Performance evaluation of CLS Brevius Stem with Kinectiv Technology

Performance evaluation of CLS Brevius Stem with Kinectiv Technology Dirk Weidmann, Ronja Bruhn, Susanne Frei, Daniel Hertig, Xavier Langlois, Paul Hulme, PhD Zimmer GmbH Background A primary goal of total hip arthroplasty is the reconstruction of the patient s personalized kinematics. Three important factors for biomechanical reconstruction of the hip are leg length, offset and avoidance of impingement. Neck modularity improves the ability to re-create the head center to achieve these goals but also raises concerns regarding fretting debris and potential fracture. The objective of this article is to summarize the extensive in-vitro and experimental testing used to predict and therefore characterize the performance of the CLS Brevius Stem with Kinectiv Technology implant system invivo. Methods Using finite element analysis, the worst case stem and neck constructs were identified. Fatigue performance bench testing was performed for ten million cycles using ISO test methods and stringent internal performance requirements used for all Zimmer GmbH monoblock stems. For the evaluation of the connection strength of the Kinectiv Technology, distraction forces were measured after implantation of the neck according to the surgical technique. Fretting wear debris was also evaluated using quantitative accelerated corrosion fatigue testing on Kinectiv Stem/Neck/Head construct. The influence of ante-/retroverted necks on the loading of the implant bone interface was analyzed based on a literature review while the effect of stem shortening on primary stability was tested on cadaver paired-femurs. Results The CLS Brevius Stem with Kinectiv Technology exceeds the fatigue strength performance criterion as defined by internal requirements based on ISO test methods. The Kinectiv Neck taper junction dis traction strength is stronger than the clinically used Zimmer 12/14 head/neck taper. The fretting corrosion of the stem is less than the clinically derived benchmark and is similar to the current Zimmer M/L Taper with Kinectiv Technology. The degree of version offered by the Kinectiv necks does not exceed that found currently clinically, or values reported to result in altered femoral loading. Stem shortening does not significantly affect the torsion or axial subsidence behavior of the stem compared to the CLS Spotorno Stem. Conclusions The CLS Brevius Stem with Kinectiv Technology successfully meets Zimmer s performance strength requirements that have proven successful for many years. It demonstrates a fretting corrosion that is comparable to the mass loss of the Zimmer M/L Taper with Kinectiv Technology, both of them being less than the mass loss of the predicate Zimmer 6 taper benchmark. The CLS Brevius Stem with Kinectiv Technology provides a secure fit of the modular components. 1

Introduction A primary goal of total hip arthroplasty is the reconstruction of the patient s personalized kinematics. Three important factors for biomechanical reconstruction of the hip are leg length, offset and avoidance of impingement. Literature has reported that leg length and dislocation are major challenges in total hip arthoplasty (THA) 1 4. When introduced in the 1980 s, hip stems with modular heads and different CCD angles facilitated a more accurate and stable biomechanical reconstruction. Although these modular designs do offer enhanced versatility, intraoperative adjustments such as these introduce additional variables. For example, an increase in head length to improve muscle tension not only changes the femoral offset, but also increases leg length, which may not be desirable. Furthermore, modular head designs, whilst a clear improvement upon earlier designs, do not facilitate changes in femoral version, which is an important consideration when one considers the implications for impingement. Clinical studies report significant differences between patients, and genders, in femoral head center location and the size and shape of the medullary canal. For example, women tend to have lower head centers, less offset, and greater anteversion than men 5 8. When using traditional implants with very limited neck-shaft angle options, these differences can make size selection and head center placement a challenge especially in patients presenting with acute varus or valgus necks and surgeons may be forced to compromise through: sacrificing the bone by making lower neck cuts to avoid leg lengthening seating the stem high to increase leg length seating the stem in varus to decrease leg length seating the stem in valgus to decrease offset inserting a stem against the natural anteversion of the femur to achieve greater prosthetic femoral anteversion. The CLS Brevius Stem with Kinectiv Technology offers more intraoperative flexibility to reconstruct personalized kinematics through modular necks which enables true independent intraoperative leg length, offset and version adjustments. Independent version adjustments after stem implantation facilitates optimal stem positioning based on the patient s proximal femoral anatomy and may reduce the risk of femoral fracture 9. Its core design is based on the original CLS Spotorno Hip Stem with its excellent clinical history (survival rate of 95% at 20 years 10 ). Optimized stem length also supports surgeons in preserving more bone and facilitates less invasive surgery. Kinectiv Technology addresses leg length and offset restoration by offering five length options in 4mm increments ( 8mm, 4mm, +0mm, +4mm, and +8mm). This simple intraoperative flexibility is possible by offering a range of modular straight, anteverted and retroverted necks (4 to 10 ) to be used only in conjunction with a +0mm femoral head. Regardless of the clear advantages brought forward by a more bone conserving modular hip stem which offers great intra-operative freedom, Zimmer s responsibility to the patient drives the company to perform exhaustive pre-clinical testing in order to prove the design meets both stringent internal strength requirements, but also those mandated by the relevant authorities. The objective of this paper is to summarize the extensive testing that was undertaken for the design of the CLS Brevius Stem with Kinectiv Technology. The paper has been divided up into five sections, each of them highlighting the research activities which were undertaken to answer the following questions: Does the CLS Brevius Stem with Kinectiv Technology have sufficient fatigue strength to withstand in vivo loading? Is the taper connection strength between the neck and stem strong enough to withstand unwanted disassociation, and so inhibit micromotion? Is fretting corrosion minimized to ensure a stable connection and minimal metal ion release? Does the use of necks with version compromise the anchorage of the stem body in the bone? 2

Performance evaluation of CLS Brevius Stem with Kinectiv Technology What is the effect of stem shortening on the primary stability of the CLS Brevius Stem with Kinectiv Technology? Mechanical fatigue strength of the CLS Brevius Stem with Kinectiv Technology Stem Introduction A hip stem must be designed to withstand the millions of load cycles that it will be subjected to during its lifetime in vivo. The majority of hip stem implants on the market today only have one modular junction between the hip stem neck and the ball head. The concept of THA has been proven through extensive clinical studies, and standard designs have proved to have low fracture rates (1.5% 11 ). The modular connection between the hip stem neck and ball head was introduced in the early 1980s 12, as an initiative to allow the surgeon to make intraoperative changes, and also to facilitate the use of different ball head materials for different clinical indications. Further modularity, whilst viewed as an important evolution in implant design, must consider its impact regarding potentially altered fatigue strength and fretting corrosion through testing and reference to the extensive body of clinical evidence which exists. As it has been highlighted, the CLS Brevius Stem with Kinectiv Technology features an additional modular junction to incorporate the Kinectiv neck. As there have been reported fractures of other modular implant systems in literature 13 15, it is critical to ensure that, similar to the standard stem with a single modular connection, the fatigue strength of the new stem can be established. Methods Prior to in vitro testing, a detailed Finite Element Analysis (FEA) identified the CLS Brevius Stem with Kinectiv Technology sizes 5 and 20, combined with Kinectiv neck DD, as being the combinations which resulted in the highest implant stresses, which would be the highest contributors towards material fatigue. The strength performance requirement imposed on the CLS Brevius Stem with Kinectiv Technology implants was based on ISO 7206-6 16 and stringent Zimmer internal requirements. This requirement included loading, orientation, cycles and number of test samples. ISO 7206-6 describes the test methodology for cyclic fatigue performance evaluation of the proximal, unsupported region of the femoral hip stems, while the acceptance criterion for testing was that all stems should survive the 10 million load cycles without fatigue failures (test set up shown in Fig. 1). Fig. 1 Fatigue test setup The requirement was the same as that for Zimmer GmbH monoblock stems, including the CLS Spotorno Stem. Results All tested stems completed the 10 millions cycles without fracture. Since all stems met the acceptance criterion, the influence of additional load increases were evaluated, to estimate fatigue limits of the design. An increase in the safety factor of 11% and 26%, over that defined by the stringent test methodology, was found for the CLS Brevius Stem with Kinectiv Technology size 5 and 20 respectively (Fig. 2). 3

The fatigue strength of the CLS Brevius Stem with Kinectiv Technology was also compared to another modular stem design reported by Grupp et al. 14. Fatigue load (100%=acceptance criteria) 130% 120% 100% 80% 60% 40% 20% 0 +11% CLS Brevius Kinectiv size 5 Neck DD +26% CLS Brevius Kinectiv size 20 Neck DD Minimum strength requirement of Zimmer monoblock stems Fig. 2 Additional safety for the CLS Brevius Stem with Kinectiv Technology size 5 and 20. Grupp et al. investigated the clinical failures of a titanium femoral implant successively combined with a titanium (Ti-Adapter) and cobalt chromium (CoCr-Adapter) neck adapter due to clinical failures of the first. The fatigue strength of the CLS Brevius Stem with Kinectiv Technology size 5 combined with the Kinectiv neck DD was 15% greater than the modular stem with a CoCradapter reported in literature and 43% greater than the original Ti-Adapter 14 (Fig. 3). Design consideration for strength There are many factors involved in modular junction design that have direct and indirect influences on the fatigue strength performance of an implant. These factors affect local reaction forces, stress states, contact pressures and fretting motion, all of which significantly influence the strength of the construct. Grupp et al., reported that by altering the material properties of the neck from Ti to CoCr, this action increased the fatigue strength by 28%. However, as shown in the Zimmer study, the CLS Brevius Stem with Kinectiv Technology with titanium modular necks affords an even greater fatigue strength than the CoCr necks. The fatigue strength of any implant is not only dependent on material properties but also is heavily influenced by the design. One design aspect of the Kinectiv neck is junction length. The simplified illustration of Figure 4 shows the relationship of the forces acting on the neck component. Fig. 4 Simplified model of forces acting on a modular neck. Fig. 3 Fatigue strength comparison between another modular stem design with Ti- and CoCr necks and the CLS Brevius Stem with Kinectiv Technology size 5 combined with Neck DD. As the length of the neck/stem taper (LTaper) decreases, the reaction forces at the proximal (FO) and distal (FR) regions of the taper increase for a given head center location (LNeck). This principle shows that a longer neck/stem taper (LTaper) leads to lower loads in the junction. The design features embodied in the Kinectiv Technology design, including shape, size and surface finish, have been carefully engineered to meet Zimmer s rigorous strength requirements while maintaining minimal fretting motion (refer to section on ACF testing). 4

Performance evaluation of CLS Brevius Stem with Kinectiv Technology Also, the amount of version provided by the modular neck has an influence on the strength of the modular construct, since more version leads to a longer lever arms and thus to higher bending moments in the stem. Kinectiv Technology offers the amount of version that successfully meets Zimmer s stringent strength requirements. While meeting the strength requirements, the version provided by the anteverted and retroverted necks was optimized to address a wide range of patient anatomies 17. For a given head center l ocation, Kinectiv Technology offers the same amount of version regardless of the neck shaft angle (Fig. 5). Fig. 5 Version provided by Kinectiv Technology. With Zimmer s innovative, exclusive use of the +0mm femoral head, Kinectiv Technology is able to meet the critical requirements of clinical need and implant strength without skirted femoral heads that can reduce range of motion. Conclusion The CLS Brevius Stem with Kinectiv Technology proximal fatigue strength meets and exceeds Zimmer internal standards based on ISO require ments and that are the same as used for Zimmer GmbH monoblock stems, including the CLS Spotorno Stem. Even the sizes showing the highest implant stress exceeded the stringent acceptance criterion by at least 11%. Compared to a predicated modular hip stem with a CoCr-Adapter (reported in literature 14 ), the CLS Brevius Stem with Kinectiv Technology has 15% higher fatigue strength. Stability of the taper junction between the stem and the Kinectiv neck Introduction The connection strength between all modular junctions must be fit for purpose and meet the in-vivo requirements to be successful long term. Taper connection strength is important so as to ensure that, during loading, the taper does not disassociate and result in interface micro-motions, which may lead to fretting corrosion. It should be noted that due to the oval profile of the neck taper, disassociation of the stem/neck assembly due to high torsional loading events, is impossible. To disassociate the neck from the stem the force must be in line with the taper axis, so as to effectively pull the neck from the stem. To evaluate the connection strength of the Kinectiv neck/stem coupling following initial assembly, the distraction forces required to separate the neck from the stem were evaluated. The Kinectiv Neck was assembled to the stem using a controlled compressive load, as outlined by ASTM F 2009 18. For assembly, the surgical technique was used whereby the neck was placed into the stem, the ball head was placed onto the neck and both together were loaded with the assembly load. The force required to distract the neck from the stem was then measured. Results Distraction forces for the Kinectiv Neck/Stem junction exceeded those required to separate the head/neck taper for all specimens (Fig. 6). As suggested by research conducted by other institutions, the Kinectiv Neck/Stem junction is expected to be even more stable after initial cyclic loading via patient activities due to progressive seating of the modular components under loads generated in-vivo 19. Nevertheless, initial stability of the modularity is still important to ensure that it does not disassociate or allow bone chips and other debris to enter into the modularity prior to the first loadings by the implant recipient. 5

Conclusion The Kinectiv Neck/Stem junction demonstrates distraction forces that exceed those required to separate a typical 12/14 head/ neck taper. Method Testing was conducted using the refined test parameters, which are summarized in the table below (Figure 7). Head/Neck junction Stem/neck junction Fig. 6 Comparison of the pull-off force between a current Zimmer 12/14 taper and the Kinectiv Necks Taper. Note: 100% is the pull-off force of a current Zimmer 12/14 titanium taper combined with a CoCr-Head. Accellerated Corrosion Fatigue (Acf) Testing Introduction Numerous design and clinical factors are associated with the fretting and corrosion performance of implant modularity 20 23. To predict the clinical performance of modular implant designs, the corrosive boundary conditions (environmental parameters and loading) were modified to accelerate any fretting corrosion mechanisms which might occur in the modular junction. Through analysis of retrieved specimens and laboratory testing, Zimmer has been able to refine the testing parameters to effectively induce the observed clinical corrosion patterns of Zimmer s original 6 degree and newer 12/14 head-stem taper designs. Using the previously developed accelerated corrosion fatigue test, the corrosion behavior of the CLS Brevius Stem with Kinectiv Technology, with focus on the stem-neck modularity, was assessed and compared to the corrosion behavior of a predicate benchmark. Loading Environment Frequency Orientation Equipment Sinusoidal waveform R = 0.1 (max/min ratio) Ringer s Solution Acidic ph Increased Temperature 5 Hz (M/L angle, A/P angle) Straight (10 o /9 o ) 5,6 Cycles 10,000,000 Closed Loop Servo-hydraulic test machines in Load control Fig. 7 Final test parameters and accelerated corrosion fatigue test setup isolating the modularity of interest from the testing solution. The sample in the test chamber was placed within a secondary, heated bath on the test machine. The isolation of the taper junction, together with regular control of the testing parameters, such as ph and temperature, provided optimal conditions for test reproducibility. 6

Performance evaluation of CLS Brevius Stem with Kinectiv Technology Quantification of fretting corrosion A mass loss approach was used to quantify the amount of fretting corrosion. Pre-test and posttest, the samples were cleaned and weighed according to a defined cleaning procedure. The difference between pre-test and post-test weight is an indicator for the amount of fretting corrosion. The mass loss quantification consisted of the following steps: 1. Precleaning and initial mass determination consisted of sonication, multiple cycles of manufacturing grade cleaning and analytical balance mass measurement. 2. Corrosion fatigue testing according to the parameters previously outlined. 3. Post-test cleaning and final mass determination consisted of sonication, multiple cycles of manufacturing grade cleaning and analytical balance mass measurement. Accelerated corrosion fatigue testing with mass loss assessment was conducted using the Zimmer modular CLS Brevius Stem with Kinectiv Technology in combination with an extra-extended offset modular neck and a +0mm offset ceramic head. In a first step, a ceramic head was used, in order to minimize head-neck mass loss and focus on the modularity of interest: the neck-stem modularity. In a second step, the CLS Brevius Stem with Kinectiv Technology system mass loss was calculated adding the mass loss of a standard 12/14 head-neck (CoCr/Ti) Data analysis and comparison The fretting corrosion behaviour of the CLS Brevius Stem with Kinectiv Technology was compared to the Zimmer M/L Taper with Kinectiv Technology and the previously tested six degree taper with a long neck length. The predicate modular Zimmer M/L Taper with Kinectiv Technology was tested previously with the same neck and head combination, and tested according to the same setup. The previously tested six degree taper was introduced to the market more than 30 years ago 24 and serves as a reasonable benchmark for mass loss. Results and conclusions The mass loss of the CLS Brevius Stem with Kinectiv Technology was comparable to the mass loss of the Zimmer M/L Taper with Kinectiv Technology. The system mass loss for both systems is the sum of the mass loss of the tested neck-stem combination and the mass loss of a +0 mm CoCr/Titanium 12/14 taper combination (Figure 8). mg/10 million cycles 120% 100% 80% 60% 40% 20% 0 CLS Brevius Kinectiv Stem benchmark mass loss M/L Taper Kinectiv Stem combined neck-stem mass loss combined head-neck mass loss Fig. 8 Accelerated corrosion fatigue mass loss results of the CLS Brevius Stem with Kinectiv Technology (left) and the Zimmer M/L Taper with Kinectiv Technology (right). Head-neck mass loss refers to the mass loss of a standard +0 12/14 taper connection (CoCr-Ti). The benchmark line refers to the mass loss results of the marketed 6 taper. The system mass loss of CLS Brevius Stem with Kinectiv Technology was less than the mass loss of the predicate 6 taper benchmark (Fig. 8, red line). The mass loss of the CLS Brevius Stem with Kinectiv Technology meets Zimmer internal test requirements. 7

Effect of neck version on the loading of the implant-bone interface Introduction A literature review was performed to establish whether the use of necks with implemented version compromises the anchorage of the stem body in the bone. The results of this investigation are organized into the following sections: 1. Natural and post-operative version angles are reported to demonstrate that the Kinectiv anteversion and retroversion angles are within previously reported version angles for natural and implanted femora. 2. A review of reported investigations (FEA) is presented to illustrate how changes in loading which is transferred to the bone may be expected with different version angles. Results 1. The version angle of the Kinectiv Technology, allowing to address a wide range of patient anatomies 5,6,8, can be altered by a maximum of ±10. This is much smaller than the variability of the natural anatomical version angle, and the observed version angle reported after total hip arthroplasty (THA) 5,6,25. 2. Various authors have reported results from Finite Element Analysis (FEA) used to investigate the influence of version on hip contact forces and/or stem anchorage. Mathematical models have shown higher hip contact forces and moments with increasing anteversion angle 26 28. In contrast, the lever arm decreases with increasing anteversion angle resulting in a decrease in the torque applied to the stem 29. Most papers concluded that an increasing anteversion angle leads to increasing strain in the implant/bone interface. However, the degree by which the version would have to be changed in order to get a significant change in loading was greater than the maximum 10 change in version offered by the Kinectiv necks 26. Thus, for situations in which stem placement would result in a change in version, the ability to correct version using a modular neck may avoid situations in which excessive version would result in changes in non-anatomical and unreasonable femoral loading. Conclusions The range of version angles made available using the Kinectiv Technology already exists in the normal anatomical diversity of femurs and has been recorded post-operatively for modular and non-modular implants. Maximum version angles of the Kinectiv Technology are less than the change in version angle required to influence the loading of the implant/bone interface and thus anchorage of the stem. Modular necks give the intraoperative flexibiliy to avoid excessive version angles which may avoid influencing the loading of the implant/bone interface. 8

Performance evaluation of CLS Brevius Stem with Kinectiv Technology The effect of distal stem shortening on primary stability Introduction Press-fit femoral stems rely on the strength and integrity of the interface between the stem and the cortico-cancellous bone (primary stability). A well designed implant will demonstrate a high resistance to excessive interface micro-motion and migration of the stem under load. During the design phase of the CLS Brevius Stem with Kinectiv Technology it was hypothesized that the very distal end of the CLS Spotorno Stem may not be essential for fixation in bone and therefore the CLS Brevius Stem with Kinectiv Technology could be designed with reduced length. Shortening must not compromise the excellent fixation behavior of the original CLS Spotorno Stem. Therefore, using 3D CT scanbased femoral bone models from a database containing over 400 femoral models, the fixation principles of the CLS Spotorno Stem was investigated by determining the amount and type of contact with the cortical bone and evaluated with respect to different levels of stem shortening (10%, 20%, 30%, 40%). Once the possible amount of stem shortening was defined, the potential drop in primary stability, incurred through a shortening of the distal stem, has been discussed based in published literature 30,31. It has been theorized that any resulting increases in micro-motion (elastic deformation) may result in generation of fibrous tissue rather than bone, affecting secondary stability 32, and could compromise the long-term success of a shortened stem. Furthermore, stem migration may result in bone resorption and osteolysis, in addition to altering the intended biomechanical joint reconstruction 33. Consequently, it was essential to undertake testing to quantify the effect that stem shortening would have on primary stability. Tests were performed using cadaveric contralateral femurs in which the CLS Spotorno Stem and a 30% distally shortened CLS Spotorno Stem were implanted and then loaded cyclically, either in torsion or under loading conditions which would induce stem subsidence. During loading, the micro-motions at the interface and also the migration of the femoral stems were measured and compared between the original design and the shortened stem design, to evaluate whether the initial primary stability of the shortened CLS Spotorno Stem was significantly different (p<0.05) from that of the CLS Spotorno Stem. Methods Two modes of stem micro-motion under load were analyzed in the current study: torsion and axial subsidence. Thirteen matched pairs of cadaveric femora (26 femurs) were analysed, five pairs in torsion and eight pairs under loading conditions required for axial subsidence. Prior to stem implantation, parameters describing specimen demographics (age and gender) and femoral morphology (bone quality assessed using CT as well as geometry of proximal femoral canal) were analysed to determine the suitability of a cementless implantation and ensure that femurs of a matched pair were morphologically equivalent. Stems were implanted by two experienced surgeons. For a matched femoral pair, a normal CLS Spotorno and shortened CLS Spotorno Stem of equal size were implanted. Intra-operative parameters describing depth of insertion, anteversion of the femoral stem, resection cut and fit within the femoral canal (intra-operative fluoroscopy) were measured and used to ensure each matched pair was implanted in a similar manner. Results Torsion: No statistical difference was detected between the torsional micro-motion or migration of the CLS Spotorno and shortened CLS Spotorno stems for any applied moment (Table 1). Given that the stem body was not altered between the CLS Spotorno and shortened CLS Spotorno Stems, it is not surprising that no statistical difference was observed between the torsional behaviour of the two stems. 9

Subsidence: No statistical difference was detected between the subsidence micro-motion or the migration of the CLS Spotorno Stem and 30% shortened CLS Spotorno Stems for all applied loads (Table 1). All micro-motions that were observed during all loading steps were less than 150µm, indicating that no formation of fibrous tissue is predicted. Conclusion Shortening a stem, which has a mostly proximal fixation philosophy, by 30% (CLS Brevius Stem with Kinectiv Technology is shortened by 20% distally) did not appear to significantly affect the torsion or axial subsidence behavior of the stem. Table 1 Results from the statistical comparison of the micro-motion and migration of the CLS Spotorno Stem compared with the shortened CLS Spotorno Stem. P values obtained from the paired t-test are reported with the number of samples per load step analysed. Torsion Micro-motion Migration Load (Nm) p value p value 10Nm 0.804 (n=5) 0.332 (n=5) 20Nm 0.579 (n=4) 0.354 (n=4) Subsidence Micro-motion Migration Load (N) p value p value 750 0.413 (n=8) 0.451 (n=8) 1250 0.679 (n=7) 0.322 (n=7) 1750 0.512 (n=8) 0.350 (n=8) 2500 0.948 (n=8) 0.312 (n=8) Summary conclusion Extensive in vitro testing of the mechanical, biomechanical, physicochemical and structural stability of the CLS Brevius Stem with Kinectiv Technology has been performed. The following is a summary of the conclusions which address the research questions posed at the beginning of this paper. The CLS Brevius Stem with Kinectiv Technology exceeds the fatigue strength performance criterion as defined by Zimmer internal requirements based on ISO test methods. The strength of Kinectiv Necks is the results of a specific geometry featuring: a long(er) neck taper a range of offset and version that successfully meets stringent Zimmer internal requirements The Kinectiv Neck/Stem junction demonstrates distraction forces that exceed those required to separate a typical 12/14 head/ neck taper The fretting corrosion behavior of the CLS Brevius Stem with Kinectiv Technology is less than the clinically derived benchmark and is similar to the current Zimmer M/L Taper with Kinectiv Technology. The degree of version offered by the Kinectiv necks does not exceed that found currently clinically, or values reported to result in altered femoral loading. In addition, modular necks give the intraoperative flexibility to avoid excessive version angles which may avoid influencing the loading of the implant/bone interface. Through cadaveric testing, it has been established that shortening the CLS Spotorno Stem by 30% (CLS Brevius Stem with Kinectiv Technology is shortened by 20% distally) did not appear to significantly affect the torsion or axial subsidence behavior of the stem compared to the original CLS Spotorno Stem. 10

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Performance evaluation of CLS Brevius Stem with Kinectiv Technology Notes: 13

14 Notes: