitals should be selected based on the surgeons that work in the department of orthopedics and their possibilities of providing revalidation. +/- If the new fixation system saves costs on the long term and preferably on the short term, insurance companies might be interested in funding. - Relatives should be well informed. Only then they can provide the best care for the patient and yield valuable information. - Society should be informed via the media when the tests are successful. 16

17 4 Goals In the previous sections the problems were defined and the stakeholders were identified. In this chapter the goals of the project will be defined. These goals are divided into main goals en subgoals. The two most important goals of the new fixation device are the reduction of bone remodeling caused by stress shielding and the prevention of fracture of the bone due to excessive loading. A secondary goal is to reduce the rate of infection in the percutaneous site. The new design seems to fulfill those goals, however, two goals remain open: The design must withstand torsion The design must withstand pull-out forces. These main goals can be complemented by sub-goals: The surgeon must be able to insert the device easily. Wear debris may not get into the bone- or soft tissue. It must be possible to replace the pin without replacing the osseointegrated bushing. Two analyses of objectives were performed to check whether the problems, as defined in chapter 2, will be solved. In the first analysis of objectives (analysis of objectives A, Fig. 4.1) it was checked whether designing a new device based on fixation by osseointegration will solve the problems of above knee level amputees. From this analysis it can be concluded that patients, family and the society will benefit from a new osseointegrated fixation system in which the risk at stress shielding, fracture and infection are reduced. In the second analysis of objectives (analysis of objectives B, Fig. 4.2) it was checked whether achieving the specific goals as defined in this chapter, will solve the specific problems that have to be solved during this project (i.e. torque transfer and prevention of pull-out) as defined in chapter 2.4. From this analysis, it can be concluded that patients and the community will benefit from enabling torque transfer and preventing pull-out. 17

18 Improved patient and community satisfaction for the new design Reduced burden to family and friends Reduced financial burden for society Reduced risk that the patient loses his employment Less psychological damage Reduced healthcare costs Reduced risk of decreased mobility or disability Reduced risk of pain Reduced chance at starting the treatment all over again Reduced risk of implant removal Reduced risk of Implant loosening Reduced risk of infection Chance that superficial infection will escalate to deep infection is decreased Excessive loads can cause bone fracture Less parts have to be replaced Chance at superficial infection is decreased Rate of bone Remodeling is decreased Less excessive loads are transferred to osseointegrated fixture Lower chance of mechanical failure Prevent bacteria from entering the tissue Stress shielding is decreased A patient is less prone to experience excessive loads on prosthesis Fig. 4.1 Analysis of Objectives A 18

19 Improved patient and community satisfaction for the new design The patient is not a burden for family and friends due to high need for care. Healthcare costs won t be high, which won t be a burden for society The implant won t have a high risk of getting damaged, which won t lead to repair costs or explantation, thereby not decreasing the mobility of the patient The patient won t experience a lot of discomfort during his activities The prosthetic leg, which is attached to the pin, will not revolve during the walking cycle The patient is able to walk and does not have an increased risk on infections. The pin cannot rotate freely in the bushing The pin won t drop out of bushing during activities Torque is withstood Pull-out forces are withstood Fig. 4.2 Analysis of Objectives B 19

20 5 Design assignment Now that the goals are set, the design assignment can be deprived. In general, different goals can be achieved with different design assignments. In this case however, the torsion loads and the prevention of pull-out are closely connected. When a certain mechanism is developed that prevents the shaft from pulling out, this will have an effect on the amount of torsion that can be withstood. The design assignment can be formulated as: Accomplish a method that enables the new osseointegrated prosthesis fixation device to withstand as much torsion as is necessary and prevent the pin from pulling out of the bushing. Demarcations The solution will be technical and will be bound to several demarcations, which limits the amount of possible solutions. The most important limitations are time and money. Only six months and a modest budget are available for designing the solution. This means that it is impossible to, for instance, develop a design based on relatively new and/or very complicated theories in physics or develop a solution that heavily relies on complicated artificial intelligence. A second demarcation involves the target group. Although the major part of the amputees consists of elderly who lost a limb due to vasculatory dysfunction, the target group for this device will be relatively young patients, aged 17-60, who lost a limb due to trauma. Elderly will not be excluded beforehand, but this group usually suffers more from complications and heals more slowly. Furthermore, elderly have in general a less active life compared to younger people and therefore have a lower need for a prosthesis that enables them to lead a very active life. Other patient criteria (and thus demarcations) include: The patient must have a maximum weight of 110 kg. The patient does not have serious vascular complications or serious complications due to diabetes. The patient reached skeletal maturity and has normal skeletal anatomy. The patient has a maximum age of 70 and does not suffer from osteoporosis. Patients are suitable for surgery. A final demarcation is that only the osseointegrated fixation device will be studied, not the external knee and the lower leg prosthesis. 20

21 6 Program of Requirements and Whishes The goals are set and de design assignment is clear. In this next step the requirements that the solution should comply to and the whishes that the design might fulfill are being quantified. In a later stage of the design process this list will be used as selection criteria for the concepts. The program of requirements and whishes is an extensive list and is therefore categorized into six categories; General, Safety, Mechanical, Materials and Other. 6.1 General Requirements and Whishes Function The final product should enable a transfemoral amputee to regain mobility. The most important function requirements for this project are the transfer of torque from the pin to the bushing and the prevention of pull-out. Requirements: 1 The device is able to withstand torque. 2 Pull-out of the pin is prevented. Whishes: 1 The device prevents overloading of the bone when high torsion or pull-out forces are applied, for instance due to stumbling or falling. Product life span The device will consist of several parts. One of these parts is the implant, which will remain into the bone due to osseointegration. This implant should have a lifelong life span. Lifelong differs per patient, but the implant should have a life span of minimally 40 years when a patient performs an average level of activity. Requirement: 3 The implant must have a life span of 40 years when a patient is averagely active. The replaceable parts have a lifespan of 20 years at minimum under these conditions. Whishes: 2 The device has a lifelong lifespan, even when the patient has a high activity level. The replaceable parts have a life span of 40 years under these conditions. Maintenance Requirements: 4 The implant does not require maintenance by the patient. Some maintenance might be performed by the surgeon when the replaceable parts are being replaced, for instance the removal of wear particles. Whish: 3 No maintenance is required. Costs The costs for one treatment which provides the patient with an implant with a lifelong lifespan may costs at maximum This is quite expensive but it is on the long term cheaper than buying sockets. To put it in perspective: the costs for an OPRA implant are about $38.000[25]. 21

22 Requirement: 5 The costs for the treatment are at maximum ,- Whish: 4 The costs per treatment are as low as possible. Dimensions The cross sectional geometry of the human femur was studied by Stephenson et al. in a donor population of 16 cadavers with an average age of 80.9 years. It was concluded that the medulla is almost cylindrical at the mid-third region and the most cylindrical between 35 and 50% of the total bone length, in which the 35% is the distal end. The medullary width is greater in anteroposterior direction (>90% of the maximum diameter) than in mediolateral direction (77-87% of maximum diameter). The average largest diameter of the medulla is about mm[26]. The cortex was also analyzed and it appeared that the anterior side is less thick than the medial and lateral sides of the cortex. The posterior side is the thickest. These findings suggest that the medulla is not situated centrally in the femur, but slightly anteriorly. The average thickness of the cortex on the anterior side was 4-5 mm, on the medial and lateral sides 5-7 mm, and on the posterior side 6-7. The cortex is the thinnest at 35% of the length of the femur and thickest at 60% of the total length[26]. The implant has to fit in the medulla and the cortex must stay as thick as possible to support the loads. Therefore as little bone as possible should be removed. Requirements 6 The maximum diameter of the bushing is 19.2 mm. 7 At maximum the thickness of the cortex of the bone is reduced by 20%. Whishes 5 As little bone as possible is removed. Weight Requirements 8 The maximum weight of the bushing and pin together is 1 kg. Whishes 6 The weight of the pin and bushing are as low as possible 6.2 Safety Requirements and Whishes Safety Requirements: 9 The fixation system can be safely installed by a surgeon. 10 The fixation system will not damage the bone (by stress shielding or fracture) or soft tissue to an unacceptable level when acceptable loads are applied to the system. 11 The chance of deep infection is at maximum 5% during the life span of the implant. 12 The fixation system does not have harmful side-effects like injuring the contralateral leg or the spine, induce sickness due to poisoning, or induce other negative effects on the patient due to blot clots, activation of the immune system etc. Whishes: 22

23 7 The fixation device does not harm the patient in any way. 8 The infection risk is (very near) 0%. 6.3 Mechanical Requirements and Whishes Strength A transfemoral amputee patient must be able to walk in a straight line on streets and on terrain, but also on ramps and on stairs. The patient should also be able to walk around corners, or circles, and turn around. A very important requirement is that the device should survive when the patient stumbles or falls. To determine mechanical requirements and whishes, it is necessary to know what forces act on the fixation system and how large these forces are. Different methods have been developed to measure the loads on the prosthesis in transfemoral amputees. Stephenson et al. used an inverse dynamic approach in which a motion analysis system and force plates were used to calculate the loads in patients who used a socket to connect the prosthesis to the body[27]. Frossard et al measured the loads in a patient with a socket and in a patient with an osseointegrated implant in separate studies [28, 29]. The authors used a force transducer situated between the socket or abutment and the prosthetic knee to directly measure the loads. Lee et al. used the direct measurement method to compare the loads in nine transfemoral amputees who used an osseointegrated implant. This study showed that managing stairs and ramps, and walking in circles does not significantly increase the load on the abutment of the OPRA system compared to straight-line level walking[30]. The results from these studies are summarized in table 6.1 Table 6.1: mean peak loads during the walking cycle for transfemoral amputees reported by various authors. F (%BW) is the force measured and M (%BW m ) is the moment measured, where BW is the body weight (N). The forces and moments are measured upon three axes, the longitudinal axis (L), the antero-posterior axis (AP) and the medio-lateral (ML) axis. M L is the torque. Various activities were measured (table shows maximum values); a: normal daily living, b: straight level walking, c: walking in a straight line on a smooth level surface, ascending and descending a slope and a set of stairs, walking around a circle, d: level walking. Various measuring methods were used; a,b: direct on abutment, c: direct on socket, d: force platform. F L F AP- F AP+ F ML M L M AP M ML- M ML+ BW mean %BW (standard deviation) %BW m (standard deviation) N (st. dev.) a Frossard et al ,0 1,8-1,2 7,2 833 b Lee et al (17) -9 (4.4) 12 (2.3) 11 (4.3) -0,6 (0.2) 2,6 (1.2) -2,5 (1.1) 1,1 (1.2) 809 (164) c Frossard et al ,3 4,9-2,0 2,6 613 d Stephenson et al ,0 4,3-18,4 14, The results from these studies show that the highest forces that the prosthesis has to withstand during normal daily activities can reach 128% of the body weight. When we set the maximum weight of a patient to 110 kg, the F L could reach about 1400N during a normal gait cycle. When the patient stumbles, even higher forces are to be expected. Bergmann et al. found that peak forces in the hip joint are approximately twice as high during stumbling as during normal activities[31]. It is to be expected that stumbling causes an increase in peak loads in the femur bone that is comparable to the increase in the hip joint. Therefore, the whish is that higher forces than the forces that are presented in table 6.1 can be withstood. The requirement is that the maximum patient weight is 110 kg. When taking the average the data of Frossard and Lee (2009 and 2007, measured directly on the abutment, in table 6.1 shown as a and b) and extrapolating 23

24 this to a 110 kg patient, the following forces have are to be expected during the normal walking cycle: F L average: ((120+83)/2)% * (110*9.8) = 1095 N F AP- average: ((-21+-9)/2)% * (110*9.8) = F AP+ average: ((19+12)/2)% * (110*9.8) = -162 N 167N F ML average: ((5+11)/2)% * (110*9.8) = 86N M L average: ((-2,0+-0,6)/2)% * (110*9.8) = -14 Nm M AP average: ((1,8+2,6)/2)% * (110*9.8) = 24 Nm M ML- average: ((-1,2+-2,5)/2)% * (110*9.8) = -20 Nm M ML+ average: ((7,2+1,1)/2)% * (110*9.8) = 45 Nm These forces are required to be withstood by the implant. During stumbling the applied loads will be higher, as shown by Bergmann, but it is not known how much higher exactly. The whish is that the loads that can be withstood without failure of the device are as high as possible. Concluding, the following strength requirements and whishes can be defined Requirements: 13 The device enables the patient to perform normal daily activities. 14 The device should survive when the patient stumbles or falls. 15 The maximum patient weight is 110 kg. The fixation device should be able to withstand the following forces: F L : 1095 N, F AP- : -162 N, F AP+ : 167 N, F ML : 86 N, M L (Torque): -14 Nm, M AP : 24 Nm, M ML- : -20 Nm, M ML+ : 45 Nm Whishes 9 The bending and normal forces, and thus patient weight and level of activity, that can be withstood are as high as possible. 10 The torque that can be withstood is as high as possible. Therefore the torque-induced stresses should be as uniformly distributed as possible. Pull-out The prosthesis may not disconnect during normal daily activities or sports. The pull-out forces that have to be withstood are not reported in literature and therefore have to be estimated. The heaviest parts of prosthesis are the knee, the lower leg prosthesis and the parts that connect these two. The weight of a lower leg prosthesis (foot+ankle) is in the range of g[32]. Knees weight about 1500g [32, 33]. Tubes and adaptors weight about 500g. This is in total (averaged) 3500g, which is about 35N. When a patient stumbles or gets his foot stuck behind an object the pull-out forces might be far higher. In a worst case scenario the foot of the patient gets stuck behind a rock and the patient pulls with his whole body weight on the prosthesis during his fall. The maximum patient weight is 110 kg, thus the device should be able to withstand 1100N. The whish is that the device can withstand a pull-out force that is as high as possible Requirements 16 The device can withstand a pull-out force of 1100N. Whishes 11 The pull-out forces that can be withstood are as high as possible. 24

25 Fatigue The average daily physical activity of healthy low active adults ranges from 5000 to 7500 steps/day[34]. Taking the lower limit as a reasonable estimate for the transfemoral amputee and an maximum implant usage time of 50 years, the implant requires to withstand 5000x365,25x50= cycles. Any replaceable part, which requires to work for 20 years, should be able to withstand 5000x365,25x20= cycles. The wish is that both the implant and any replaceable parts can be used for 60 years on an average activity level, which is about steps/day[34]. Then the system must withstand x365,25x60= cycles. Requirements 17 The implant must withstand loading cycles. 18 Replaceable parts must withstand cycles. Whishes 12 The number of cycles that can be withstood by the implant and the replaceable parts are as high as possible. 6.4 Materials Requirements and Whishes Materials In the design the main components of the fixation system will most probably be PEEK and a titanium alloy. These and other materials that will be used have to fulfill certain requirements and whishes. Requirements: 19 The materials are already used in orthopedics. 20 The materials are biocompatible. 21 The materials do not induce infection when the patient treats it according to a protocol. 22 The materials do not produce a hazardous amount of wear debris. 23 The materials do not fail under the forces and moments determined in chapter The fixation system can be sterilized. Wishes: 13 The materials do not induce infection at all. 14 The materials do not produce wear debris at all. 6.5 Other Requirements and Whishes Manufacturing Requirements and Whishes Requirements 25 It must be possible to manufacture the device 26 No new production facilities have to be built. 27 Packaging must be performed cheap and sterile. Whishes 15 Manufacturing is as cheap as possible. 16 Packaging is as cheap and sterile as possible. 25

26 Testing Requirements and Whishes Requirements: 28 The fixation system must be tested in vitro and pass. 29 The fixation system must be tested in vivo in an animal model and pass. 30 The fixation system must be FDA and/or EMA approved. 31 The fixation system must be tested in clinical studies and pass. 26

27 7 Function analysis In the previous sections the specific functions that should be performed by the device were formulated as: Make sure that torque is transferred and pull-out is prevented. To make sure that as many solutions as possible are found for these problems, the problem will be made more abstract. In this function analysis the basic functions of the device will be analyzed. Energy During the walking cycle the patient transports, absorbs, stores and converts energy by using his tendons, muscles etc. These actions exert loads on the fixation device. When the patient for instance converts potential energy that is stored into the stretched tendons to kinetic energy, a bending moment will be exerted on the fixation device. Storage, absorption and conversion of energy is already performed by the patient and the leg prosthesis. The fixation device only has to transport energy from the patient to the leg prosthesis and from the leg prosthesis to the patient. In figure 7.1 a schematic view of the function analysis is shown. Since the device does not have to perform any functions that are based on material or information, all the flow arrows represent energy. 27

28 Patient Fixation device Prosthesis. Store Transport Fig. 7.1: Schematic view of the function analysis. All arrows represent energy flows. 28

29 Part 2: Synthesis I In the Synthesis I phase a lot of ideas for solutions are generated and the best ideas are selected. The phase consists of the following sub-phases: Brainstorm: During the brainstorm lots of ideas are generated. Pre-concepts: The best ideas are further detailed and become pre-concepts Selection of the best pre-concepts :The best pre-concepts are selected 29

30 8 Brainstorm Now that the analysis phase is completed, it is known what the problem is, what the goals are and what the design assignment is. The requirements and whishes are known and the fundamental function is clear. This means that it is time to move on to the Synthesis I phase. In this phase many ideas will be generated and then brought back to about 10 pre-concepts. 8.1 Applied creativity method To gather as many ideas for solving the torsion and the pull-out problem as possible a brainstorm session was organized. Seven students with backgrounds in biomedical engineering, medicine, construction engineering, and Industrial Design attended to this brainstorm session. It was decided that the fundamental function as described in chapter 7, energy transport, was too broad for the brainstorm session. Instead two separate sessions with a bit more focus were held. In the first session the goal was to find ideas for the torsion problem. In the second session solutions were found for the pull-out problem. In both sessions it was allowed to contribute ideas that solve both problems at once. The results of these sessions are shown in table 8.1. All the ideas were sketched to make a better estimate of the feasibility. 8.2 Results Brainstorm Table 8.1: List of ideas as obtained from the brainstorm sessions Torsion 1 High friction due to relief (might be microscopic). 2 Velcro 3 Rubber-rubber interface 30

31 4 Bone tissue pin through metal pin. Breaks when too much force is applied to the cortical bone. 5 Mechanism like dotted tear-off line. It is strong but not in a certain direction, than it breaks when loads are applied. 6 Triangular pin 7 Square pin 8 Hexagonal pin 9 Octagonal/decagonal pin 31

32 10 Oval or triangular pin. Fail-safe when surrounded by springs in the bushing or elastic coating of the bushing. 11 Bone cement to fixate pin 12 Artificial ligaments connect pin to bushing but break when the torque exceeds a pre-defined limit to protect the bone from fracture. 13 Screw-thread on spheres of the pin. 32

33 14 A pin with a knob that can be pushed inward, is slid into a bushing with a hole. The pins can be pulled inward by a surgeon to disconnect the parts. The more pins, the better the torque is distributed over the bushing. 15 Springs that are tight to a pin with pads, to prevent pull-out and torsion, but release the pin at a certain torque. The pin could be either circular or non-circular. When the pin is noncircular, pads can be used. When the pin is circular, balls could be in between with little holes in the shaft and in the pin. 16 System similar to a torque wrench. 17 Coaster brake system 33

34 Pull-out 18 Hollow wall plug system, as used when something has to be anchored to a gypsum wall. Screwing can be performed at a certain distance from the protrusions. (easy to access for the surgeon) 19A system which is used in umbrellas. When the button is pushed, the ring disconnects. The button can be replaced by an Allen key. 20 A click system as used in phone chargers to click the pin into the bushing. A click system as used in bags/backpacks to click the pin into the bushing. 34

35 21 Sliding a knob through a rail. 22 A circular click connection like a water hose 23 System similar to a click pedal or ski-shoe release system 24 A cable to unfold protrusions like a ship in a flask or a stent in a blood vessel. 35

36 25 Tie-rip. 26 Rattle like drill or ratchet. 27 Expanding/contracting sphere based on heating/cooling or current. Sphere fits through hole in contracted state, but not in expanded state. 28 Memory metal pin that does fit through opening when deformed, but does not fit when returned back to original shape. 36

37 29 Filling a air-tight structure into the pin with air, to prevent it from pull out. 30 Deform the bushing with heating tools after the pin is inserted. 31 Claw that grabs a ball. The ball and the claw have recesses in which balls are located. They start to roll at a predefined torque due to flexibility of the claw. 32 Little hairs like a brush. 33 A click system as used in seats to change the height of the armrest to click the pin into the bushing. 37

38 34 teeth or spikes that hold the pin. 35 Elastic hinge 36 Screw or spring on top of the pin into the bushing 37 Put epoxy wax/glue/pur foam on bushing to prevent the pin from pull out. All of the ideas from the brainstorm sessions where categorized as either useful or not useful and they were divided in groups, as shown in table 8.2 and table 8.3 in which the ideas that were not useful are left away. The solutions for the torque problem and for the pull-out problem appeared to be dividable into two categories: based on multiple components and based on shape/deformation. There were also solutions that solve both the torsion and the pull-out problem, as showed in table 8.4. Table 8.2: Useful ideas for transferring torsion Based on multiple components High friction due to microscopic relief Bone tissue pin through metal pin. Mechanism like dotted tear-off line. Balls with spring that rotate. Based on shape Triangle pin Square pin Hexagonal pin Octagonal/decagonal pin Oval or triangle pin. Fail-safe when surrounded by springs in the bushing or elastic coating of the inside of the bushing. Bone cement to fixate pin. Table 8.3: Useful ideas for preventing pull-out Based on multiple components which Based on deforming component connect/disconnect Hollow wall plug system Expanding/contracting sphere based on heating/cooling or current. Sphere fits through hole in contracted state, but not in expanded state. A system which is used in umbrella s. Memory metal pin that does fit through opening when deformed, but does not fit when returned back to original shape A click system as used in phone chargers/bags/backpacks. Filling a air-tight structure into the pin with air, to prevent it from pull out Sliding a knob through a rail, like a tea-egg Deform the bushing with heating tools after the A circular click connection like a water hose System similar to a click pedal or ski-shoe release system A cable to unfold protrusions like a ship in a flask or a stent in a blood vessel Little hatch pin is inserted Put epoxy wax/glue/pur foam on bushing to prevent the pin from pull out. 38

39 Table 8.4: Useful ideas for both transferring torsion and preventing pull-out Useful ideas for both transferring torsion and preventing pull-out Artificial ligaments connect pin to bushing. screw-threads on spheres of the pin. A pin with a knob that can be pushed inward which is slid into a bushing with a hole. Springs that are tight to a pin with pads, to prevent pull-out and torsion, but release the pin at a certain torque. Claw with ball. Elastic hinge. Screw or spring on top of the pin into the bushing. Little hairs like a brush. Morphological scheme It is usual to make a morphological scheme in which the ideas per function or part are summarized and where sub-solutions are combined into different solutions for the whole problem. In this research project however, almost all of the torsion solutions can be combined with all of the pull-out problems, which generates an enormous amount of possibilities. Therefore it was decided that it is more practical to first develop the separate solutions further and then combine the most viable solutions later in the design process. This scheme can be found in chapter 10.2, pre-concepts. 39

40 9 From useful ideas to Pre-concepts The useful ideas are in a very early stage of development and are not ready yet to be scored based on the program of requirements and whishes. A selection had to be made though. Selection criteria included: feasibility, functioning and potential. Feasibility means that the solution will work as it should do and that it can be made with the current technologies. Functioning means that the solutions withstand the loads (i.e. torque or pull-out). Potential includes the expectancy of the solution to be a success. Scoring was performed with a 1-5 scale where 1 is very low and 5 is very high. The summarized scores of 9 and above are shown in bold and will be used as a preconcept. It should be noted that these selected ideas are only a solution to a sub-problem. When the pre-concepts are designed, multiple ideas or parts of ideas might be used to compose one preconcept. Table 9.1: Scoring of the useful ideas. The total scores of 9 and above are shown bold and will be used as a pre-concept. Feasibility Functioning Potential Total Torsion High friction due to microscopic relief. Bone tissue pin through metal pin. Mechanism like dotted line. Balls that start to rotate at certain torque. Triangle pin Square pin Hexagon pin Octagon/decagon pin Oval or triangle pin Fail-safe when surrounded by springs in the bushing or elastic coating of the bushing. Bone cement to fixate pin Pull-out Hollow wall plug system. Umbrella click system Phone charger click system. bags/backpacks click system. Sliding a knob through a rail. A circular click

41 connection like a water hose. Clipless bicycle pedal or ski-shoe release system. Cable/screw to unfold protrusions. Little hatches in bushing. Expanding/contracting sphere. Memory metal pin Air-tight inflatable structure into the pin. Deform the bushing with heating tools after the pin is inserted. Put epoxy wax/glue/pur foam on bushing to prevent the pin from pull out. Both Torsion and Pullout Artificial ligaments connecting pin to bushing. screw-thread on spheres of the pin. Pin with a knob that can be pushed inward, slid into a bushing with a hole. Springs push balls or pads to a non-circular pin in a circular bushing. Safety pal Claw and Ball Elastic hinge Screw or spring on top of the pin into the bushing. Little hairs like a brush It appeared that 22 solutions have a score of 9 and above. All of these ideas will be used as a preconcept. Some ideas however might be merged, because they have properties that can be combined or have a lot of features in common. 41

42 10 Pre-concepts Pre-concept 1: Transfer of torque from pin to bushing using non-circular shapes Various non-circular shapes can be used to transfer torque from the pin to the bushing. The two spheres in the design are the only parts of the pin that are in contact with the bushing, as can be seen in figure Therefore, the only parts of the pin that are made non-circular are the spheres, thereby leaving the distribution of forces as was intended with this design intact. Fig. 10.1: Endo/Exo prosthesis as invented by the Departments of Biomedical Engineering and Orthopaedics of the University Medical Center Groningen and the University Medical Center Radboud in Nijmegen. Possibilities for the non-cylindrical shapes include triangle, square, hexagonal, octagonal, oval and a star shape with blunted arms (Fig and Fig. 10.3). When a shape has more corners, the torsion forces will be distributed more uniform over the bushing. This would make an octagon preferable compared to a triangle. More corners however, increase the chance of rotation of the pin in the bushing, which causes wear particles to form. The amount of torque at which the pin starts to slip depends on the elasticity of the bushing. FEM analysis might yield the optimum number of corners. An overload mechanism can be installed at the lower end of the pin. Small torque limiters are commercially available. Fig. 10.2: several non-circular cross-sections of the pin that can be used to transfer torque; triangular, square, hexagonal, octagonal, oval, star with blunted arms 42

43 Fig. 10.3: Only the two spheres will be made non-circular. Table 10.1: Advantages and Disadvantages Pre-concept 1. Advantages Disadvantages Simple and relatively cheap Contact surface on pin might be to small High feasibility No solution for pull-out Little chance of failure No failsafe mechanism Uniform distribution of torsion forces 43

44 Pre-concept 2: Transfer of torque by using a ledge Torque can be transferred from the pin to the bushing by using a ledge or several ledges on the bigger sphere. The ledge runs parallel with the longitudinal axis and fits in a socket in the bushing. This prevents the pin from rotating freely in the bushing. The torsion forces are however very unevenly distributed over the bushing. More ledges can be added to improve the distribution. The ledge can also be used as a failsafe mechanism. When the torque exceeds a preset value, than the ledge breaks. This protects the femur bone from overload, but then a broken pin is situated in the bushing, which also is a problem. Fig. 10.4: Pre-concept 2. Left side: bushing with the notch in which the ledge fits. Middle: pin with ledge. Right side: a cross section of the pin in the bushing. It can be seen that the ledge fits in the notch, which will transfer torsional loads. Table 10.2: Advantages and Disadvantages Pre-concept 2. Advantages Disadvantages Simple and relatively cheap Pin might break High feasibility Uneven distribution of forces 44

45 Pre-concept 3: Transfer of torque and limiting maximum torque by using little balls Torsion can be transferred by using little balls that are situated between the pin and the bushing. The pin and the bushing both have small recesses. The more balls are used, the better the torque is distributed. However, the other forces (anterior-posterior, medio-lateral, and the compressive forces) will also get distributed over the balls. Depending on the elasticity of the bushing, this mechanism can also be used as a failsafe. When a certain torque is exceeded, the pin starts to roll. This however might yield wear debris. The torque at which the balls start to roll depends on the elasticity of the bushing. This might also prevent pull-out, but pull-out forces differ from the torsion forces, implying that the recesses must be designed very precise. Fig. 10.5: Pre-concept 3. Left: Bushing with the small recesses in which the small balls fit. Middle: Pin with recesses and balls in the recesses. Right: cross section of the pin in the bushing. Table 10.3: Advantages and Disadvantages Pre-concept 3. Advantages Disadvantages Innovative Many parts, thus expensive Accurate Wear debris Cell ingrowth should be prevented, since this can prevent the balls from rolling, and thereby enabling overloading of the femur bone.. 45

46 Pre-concept 4: Preventing pull-out by using a circular connection A circular connection can be used to prevent pull-out. The bigger sphere of the pin is replaced by two rings of which the upper ring is a little smaller than the lower ring. When firmly pushed, the pin clicks into the bushing and is able to withstand the pull forces applied to it during normal daily activities. When firmly pulled the pin disconnects, which might serve as a failsafe for preventing the femur bone from fracture. A click design like a gas hose might also be used to disconnect the pin. Fig. 10.6: Pre-concept 5. Table 10.4: Advantages and Disadvantages Pre-concept 4. Advantages Disadvantages Low-cost solution Not very accurate Firm, solid, mechanism. Won t break very easy Susceptible to wear Wear debris 46

47 Pre-concept 5: Preventing pull-out by using a click system, inspired by SPD clipless pedals Pull-out can be prevented by using a click system inspired by ski shoe bindings and clipless pedals used in cycling racing and mountainbiking. These bindings are clicked in easily and only release when the device is twisted. A similar device can be used to prevent the pull-out of a prosthesis. The surgeon clicks the upper part of the pin into the bushing, thereby locking it. The pin can be removed, for instance if it is bent, by twisting the pin. This can be prohibited with a pin or a switch. The surgeon hits the switch and twists the pin out easily. This system can also be used on the lower side of the pin easily don and doff the prosthetic leg from the pin. Fig. 10.7: Pre-concept 5. Left: Clicking mechanisms both in the top of the bushing and in the bottom of the pin. Upper right: side view, clicking mechanism works with a spring. Lower right: bottom view, device can be released when twisting. Table 10.5: Advantages and Disadvantages Pre-concept 5. Advantages Disadvantages Extensively tested system, and thus high safety Small scale compared to clipless pedals can be acquired. Quick doffing and donning of prosthesis Susceptible to wear Might induce peak stress 47

48 Pre-concept 6: Preventing pull-out by deformation of the pin or bushing Pull-out can be prevented by deformation of the pin or the bushing. The bigger sphere for example can be enlarged, thereby locking it into the bushing (Fig. 10.8). This might be possible by inflating the sphere, which can be achieved by surrounding the bigger sphere with a balloon. Another method is to cool the pin down, put it into the bushing en wait for it to warm up, after which it is stuck in the bushing. A third method for enlarging the bigger sphere is by using memory metal like Nitinol (NiTi). This can be deformed while cooled down after which it is inserted. The high body temperature enables the Nitinol to transform back to its old shape. Alternatively the bushing can be deformed, for instance under the bigger sphere or between the spheres, to prevent it from pull-out (Fig. 10.9). Deformation can be done by adding material when the pin is inserted. Material can be added by drilling a hole through the bone and the bushing or from inside the pin. It is however difficult to remove the pin when the material is added. Then a material which is solid but can be dissolved easily has to be used. Fig. 10.8: Pre-concept 6. The lower part of the pin is enlarged after insertion. This can be done by using a hot/cold transition, inflating the sphere or by using a memory metal like NiTi. Fig. 10.9: Pre-concept 6. An alternative way of using deformation is by deforming the bushing instead of the pin. Table 10.6: Advantages and Disadvantages Pre-concept 6. Advantages Disadvantages Strong connection Difficult to detach pin Low number of parts Bone may not damage, so bushing should absorb temperature differences. Low chance of failure In case of NiTi: fatigue might occur. 48

49 Pre-concept 7: Transfer of torque and prevention of pull-out by using a mechanism inspired by a shoulder joint The pin consists of two parts of which the top of the lower part consists of a ball. This ball is grabbed by a claw. The claw prevents the ball from rotation and tightly fits into the bushing, thereby transferring torsion loads from the pin to the bushing. In addition the claw and bushing might have some relief to achieve a better fit. Pull-out is also prevented. However, when the pullout forces or torsion loads are excessive, the ball pops out of the claw or starts to rotate, thereby preventing femur bone fracture. In addition, ball bearings can be used to facilitate rotation and pull-out. The tightness of the connection between the balls and the two parts of the pin (depending on the materials used and the thickness of the parts) determine when the ball pops out or starts to rotate. Fig : Pre-concept 7. Left: overview of the two parts of the pin, the balls and the bushing. Middle: the claw which is part of the upper part of the pin. Right: sideview of the two parts of the pin with a ball in between. The ball rolls when a lot of torque or pull-out forces are applied. Table 10.7: Advantages and Disadvantages Pre-concept 7. Advantages Disadvantages High safety, very low chance at bone fracture Difficult to manufacture Difficult to manage both torsion and pull-out fail safe. Might be easier to incorporate a failsafe in another part of the prosthesis. Expensive due to small parts 49

50 Pre-concept 8: Preventing pull-out and transferring torque by protrusions from the pin Protrusions can be used to lock the pin and prevent pull-out. These protrusions have to be in the pin when it is inserted and extend from the pin when it is in the correct position. The control over the extension and pull-in of the protrusions into the bushing can be performed using a long screw, air pressure or another system. The protrusions should be able to move back into the pin, for instance when the pin has to be removed. The protrusions can be little rods, but also balls or soft rubber pads can be used. Alternatively a piston as is used in an umbrella can be used. It should be found out which method enables the best force distribution. Fig : Pre-concept 8. Left: little rods are pushed out of the pin as the screw is twisted. Middle: a system similar like a gypsum wall plug is used to screw triangular protrusions out of the pin. In both systems the bushing has premade notches. Right: An umbrella-like system with a piston can also let the protrusions protrude. Table 10.8: Advantages and Disadvantages Pre-concept 8. Advantages Disadvantages Reliable system Risk at peak stresses on protrusions Quite cheap Surgeon can remove pin easily 50

51 Pre-concept 9: Preventing pull-out and transferring torque by various clicking mechanisms Various clicking mechanisms were invented during the brainstorm and many of them can be used as a pre-concept. The most promising and feasible systems include knobs and holes (Fig ), bag clicking mechanism (Fig ) and a mechanism which looks a bit like the system that is used to unfold a ship in a glass flask (Fig ). The knob and hole solution consists of one or more knobs in the pin that click into holes into the bushing. This can be performed using a spring or elastic rubber to push the knob. Another important aspect is removal of the pin. In order to remove the pin from the bushing, the knobs have to be pulled into the pin. Fig : Pre-concept 9. Left: overview of the knobs and hole system. The knobs can be pulled inward using the piece of wire. Right: knob and hole principle. The knob is pushed inward until it reaches a place where it can pop out. A second clicking mechanism consists of triangular shaped protrusions that extend from the pin and holes in the bushing. When the pin is slid into the bushing, the protrusions click into the holes. The click systems works similar to the closing mechanism of bags, backpacks etc. The protrusions are somewhat elastic and are pushed inward until the pin arrives at its location in the bushing. Then the stored energy causes the protrusions to extend out of the pin. Instead of elastic parts, a spring can also be used to push the protrusions to the outside of the pin (Fig ). Another clicking mechanism is shown in figure Several rods are connected to one piece of wire which can be pulled to click the rods in notches in the bushing. 51

52 . Fig : Pre-concept 9, mechanism 2. Left: triangular protrusions click due to a spring and can be pulled into the pin with a scissor-like system. Middle: triangular protrusions click due to elasticity of the triangles and can be pulled back into the pin with a wire. Right: cross section of the protrusions which have a triangular shape in a side view, but have a round shape in the cross sectional view. It can be seen that the protrusions fit into the bushing, thereby prohibiting pull out and ensuring torque transfer. Fig : Pre-concept 9, mechanism 3. Several rods are connected to one piece of wire which can be pulled to click the rods in notches in the bushing. Table 10.9: Advantages and Disadvantages Pre-concept 9. Advantages Disadvantages Easy for surgeon to place pin in correct Risk at peak stresses at protrusions position due to click. Quite cheap Might be difficult to get the pin out of the bushing when the material fails. Very quick installation (and removal) of pin 52

53 Pre-concept 10: Preventing pull-out by a long screw into the top of the bushing When a long rod with a wide end is inserted all the way through the pin and attached to the top of the bushing, it can prevent pull-out. There is a risk at peak stresses in the top part of the bushing, but if the connection between the pin and the bushing is made a bit bendable (and fatigue resistant), it will not absorb the bending forces, but only the pull-out forces. The rod can be screwed into the top of the bushing. The system can be made bendable by inserting a spring a elastic polymer between the rod and the screw which attaches the rod to the bushing. Fig : Pre-concept 10. A long screw with a broad end at which the pin rests is inserted through the pin into the top part of the bushing, where it is connected with a bendable rod or a spring. Table 10.10: Advantages and Disadvantages Pre-concept 10. Advantages Disadvantages No peak stresses around the bigger sphere due Risk at peak stresses in screw at top part if not to the lack of protrusions well connected Movement possible along longitudinal axis Risk at damaging bushing when overloaded 53

54 Pre-concept 11: Withstanding torque and preventing pull-out by high resistance in mediolateral and proximal-distal directions with a brush When the pin is covered in a brush of which the hairs a pointing towards two different directions, torsion forces can be transferred and pull-out can be prevented due to high friction. Half of the hairs of the brush should point in mediolateral direction and half of the hairs in lateromedial direction. All of the hairs should point a little in proximodistal direction to prevent pull-out. The inner surface of the bushing should not be smooth, but it should have some relief. Due to the directions in which the hairs are pointing towards, the pin can be inserted quite easily. It is a problem however to remove the pin, when necessary. This problem might be solved by constructing the brush of a material that can be dissolved quite easily. A solvent can then be inserted by the surgeon through a little canal. Fig : Pre-concept 11: Left: Hairs on the pin prevent rotation and pull-out. Right: a cross section of the pin in the bushing. The hairs, which are pointing in two directions cause a lot of friction. Table 10.11: Advantages and Disadvantages Pre-concept 11. Advantages Disadvantages No peak stresses around the bigger sphere due Difficult to remove pin to protrusions Little movement possible along longitudinal Risk at damaging bushing and brushes when axis overloaded 54

55 10.2 Combination of pre-concepts In chapter 10.1 pre-concepts were selected out of selected ideas from the brainstorm session. These pre-concepts are however only part of the final solution, which has to both solve the torsion and the pull-out problem. Therefore, it is useful to combine the ideas that solve the torsionproblem with the ideas that solve the pull-out-problem. In table 10.1 an overview of the possible combinations is shown. The potential concepts are marked green. The combinations that are not feasible or not logical are marked red. Pre-concepts 7 (ball and claw) and 11 (brush) can be used to solve both the torsion and the pull-out problem at once and do not have to be combined with any of the other pre-concepts. An overview with descriptions of the combined pre-concepts that are possible concepts, based on the results from table 10.1, is shown in table A concept should solve both the torque problem and the pull-out problem. Table 10.1 Overview of possible combinations of pre-concepts that solve the torsion problem and preconcepts that solve the pull-out problem. Green indicates potential concepts, red indicates unfeasible combinations. Pre-concepts that solve the torsion problem 1 Non-circular shapes 2 Ledge 3 balls in recesses between pin and bushing Pre-concepts that solve the pull-out problem 4 circular connection 5 click system, inspired by SPD clipless pedals 6 enlargement of the bigger sphere 8 protrusions from the pin 9 clicking mechanisms 10 long rod into the top of the bushing Table 10.2 Overview of possible concepts, derived from table 10.1 and chapter 10.1 Preconcept Description 1+4: Non-circular cross section to transfer torque with circular connection to prevent pull-out 1+5: Non-circular cross section to transfer torque with Clipless pedals-like click system 1+8: Non-circular cross section to transfer torque with protrusions to prevent pull-out 1+9: Non-circular cross section to transfer torque with clicking mechanism to prevent pull-out 1+10: Non-circular cross section to transfer torque with long rod to prevent pull-out 2+4: Ledge to transfer torque with circular connection to prevent pull-out 2+5: Ledge to transfer torque with Clipless pedals-like click system 2+6: Ledge to transfer torque with enlargement of bigger sphere to prevent pull-out 2+9: Ledge to transfer torque with clicking mechanism to prevent pull-out 2+10: Ledge to transfer torque with long rod to prevent pull-out Balls in recesses between pin and bushing to transfer torque with circular connection to prevent pull-out 3+4: 3+10: Balls in recesses between pin and bushing to transfer torque with long rod to prevent pull-out 7: Transfer of torque and prevention of pull-out using a mechanism inspired by a shoulder joint 11: Withstanding torque and preventing pull-out by creating a high resistance in mediolateral and proximodistal directions using a brush 55

56 10.3 Selection of combined pre-concepts The combined pre-concepts and the two pre-concepts that solve both the torsion and the pull-out problem were scored to find the best concepts. Scoring was based on the program of Requirements and Whishes (chapter 6). De pre-concepts are not well enough developed to use all requirements and whishes. Therefore, a selection of the most important requirements was made. Some of these criteria are more important than others. Therefore weight factors are used. The weight factor is a multiplication factor used on the original score. The weight factor ranges from 1-5 of which 5 corresponds with the highest importance. The scoring scale of a combination of two pre-concepts also ranges from 1 to 5 of which 5 corresponds to meeting the specific requirement very well. The requirements that will be used for scoring are: R1 R2 R3 R4 R5 R6 R7 Functioning: The device should transfer and withstand torque and prevent pull-out as calculated in chapter 6.3. A weight factor of 5 was assigned Safety: The device does not damage the bone or soft tissue and does not have harmful side-effects. A weight factor of 4 was assigned Installation: The device can be installed easily by the surgeon. A weight factor of 2 was assigned Mechanical: The device will not break, wear or fatigue. A weight factor of 3 was assigned Costs: The costs of the device and the surgical procedure do not exceed ,- A weight factor of 3 was assigned Life Span: The life span of the implant and the pin are 40 and 20 years respectively. A weight factor of 3 was assigned Feasibility: The feasibility of the idea. A weight factor of 4 was assigned 56

57 Table Scoring of combined pre-concepts. Scoring was performed based on the most important requirements. A score ranging from 1 to 5 was given to a pre-concept and a weight factor was added to differentiate between the importances of the requirements. Description Preconcept R1 R2 R3 R4 R5 R6 R7 Weight factor Non-circular cross section with circular connection Non-circular cross section with 1+5 Clipless pedals-like click system Non-circular cross section 1+8 with protrusions to prevent pull-out Non-circular cross section 1+9 with clicking mechanism Non-circular cross section 1+10 with long rod to prevent pullout Ledge to transfer torque with circular connection Ledge with Clipless pedals-like click system Ledge with enlargement of bigger sphere Ledge with clicking mechanism Ledge with long rod Balls in recesses between pin 3+4 and bushing with circular connection Balls in recesses between pin and bushing with long rod Mechanism inspired by a shoulder joint Brush on the pin Total Score 10.4 Conclusions It can be concluded that the highest scores (>90) are given to pre-concepts 1+8, 1+9, 1+10, 2+10 and 11. These pre-concepts will be used as a basis for the concepts, which will be developed in the next step of the design process; the Synthesis II phase. 57

58 Part 3: Synthesis II In the Synthesis II phase the selected pre-concepts are further detailed and become concepts, after which the best concept is selected. The phase consists of the following sub-phases: Concepts: The concepts are described into detail. Selection of the best concept. The best concept is selected based on the requirements and whishes. 58

59 11 Concepts 11.1 Concepts In the previous chapter five possible concepts were derived from the list of pre-concepts. In this chapter, these pre-concepts will be further improved and detailed in order to select a final concept. Some of the five selected pre-concepts show some similarities and might be grouped together as one concept. The derived concepts include the following pre-concepts: 1+8: Non-circular cross section to transfer torque with protrusions to prevent pull-out. 1+9: Non-circular cross section to transfer torque with clicking mechanism to prevent pull-out. 1+10: Non-circular cross section to transfer torque with long rod to prevent pull-out. 2+10: Ledge to transfer torque with long rod to prevent pull-out 11: Withstanding torque and preventing pull-out by high resistance in mediolateral and proximodistal directions with a brush Pre-concepts 1+8 and 1+9 can be grouped together because they are both based on a noncylindrical shape with protrusions, which come out of the pin by either stored energy (spring, hinge) or by mechanical work (screwing, pushing, pulling). This difference is acceptable enough to combine the two concepts into one concept. Pre-concept 1+10 and pre-concept 2+10 are both based on a pin which is penetrated by a long rod. The pin is not cylindrical due to ledges or due to a non-cylindrical shape. Therefore, these pre-concepts can be combined into one concept. This leaves three concepts. 59

60 Concept 1 Octagonal shaped pin with balls that click in recesses in the bushing Concept 1 is derived from pre-concepts 1+8 (Non-circular cross section to transfer torque with protrusions to prevent pull-out) and pre-concepts 1+9 (Non-circular cross section to transfer torque with clicking mechanism to prevent pull-out) and consists of a partly octagonal shaped pin with protruding balls that fit into holes in the bushing. The octagonal shape enables transfer of torque from the pin to the bushing in such a way that the torque is distributed over a relatively large area of the bushing. In the part of the pin that is octagon- shaped, two balls protrude. The balls are situated in a shaft, which contains, apart from the two balls, two elastic pieces of rubber and two titanium rods that can slide in the shaft and a third long rod that is situated partly in the shaft and partly in another shaft forming a T-junction in the middle of the octagon-shaped part of the pin (Fig.11.1). The long rod can be lowered into the pin, thereby enabling the balls to roll into the shaft and push the piece of rubber and the titanium rod into the shaft (Fig. 11.2). Alternatively, the three parts in the shaft (the ball, the piece of rubber and the small titanium rod) can be replaced by one part with the same shape which is slightly flexible. The balls might also be replaced by an ellipse-shaped unit for a better distribution of pull-out forces. Another alternative is to place the balls in the upper sphere instead of the lower sphere, because more PEEK is available around the upper sphere. Fig. 11.1: Concept 1 Fig. 11.2: The balls roll into the pin when the rod is pulled into the shaft 60

61 Fig. 11.3: Concept 1: Cross section. Left: The pin has an octagonal shape with a width of 12,5 mm. Right: The bushing has a diameter of 20 mm. The device is installed by first inserting the bushing into the medullary canal of the femur. Then the pin is inserted into the bushing. When the pin is placed at its correct position the long rod is screwed into the pin, thereby pushing the short rods, the pieces of rubber and the balls outward. The balls fit into the holes in the bushing, thereby preventing pull-out. The same procedure can be followed when the pin has to be removed. Bone damage due to excessive loads on the fixation device has to be prevented. Therefore the prosthetic leg should be equipped with a torque limiter and a mechanism that breaks when the pull-out forces or the bending moment are too high. Industrial torque limiters and failsafe mechanisms already are widely available. Distribution of forces The loads applied to the pin when a patient is walking and the distribution of axial and bending stresses are described in chapter 6.3. The axial stresses are directly transferred to the bone through the collar of the pin. A flexible layer is situated on top of the collar, which might cause a peak load in the balls when axial loads are applied. This is prevented by leaving a certain amount of space between the balls and the bushing and using a flexible material in the shaft next to the ball. This will enable the ball to be pushed a little into this piece of flexible material. When bending stresses are exerted, it is to be expected that a certain fraction of these stresses is also transferred to the bone via the collar. However, a substantial share of the bending stresses is transferred to the part of the bigger sphere of the pin that touches the bushing, which might cause a peak stress in that area. In concept 1, this potential peak stress will be greatly reduced due to the larger contact area between bushing and pin caused by replacing the bigger sphere of the pin by a flat sided octagon (as shown in Fig. 11.2). The corners of the octagon should be a bit curved, to ensure a 61

62 better distribution of bending stresses to the bushing. Peak stress on the balls will be prevented by the space between the balls and the bushing. Flexural stresses Because the bending moment that can be exerted on the fixation device is quite high, it is wise to calculate the flexural stresses in the pin, to be able to estimate the chance of failure of the pin. The calculation was done for two different shapes of the pin, rectangular and circular, to see how much influence the shape has on flexural stresses. From table 6.1 it can be concluded that the highest measured moment is about 45 Nm, which is the M ML Flexural stress can be calculated using the formula: My x I This equation is known as the flexure formula. σ x is the flexural stress, M is the bending moment (M ML ), y is the distance between the neutral axis and the point at which the stress is sought and I is the moment of inertia. To simplify things a bit, the flexural stress of a rectangular pin will be calculated. For a rectangular pin: 3 bh I 12 In the design, the area of the cross section of the part of the pin where the bending moment is exerted, is π 0,006 2 = 1, m 2, The dimensions of a square cross section with the same area as the circular cross section will be 4 1,13 10 =0,0106 x 0,0106 m. Using M=45 Nm and b=h=0,0106m the equation yields σ x = this is compared to a circular shaft, in which: 4 r I 4 45 ( / 2) =227 MPa. When Using M=45Nm and r=0,006m, the flexure formula yields 45 (0.006) = 265 MPa From these calculations it can be concluded that the flexural stresses, caused by the bending moment are a bit larger in a pin with a circular cross section, compared to a pin with a square cross section. It has to be mentioned though, that a square cross section means that the thickness of the PEEK has to be decreased in order to fit the implant into the medullary canal of the femur. Since titanium (Ti-6Al-4V) has a compressive strength of 758 MPa and a tensile strength of 896 MPa, the calculated values will be withstood by the titanium. The safety factor is

63 Shear stress Torque will yield shear stress. The amount of shear stress that has to be withstood by the pin, the bushing and the bone can be calculated using the torsion formula: Mr (1) J In this equation τ is the shear stress, M is the applied torque, r is the radius and J is the polar moment of inertia, which can be expressed for a solid shaft (a) and for a hollow shaft (b) as: r ( a) J o ( r ) ( ) o r b J i (2) 2 4 In these equations J is the polar moment of inertia, r o is the outer radius of the solid or hollow shaft and r i is the inner radius of the hollow shaft. The shear stress in the pin can be calculated using equation 1 and equation 2(a). Using M L =45Nm and r=r o =0.006m, the shear stress will be τ=133mpa. When the torque is transferred from the pin to the bushing the shear stress in the bushing can be calculated using equation 1 and equation 2(b). Using M L =45Nm, r=r o =0.010m and r i =0.006m, the shear stress will be τ=34mpa. When the torque is transferred from the bushing to the bone the shear stress in the bone can be calculated using equation 1 and equation 2(b). Using M L =45Nm, r=r o =0.012m and r i =0.010m, the shear stress will be τ=31.5mpa.these stresses are acceptable for Titanium, PEEK and bone. This will be calculated into more detail in chapter 13.3, the Finite element model analysis. Pull-out forces Pull-out forces have to be withstood by the balls and the bushing, which is in contact with the balls. The pull-out forces are not known and therefore were estimated to be 35N in chapter 6, the requirement and whishes. This force is acting upon the balls when the leg is lifted. The area of the bushing that is in contact with the balls is about 1 mm 2 per ball. When two balls are used, the total area is about 2 mm 2. The pressure that is exerted on the balls and on the bushing will be 35N/2 mm 2 = 17.5MPa. This is far beneath the compressive strength of PEEK (118). Since the endurance limit of PEEK is about cyclic loading will not damage the PEEK. When a patient stumbles or gets his foot stuck behind an object the pull out forces might be far greater. In a worst case scenario the foot of the patient gets stuck behind a rock and the patient pulls with his whole body weight on the prosthesis during his fall. In chapter 6 (requirements and whishes) it was calculated that the device should be able to withstand 1100N of pull. The pressure on the titanium balls will then be 1100/2 mm 2 = 550 MPa. This is quite close to the maximum compressive strength of titanium. Most probably a failsafe mechanism has to be developed in order to prevent the balls and the peek surface underneath from plastic deformation. 63

64 Concept 2: Circular pin with fins, attached in the upper part of the bushing Concept 2 is derived from pre-concepts 1+10 (Non-circular cross section to transfer torque with long rod to prevent pull-out) and pre-concepts 2+10 (Ledge to transfer torque with long rod to prevent pull-out). It consists of a circular pin and a bushing. The pin contains four fins on the bigger sphere that fit into recesses in the bushing, thereby transferring torque from the pin to the bushing. (Fig. 11.4) The pin is attached to the bushing via a long metal rod or wire which runs from the basis of the pin up to the top. The wire or rod is connected with a bold or a flange to the top of the bushing. (Fig. 11.5) The wire is also connected at the basis of the pin. (Fig. 11.6) This system prevents pull-out, but enables a little movement in longitudinal direction. To prevent a peak stress in the top of the rod or wire when bending forces are applied, the top part of the rod or wire should be a bit flexible. Fig. 11.4: Concept 2 Fig. 11.5: Top part of the pin and the bushing. A bolt prevents pull-out of the pin 64

65 Fig. 11.6: The wire or rod is also connected to the bottom of the pin, for instance with a nut Fig cross section of the pin with fins in the bushing 65

66 Distribution of forces The loads applied to the pin when a patient is walking and the distribution of axial and bending stresses are described in chapter 6.3. The axial stresses are directly transferred to the bone through the collar of the pin. The flexible layer on top of the collar might induce a peak stress in the fins. This can be prevented by enabling the pins to slide a few mm through the recesses in the bushing. The same sliding principle can be used when high bending forces are applied. This will ensure that the fins only have to withstand the torsion forces. A disadvantage of this system is that wear particles might be formed. An optimal shape of the fins has to be found in order to distribute the torsion forces as effectively as possible. In chapter 6.3 it was calculated that the required maximum torque that has to be withstood is 14 Nm. Concept 2 consists of 3 fins, which protrude for 2 mm and have a contact surface with the bushing when torque is applied of about 2 mm x 20 mm =40 mm 2. The centre of the fins is located at 6,5 mm from the centre of the pin (Fig. 11.7). 20 Nm therefore results in a pressure on the fins of (20Nm/6,5mm)/(3x40mm 2 )=25,7Mpa. This is acceptable for titanium fins, since the endurance limit of the titanium alloy is 613MPa. It is also acceptable for the PEEK bushing, which has an endurance limit of 28-41MPa. Pull-out forces have to be withstood by the rod or wire. It was calculated in chapter 6.3 that the pull-out forces during normal day activities are about 35N, but can reach 1100N during a fall. The area of the metal wire is about π* =1.77mm 2. The pull on the wire during normal walking will be 35N/1.77mm 2 =19.8 MPa. A titanium rod or wire can withstand these forces easily. When the whole body weight is applied, the pull on the wire will be (1100N)/1.77mm 2 =621 MPa. This is quite high since the tensile strength of titanium is MPa. [35]. A failsafe mechanism should be used to prevent failure of the wire or rod. 66

67 Concept 3: High friction between the pin and the bushing by using a brush Concept 3 is derived from pre-concept 11. (Withstanding torque and preventing pull-out by high resistance in mediolateral and proximodistal directions with a brush) This concept consists of a pin covered with short hairs that point in mediolateral and lateromedial directions and a bushing with some relief (Fig. 11.8). The connection between the pin and the bushing causes a lot of friction, thereby transferring torque from the pin to the bushing. Rotation is not possible due to this high friction. The hairs are pointing about 20 in proximodistal direction, thereby preventing pull-out as well. A small hole runs through the pin, which can be used to transport liquid from the basis of the pin up to the top. The pin can be removed by transporting a liquid through the pin that dissolves or lubricates the hairs on the pin, thereby greatly reducing the friction between the pin and the bushing. Fig. 11.8: Concept 3. The pin is covered with small brushes that point in both mediolateral and lateromedial directions. On the right the cross section of the lower sphere is shown. Alternatively, the pin is separated into two parts of which the upper part is only covered with hairs that point in proximodistal direction. This part prevents pull-out. The lower part transfers torsion (for instance with fins or by using a multicornered shape) from the pin to the bushing. The 67

68 upper part can be rotated separately from the lower part by using an axle that runs through a hole in the lower part. When the upper part is rotated, the hairs will be pointing in mediolateral direction, thereby enabling pull-out. Distribution of forces In chapter 6.3 it was calculated that the pull-out forces are about 35N during normal walking and can reach up to 1100N when falling/stumbling. To prevent pull-out the maximum magnitude of static friction f max should be at least 35N, as is showed in figure The f max is calculated using f N max s In this equation f max is the maximum magnitude of static friction coefficient, μ s is the coefficient of static friction and N is the normal force. To withstand higher pull-out forces the coefficient of friction μ s should be as high as possible which can be achieved using the hairs. Also, the normal force should be as high as possible. This is achieved by making the fit of the pin into the sleeve as tight as possible. The same principle applies for the transfer of torque. A higher μ s and N result in a larger amount of torque that can be transferred without rotating the pin in the sleeve. Fig The static friction f max should have at least the same magnitude as the pullout force F in order to prevent pull-out. 68

69 11.2 Selection of the best concept The best concept was selected using the program of requirements and wishes. All of the requirements and wishes were given weight factors, ranging from 1-5, which were determined by examining the goals en design assignment. The most important requirements and whishes were given high weight factors. Every concept was examined for the level of which it meets every requirement. Again a 1-5 scale was used. In table 11.1 the results of the scoring are summarized. Detailed scoring can be found in appendix 2 Table 11.1: Results selection of concepts. For every concept a score was assigned for all of the requirements and whishes, which all had different weight factors (see appendix 2). The scores were added up for the requirements and whishes separately and were then summarized into a total score. This score was divided by the amount of requirements and whishes and their weight factors, resulting in an average score per concept, ranging from 1-5. It can be seen that Concept 1 has the best score. Concept Score Requirements Score Wishes Total score Average score Concept 1: octagonal shaped pin with protruding balls Concept 2: fins on the pin, connected to the bush in the top Concept 3: brush It can be concluded that Concept 1, the octagonal shaped pin with balls that click in recesses in the bushing, meets the requirements and whishes the best. The most important requirements and whishes on which concept 1 scores better than concept 2 include the distribution of torsion, the lifetime (number of cycles) and the manufacturing process. Concept 2 scores a bit better on withstanding pull-out though. This implies that the withstanding of pull-out of concept 1 might be improved by combining it with concept 2. The scores of concept 3 were at most requirements and wishes equal or lower than the scores of concept 1 and 2. 69

70 Part 4: Synthesis III In the Synthesis III phase the selected concept is further detailed and becomes the final concept. Several alternatives of the final concept are considered and the final concept is tested using a Finite element model. Also a prototype is made and tested in vitro. The phase consists of the following sub-phases: Alternatives of the final concept. Several alternatives of the final concept are considered. Final concept: A final concept is developed based on the alternatives. Aspects like the materials which are used, the methods of production, the costs etc. are described into detail. FEM analysis: A finite element model analysis is performed to measure if the stresses that are generated in the final concept when loads are applied are acceptable. Prototype: A prototype is build Testing: The prototype is used in in vitro tests. 70

71 12 Final Concept: The alternatives 12.1 The alternatives The final concept consists of a PEEK bushing, which will be inserted in the femoral medullary canal, and a titanium pin, which will be inserted in the bushing. In the previous chapter concept 1, in which torque was transferred from the pin to the bushing using an octagonal shape and pull-out was prevented by protruding balls, was found to be the best concept. The main disadvantage of this concept was that the mechanism that prevents pull-out of the prosthesis might be difficult to realize, although theoretically feasible. The pull-out prevention system of concept 2 on the other hand scored quite high on withstanding pull-out forces. Therefore it might be interesting to look a bit more to concept 2 when it comes to preventing pull-out. To find the best solution, several alternatives of the final concept have been considered. Alternative 1 Alternative 1 consists of a non-circular shaped pin and bushing to transfer torque and protruding balls in the pin to withstand pull-out, as was derived from concept 1. (Fig. 12.1). A rod is used to push the balls outwards. When this rod is pulled down, the balls click into the pin again. Two titanium rods are the spacers between the central rod and the balls. In figure 12.2 the main dimensions are presented. In the synthesis II phase this concept scored high on torque transfer but relatively low on pull-out. Therefore more research concerning the feasibility was performed. It was calculated in chapter 11 that the pull-out stresses in the balls and the bushing reach about 18 MPa during normal walking and can reach up to 550MPa during stumbling. When the excessive loads due to stumbling are eliminated by using a failsafe mechanism in the prosthetic leg, the stresses in the balls and bushing are acceptable, as was explained in chapter 11. However, the balls are very small (diameter is 1 mm) and the bushing is a bit flexible. One extra risk that was found in this system includes the possibility that the ball pops out of the notch into the very limited space between the bushing and the pin, for instance during bending during which the bushing might elastically deform a bit. It was also checked whether it was possible to produce a prototype of this implant in the research instrumentmakerij (workshop) of the University Medical Centre Groningen. This was found to be possible, though it would be difficult to make the balls perfectly round. 71

72 Fig. 12.1: Final concept Alternative 1 Fig. 12.2: Dimensions Alternative 1 Fig. 12.3: Dimensions cross section at B (Fig 12.2) of Alternative 1 72

73 Alternative 2 Because the scores that were assigned to the requirements concerning pull-out prevention were relatively low, alternative solutions for pull-out prevention were developed. In alternative 2, the torque is transferred to the bushing using a non-circular shape again, but the pull-out is prevented by using a ball which can be screwed upon a rod. The ball has a large surface area and therefore distributes the pull-out forces over the bushing much more uniform than the small balls do in alternative 1. The bushing has to be opened in order to let the ball in, but can be closed off using a cap, which can be screwed or glued into the top of the bushing. A rod that runs through the whole pin is screwed into the ball and is connected with a bolt to the bottom of the pin, thereby enabling the surgeon to decide the amount of pressure that will be exerted on the osteotomy site of the bone. This design however has two disadvantages. The first is that the ball will cause shear stress at the ball-bushing interface. This stress will not be very high during normal daily activities, but it is there during every step. The wear properties of PEEK and titanium are quite good, but since the bushing will be in the bone permanently, wear particles might start to appear after a certain amount of time. A second disadvantage is that the pin is a bit shorter because of the relatively large amount of space that the ball occupies in the bushing. A Fig. 12.4: Final concept Alternative 2. On the left side the longitudinal section is shown. On the right side a cross section is shown at the height of A 73

74 Alternative 3 The third alternative is quite similar to the second alternative. It enables torque transfer by an octagonal shaped pin and bushing and pull-out is prevented by a rod which runs through the entire pin and is fixed in the top of the bushing. The difference with alternative 2 is that the ball is replaced by a screw with a flat head. While a ball shape as was used in alternative 2 generates a lot of shear stress, and thereby increases the chance at wear particle formation, a flat head generates normal stress during pull-out. Normal stress will generate less wear particles, which is an advantage compared to alternative 2. The contact area of the flat head with the bushing is however smaller than the contact area of the ball and the bushing. A Fig Final concept, alternative 3. On the left side the longitudinal section is shown. On the right side a cross section is shown at the height of A. 74

75 12.2 Comparison of the three alternatives Now that three alternatives of the final concept have been developed, one alternative has to be selected to be used as a prototype. It appeared that the ball-bearing system from alternative 1 should work in theory, but in practice there may be problems with manufacturing the small titanium balls. Also there are uncertainties about the risk of losing the balls when the bushing bends a bit. In alternative 2 and 3 an alternative mechanism was used to prevent pull-out. The usage of a rod which is fixed in the top of the bushing and in the bottom part of the pin might work very well. Calculations showed that the rod will be able to withstand the pull-out loads. The advantage of alternative 2 compared to alternative 3 was that the surface contact area between the ball and the bushing is larger than the surface area in alternative 3. The pressure which will be exerted on the PEEK in alternative 3 is, at a pull-out force of 1100N: 1100/ (π*(4,5mm- 1,5mm) 2 )=38,9MPa. The compressive strength of PEEK is about 118MPa, so a load of 1100N is safe. However, the endurance limit of PEEK is 28MPa, so the load of 1100N may not be applied to often. In fact it would be better to optimize the surface contact area. When the diameter of the flat head is increased from 9 mm to 12 mm, then the pressure on the bushing at a load of 1100N is: 1100/(π*(6mm-1,5mm) 2 )=17,3 MPa. This is a very safe value. A larger surface area is not necessary and therefore the advantage that alternative 2 has expired. Conclusion Since alternative 1 might cause problems with feasibility and Alternative 2 might generate to much shear stress, Alternative 3 was considered to be the best alternative. Therefore this alternative will be further developed to become the final concept and eventually the prototype. There is however one question that remains. All of the alternatives used the octagonal shaped pin and bushing for torque transfer. It is however not clear if this is the most favorable shape. Therefore it is wise to calculate which shape is the most optimal. Optimal means in this case that the forces should be as uniformly distributed over the bushing as possible. The optimal shape was calculated using Finite Element Model (FEM) analysis, which will be described in the next chapter. 75

76 rectangular pin 33-key circular pin 12.3 Shape of the torque transferring part The torque transfer of all of the three alternatives is realized by using a non-circular shape. To find out which non-circular shape is the most favorable, several shapes were tested using Finite Element Model analysis. These models consisted of a pin embedded in the bushing, both of length, l=35 mm. The pin was loaded with torque, T=10Nm. The bushing was clamped. The following material properties were assigned: E pin = 114GPa and E bushing = 4GPa. The poisson ratio of the pin was set to 0.3 and the poisson ration of the bushing was set to 0.4. The results are summarized in table The following properties were measured in the bushing: The peak normal stresses ( ), the shear stress ( ), the strain ( ) and the equivalent Von Mises stress, N which is a measure for the combined normal and shear stress. In the pin only the equivalent von Mises Stress was measured. Table 12.1 Results from the FEM analysis of the different shapes used for transferring torque. In the left column the technical drawing is shown. In the middle column the maximum stresses are shown. In the bushing the equivalent Von Mises stress, The peak normal stresses ( ), the shear stress N eq ( ) and the strain ( ) were measured and in the pin only the von Mises equivalent stress was measured. On the right side the stress distribution in the front side of the bushing is shown. Bushing: eq =108.8 MPa =68.5 MPa N =44.2 MPa = Pin area: A=98.8 mm 2 Pin: eq = MPa Bushing: eq =68.9 MPa =37.1 MPa N =36.0 MPa = Pin area: A=75.9 mm 2 Pin: eq = 94.6 MPa 76

77 Octagonal pin Hexagonal pin rectangular pin triangular pin Bushing: eq =111.8 MPa =70.7 MPa N =50.3 MPa = Pin area: A=69.3 mm 2 Pin: eq = MPa Bushing: eq =132.1 MPa =23.4 MPa N =61.5 MPa = Pin area: A=69.0 mm 2 Pin: eq = MPa Bushing: eq =76,8 MPa =16,8 MPa N =37,6 MPa =2, Pin area: A=73.8 mm 2 Pin: eq = 105 MPa Bushing: eq =116 MPa =12.8 MPa N =50.2 MPa = Pin area: A=75.8 mm 2 Pin: eq = 145 MPa 77

78 ellipse Pin area: A=77.0 mm 2 Bushing: eq = 129 MPa =28.0 MPa N =60.6 MPa = Pin: eq = 152 MPa From table 12.1 it can be concluded that there seems to be an optimum in the amount of corners when it comes to the von Mises equivalent stress. The von Mises stress in the triangle is high, the rectangle and the hexagon are lower and then the octagon is higher again. It seems that when there are more corners, the maximum normal stress is lower and thus the distribution of normal stress is better when more corners are used. The shear stress however works the other way around. More corners generate a higher shear stress and this leads to a higher von Mises equivalent stress. This explains why there is an optimum to the amount of corners used. It also seems that the higher the fillets are, the more shear stress is generated. This can be seen when both of the rectangular pins are compared. The radius of the fillets is increased from 1 to 3 mm and the maximum shear stress increases from 36MPa to 61,5MPa. Also an ellipse generated a lot of shear, resulting in a high von Mises equivalent stress. The triangle and the 3 key pin also generated high von Mises equivalent stresses. This was due to high normal stresses, which are caused by the tight corners. This leads to small contact surfaces and thus to relatively high peak stresses in these corners. Because shear stress is known to increase the chance at the generation of wear particles and wear particles are known to be undesirable because of their properties to induce bone resorption[36], it is best to choose a shape which generates low shear stresses. Another disadvantage of wear particles is that the play in the bushing increases, which might cause the pin to wobble a bit. Also, the normal stresses and thus the von Mises equivalent stresses should be as low as possible. This findings taken in consideration, it can be concluded that the rectangular and the hexagonal shape with a small radius are the most favorable shapes. Although the von Mises equivalent stress is a bit higher in the hexagonal shape compared to the rectangular shape, the hexagonal shape is considered to be the best choice for the purpose of this project. This choice was made because the shear is only a bit higher in the hexagon, but the normal forces are a lot lower. Also the rectangular shape takes a bit more space, which leads to a thinner wall of the bushing. This is undesirable since it weakens the bushing. 78

79 13 Final concept In the previous chapter three alternatives were considered and based on the comparison of the alternatives (chapter 12.2) and the FEM analysis of the torque models (chapter 12.3) a final concept was developed. The temporary name was chosen to be Profix; PROsthesis FIXation. The main parts of the final concept include a pin and a bushing. The bushing is made of PEEK, a relatively flexible material, and is inserted in the medullary canal of the femur completely. The pin is made of a titanium alloy, which is stiff, and is inserted in the bushing. Due to the combination of the stiff pin and the flexible bushing, the implant is very strong, yet distributes the loads relatively uniform over the bone. The pin has a collar which ensures that a large part of the normal forces is exerted on the osteotomy site of the bone. Because the bone is relatively distally loaded with this implant, the stresses in the bone resemble the natural physiological stresses. This should reduce severe bone remodeling, a problem that was explained in chapter 2. The pin is implanted in two steps. During the first surgery the bushing is inserted into the medullary canal of the remnant femur. During the second surgery the pin is inserted in the bushing. The time gap between the two surgeries is approximately 2 months. This time gap is needed for wound healing and early osseointegration. After the second surgery, during which the pin is inserted, the implant has to be loaded by the patient gradually. The length of the intramedullary part of the implant is 73 mm. The diameter of the hole which has to be drilled in the medullary canal has to be 19.2 mm. Because there is quite a lot of variation in the thickness and the length of the femur bone between individual patients[26], it might be necessary to develop the implant in different sizes. Bushing 19,2 mm Pin 73 mm 108 mm Fig. 13.1: The final concept 79

80 13.1 Design of the final concept The final concept consists of 6 parts, as is shown in figure These parts are the top screw (1), the bushing (2), the rod (3), the pin (4), the PEEK ring (5) and the nut (6). Together these parts ensure that the torsional loads can be withstood and that pull-out is prevented. Torsional loads are transferred to the bone via the hexagonal shape of the pin (4) and the bushing (2). This prevents free rotation of the pin in the bushing. Pull-out is prevented by the rod (3), the top screw (1) and the nut (6). In figure 13.3 it is shown how these three parts are connected. The top screw is screwed in the bushing. Then the rod is screwed into the top screw and the nut is screwed to the lower part of the rod. The more the nut is tightened, the more pressure is exerted by the collar (with the PEEK ring) on the osteotomy site of the bone. A complete set of technical drawings of the whole implant (that were sent to the workshop to discuss the final concept) can be found in Appendix 3. The bushing is inserted in the medullary canal of the femur completely, as can be seen in figure Top screw 2 Bushing 3 Rod 5 PEEK ring 4 Pin 6 Nut Fig. 13.2: Exploded view of the final concept. 80

81 A Fig Technical drawings of the final concept. Left: Longitudinal section of the final concept. The bushing is located in the medullary canal of the femur bone and the osteotomy site rests on the collar. On the right side a cross section at the height of A is shown. Bushing The bushing consists out of 1 part and is made of PEEK (polyetheretherketone). PEEK is about 4 times as flexible as cortical bone, which enables some elastic deformation of the bushing. This property is used to ensure a uniform stress distribution when high loads are applied. The lower part of the bushing has an hexagonal shape. The pin fits exactly in this shape, thereby enabling torque transfer. In this final concept the hexagonal part has a length of 30 mm. This was the maximum length that can be acquired in the workshop, because the fraise that cuts the hexagonal shape into the bushing should be very thin, since the radius of the fillets is 2 mm. The maximum length of a fraise that thin is 30 mm. The top part of the bushing in which the top screw is screwed has M12 0,5 mm thread. This yields eight pitches of thread which helps the flat surface underneath the top screw to withstand the pull-out forces. The outside of the bushing has V- shapes to ensure a better osseointegration process. The bone can grow into these recesses, which protects the bone and the fixation device from pull-out. The bushing will be coated with two coatings. On the outside of the bushing a trabecular titanium coating will be plasma spayed. This coating is known to improve the biocompatibility and osseoconductive properties of PEEK[37]. A hydroxyapatite coating will be sprayed upon the titanium coating the speed up the early stage of the osseointegration process[38]. More information about the coatings will be given in chapter The bushing has two openings and both openings should be sealed to ensure that biological fluids and cells cannot enter the bushing and wear particles cannot leave the bushing. This seal could be made of two silicone rings; one is situated in the 1 mm protrusion around the top screw and one is situated between the collar and the lower end of the bushing. 81

82 A Fig. 13.4: Technical drawings of the bushing. In the upper left corner the isometric view is shown and the location of the V-shapes is specified. In the bottom left corner the cross section is shown at a height of A. On the right side the longitudinal section A-A of the bushing is shown. Pin The pin will be made of the titanium alloy Ti-6Al-4V, also known as grade 5 or medical grade titanium. This alloy is frequently used in orthopedic surgery and is known for its high compressive and tensile strength and its good biocompatibility [4]. The part of the pin that fits into the bushing consists of two sections (Fig. 13.5). The hexagonal shaped section fits in the hexagonal shaped section of the bushing, thereby enabling torque transfer. This section cannot be longer than 30 mm due to limitations to the production process of the bushing as was described earlier. The top part therefore is cylinder shaped. In the original concept two spheres were used to distribute the loads. It was chosen to flatten these spheres, because the contact area of the spheres with the bushing is too small. The collar is used to transfer the normal forces directly to the cortex of the bone, instead of to the top of the implant. Because the cortex of the bone is loaded, resorption of the cortex is prevented. A hole through which a rod is inserted is drilled through the entire pin. The rod is attached to the top of the bushing and to the bottom of the pin. In the top of the pin the hole is made conical, which was done to prevent the rod from getting loaded during bending of the pin. These loads should be either transferred to the osteotomy site of the femur bone or to the tip of the pin, but not to the rod. 82

83 Fig. 13.5: Technical drawings of the pin. In the upper left corner the isometric view is shown and in the bottom left corner the top view is shown. On the right side the longitudinal section A-A of the pin is shown. Rod, Screw and nut The rod, the screw and the nut (Fig. 13.6) have to prevent pull-out. This is done by screwing the top screw into the bushing, screwing the rod into the top screw and twisting the nut onto the bottom part of the rod. The rod runs through the pin and the nut prevents the pin from sliding of the rod. The rod, the screw and the nut will be made of the Ti6Al4V alloy, just like the pin. The top screw has M12 0.5mm thread to screw it into the bushing. It has to be taken into account that the top screw is screwed into the bushing until the bottom of the screw touches the bushing firmly. If the screw is not tightened then all of the pull-out forces are applied to the screw thread instead of the plateau in the bushing. The rod has M4 0.5 mm tread on the top site and M3 0.5 mm on the bottom part. This ensures that the connection on the top part is stronger than the connection on the bottom part. This was done as a last-resort fail mechanism. If the pull-out forces are too high, then the lower thread with the nut is the weakest link and will fail. This is better than when the 83

84 thread inside the top screw fails, since the top screw cannot be replaced ever and the rod and the nut can be replaced quite easily. All of the treads have the same alignment. This means that if the nut is tightened, then the other two screw connections also tighten. The surgeon can tighten the nut with a torque wrench to set the amount of pressure that is exerted on the osteotomy site (by the collar) precisely. It has to be found out what the optimal preload on the bone is. It is known that the muscles and ligaments also exert a preload on the bone. The nut has a relatively flexible PEEK interior, which prevents loosening of the nut due to vibrations caused by using the implant. The nut can only be used once. If the surgeon wants to loosen it a bit, the PEEK inside the nut will be worn and the nut has to be replaced. Fig. 13.6: Technical drawings of the top screw, the rod and the nut. On the left side the isometric view is shown and in the middle the longitudinal section AA is shown. On the right side the bottom view of the rod (upper drawing) and the screw (lower drawing) are shown. PEEK ring A PEEK ring is placed upon the collar of the pin. The main function is to distribute the stresses more uniformly to the bone than the titanium collar can do. This can be performed due to the flexibility of the PEEK ring, which has the same flexibility as the bushing. The ring will osseointegrate to the osteotomy site. This is achieved by not connecting the collar of the pin to the 84

85 PEEK ring, which is press-fitted on the pin. By doing this, only the normal compressive forces are transferred from the collar to the bone, thereby strengthening the bone. The tensile forces which for instance may arise due to bending will not be transferred to the peek ring, because it is not connected to the collar. This prevents the peek ring from tearing off from the osteotomy site. Fig. 13.7: Technical drawings of the PEEK ring. In the upper left corner the isometric view is shown and in the bottom left corner the top view is shown. On the right side the longitudinal section A-A of the pin is shown. Surgical procedure It was already mentioned that two surgical operations are necessary for implanting the implant. In the first stage the bushing with the top screw are inserted and in the second stage the pin and the rod are inserted. Before the first stage operation, the remnant femur bone has to be analyzed using a 3D CT scan to see if it is suitable for operation and to select an appropriate implant size. The rest of the surgical preparation is that of any other major orthopedic procedure where a prosthetic material is implanted. Intravenous antibiotics will be administered perioperatively together with low molecular weight heparin[4]. During the first surgery the bushing with the top screw is implanted. First the scar is reopened and the medullary canal is exposed. Then, after the position of the boring device is checked with the CT image, the medullary canal is carefully bored out. There are surgical tools available that can bore a hole perpendicular to the osteotomy site. The alignment of the bore has to be checked frequently to make sure that the hole is straight. When the hole is drilled the bushing is inserted 85

86 up to 3 mm from the osteotomy site. A silicone plug will be inserted in the bushing to prevent leakage of biological fluids and cells into the bushing. Also the PEEK ring will be attached to the bone already, to let the bone osseointegrate into the ring. The second stage surgery will be performed approximately two months after the first stage. In these two months the wound is healed and the osseointegration process is started. It is not yet clear what the optimum time between the two stages of surgery is. For comparison: The OPRA system starts the second stage after six months and the percutaneous part of the ISP Endo/Exo prosthesis is inserted after only four to six weeks. The optimum time is dependent on the healing process and the speed of the osseointegration process. This has to be tested first in for instance an animal study. During the second stage surgery the wound is reopened and a hole is made in the skin for the pin. The silicone plug is pulled out of the bushing and the rod is screwed into the top screw. Then the pin is inserted into the bushing. The nut is applied and tightened with a torque key and the surgeon can decide how much torque he will apply. The more torque is applied on the nut, the more pressure is applied on the cortex of the bone by the collar. Optimal values for these loads have to be found during the test phase of the product. 86

87 13.2 Materials used Titanium alloy The pin, top screw, rod and nut will be made of a titanium alloy Ti6Al4V, also known as grade 5 titanium or medical grade titanium. This alloy consists for 90% of titanium, 6% Aluminium and 4% Vanadium. It is known for its high compressive and tensile strength ( MPa and MPa respectively [35]), its high endurance limit ( MPa [35]) and its great biocompatibility and it has been used in orthopedic surgery for decades [4]. Because of its high strength, good biocompatibility, good wear resistance and because it is tested extensively and well known among surgeons, titanium is the best choice to use as a material for the pin, top screw, rod and nut. More information about the mechanical properties of the titanium alloy can be found in appendix 4 An alternative for the titanium alloy could be a cobalt chromium molybdenum alloy. This alloy is also widely used in orthopedics, especially in prostheses for the hip, knee and shoulder [39]. CoCrMo also has excellent mechanical properties and is biocompatible. The main difference between the titanium alloy and the CoCrMo alloy are the Young s modulus and the wear resistance. The Young s modulus of titanium is 114GPa and the Young s modulus of the CoCrMo alloy that is mostly used in orthopedic surgery is 248GPa [40]. This means that CoCrMo is about twice as stiff as titanium. The wear properties of CoCrMo are reported to be superior to titanium [39]. For the purpose of this implant good wear properties are a pre, but the high stiffness might be too high. When the pin does not bend at all, it is too stiff and it might damage the bushing when high stresses are applied. Therefore the titanium alloy is preferred to cobaltchromium. One disadvantage of titanium however is its relatively high friction coefficient. This coefficient differs with the method of production, but can reach up to 0.8. Therefore it might be necessary to coat the titanium parts. This will reduce wear of the bushing and of the titanium itself. A coating which is used already in orthopedics is the Diamond-like-coating (DLC)[41]. This can be sprayed onto the titanium parts if necessary to reduce friction and prevent the bushing from wear. PEEK The bushing and the ring of the implant will be made out of polyetheretherketone (PEEK). PEEK was chosen because it has a suitable Young s modulus ( GPa), a high compressive strength ( MPa), a high tensile strength (70-103MPa), a good endurance limit (28-41MPa), good wear properties, it is biocompatible and it already is extensively used in orthopedics [35, 42]. It is also easy to machine, which has a positive effect on the accuracy of the clearance between the pin and the bushing. More information about PEEK can be found in appendix 5 Another plastic that is often used in orthopedics is ultra high molecular weight polyethylene (UHMWPE). This plastic has a Young s modulus of GPa and is thus more flexible than PEEK. The effect of this higher flexibility on the force distribution in the bushing is not known, but could be tested in FEM. However, UHMEPE also has a lower compressive strength ( MPa), a lower tensile strength ( MPa) and a lower endurance limit ( MPa) [35]. These properties make UHMWPE less suitable for usage in the bushing than PEEK. Although the wear properties of UHMWPE are about the same as PEEK s, the UHMWPE would generate wear particles more easily because the endurance limit (and possibly also the compressive strength) will be exceeded occasionally, thereby damaging the material. 87

88 It was already mentioned that titanium has a high friction coefficient. If the pin causes a lot of friction inside the PEEK bushing, wear particles can be formed. The pin can be sprayed with a DLC coating to reduce friction, but if this is not sufficient and in vitro tests show that wear particles are forming, than it might be necessary to use carbon fiber reinforced PEEK (CFR- PEEK). CFR-PEEK has better wear properties than PEEK (although PEEK by itself already has good wear properties) but it is also less flexible. CFR-PEEK has a Young s modulus of about 12 GPa [42] and thus approaches the flexibility of cortical bone. This might have a negative effect on the stress distribution, which might get less uniform. It should be tested if it is necessary to use CFR-PEEK instead of PEEK Titanium coating Titanium is known to have good osteoconductive properties and is therefore extensively used in orthopaedics[4]. The osteoconductive properties of PEEK are known to be suboptimal [42] and therefore it was decided to use a Ti coating to improve these properties. A titanium coating can be applied to PEEK using electron beam deposition [37] or plasma spay deposition [43]. In vivo results show that Ti-coated PEEK implants have better osteoconductive properties than uncoated PEEK [37]. The mechanical properties of PEEK do not change much. The tensile strength is reduced by 4,6%, flexural strength is reduced by 7,0% and the Young s modulus is increased by 17% [43]. The Ti coating will have a thickness of about 0,2 mm. Hydroxyapatite coating On top of the titanium coating a Hydroxyapatite (HA) coating will be sprayed. The HA-coating is known to enhance bone apposition and bone ingrowth and is extensively used in orthopedic surgery. Especially on the short term, the osseointegration rate is increased [38, 44]. After some time the HA will be replaced by bone, and then this bone can attach to the titanium coating, which is located underneath the HA coating. The HA coating of PEEK has minor effect on the material properties. The tensile strength is reduced by 5,5% and the Young s modulus is increased by 13% [45]. 88

89 13.3 Finite Element Model (FEM) Analysis In the requirements and whishes, chapter 6, several requirements with respect to withstanding forces were defined. It is important to know the magnitude of the stresses that are generated when the device is loaded. To analyze the distribution of stresses in all of the parts of the final concept a finite element model (FEM) analysis was performed. The device was loaded with torque, pull-out and bending. Torque and pull-out were applied because withstanding these two forces was the main goal of the thesis. Bending was tested because this is the most demanding load that the implant has to face when it is used in the clinic. After analyzing the results a second version of the implant was developed and this version was tested again using FEM. In a final FEM the second version was tested in a bone with a realistic load case The model version 1 All of the parts of the final concept were used in the model. Only the rod and the top screw were fused to make the model a bit less straining for the computer server. This did not have any effect on the usefulness of the model since the connection between the top screw and the rod is not the weakest link. The parts that are most prone to fail due to pull-out are the nut and the bushing. The model consisted of around noded tetrahedral elements, which were distributed equally among the model (Fig. 13.8). The material properties of the titanium parts were as follows. The Young s modulus was set to E=114GPa and de Poisson s ratio was set to 0.3 For the PEEK parts the Young s modulus was set to E=4.0 GPa and de Poisson s ratio was set to 0.4 The parts all had frictional contact, except for the top screw and the rod, which were glued to the top of the bushing (This resembles the screw-connection) and the nut, which was glued to the rod. The friction coefficient between titanium and titanium was set to 0.8 and between PEEK and titanium 0.5. PEEK parts are never in contact with each other. The software package that was used was Marc (MSC Software Corporation, Santa Ana, CA, USA) Torque The model was loaded with 20Nm of torque, which is a bit higher than the required torque (which is 14Nm) The torque was applied to the lower end of the pin and the bushing was fixed. The results showed that all of the parts can withstand the load. The maximum von Mises equivalent stress, which is a measure for the combined normal and shear stress at a certain point, was found in the pin and was 277MPa (Fig. 13.8). The titanium pin should be perfectly capable of withstanding this, since the tensile and compressive strength of the titanium alloy are MPa and MPa respectively. This is also below the endurance limit of the titanium alloy, which is MPa. The maximum von Mises equivalent stresses in the rod+top screw and in the nut were 69 MPa and 0.90 MPa respectively. The most vulnerable part during torque would be the bushing. From the model it can be seen that the load is uniformly distributed over the six sides of the hexagonal shape (Fig. 13.8). The maximum von Mises stresses that was found in the bushing was 45 MPa. This is safely below the maximum tensile and compressive strength ( MPa and MPa respectively). It exceeds however the endurance limit slightly. The endurance limit of PEEK is MPa. Although 45MPa is only a peak stress (the average von Mises stress in de bushing ranges from MPa), there is a danger of deformation when the stress is applied often. A torque of 20Nm however is quite a lot and it is not expected the device will be loaded with 20Nm that often. 89

90 Fig Results FEM analysis of the final concept with a load of 20Nm torque. In the upper left corner an overview of the model is presented, in which it is shown that the highest stresses (yellow) are located in the pin (longitudinal section, side view). In the upper right corner the pin is shown without the rest of the model (longitudinal section, side view). In the lower left corner the rod is shown. It can be seen that the stresses are relatively low and concentrated at the level of which the pin is also the most heavily loaded (longitudinal section, side view). In the lower right corner a longitudinal section of the hexagonal part of the bushing is shown. It can be seen that the loads are distributed uniformly (longitudinal section, slightly tilted side view). The nut was not displayed because the stresses were very low. 90

91 Pull-out The pull-out model was slightly different from the other models. It was chosen to split the model into two models, as is showed in figure 13.9 and figure In the first model only the bushing and the rod+top screw are loaded and analyzed. In the second model only the pin, the rod+screw and the nut are loaded and analyzed. The model was split because it s more practical to analyze both of the parts that are most prone to fail separately. Also, the data is better to interpret when the model is less complex. The parts that have the highest risk of failure are the screw-thread on the bottom side of the rod and the top part of the bushing. These parts do not share the load, they both have to withstand all of the pull-out forces that are applied. This enables the models to be split. The models were loaded with a force of 1100N of pull-out, as was defined in the requirements and whishes. In the first model, with only the bushing and the rod+ top screw, the pull-out force was applied to the lower end of the rod and the bushing was fixed. The connection between the bushing and the rod was a glue-connection. In the second model the force was also applied to the lower end of the rod, but this time in reverse direction. This has of course the same effect on the nut as a pull-out force that is applied to the pin, since the pin exerts pressure on the nut then. The nut again was glued to the rod and the pin and the rod were in normal friction contact with a friction coefficient of 0.8. The results of the first model (only bushing and top screw) showed that both parts can withstand the load. The maximum von Mises equivalent stress in the model was found in the rod (Fig ) and was 171MPa. This can be withstood easily by titanium. The maximum von Mises equivalent stress in the top of the bushing was 9.15 MPa. This is far less than the maximum tensile and compressive strength of PEEK ( MPa and MPa respectively) and thus these stresses are acceptable. It has to be noted though that the connection between the top screw and the top part of the bushing was assumed to be a glue-connection, while in the concept it is a screw connection. In figure it can be seen that a large part of the stresses in the bushing are localized on the edge of the bushing, instead of the plateau. For an optimal stress distribution the top screw should be screwed down into the bushing as far as possible, so that the top screw rests on the plateau in the bushing. If the top screw is a bit up, all of the pull-out forces have to be withstood by the screw-thread in the top of the bushing. Fig. 13.9: Results of the first pull-out model. The von Mises equivalent stresses in the rod are at maximum 171MPa (left, longitudinal section) and the von Mises equivalent stresses in the bushing (right, top part of the bushing, longitudinal section) are at maximum 9.15 MPa. 91

92 The results of the second pull-out model show that the nut can withstand the pull-out force. The van Mises equivalent stresses in the nut are at maximum 156MPa (Fig ). It should be noted that the nut was glued to the rod and not screwed, as it is in the real situation. This caused that there was no stress transferred to the pin. The maximum von Mises equivalent stress in the rod again is 171MPa. Fig Results of the second pull-out model. The von Mises equivalent stresses in the rod are at maximum 171MPa (left, longitudinal section) and the von Mises equivalent stresses in the nut (right, isometric view) are at maximum 156 MPa. Bending The model was loaded with a bending moment of 50 Nm which is a bit higher than the required bending moment (which is 45Nm). Also the femur bone was not included in the model, which lead to an extreme loading case because in the real situation the collar transfers a big share of the resulting stresses from the bending moment to the osteotomy site of the bone. Now that the bone is absent, all of the forces will be transferred through the pin into the bushing. The bending moment was applied to the lower end of the pin and the bushing was fixed. The results showed that all of the parts can withstand the load. The maximum von Mises equivalent stress was found in the pin and was 749MPa. (Fig ). This is almost as high as the maximum tensile and compressive strength of the titanium alloy ( MPa and MPa respectively) and higher than the endurance limit, which is MPa. In that respect the stress of 749 MPa is quite high, but it should be taken into account that a large share of these stresses are transported to the osteotomy site. The maximum von Mises equivalent stress in the bushing was found in the lower part of the hexagonal shape and was 81 MPa, which is quite high since the maximum tensile and compressive strength are MPa and MPa respectively. Also it would be better if this stress is more localized in the higher part of the bushing since this is further away from the bending point, and thus yield lower stresses. The maximum von Mises equivalent stresses in the rod+top screw and in the nut were 207 MPa and 52 MPa respectively (Fig ). These stresses are acceptable. From the rod it can be concluded the cone shape of the hole in the top of the pin works. There are no peak stresses in that part of the rod. 92

93 Fig : Results of the bending model. In the left corner an overview of the model and the stress distribution is shown (longitudinal section, side view). The maximum von Mises equivalent stress was found in the pin and was 749 MPa (right corner, side view). The maximum von Mises equivalent stress in the bushing was found to be 81 MPa and was concentrated in the lower part of the bushing. (bottom, longitudinal section, slightly tilted side view) 93

94 Fig : Results of the bending model. The maximum von Mises equivalent stress in the rod was 207 MPa (left, side view) and the maximum von Mises equivalent stress in the nut was found to be 52 MPa (right, isometric view) Conclusions version 1 The stresses that were generated in the model when the loads were applied are summarized in table The stresses caused by torque and by pull-out were all acceptable, only the stresses in the bushing were a bit high. The stresses in the pin and in the bushing that were caused by bending were all quite high. This was most likely due to the absence of bone. The stresses were also not localized very well. The bending loads should be transferred to the top of the bushing instead of the lower part of the bushing. Table 13.1: Summary of the stresses in the pin, bushing, rod+top screw and the nut caused by the torque, pull-out and bending forces. Pin (MPa) Bushing (MPa) Rod+Top screw Nut (MPa) (MPa) Torque (20Nm) Pull-out (1100N) Bending (50Nm) Several improvements of the design of the final concept were suggested 1. The diameter of the top part of the pin was increased from 7 to 9 mm. This was done to try to transfer the forces better to the top of the bushing. 2. The clearances in the model were reduced from 0.1 mm to 0.05 mm, to ensure a better fit and less play between the parts. 3. The diameter of the bushing was also increased from 7.2 to 9.1 mm to fit to the pin 4. The fillets of the hexagonal part of the pin were reshaped (Fig ) 5. The fillets of the bushing were reshaped in the same way 6. The lower part of the pin was made a bit broader to give more space to tighten the nut 7. The length of the rod was decreased, so that it did not protrudes from the pin. 8. The diameter of the collar was reduced from 35 to 30 mm, because this was more practical for the workshop and had no effect on the working of the device. 9. The length of the threaded on the bottom of the rod was reduced to 10mm, because this is enough. More thread only results in more friction between the rod and the hole in the pin. 94

95 The technical drawings of the second version can be found in Appendix 6. To see if the suggested improvements had the desired effect, a second version of the concept was made and the new concept was tested again using FEM. Fig : The fillets of the rod were reshaped to make the transition from the cylinder to the hexagonal shape more fluid Final concept version 2 After all the improvements were made, two new FEM models were constructed to test the effects of the improvements. Both of the models consisted of the bushing, the pin and the rod. In the first model a torque of 20Nm was applied to lower end of the pin. The bushing was fixed. In the second model the torque was replaced by a bending moment of 50Nm. It was decided not to make a FEM model for pull-out since no changes have been made to the concept that could affect the pull-out stress distribution in the model. Torque The results of the FEM models showed that the torque causes higher stresses in the bushing and in the pin than in version 1. The peak von Mises equivalent stress in the bushing was increased from 45 MPa to 60.2 MPa and the peak von Mises equivalent stress in the pin was increased from 277MPa to 391 MPa. These values were unexpected of course. The most plausible explanation is that the meshing has some errors. In figure it can be seen that all of the stresses in the bushing are concentrated on the vertices of the hexagon, while in the previous version this was not the case. The fillets were connected a bit more fluid to the cylindrical part of the pin, so it was expected that the stress distribution would be more uniform. Therefore it was thought that there are some meshing issues. The elements in the points of the hexagon were probably protruding a bit too much. Also the stresses in the pin were higher. This however was also due to a meshing issue. When this was left out, the maximum von Mises equivalent stress was 278 MPa. The peak stresses again were located in the hexagonal vertices. The stress distribution in the rod showed an artifact. One element was loaded with 516 MPa. The rest of the stress distribution in the rod remained the same. (69 MPa) 95

96 Fig Results of the torque model of version 2 of the final concept. High stresses are generated in the pin (left, overview of the model, longitudinal section, side view). The stresses in the bushing are not distributed uniformly. (Middle, longitudinal section, slightly tilted side view). An artifact was observed in the rod (right, side view) Bending The results of the bending model showed that the peak von Mises equivalent stress in the bushing was reduced from 81 MPa to 56 MPa. The peak von Mises equivalent stress in the pin was increased from 749 to 813 MPa. Again an artifact was observed in the rod. The maximum von Mises equivalent stress was increased from 207 MPa to 502MPa in 1 element. The rest of the rod showed the same stress distribution as in version 1 (maximum of 207 MPa). The reduction of the peak stress in the bushing was a good result. Apparently the bending stresses were distributed more uniform. (Fig ) The increase of peak stress in the pin was bad news. This could be caused by the increase of the thickness of the pin. This might have made the pin less flexible, which causes higher peak stresses. Also 813 MPa is in the range of the maximum tensile and compressive strength of titanium. ( MPa and MPa respectively). Therefore it was interesting to see how the implant behaves in bone during bending. 96

97 Fig Results of the bending model of version 2 of the final concept. High stresses are generated in the pin (left, overview of the model, longitudinal section, side view). The stresses in the bushing are still concentrated in the lower part of the bushing (middle, longitudinal section). An artifact was observed in the rod (right, side view) Final concept version 2 tested on bending in bone Because the stresses that were caused by the bending moment were quite high in the pin and in the bushing, it was interesting to see what it would look like in the bone. Therefore a PEEK collar and a finite element model of bone were added, of which the bone was derived from Tomaszewski et al. [22]. The geometry of the bone was determined from CT scans (slice thickness 3 mm) of a male femur bone with normal mineral density. The amputated femur model represented the most common osteotomy level of 250 mm above the knee. The model was loaded in the same way as the previous bending model i.e. with a bending moment of 50Nm which was applied to the lower end of the pin. The bone was fixed and the bushing was bonded with the bone, as if it were osseointegrated. The results show that the stresses in the implant are lower than the stresses that were measured during bending without the bone. The maximum von Mises equivalent stress in the pin was reduced from 813 MPa (749 MPa in version 1) to 347 MPa. This is quite a large reduction and the titanium alloy can withstand these stresses easily since the endurance limit of titanium is MPa. The maximum von Mises equivalent stress in the rod was reduced from 207 MPa to 91 MPa. The maximum von Mises equivalent stress in the bushing was reduced from 56 MPa (81 MPa in version 1) to 35 MPa. This is also a large reduction, although it is still slightly within the range of the endurance limit of PEEK, which is MPa. Therefore the device should be tested thoroughly on wear and fatigue during the in vitro test phase. It was observed that the stress distribution in the bushing was still localized mainly in the lower part of the bushing (Fig ), but some stresses were also transferred to the higher part. The maximum von Mises equivalent stress in the top part was about 2 MPa. The PEEK ring showed a maximum von Mises stress of 61 MPa. This is below the maximum tensile and compressive strength of PEEK ( MPa 97

98 and MPa respectively) but slightly above the endurance limit. In the real situation the osteotomy site will osseointegrate with the PEEK ring and then the stress distribution might be a bit different. This might be studied in an in vivo study. The maximum von Mises stress on the osteotomy site of the bone was found to be 85MPa. It is not known whether these stresses are too high for the bone, but it is expected that it would be better to reduce these stresses a bit. This could be achieved by transferring the stresses a bit more up into the implant. It should be mentioned that the results showed that the stresses were distributed over the cortex quite uniformly. The stresses in the medullary canal were very low. (Fig ) Fig Results of the bending model of version 2 of the final concept in bone. The stresses were distributed quite uniformly over the bone cortex. (upper left, longitudinal section, side view). The stresses in the pin were much lower than without the bone. (upper right, longitudinal section, side view). The stresses in the bushing were acceptable but should be transferred more up into the implant. (lower left, longitudinal section, side view ) The stresses in the PEEK ring were quite high, but safely below the maximum compressive strength of PEEK (middle, isometric view) The stresses in the rod were relatively low. (lower right, longitudinal section, side view) 98

99 A realistic load case The second version of the implant was tested in a realistic load case. The case was derived from measurements performed with amputees using the OPRA device [30] and was taken at 25% of the walking cycle (heel strike). The loading conditions are presented in table Table 13.2: Overview of the load case applied to the FE model. The patient of which the measurements were taken had a body weight of 61Kg. F SI is the superior-interior axis (superior being positive), F AP is the antero-posterior axis (anterior being positive) and F ML is the medio-lateral axis (lateral being positive). F SI (N) F AP (N) F ML (N) M SI (Nm) M AP (Nm) M ML (Nm) The results showed that the von Mises equivalent stresses were relatively low compared to the previous FE models. This was not unexpected, since the patient has a body weight of 61kg, which is much lower than the required body weight. The maximum von Mises equivalent stress in the bone was 75 MPa, which was located on the osteotomy site (Fig ). It is not known whether these stresses are too high for the bone, but it is expected that it would be better too lower these stresses a bit. The maximum von Mises equivalent stress in the pin and in the Rod+top screw are 187 MPa and 72 MPa respectively. This is safely below the endurance limit of titanium which is MPa. The maximum von Mises equivalent stress in the bushing and in the collar are 34 MPa and 46 MPa respectively (Fig ). This is within the range of the endurance limit of PEEK, which is MPa, thus it is important to test the endurance of the implant during the in vitro tests. Fig Results of the realistic loading condition applied to version 2 of the final concept in bone. The stresses were distributed quite uniformly over the bone cortex. (left, partial longitudinal section, side view). The stresses in the pin were concentrated in the lower part of the pin (middle longitudinal section, side view) and the stresses in the rod were quite uniformly distributed (right, longitudinal section, side view) 99

100 Fig Results of the realistic loading condition applied to version 2 of the final concept in bone. The stresses in the bushing were acceptable but too concentrated in the vertices of the hexagonal shape. (left, longitudinal section, side view). The stresses in the PEEK ring were concentrated on the part which was pushed towards the bone as a result of the bending moment. The results of the load case were compared with the results of an intact femur bone, the OPRA implant and the ISP implant, which were all loaded with the same load case as was used in this study by Tomaszewski et al. [22] A visual comparison in which the von Mises equivalent stresses were shown is presented in figure Unfortunately no time was found to get into more detail with this comparison. Also the priority to compare the results very precisely already was not very high since the new implant was not yet tested in vitro, which could yield new improvements. It is more efficient to fully compare the three implants when the new implant is completed. From the visual comparison it can be concluded that the Profix distributes the stresses more physiological than the OPRA and the ISP. These implants exert the stresses primarily via the top of the implant into the upper part of the medullary canal. In the Profix a large part of the stresses are transferred to the cortex of the bone. Because the cortex of the bone is loaded, the stress shielding and thus bone remodeling should be reduced, compared to the OPRA and the ISP implants. 100

101 Profix Fig : Comparison of the von Mises equivalent stress distribution in the intact bone, the OPRA and ISP implants and the Profix implant. The Profix distributes the stresses more physiological than the OPRA and ISP implants. The intact bone is heavily loaded due to preloading of the muscles and ligaments Conclusions version 2 The results of the analysis of the FE models of version 1 and version 2 (both with and without bone) are summarized in table It can be concluded that the distribution of torque-induced stresses has not improved in the second version, but it is assumed that this is due to meshing issues with the vertices of the hexagonal shape. The distribution of bending stresses in the bushing is improved, since a reduction of the maximum von Mises equivalent stress of 31% was found in the bushing. The maximum von Mises equivalent stress in the pin was slightly increased. When the implant is inserted in the bone, the bending stresses in all of the parts are greatly reduced because a large share is transferred to the cortex of the bone by the collar. In the real loading case the stresses in all parts are relatively low. The maximum von Mises equivalent stresses in all of the parts were reduced, but the reduction in the bushing and in the bone was only small. This should be studied further. Apart from the table it can be concluded that the stresses are not yet optimally transferred to the top of the bushing. This might be caused by shortcomings of the FEM model, which did not have clearances between the parts. Another reason could be that the design is not optimal yet. This issue can be further analyzed in an in vitro study. 101

102 Table Results of the analysis of the FE models. Pull-out was left out of the table because this was not analyzed in version 2, because no parts that influence pull-out were improved, as was explained in chapter and thus cannot be compared with version 1. Pin (MPa) Bushing (MPa) Rod+top Screw (MPa) Collar (MPa) Version 1 Torque (20 Nm) Version 2 Torque (20Nm) Version 1 Bending (50Nm) Version 2 Bending (50Nm) Version 2 Bending with bone (50 Nm) Load case Bone (MPa) 102

103 13.4 Prototype The final part of this master s thesis was the production of a prototype, which can be used for demonstration purposes like conferences and meetings with surgeons. The production of the prototype was performed by the research instrumentmakerij (workshop) of the University Medical Centre Groningen. The prototype was made of PEEK (bushing and PEEK ring) and Titanium grade 2, which is pure titanium (rod, nut, pin, top screw). The intention was to use titanium grade 5, but due to miscommunication with the workshop about the term medical grade, accidentally grade 2 was used. This was a pity but acceptable since it looks the same as titanium grade 5 and the material properties are only slightly different. (detailed material properties are found in appendix 7. The production process took about 80 working hours and the result is presented in figure All of the parts were produced by machining using computer numerical control (CNC). All of the parts matched the drawings and the assembly and disassembly process went as expected. The only difference between the prototype and the drawings was the outside of the bushing. In the drawings it was smooth (except for the V-shapes), but in the description it was mentioned to be rough. To make it rough it was chosen to create a lot of small rectangles on the implant by carving 60 lines in proximodistal direction and a long threaded line around the longitudinal axis. The titanium nut was provided with a PEEK layer on the inside to prevent loosening of the nut due to vibrations. During the waiting time for the prototype the idea was launched to do already a few in vitro tests, to see how the implant behaves when torque, pull-out and bending loads are applied. The results of the tests are discussed in chapter 16. Rod Nut PEEK ring Pin Top screw Bushing Fig : Prototype disassembled (left) and assembled (right). The rod, nut, pin and top screw are made of titanium grade 2. The bushing and the PEEK ring are made of PEEK. 103

104 13.5 Methods of production Titanium parts The titanium parts (the pin, rod, top screw and nut) can be machined very well. An alternative is to cast the parts, but this is only profitable if a lot of implants are being produced, because of the high costs of the mold. Since the take-off of the implant will not be high, especially not during the testing period, casting the titanium is not profitable. Another argument against casting is that it might be necessary to produce different sizes of the implant. Then even more molds are needed. It is cheaper to just machine the titanium parts. Because of the broad collar, a relatively thick rod of titanium has to be used, but the debris can of course be recycled. Machining can be done using CNC (computer numerical control) during which the design is uploaded in the computer, which operates a turning lathe. If the program for the CNC is created, the parts can be machined at relatively low costs. PEEK parts The PEEK parts (the bushing and the PEEK ring) can be, just like the titanium parts, machined very well using CNC. It is also possible to injection mold the PEEK parts, but just like the titanium parts it is expensive to build several molds. Also when PEEK is injection molded the material is already pre-stressed a bit and the fibers might get aligned slightly, leading to possible anisotropy. This makes the bushing slightly more unpredictable. Thus since machining will be cheaper than injection molding and it gives more reliable material properties, the method of production of choice for PEEK is machining Costs The costs of the prototype were as follows: Materials Titanium grade 2 bar 30 mm Titanium bar 5 mm and bar 15 mm PEEK Test setup working hours 80 hour x Total Since the production costs in the workshop of the UMCG cannot be compared with the production costs of a commercial partner and the amount of implants that might be produced in the future is very difficult to estimate, it is not possible to give an accurate estimation of the production costs when the scale is enlarged. When a rough estimation is made however, it can be concluded that the production costs on a higher production scale with a commercial partner will be quite similar to the costs in the workshop. The materials will be slightly cheaper because of the higher production scale and the test-setup can be skipped. It should be noted that grade 5 titanium should be used instead of grade 2. Grade 5 is about 15-30% more expensive and according to the Cambridge engineering selector 104

105 V4.5 the materials are equally expensive [35] and are estimated at about per kilo. PEEK is a bit more expensive with per kilo. These prices however are unrealistically low for the relatively small amounts that are used for the implant. The total material costs per product can be estimated at about 150, ,- The working hours will of course be more expensive than in the UMCG. However, a lot of effort was put into making programs for the CNC. These programs are already made, so the amount of working hours will be drastically lower for future products. If all the CNC programs are pre-made and the device only has to be machined, than this should take less than a day. This should cost no more than 300,-, which gives a total cost estimation of about = 500,- 105

106 14 Failure Mode and Effect Analysis (FMEA) If the implant is used by a patient and it fails, this might have serious consequences for the patient. To make an assessment of the risks and the consequences a standard failure mode and effect analysis (FMEA) was performed for the functions transfer torque, preventing pull-out and transfer bending loads. For these functions the failure mode, causes of failure and the effect of the failure were analyzed. A rating ranging from 1 to 10 was given for the probability, the severity and the detection number. The detection number is a measure for the risk that failure will escape detection. The higher the number, the lower the chance that failure is detected. An overview of the ratings and their meanings is given in table The ratings were multiplied (indicated by X) and it was decided that recommendations to decrease X should be made when to score was higher than 40. The FMEA table is presented in table Several X-scores above 40 were detected and these require action. The actions that will be taken are: 1 A torque limiter will be applied to decrease the chance of sudden excessive torque on the implant. Torque limiters are widely available and are already used in orthopedics. 2 A failsafe mechanism has to be incorporated in either the implant or on the prosthetic leg. This mechanism prevents excessive pull-out forces and excessive bending moments. 3 X-ray photos should be made regularly to analyze bone remodeling. This should decrease the chance at fracture. One risk that was not very high but was reported in the FMEA was wear. The total score (X) of wear-related risks is not very high, but repeated loading tests might show that wear does occur. Then measures like applying a DLC coating on the pin and/or the bushing or using CFR-PEEK instead of PEEK might be necessary. Table 14.1: Ratings (R) and the corresponding probability, severity and detection number R Probability Severity Detection number 1 Very low (1x in 100 year) Possibly not detected Failure is immediately visible when it happens 2 Low (1 x in 10 year) Low (Visible but does not harm at all) Failure is detected within minutes when it happens 3 Less low (1 x in 5 year) Less severe (can cause pain for patient but not serious) Failure will be detected within one hour 4 Below average (1x in 3 years) Below average (causes pain for patient in direct environment) Failure will be noticed within one day 5 Average (1x per year) Average (Serious injuries, doctor needed) Failure will be noticed within one month 6 Above average (1x in 6 months) Above average (Wounded patient, hospital needed) Failure will be noticed within one year 7 Rather high (1x in 2 months) Rather serious (Wounded patient, hospital needed) Failure is only discovered at routine inspection 8 High (1x per month) High (Wounded patient and a long stay in hospital needed) Failure is only discovered during special inspection 9 Very high (1x per week) Very high (patient dies) Failure is only discovered with special equipment 10 Sure (1x per day) Catastrophic (several deads) No possibility for detecting failure 106

107 Table 14.12: Failure mode and effect analysis. P=probability, S=severity, D=detection number and X is the multiplication of P, S and D. Function Failure mode Causes of failure Effect of failure Rating Action status P S D X Recommendations Transfer torque pin slips in bushing sudden extreme high load implant has to be removed, torque limiter endurance limit exceeded remnant limb will get shorter wear surgery + long revalidation fracture of bushing sudden extreme high load low quality PEEK implant has to be removed, torque limiter remnant limb will get shorter rotation in bone failed osseointegration due to: failing HA coating failing Ti coating implant has to be removed surgery + long revalidation Preventing pullout bone fracture Top of bushing fails sudden extreme high load stress shielding extreme pull-out forces endurance limit exceeded implant has to be removed, torque limiter remnant limb will get make regularly X-ray shorter photo surgery + long revalidation implant has to be removed, failsafe mechanism remnant limb will get shorter

108 low quality PEEK surgery + long revalidation rod fails extreme pull-out forces new rod has to be placed failsafe mechanism Transfer bending loads nut or thread rod fails bushing fails extreme pull-out forces sudden extreme high load wear new rod has to be placed failsafe mechanism implant has to be removed, failsafe mechanism remnant limb will get shorter surgery + long revalidation pin bends sudden extreme high load replace pin and rod failsafe mechanism endurance limit exceeded pin breaks sudden extreme high load replace pin and rod failsafe mechanism endurance limit exceeded rod bends sudden extreme high load replace pin and rod failsafe mechanism pin breaks failsafe mechanism rod breaks sudden extreme high load replace rod failsafe mechanism 108

109 15 Test protocol Before the new implant can be used in the clinic an extensive test protocol should be followed, which includes FEM studies, in vitro studies, in vivo studies and clinical studies. After each study it should be analyzed how the implant behaved and if any improvements have to be made. These improvements have to be tested again with FEM, with in vitro and possibly also in vivo. The whole test protocol is schematically shown in figure Finite element model analysis The first step in the test protocol is the Finite element model (FEM) study. In the FEM study the stresses in the model are calculated when loads are applied to it. If realistic loads are used, the model can give valuable information about the behavior of the different parts and materials. Based on the FEM analysis it can be decided to change the design or the materials. When the model is found to be able to withstand the loads that it requires to withstand prototypes can be made. In vitro The prototype can be tested in vitro on different aspects. In this particular implant the implant should be tested on 1 Withstanding the torsional, pull-out and bending loads that the implant requires to withstand. 2 Failure; the torsional, pull-out and bending loads at which the implant fails should be studied 3 Endurance; the behavior of the implant during repeated loading should be studied. Especially the presence of wear particles should be closely monitored. When the implant is able to withstand the required forces and no unacceptable damage or wear is found in the endurance tests, the implant can be tested in vivo. If the implant is not able to withstand the required loads or excessive wear is detected, the implant should be improved. This can be done by altering the design, but also by using different materials, like Carbon fiber reinforced (CFR)-PEEK instead of normal PEEK. CFR- PEEK has better wear properties, but also has a higher Young s modulus and thus is stiffer than PEEK. The consequences of the higher stiffness should be tested in FEM before new prototypes are made. Another option for decreasing wear is to apply a coating to the bushing-pin interface. A diamond like coating (DLC) has proven to be able to reduce friction [41]. In vivo After passing the in vitro tests, improving the design and testing it again in FEM and possibly in vitro, the in vivo tests can be commenced. The most important aspect that should be studied in vivo is the bone remodeling. This can be studied in for instance goats. Although the stress distribution in a goat femur is slightly different from a human femur, a lot of information about the remodeling process can be acquired. Even more information can be acquired when also the OPRA and ISP implants are tested in the same animal. Because the bone remodeling properties of these implants in humans are known, more information about how the new implant would behave in human bone can be gained by observing how the OPRA and ISP implants behave in goats. However, the value of this extra bit of information should be weighed against the suffering of more animals. Other aspects that can be tested in vivo are the osseointegration of the bushing and the PEEK ring. The results of the in 109

110 vivo tests cannot be directly extrapolated to humans, since the loads on the implant will be higher in humans. The stresses in the PEEK ring due to bending for example will be lower in a goat than in a human. This might have an effect on the osseointegration properties of the PEEK ring. Clinical trials When the in vivo studies show positive results, the clinical trials can start. Since the implant cannot be tested in healthy subjects, the classical 4 phases of clinical trials cannot be performed. The first phase should be performed with a small group of healthy and relatively young subjects. This patient group has the strongest bones and the lowest risk of complications. During the study the bone remodeling should be monitored closely and also fracture, loosening, etc. should be reported. If the study shows better results than the OPRA and the ISP implants, then it can be used for this group in the clinic, after FDA and/or EMA approval. If the results on the study with healthy subjects is successful, then it should also be studied if other patient groups, like elderly or diabetics, can use the implant. 110

111 Larger patient group/clinic pass fail Clinical phase: small group of less healthy/older patients Patients are satisfied with the prosthesis Implant does not fail Larger healthy young patient group/clinic pass fail Clinical phase: small group of young and healthy patients Patients are satisfied with the prosthesis Bone remodeling is reduced compared to OPRA and ISP Implant does not fail pass fail In vivo Bone remodeling is reduced in comparison to the other implants Implant is well osseointegrated Implant does not fail pass fail In vitro Required Torque, Pullout and Bending loads are withstood Failure analysis on maximum Torque, Pull-out and Bending loads Required endurance is withstood. Fig. 15.1: Test protocol FEM study Redesign 111

112 16 In vitro tests In order to see if the prototype could withstand the loads that were defined in the Requirements and Whishes (chapter 6), a test setup was designed. Two goals were defined. 1 Make sure that the prototype can still be used for demonstration purposes afterwards 2 Test if the prototype can withstand the following loads: -Torque: 10Nm -Pull-out: 550 N -Bending: 10 Nm These loads are slightly lower than the loads that were defined in the requirements and whishes and the loads that were used in the FEM analysis. Lower loads were applied because of the lower grade of the titanium that was used. The compressive strength, tensile strength and endurance limit of grade 2 titanium are MPa, MPa and MPa respectively [35]. More information about the material properties of titanium grade 2 is found in appendix 7. These values are lower than the compressive strength, tensile strength and endurance limit of grade 5 titanium ( MPa, MPa and MPa respectively). The loads that were used in the FE models were: torque: 20Nm, Pull-out: 1100N and Bending: 50Nm. The maximum von Mises equivalent stress that was generated in the titanium parts was 277 MPa. This is higher than the maximum compressive strength of titanium grade 2 and therefore the torque in the test setup was reduced to 10Nm. The maximum von Mises equivalent stress during pull-out was 171 MPa. This should be acceptable, but to be certain also the pull-out load was divided into halve. The maximum von Mises equivalent stress that was found in the titanium parts during bending was 813 MPa. Therefore it was chosen to reduce the bending load to 10Nm. Test setup To achieve the goals a test setup was built, which is shown in figure It consisted out of the prototype, a vise to clamp the bushing, a tensile test machine (Zwick Roell, i2,5kn), a laptop and custom made parts to connect the prototype to the tensile testing machine. The custom made parts were developed with solid works and produced in the research instrumentmakerij (workshop) of the University Medical Centre Groningen Fig Test setup. 1: tensile testing machine, 2: laptop, 3: prototype, 4:vise 112

113 Torque The setup for torque is shown in figure Part 1 was fixed to the pin with four screws and functions as an extension of the pin, onto which the arm (part 2) could be screwed. The arm has a length of 5 cm from the middle of the point where the torque is applied to the centre of the long axis of the pin. Part 3 was used to support part 2, to make sure that only torque is applied and not also a bending moment. Parts 1,2 and 3 were made of stainless steel. Part 4 is the vise which clamps the bushing, thereby preventing it from rotating. Part 5 is the extension if the tensile testing machine which looks a bit different in real life. 200 Newton was applied to the point 5 cm from the long axis. This corresponds with 10Nm. Technical drawings of parts 1 and 2 are shown in figure Fig. 16.2: Torque test setup. 1: extension of the pin that enables the connection with the arm (2). Part 1 is supported by part 3 to ensure that only torque is applied. Part 4 is the vise that clamps the bushing and part 5 is the tensile testing machine. Fig technical drawings test setup: Left: isometric view of the arm. Middle left: top view of arm. Middle right: side view of pin extension. Right longitudinal section (A-A) of pin extension. 113

114 Bending The setup that was used for bending was almost the same, only the bending moment was not applied at 5 cm from the axis but at 5 cm from the bushing, right on the long axis. (Fig. 16.4) Fig Bending test setup. 114

115 Pull-out The test setup for pull-out (Fig. 16.5) was slightly different from the other test setups. Part 1, which was fixed to the pin with four screws, remained the same and was used to connect the pin to the tensile testing machine. Part 2 is a stainless steel cage that fits the pin and prevents the bushing from moving in upward direction. It is connected to the tensile testing machine. Part 3 is the moving component of the tensile testing machine and part 4 is the lower side of the tensile testing machine. A pull-out force of 550 N was applied to part 1 by part 3. A technical drawing of part 2 is shown in figure Fig. 16.5: Pull-out test setup. 1: extension of the pin that enables the connection with the tensile testing machine. Part 2 prevents the bushing from moving up and is connected to part 4, the bottom of the tensile testing machine. Part 3 is the moving head of the tensile testing machine. Fig. 16.6: Technical drawing of the stainless steel cage. Isometric view 115

116 Results The results of the in vitro tests were positive. The prototype was able to withstand all the loads and no plastic deformation was seen. The results are shown in figure The results from the torque test show a maximum travel of the moving part of the tensile testing machine of 1.1 mm at 200N (corresponds with 10Nm). Since the arm was 5 cm and the travel was 1.1 mm the angle of rotation was tan -1 (1.1/50)=1,26. The line rises linearly, which means that there is no plastic deformation, only elastic deformation. The results from the bending test show that there was a deflection of tan -1 (0.7/50)=0,80. The graph is almost linear but at 200N (which corresponds again with 10Nm) a bit of flattening of the graph was observed. This is however minimal (the scale is very small) and might be caused due to a bit of movement in the vise that clamped the bushing. The result from the pull-out test also shows a straight line. The travel at 550N was only 0.2 mm and this might be caused by the small amount of movement that is possible between the three thread connections in the implant. Torque Bending Pull-out Fig 16.7 Results in vitro tests. 116

117 Conclusions in vitro tests The results of the in vitro tests showed that the prototype was able to withstand 10 Nm of torque, 10Nm of bending and 550N of pull-out. No plastic deformation or failure of the material was observed. This is already promising but further testing is needed. When titanium grade 5 is used, the loads that a applied to the prototype should be higher, as was defined in the requirements and whishes. Also endurance tests should be performed. It was not possible to do this during this study, since there were no possibilities to analyze the prototype after testing. To see if the inside of the bushing is damaged, it should be cut in halves. If this is done then the prototype cannot be used anymore for demonstration purposes. In a future in vitro study also more prototypes should be used, to be able to do a statistical analysis and to see if the quality of the implants is equal. 117

118 17 Conclusion During this master s thesis an osseointegrated upper leg prosthesis fixation system was developed. The goal was to improve the existing concept so that it could withstand torsion an pull-out forces, while preserving the advantages of the existing concept. The methodical design process was used to develop a solution and this solution was tested and improved using FEM and in vitro tests. The FEM analysis showed that the new concept has a more physiological stress distribution than the existing implants (OPRA and ISP) and also showed that torsional loads and pull-out loads are distributed uniformly. These properties of the implant should lead to a reduction in bone remodeling, thereby decreasing the risk at fracture. The FE model however also showed that the peak stresses during bending are quite high in some areas in the bushing, on the PEEK ring and in the osteotomy site of the bone. The solution would be to transfer the stresses a bit more up into the implant. During bending the upper part of the bushing should take the biggest share of the stresses and not the lower parts. This could be achieved by changing the clearances in the model. The space between the pin and the bushing was 0 mm and this might be changed into 0.05 mm. Another improvement would be to make the pin a bit more flexible by reducing the thickness of the part that is situated above the collar. A third improvement could be to make the whole pin hexagonal. This was not tested with an FE model because the workshop is not able to produce a hexagonal bushing with such a length. When these improvements are considered and tested in FEM and the stresses that are resulting from the bending loads are distributed more uniform, I think that this implant really can compete with the existing implants. Before that happens however, a lot of testing and improving has to be done, and a lot of investments have to be made to get FDA and/or EMA approval. I hope that the implant that was developed during this thesis will reach this stage of development and it would be great if patients could benefit from this master thesis within the next ten years. 118

119 18 Evaluation In the past seven months I had a very busy yet very exciting time. For the first time in my academic career I came to the point were a prototype was produced. In the past five years I always enjoyed projects during which a new device was developed, like the courses ontwerpen I and 2, my bachelor s thesis and the course multidisciplinary project but the last phase in these projects was always the point were the technical drawings were made. During this project I worked full-time for seven months on a concept and this led, finally, to the production of a prototype. The prototype looked and felt really good and survived the in vitro tests. This really gave a good feeling. Project management At the start of the thesis a planning was made (Appendix 1). The first six weeks (the analysis phase until the selection of the final concept) went according to the planning. Then the detailing of the final concept and building all of the alternatives in Solid works took one week more than planned. Then in week the FEM analysis was planned. This however took more time than four weeks. This was caused primarily by the fact that the software I used was not particularly user friendly and that a lot of models had to be rebuild several times before they worked. Also the computer I used was not very up-to-date, which also slowed the process down quite a lot. Furthermore I made more models than I planned. It was not planned for example to test the different non-circular shapes with FEM and it was also not planned to use a bone in the model, to which the model of the implant had to be fitted. During the FEM study Pawel Tomaszewski helped me a lot and this was highly appreciated. Another part which took a lot of time then expected was the production of the prototype. It took about 80 hours to build it, but I also had to wait for about 5 weeks before the workshop had time to start building it. Then also the test setup had to be built, which was finished in the end of June, just before the final presentation. I filled the time that I had to wait with building more FEM models and working on this report. I planned a few weeks to write the report but this time was now used for building FE models, designing the test setup, performing the in vitro tests etc. This however led to a shortage of time for writing the report. Fortunately I had the opportunity to work two weeks longer on the thesis, which exactly was enough to finish the report. Methodical design process The different phases of the methodical design process have been successfully completed, but I did encounter some difficulties during the process. The main difficulty was the lack of time I had for doing the FEM study. I made models of both of the versions of the final concept, but the results of the FE models of version 2 showed that there were some meshing issues. There was however no time to make new models, because if the model of version 2 had to be changed, then the models with the bone also had to change. Also during the end of the project, we came up with more ideas for improvements, which I would have liked to test with an FE model. There was however simply no time for that. This leaves me with the feeling that the detailing of the final concept could have been better. On the other hand, designing a prosthesis is an iterative process anyway. New ideas for improvements come up all the time, and it is simply not possible to test them all, especially not in this short amount of time. I am however pleased with the final concept and the prototype and I truly think that this solution might help the department of biomedical engineering with designing an implant that it superior to the OPRA implant and the ISP implant. 119

120 Team work When doing a master s thesis at a university, students usually get an assignment and work on that assignment alone for half a year or more. Therefore teamwork seems to be misplaced in an evaluation of a master s thesis. There was however quite a lot of teamwork involved in the process of my thesis, although it was not traditional teamwork. The tasks were not divided among team members, but a lot of people were involved in making this assignment a success. Several students contributed to the brainstorm and we also discussed problems of each other s projects during coffee- and lunch breaks. Also staff members were really eager to help when I had a question or a problem. Especially Pawel Tomaszewski helped me a lot with the FE models and gave a lot of advise. The staff meetings, coffee breaks, lunch breaks, discussions with staff members and students combined with the pleasant working environment gave me a team feeling. During the project I learned that developing a medical device with a team is better than developing it alone. I appreciated the help of students and some staff members a lot. If people are working on a project as a team than the members can inspire each other, give each other ideas during the brainstorm sessions and when the background of the members is different than they can really accomplish more than the individuals would develop when they are alone. Conclusions Looking back on the past seven months the main conclusion to draw is that the master s thesis project went very well and that I had a good time doing it. The design assignment is fulfilled and both of the goals, transferring torque and preventing pull-out, are achieved. Some stages in de process took a bit more time than expected, but this was necessary to come to a good product. I had a busy time working on this thesis, especially in the final weeks, but I m content with the result. 120

121 19 Acknowledgements As I already mentioned in the evaluation, I had a great time working on this project. There are a few people that I would like to thank for their contribution to this master s thesis. First of all I would like to thank Prof. dr. ir. G.J. Verkerke and Prof. dr. S.K. Bulstra for giving me the opportunity to perform this master s thesis and for their supervision. I found the weekly meetings with Bart very useful and I think that it s quite unique that a supervisor finds that much time to spend in the research of all of his students, thereby helping them to stay focused and develop a better product. For helping me with the brainstorm session and providing me with lots of ideas I would like to thank: Aukje Zijlstra, Esther Vredeveld, Lútzen Kuiper, Marcel Schouten, Tryanda de Jong, Ward Sikkema and Willemijne Schrijver. For their always honest opinions about my ideas and their useful advises and ideas I would like to thank: Ward van der Houwen and Ed de Jong. For providing me with a great working environment and giving me advise or an opinion occasionally, I would like to thank: Aukje, Berend Pieter, Bob, Charissa, Chris, Esther, Gabor, Jason, Lútzen, Marcel, Onno, Tryanda, Ward S., Willemijne, Adhi, Hendi, Marten, Oana, Pawel, Stephan, Shanti, Bart, Ed, Ellen, Gerhard, Gu, Reindert, Tjar, Ward v/d H. Special Thanks go to Pawel Tomaszewski, who did a great job in tutoring me during the project. Your advises, ideas, opinions and the coffee-talk were appreciated a lot, just as your help with the not always very user-friendly FEM software. Thanks for showing me how the cookie crumbles! A different kind of special thanks go to Marlies, who did a great job in supporting me during the project and dragging me away from the computer now and then to get some fresh air, sunlight and all the other good things in life. 121

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124 30. Lee, W.C.C., et al., 'Kinetics of transfemoral amputees with osseointegrated fixation performing common activities of daily living'. Clinical Biomechanics, (6): p Bergmann, G., F. Graichen, and A. Rohlmann, 'Hip joint contact forces during stumbling'. Langenbecks Archives of Surgery, (1): p Össur Europe, Prothesen Product Catalogus Otto Bock HealthCare, C-Leg Product Line Tudor-Locke, C., et al., 'Revisiting "How Many Steps Are Enough?"'. Medicine and Science in Sports and Exercise, (7): p. S537-S Cambridge Engineering Selector V Goodman, S.B., et al., 'Effects of orthopaedic wear particles on osteoprogenitor cells'. Biomaterials, (36): p Han, C.-M., et al., 'The electron beam deposition of titanium on polyetheretherketone (PEEK) and the resulting enhanced biological properties'. Biomaterials. 31(13): p Dumbleton, J. and M.T. Manley, 'Current concepts review - Hydroxyapatitecoated prostheses in total hip and knee arthroplasty'. Journal of Bone and Joint Surgery-American Volume, A(11): p Marti, A., 'Cobalt-base alloys used in bone surgerykobalt-basislegierungen in den knochenchirurgiealliages à base de cobalt utilisés en chirurgie osseusealeaciones con base de cobalto utilizadas en cirugía ósea'. Injury, (Supplement 4): p. D18-D Chandra, A., et al., 'Stress assisted dissolutlion of biomedical grade CoCrMo: Influence of contact loads and residual stresses'. Cirp Annals-Manufacturing Technology, (1): p Roy, R.K. and K.R. Lee, 'Biomedical applications of diamond-like carbon coatings: A review'. Journal of Biomedical Materials Research Part B-Applied Biomaterials, B(1): p Kurtz, S.M. and J.N. Devine, 'PEEK biomaterials in trauma, orthopedic, and spinal implants'. Biomaterials, (32): p Robotti, P., 'Plasma spray deposition of Titanium and Hydroxyapatite on PEEK and corbon fibre reinforced PEEK'. Society for Bioamterials Annual Meeting, Shepperd, J.A.N. and H. Apthorp, 'A contemporary snapshot of the use of hydroxyapatite coating in orthopaedic surgery'. Journal of Bone and Joint Surgery-British Volume, B(8): p Robotti, P., 'Effects of Plasma Spray HA coating process onto mechenical properties of PEEK and Carbon Fiber reinforced PEEK'. 8th World Biomaterials congress Lee, W.C.C., et al., 'Magnitude and variability of loading on the osseointegrated implant of transfemoral amputees during walking'. Medical Engineering & Physics, (7): p

125 Appendix I Project Planning Planning Master s Thesis: Osseointegrated fixation of upper leg prostheses Mike van Diest Time frame: 7 december juli 2010 The thesis will consist of 40 EC, which corresponds to 28 weeks. During these weeks a fixation system will be designed and a Finite element method (FEM) analysis will be performed. Then a prototype will be made, and this prototype will be tested in vitro, if there is enough time to do so. Phase week Subphases Date Analyses phase 1,2 Problem definition 7 Dec Dec. Goals Specify design assignment Program of requirements and whishes Function analysis SynthesisI 3,4 Brainstorm 4 Jan. 15 Jan. Pre-concepts + selection Synthesis II 5,6 Concepts + selection 18 Jan. 29Jan. Synthesis III (1): 7,8 Final concept 1 Feb. 12 Feb. final concept FMEA Synthesis III (2): 9-12 Torque 15 Feb Mar. FEM study Pull-out Synthesis III (3): Redesign, prototype and testing Redesign Develop test setup Order prototype+test setup Testing prototype 15 Mar. -28 May Extra time + report Improvements May-2 Jul. 125

126 Appendix 2 Selection of the final concept 126

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128 Appendix 3 Technical drawings final concept version 1 (for the workshop) 128

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134 Appendix 4 Material properties Ti-6Al-4V 134

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136 Appendix 5 Material Properties PEEK 136

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138 Appendix 6 Technical drawings final concept version 2 (for the workshop) 138