POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED 3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS POST INSERTION IN THE BEAGLE DOG

Transcription

1 POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED 3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS POST INSERTION IN THE BEAGLE DOG Damen M. Caraway, B.S., D.D.S. An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2007

2 Abstract Introduction: The use of miniscrew implants for orthodontic anchorage has raised questions concerning their limitations. Specifically, the maximum shear force that an immediately loaded miniscrew can withstand has not been investigated. The specific aim of this study is to determine the maximum shear resistance of miniscrew implants. The effect of immediate loading on the maximum shear resistance of miniscrew implants will be compared between implants of two different lengths, and with three different applied force loads. A comparison of shear force at failure will also be made according to the depth of the miniscrews in bone. Methods: The sample was derived from five skeletally mature beagle dogs that had 60 miniscrews placed at predetermined locations in the palate and buccal surface of the mandible. Miniscrews were immediately loaded with either 0 (control), 600, or 900 g. After six weeks of continuous force application, 45 of the miniscrews remained in place. The dogs were then sacrificed, and bone samples from the maxilla and mandible were dissected such that each contained one orthodontic miniscrew. The bone specimens were mounted in dental stone for testing purposes and secured in a universal vice for mechanical testing. 1

3 Testing was performed by the application of a shear force in the same direction as the original force until failure of the implant. The maximum force sustained by the implants prior to failure was recorded in Newtons (N). Results: The mean shear force at failure of 6 mm miniscrew implants was significantly higher (53.0 N ± 8.3 N) (Mean ± SE) than that of 3 mm implants (31.9 N ± 4.1 N). No significant difference in force at failure was noted between implants that were immediately loaded, and those that served as controls. A significant difference was determined to be present between the groups formed by extent of bony purchase. The groups with 2-3 mm and 3 mm+ of bony purchase showed a significantly higher shear force at failure than the groups with 0-1 mm or 1-2 mm of insertion depth. Shear force at failure showed a moderately strong correlation (r=0.57) with the depth of the miniscrew in bone. Conclusions: Immediate loading of miniscrews has no significant effect on maximum shear force at failure. Complete cortical engagement by miniscrews may result in significantly higher shear resistance. Miniscrews as short as 3 mm can withstand shear forces well beyond levels typically used in orthodontics. 2

4 POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED 3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS POST INSERTION IN THE BEAGLE DOG Damen M. Caraway, B.S., D.D.S. A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2007

5 COMMITTEE IN CHARGE OF CANDIDACY: Professor Rolf G. Behrents, Chairperson and Advisor Assistant Professor Ki Beom Kim Assistant Professor Donald R. Oliver i

6 DEDICATION To my lovely wife, Tiffany, for her unwavering support during my years of formal (and informal) education; I thank you for your love, patience, sacrifice, and understanding in my pursuit for something better. I love you more than words can describe. To my wonderful children, Avery, Gavin, and Sydney; the time not spent with you during my professional training will ultimately allow me to spend more time with you in the future; I do this all for you. To my parents, who inspired me to be a little better and supported me every step of the way. To all the teachers in my many years of education; I express to you my gratitude, and hope you always feel that your efforts are appreciated. ii

7 ACKNOWLEDGEMENTS I would like to acknowledge the following individuals: Dr. Rolf Behrents for chairing my thesis committee. Thank you for your guidance, insights and time. You have fulfilled your responsibility in teaching me how to think. Dr. Don Oliver for serving on my thesis committee. You are a great teacher and mentor. Your love for orthodontics and teaching are obvious. Thank you for your time and suggestions. Dr. Ki Beom Kim for serving on my thesis committee. I truly appreciate your assistance with the writing of my thesis. I would also like to thank the following individuals for their help and expertise: Dr. Micah Mortensen for his diligence and assistance throughout the entire process of developing and completing this thesis. Dr. Heidi Israel for her assistance with the statistics in this project. The Orthodontic and Education Research Foundation for contributing to the funding of this project. iii

8 TABLE OF CONTENTS List of Tables...v Chapter 1: Introduction...1 Chapter 2: Review of the Literature Anchorage in Orthodontics...4 In Search of Skeletal Orthodontic Anchorage...5 Osseointegrating Titanium Implants...6 Alternative Forms of Skeletal Anchorage...8 Palatal Implants...9 Palatal Onplants...10 Miniplates...11 Orthodontic Miniscrew Implants...11 Design...12 Sites for Placement...15 Time of Loading...16 Applied Force...18 Miniscrew Implant Stability...19 Testing of Bone Screws...21 Management of Bone Samples...21 Pull-Out Tests...22 Orthopedic Screws...23 Maxillofacial Rigid Fixation Screws..23 Orthodontic Miniscrews Implants...24 Shear Tests...26 References...28 Chapter 3: Journal Article Abstract...37 Introduction...39 Methods and Materials...44 Sample Selection...44 Miniscrew Implants...44 Preparation of Samples for Testing...45 Mechanical Testing...46 Statistical Analysis...47 Results...49 Miniscrews Failures...49 Maximum Shear Force Measurements...50 Discussion...53 Conclusions...61 References...63 Vita Auctoris...67 iv

9 LIST OF TABLES Table 3.1: Type, Location and Load Category of Surviving Miniscrews...50 Table 3.2: Maximum Shear Force at Failure in Newtons (N)...51 Table 3.3: Mann-Whitney U Test Results of Loaded Versus Control Implants...52 Table 3.4: Kruskal-Wallis and Scheffe Post Hoc Results of Maximum Force at Failure by Depth of Screw in Bone...52 v

10 CHAPTER 1: INTRODUCTION In orthodontics, malpositioned teeth are moved into proper alignment by the application of force. This force originates from wires, elastics and other appliances attached to the teeth. Often, teeth that are in proper alignment are used to provide the force to move those that are not, and are referred to as anchorage teeth. In accordance with Newton s third law, there is a reactive or equal and opposite force for every applied orthodontic force. Unfortunately, these reactive forces often result in undesirable movements of the teeth serving as anchorage. Anchorage, broadly defined as the degree of resistance to displacement, is a critical component to successful orthodontic treatment. As a result, orthodontists have historically used a variety of appliances and strategies to enhance anchorage, particularly when minimal movement of the teeth providing the anchorage is desired. This allows the movement of malaligned teeth while leaving teeth that do not need to be moved relatively undisturbed. Anchorage enhancing appliances, such as headgear, are highly dependent upon patient compliance for success. 1

11 In an effort to establish anchorage without significant reliance on patient cooperation, other forms of anchorage have been investigated. Restorative dental implants, despite their stability in bone, have limited use in orthodontics due to cost, an extensive healing period after surgical placement, and anatomic placement limitations. Still, the use of these titanium dental implants as a form of anchorage has provided the potential for absolute, compliance independent, orthodontic anchorage. Orthodontic miniscrew implants have been designed to circumvent the limitations posed by restorative dental implants. These smaller bone screws are significantly less expensive, are easily placed and removed, and can be placed in almost any intra-oral region, including between the roots of the teeth. Some basic questions remain, however, concerning the limitations of miniscrews. Specifically, what is the maximum amount of lateral or shear force that can be applied to these miniscrews before they fail? How does a force that is applied immediately after the miniscrew is placed affect the maximum holding power of the implant? To what extent does the total length of screw engaged in bone affect the maximum shear force the implant can withstand? These are questions that remain unanswered in the literature. 2

12 The present study intends to provide information on the maximum shear force that immediately loaded orthodontic miniscrew implants can withstand before failure. In a preliminary study, 1 3 and 6 mm orthodontic miniscrews were placed in the maxilla and mandible of the beagle dog and immediately loaded. Two levels of force were applied (600 and 900 grams). After a period of six weeks, the dogs were sacrificed. These implanted miniscrews were utilized in the present study to determine the maximum shear force that can be applied prior to implant failure. Comparisons of the maximum force at failure will be made according to implant length (3mm, 6mm), applied force (0g, 600g, 900g), location (maxilla, mandible) and depth of the screws in bone. 3

13 CHAPTER 2: REVIEW OF THE LITERATURE Anchorage in Orthodontics The attainment and control of anchorage is fundamental to the successful practice of orthodontics and dentofacial orthopedics. According to Newton s well-known law of physics, action and reaction forces are equal and opposite. In orthodontics, anchorage is used to describe resistance to reaction forces. 2 Teeth are the usual source of anchorage and, in the typical orthodontic biomechanical situation, are pitted against one another to produce tooth movement. The teeth serving as the anchorage unit, by virtue of their number, position, size and number of roots, intend to offer resistance to movement so as to bring about the movement of the other teeth. A threshold value of force to initiate tooth movement has not been identified, 3 but appears to be very low. 2 For example, tooth movement has been detected with as little as 4 gm of force. 4 Considering this principle, is has been concluded that the practice of pitting more teeth with a larger root surface area against fewer teeth with less surface area in intraarch mechanics may not be sufficient to prevent movement of anchor teeth. 5-7 Therefore, in order to achieve increased 4

14 anchorage control, a supplemental form of anchorage is often required. Traditionally, headgear and intermaxillary elastics have been used as forms of supplemental anchorage. 2 While this form of supplemental mechanics may be effective in increasing anchorage, effectiveness depends upon the cooperation of the patient. Consequently, orthodontic anchorage control has historically been contingent on patient compliance. Due to the inconsistent nature of such compliance, 8 orthodontists often note the unfavorable reciprocal movement of the intra-arch and inter-arch anchor teeth. In Search of Skeletal Orthodontic Anchorage Orthodontists have recognized that stability of reactive anchorage units could be significantly increased if orthodontic anchorage could be provided by the skeletal bone itself, 9 and in the 1940s began to conduct research on the subject. An early study by Bernier and Canby suggested that surgical vitallium bone screws were inert and stable in bone. 10 However, when Gainsforth and Higley 9 attempted to use these screws as a source of orthodontic anchorage they were largely unsuccessfully. 5

15 Subsequently, many other investigators have attempted to identify a successful form of skeletal anchorage through the use of a variety of endosseous implants and bone plates. In addition to the original study using surgical vitallium screws, research was conducted during this early era on each of the following: blade implants, 11 vitreous carbon, 12,13 bio-glass coated aluminum oxided, 14 and vitallium implants. 15,16 All of these implant types have exhibited some degree of success in terms of implant stability when subjected to orthodontic forces. However, each implant system exhibited some form of weakness such that stability was unpredictable. Consequently, none of these implant systems have gained widespread clinical acceptance. It was not until the development of osseointegrating titanium implants that a reliable source of skeletal anchorage was established and found widespread clinical application. Osseointegrating Titanium Implants In the 1960s, Brånemark discovered the unique healing response exhibited by bone when it was exposed to titanium. He and his colleagues later described the 6

16 process by which a titanium fixture could be embedded and incorporated into bone. 17 This phenomenon became known as osseointegration, and was introduced to restorative dentistry in From this principle came the development of titanium dental implants, which resemble the root of a tooth (approximately 4 mm X 9-15 mm) and provide for the replacement of missing teeth without compromising adjacent teeth. Dental implants, having been proven highly successful, 18 have been referred to as the most influential change in dentistry during the last half-century. 19 Despite the discovery of osseointegration in the 1960s and rapid development of traditional titanium dental implants, such devices were not evaluated for use as orthodontic anchorage until the 1980s In an early study by Roberts and associates, they demonstrated that implant osseointegration and stability persisted despite the application of an orthodontic force. 20 Consequently, osseointegrating titanium dental implants were considered an effective source of skeletal orthodontic anchorage Endosseous titanium dental implants have been used to provide anchorage independent of patient compliance and without the need to accept unfavorable reciprocal movement of anchor teeth. Unfortunately, dental implants are associated with some distinct and significant disadvantages 7

17 that limit routine use in orthodontics. For example, the size of restorative implants (approximately 4 mm X 9-15 mm) limits the anatomic sites available for placement (e.g., edentulous areas or the retromolar region). Furthermore, use of these implants is highly dependent on a precise 2- stage surgical protocol and a healing time of 3-6 months prior to the application of orthodontic force. 18,20,22,30 Considering the time required to complete orthodontic treatment alone, this additional time for healing is considered a significant deterrent in terms of the use of dental implants. Such limitations have motivated a search for alternative forms of orthodontic anchorage via implants. Alternative Forms of Skeletal Anchorage Based on the need to develop a form of skeletal anchorage in orthodontic patients, alternate forms of implant anchorage have been developed. These devices include palatal implants, palatal onplants, surgical miniplates, and miniscrews. 8

18 Palatal Implants Designed after a traditional restorative implant, palatal implants are intended to osseointegrate and provide a rigid point of attachment for the teeth. The Straumann Orthosystem (Institut Straumann AG, Waldenburg, Switzerland) is an example of a palatal implant. This type of implant consists of three parts: a self-tapping endosseous body, a smooth cylindrical collar, and an octagonal head used to connect attachments. 31 In terms of placement, after the removal of a small circular section of palatal mucosa and the preparation of an appropriate pilot hole, the implant is inserted, covered, and allowed to heal. 32 After a healing period of at least 3 months 32,33 a second surgical procedure is performed to uncover the implant and place an apparatus that allows attachment to the teeth. Despite the established success of palatal implants in providing anchorage, there are, again, certain drawbacks to this system that have limited clinical acceptance. The size of the implant and the invasive surgical procedures required for placement, use and removal are likely the most significant disadvantages. In addition, the healing time required prior to loading, and the time and cost associated with fabrication of custom attachments make this type of 9

19 implant even less appealing. Lastly, because these implants are not intended to be permanent, there are potential problems associated with their removal from the palate. If the degree of osseointegration cannot be overcome by the use of a hand ratchet, trephination of the implant and the surrounding bone must be performed. This procedure leaves a void in the palatal bone which is left to granulate and heal over time. 32 Palatal Onplants The palatal onplant, developed by Block and Hoffman, is a unique device consisting of two parts: a dome shaped disk (7 mm diameter x 3.5 mm thick) and an abutment that screws into the center of the disk. 34 This fixture is designed to lay against the bone of the palate under the periosteum. This is in contrast to palatal implants which penetrate the bone. When surgically placed subperiosteally, bone grows into the hydroxyapatite-coated surface of the disk resulting in osseointegration. This process of osseointegration requires at least 10 weeks of healing, 35,36 after which an incision is made and the abutment is attached and left protruding through the soft tissue. The palatal onplant shares similar disadvantages with the palatal implant. The possible locations for 10

20 placement are limited, and three surgeries and an extended healing time are required. Miniplates Miniplates are derivatives of rigid fixation plates used in maxillofacial surgery. They are secured to the bone with two or three bone screws and have an extension arm designed to extend through the mucosa into the oral cavity. The arm, which measures mm, serves as a point of attachment for the orthodontic appliance. Unlike the previously described palatal implants and onplants, miniplates can be placed in various locations including the zygomatic buttress, the periform rim, and the lateral border of the mandible. 37 A surgical flap is required to place miniplates, and a healing period is reccommended. 38 A second surgical procedure is required to remove the plates when they are no longer needed. Orthodontic Miniscrew Implants The development and improvement of dental implants and maxillofacial fixation methods brought about the evolution of orthodontic miniscrew implants. They have 11

21 been designed to circumvent the shortcomings of other forms of skeletal anchorage in the context of orthodontic anchorage. The first successful screw shaped implant used exclusively for orthodontic anchorage was reported in In this report maxillary incisor intrusion was accomplished in a deep-bite patient with a miniscrew for anchorage. 15 Since that time many miniscrew designs have been developed, and there has been a dramatic increase in use and popularity. It has been argued, however, that their utilization has preceded a thorough understanding of the biology involved and their mechanical potentials. 39 Design In recent years, many different miniscrew implants have been designed and manufactured for orthodontic use. Today, the material of choice for miniscrews is titanium. Titanium allows for the small size and weight of the miniscrew without compromising strength and biocompatibility. 40 The common shape of these designs is a threaded cylindrical body with a conical tip. Variations center on the basic features of the screw portion in terms of diameter, length, thread width and pitch, and head design. Another important variation involves the screw being self-tapping and self-drilling. The sharp threads of 12

22 a self-tapping screw cut into the bone and advance the screw as it is turned. All screws are self-tapping. All screws are not self-drilling, however. Those that are not require the preparation of a drilled pilot hole prior to insertion. Self-drilling screws have a drill-shaped point and a specialized cutting flute that allows insertion without prior drilling. This type of miniscrew, sometimes called drill-free, has been shown to exhibit more bone-tometal contact and less mobility than miniscrews placed with a pre-drilled pilot hole. 41 The design of a miniscrew can significantly affect its function and stability. Increased length, which can provide for bicortical placement, improves primary, or initial, stability. 42 Numerous studies have reported successful use of miniscrews 6 mm in length When Deguchi et al. loaded 96 implants (1 mm x 5 mm) with g of force, 93 of the miniscrews were still stable after 3 months. 48 There are no reports in the literature, however, of the stability of miniscrews shorter than 4 mm. The diameter of a miniscrew is another design feature that seems to play an important role in stability. Miniscrew diameters vary widely among, and within different manufacturers. Miniscrews currently on the market range in diameter from 1.2 to 2.0 mm. Various diameters of 13

23 miniscrews have been shown to be successful in providing anchorage. Park et al. reported on 227 miniscrews of four types with diameters of 1.2 mm and 2.0 mm. 49 The overall success rate was 91% and no difference was noted between the implants of different diameters. In a case report describing the use of miniscrews in non-extraction treatment, miniscrews measuring 1.2 mm X 6 mm were used with success to move an entire arch en masse. 50 There appears to be a limit, with regard to the diameter of miniscrews, below which success is compromised. In a study by Miyawaki et al., 51 all 1.0 mm diameter screws failed, but the 1.5 mm and 2.3 mm diameter screws showed no significant differences with success rates of 83.9% and 85%, respectively. The authors concluded that a diameter of less than 1.0 mm was a significant criterion associated with failure. The advantage of a thinner screw is that it can be placed in more locations, such as between the roots of teeth. The drawback, however, is the greater potential for screw fracture. 52 Cope has stated that the minimum diameter to avoid metal failure should be 1.5 mm

24 Sites for Placement Numerous studies and clinical reports show a variety of implant placement sites including the retromolar pad, 54 palate, 55,56 and the maxillary and mandibular buccal cortex. 51 In addition, it has been shown that implants can be inserted into the anterior nasal spine, the symphysis 57 and are the only type of implant that can be placed between the roots of the teeth. 58 In studies of miniscrew implants that have been performed on dogs, implants have been placed in the palate, 43,59,60 the lingual cortical plate of the mandible, 45 and the maxillary and mandibular buccal cortex. 43,44 The literature is conflicting when comparing the stability of implants placed in the maxilla versus those placed in the mandible. Cheng and colleagues found that miniscrews in the posterior mandible were susceptible to increased failure rates when compared to the anterior mandible, anterior maxilla, and posterior maxilla. 61 Tseng et al. also found higher failure rates of miniscrews in the mandible. 62 Relating bone contact to stability, Wehrbein and associates found a 79% bone-to-implant contact in the maxilla as compared to 68% in the posterior mandible

25 In contrast to the aforementioned studies, two additional studies suggest a higher degree of stability for implants placed in the mandible. Deguchi et al. reported that implants placed in the mandible were found to have higher bone-to-implant contact, 48 while Bischof et al. showed that 3 months after placement, implants in the mandible were more stable than those placed in the maxilla. 64 Time of Loading Suggested healing times for orthodontic miniscrew implants cover a broad range. One of the earliest studies on these implants recommended a period of 9 months prior to force application. 54 The lack of consensus is evidenced by a more recent study. Using the beagle dog as a model, Ohmae et al. tested the efficacy of miniscrews for orthodontic intrusion. 65 After allowing six weeks for the 4 mm long miniscrews to heal, they were loaded with a 150 g of force. At the end of the 18 week loading period, all 36 of the implants were stable in the bone, and 4.5 mm of intrusion had been achieved. Despite this success, the author suggested that the 6 week healing time may still have been too short. 16

26 Immediate loading of miniscrews has become more common, with several reports in the literature to support the practice. 57,66-68 Doi conducted a study in which 48 miniscrews (6 mm) were placed in the jaws of four beagle dogs. Immediately after placement, two miniscrews were connected to each other by nickel titanium coil springs that produced either 300 or 600 g of force. Two of the 48 miniscrews were placed near erupting teeth and failed shortly after placement. This required their removal, and the exclusion of the other miniscrews to which they were connected. The force remained active for 5 weeks. At the end of the testing period 5 out of the 44 remaining miniscrews demonstrated significant mobility. The author concluded that miniscrews can be loaded immediately with orthodontic, and even orthopedic, force levels with a success rate of 100%. 43 Another study by Owens was designed to place 56 miniscrews (1.8 mm X 6 mm) into the jaws of 7 beagle dogs. 69 Twenty-one of the implants were immediately loaded with either 25 or 50 g of force. Even though three of the immediately loaded miniscrews failed within 21 days of placement, a comparison of the delayed vs. immediately loaded miniscrews showed no differences in failure rate. This data suggests that the success of miniscrew implants 17

27 is not dependent on timing of implant loading. There are no studies in the literature, however, that have described the affect immediate loading has on the maximum amount of force an implant can withstand before failing. Applied Force The literature supports the view that a wide variety of force loads can be applied to miniscrew implants without the implant failing. In an early study, miniscrews were loaded with 60, 120 and 180 g of force. 16 After 28 days, the implants showed no significant movement at any of the force levels described. Another author described the placement of 96 miniscrews into the buccal and lingual cortical plates of 8 mature beagle dogs. 45 The implants were immediately loaded with 25, 50, or 100 g of force which remained active over 98 days. One of the 96 miniscrews, which was loaded with 100 g of force, failed after 50 days, but all others remained stable in the bone. In another study, after a healing period of 12 weeks, a group of 20 miniscrews were loaded with 250 to 350 g. 70 All of the miniscrews withstood the force until they were removed 3 months later. In another study, Doi immediately loaded miniscrews with either 300 or 600 g and noted significant mobility in 5 of the 44 implants. He also 18

28 measured the displacement of the miniscrews and reported an average of less than 0.5 mm per implant loaded with 600 g. Turley et al. placed implants in the zygomatic buttress of dogs, and allowed them to heal for 20 weeks. 22 The implants successfully withstood a load of 1000 g for 18 weeks. Though these studies have demonstrated a range of force loads that can be applied to miniscrews without significant failure, there is no research delineating the maximum force load that can be sustained by orthodontic implants. Miniscrew Implant Stability In contrast to osseointegrating dental implants, miniscrew implants are intended to be temporary. Thus, the screws are intentionally not subjected to surface treatments (e.g., sandblasting, etching, plasma spraying) designed to increase the percentage of bone-to-implant contact. 60 At the time of miniscrew removal, integration is overcome by hand with a surgical driver. Upon placement, the ability of a miniscrew to provide anchorage depends on the mechanical retention provided by intimate contact between bone and the surface of the implant. This mechanical retention, also known as 19

29 primary stability, is critically important for orthodontic anchorage to be successful, 71 and is influenced by several factors. For instance, implants with greater diameter and thread depth will have a larger surface area in contact with bone, and theoretically, more primary stability. The quality and quantity of the bone into which miniscrews are placed also influences their primary stability and subsequent success. 72,73 Cortical bone is much denser than cancellous bone, and provides for more intimate contact between the bone and the threads of the miniscrew. A recent study found a weak but significant positive correlation (r = 0.39) between cortical bone thickness and miniscrew implant pull-out strength in dog bone. 59 Indirectly, Miyawaki and colleagues related cortical bone thickness to implant failure by noting that patients with high mandibular plane angles were more likely to experience implant failure. 51 The link was provided by Tsunori et al. who quantified a thinner cortical plate in patients with high mandibular plane angles. 74 The extent to which miniscrew design and bone quality influence the stability of miniscrews and the maximum force they can withstand is not completely known or described in the literature. It has been suggested that orthodontists are still in search of the formula for 20

30 ultimate miniscrew stability. 39 As different variables that affect miniscrew stability are investigated, such as miniscrew design, stability can be quantified for comparison. Quantifying the stability of miniscrews is accomplished by performing the same tests on miniscrews that are used to evaluate orthopedic bone screws. Testing of Bone Screws Before World War II, selection of screws for orthopedic implantation was based primarily on the ease of insertion. 75 Later, stability became the primary selection factor and tests were designed to determine the differences between various screws. The most common test performed on bone screws of any type is the pull-out test. An alternative to the pull-out test is the shear test, which examines the effects of tangential or lateral forces. Management of Bone Samples Many of the tests performed on bone screws utilize non-living bone. It is important to note that for accurate correlation of fresh bone tests to living bone, proper preservation techniques used during preparation and testing 21

31 of bony samples are essential. 76 Various methods that have been used to adequately preserve bone have been described in the literature. In early tests, bone samples were wrapped in wet paper towels, enclosed in plastic, and stored at 7 C for no more than 48 hours. 77 More recently, fresh bone samples have been stored in refrigerated physiologic saline, 78 wrapped in saline soaked gauze and stored at -15 C, 59 or simply sealed in plastic bags and stored at -25 C prior to testing. 79 It has been shown that the freezing process does not have an adverse effect on the elastic properties of bone. 76,80,81 Pull-Out Tests The pull-out test is considered an accurate method of evaluating the relative strength or holding power of surgically placed bone screws. 77,82 Holding power is defined as the maximum uniaxial tensile force needed to produce failure in the bone. 83 Pull-out tests measure holding power against tension applied along the longitudinal axis of the screws. Results of pull-out tests have been reported on numerous types of bone screws. 22

32 Orthopedic Screws Several authors have reported specific results of pull-out tests on screws used in orthopedic surgery. 77,84,85 Koranhi et al. placed large diameter screws in canine and bovine femurs to test the difference between two thread types. 77 No difference between the screws was detected, but the results showed a linear relationship between pull-out strength and cortical bone thickness. In another study, screws intended for use on the spine were inserted into porcine (pig) vertebral bodies. 84 Despite differences in diameter ( mm), length (25-35 mm) and thread depth (1-1.8 mm), no significant differences were noted in axial pull-out strength which measured an average of 268 lbs (1194 N). The authors concluded that the shorter test screws with increased thread depth could provide as much holding power as the routinely used longer screws. Maxillofacial Rigid Fixation Screws The designs of currently available orthodontic miniscrew implants are similar to rigid fixation bone screws. Rigid fixation bone screws, used in orthognathic surgery to affix rigid plates to the bones of the face, have been tested by means of pull-out tests. 78,82,86-88 Foley et al. tested five different types of fixation screws in 23

33 the long bones of a mongrel dog and found a mean pull-out tension of 44.5 kilograms (436 N). 82 In another study, five additional types of fixation screws were tested using bones from the skull and mandible of human cadavers. 86 The screws varied in diameter and length and demonstrated pull-out strengths of up to 620 N (1 N = 102 g). In pull-out tests of 2.0 mm diameter screws in porcine rib with a cortical thickness of between 0.5 and 2.0 mm, Boyle et al. found a mean pull-out force of 21 kg (205 N). 88 In a similar test by the same author another group of 2.0 mm diameter screws showed pull-out strengths ranging from kg ( N). 78 It should be noted that the results of tests performed on screws in animal bone may not directly correlate to human orofacial cortical bone due to differences in mechanical properties. Specifically, in the dog model the alveolar process has a higher density than the equivalent structure in the human. 89 Orthodontic Miniscrews Implants Recently, three studies have been conducted on the pull-out strength of orthodontic miniscrew implants. Pickard placed miniscrews in cadaver mandibles and evaluated the effects of implant orientation on stability 24

34 and resistance to failure. 90 The study also intended to identify the maximum holding power of miniscrew implants in the human mandible. Pulling on the implants at various angles, he determined that a direction of force along the long axis of the implant offered the greatest resistance to failure. The results of his axial pull-out tests showed a maximum force at failure of 342 N ± 80.9 N (Mean ± S.D.). Huja and colleagues have conducted two studies on the pullout strength of miniscrews. Both of the studies were designed to determine the maximum pull-out strength of miniscrews in the maxilla and mandible of beagle dogs. In the first study, the implants were placed and tested immediately after the dogs were sacraficed. 59 Average pullout strength of all implants measured 222 N. The second study tested the screws in the same manner after they were allowed to heal, unloaded, for 6 weeks. 60 Average pull-out strength of implants after 6 weeks of unloaded healing was 245 N. There was no significant difference in the pull-out strengths between the two time periods. The authors did, however, show a weak but positive correlation between pullout strength and cortical plate thickness in both studies. 59,60 25

35 Shear Tests Although tests of the stability of bone screws have been primarily focused on pull-out, it is important to recognize that these pull-out tests alone are not totally adequate to measure the anchorage potential of bone screws. They do not, for example, address shearing forces which are present in a clinical setting. 82 There is a limited number of reports on shear testing of bone screws or implants in the literature. 84,90 Glatzmaier et al. tested the shear strength of a bioresorbable polylactide implant in vitro. 91 These experimental implants showed a shear force at failure of 50 N. Pierce et al, working with instrumentation screws used in the vertebral bodies of the spine, conducted pullout and shear tests on screws with diameters of between 6.5 and 7.5 mm. 84 The results showed an average maximum shear force at failure of 786 N. Pickard has conducted the only shear tests on orthodontic miniscrew implants. 90 As described above, he studied the effect of orientation of miniscrew implants on resistance to failure. Placing miniscrews in human cadaver bone and immediately afterwards performing shear tests, he determined that the mean shear force required to cause failure (123 N) is roughly a third of the mean axial pullout force (342 N). 26

36 There are no reports in the literature of shear tests on miniscrew implants that have been placed in living bone and allowed to heal, nor have there been shear tests performed on miniscrews that have been immediately loaded. The purpose of this study is to determine the affect of immediate loading on the maximum shear resistance of miniscrew implants, and to compare this effect between implants of two different lengths, and with three different applied force loads (0, 600 g, 900 g). This will be accomplished in a dog model using a sample involving 3 mm and 6 mm miniscrews that were immediately loaded or unloaded (control) for a period of six weeks, and then subjecting both loaded and control implants to shear force testing. The goal of shear force testing is to imitate, as closely as possible, the conditions that exist when an orthodontic miniscrew is subjected to lateral forces in the mouth. Many variables are responsible for the maximum shear force a miniscrew can withstand. By studying the expression of these variables, this study may give clinicians a better indication of what can be expected of orthodontic miniscrews in clinical practice. 27

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