Introduction Extensor mechanism disruption is a debilitating complication that can occur following total knee arthroplasty. High primary repair failure rates have made reconstruction the treatment of choice. The literature describes two methods of reconstruction with synthetic mesh, however these methods have not been biomechanically compared. Methods Synthetic mesh constructs were implanted in six matched pairs of cadaveric tibiae using the tuberosity technique, where mesh is anchored at the tuberosity, and the intramedullary technique, where the mesh is cemented within the medullary canal underneath the tibial component. During biomechanical testing, joint reaction forces were applied to the tibial base plate while axial tensile loads were applied to the mesh construct to measure stiffness, yield force, and ultimate force to failure. Displacements of the tibial tray and the point of load were tracked using an infrared motion analysis system. Results There were significant differences between the two constructs. Overall, the intramedullary technique was 3 times stiffer (107 ± 11 N/mm vs 42 ± 12 N/mm, P = 0.03) and 54% stronger (1143 ± 31 N vs 741 ± 85 N, P = 0.03) than the trough technique. There was also an increased yield strength (729 ± 9 N vs 542 ± 78 N, P = 0.03). Failure of the trough method occurred at the mesh insertion site whereas failure of the intramedullary technique was between the insertion site and patellar level. Conclusions This study shows a potential biomechanical advantage when implanting under the tibial base plate. While our model does not incorporate soft tissue ingrowth, it does compare insertion techniques where ingrowth has minimal effects.

Background: While traditional metallic cerclage remains the primary method in clinical application, non-metallic cerclage systems have recently gained popularity due to low risks of soft tissue irritation and bone intrusion. The objective of this study was to assess the performance of a novel non-metallic suture-based cerclage in comparison to traditional metallic cerclage cables for fixation of periprosthetic femoral fractures. Methods: An extended trochanteric osteotomy was performed on eight pairs of cadaveric femora, followed by reduction using either metallic cerclages (Group I) or the suture-based cerclage (Group II). A modular tapered fluted stem was then implanted in each specimen. The fragment translation during canal preparation and stem implantation was quantified using laser-scanning. Subsequently, each specimen underwent 500 cycles of multiaxial loading, with fragment translation and stem subsidence measured using a motion capture system. Findings: Following stem implantation, specimens in Group II exhibited a significantly greater lateral fragment translation (466 mu m vs 754 mu m, p = 0.017). However, there were no significant differences in anterior and distal translation between groups (p > 0.05). During multiaxial loading, the average stem subsidence in Group I was 0.36 mm (range, 0.04-1.42 mm), compared to 0.41 mm (range, 0.03-1.29) in Group II (p > 0.05). No significant difference was found in fragment translations between the two groups (p > 0.05). Interpretation: The suture-based cerclage system exhibited comparable biomechanical performance in fixation stability to conventional metallic cerclage cables. Yet, it was associated with a larger residual lateral gap between the fragments following stem implantation. Ultimately, the choice of fixation method should account for multiple factors, including patient characteristics, surgeon preference, and bone quality.

OBJECTIVES:Prophylactic cerclage cables are often placed intraoperatively about a fracture to prevent propagation. However, biomechanical data supporting optimal cable placement location are lacking. The objective of this study was to evaluate the impact of prophylactic cerclage placement location on the propagation of femoral shaft fractures. METHODS:The diaphysis of 14 fresh-frozen cadaveric femora were included. Volumetric bone mineral density in the femoral shaft was obtained from quantitative computed tomography scans. For each specimen, a 5-mm longitudinal fracture was created proximally to simulate a pre-existing fracture. After reaming of the femoral canal, a 3 degrees tapered wedge was advanced with an MTS machine at 0.2 mm/s until failure. The tests were conducted with a CoCr cable placed at varying distances (5 mm, 10 mm, 15 mm, 20 mm, and cableless) from the distal tip of the initial fracture. A compression loadcell was used to measure the cable tension during the tests. The axial force, displacement, and cable tension were monitored for comparison between groups. RESULTS:In the cableless group, the mean force needed to propagate the fracture was 1017.8 ± 450.3 N. With the addition of a cable at 5 mm below the fracture, the failure force nearly doubled to 1970.4 ± 801.1 N (P < 0.001). This also led to significant increases in stiffness (P = 0.006) and total work (P = 0.001) when compared with the control group. By contrast, in the 15 and 20 mm groups, there were no significant changes in the failure force, stiffness, and total work as compared with the control group (P > 0.05). CONCLUSIONS:Propagation of femoral shaft fracture was effectively resisted when a prophylactic cable was placed within 5 mm from the initial fracture, whereas cables placed more than 10 mm below the initial fracture were not effective in preventing fracture propagation.

BACKGROUND:Historical MCL (medial collateral ligament) reconstruction (MCLR) techniques have focused on the superficial MCL (sMCL) to restore valgus stability while frequently ignoring the importance of the deep MCL (dMCL) in controlling tibial external rotation. The recent recognition of the medial ligament complex importance has multiple studies revisiting medial anatomy and questioning contemporary MCLR techniques.PURPOSE:To assess whether (1) an isolated sMCL reconstruction (sMCLR), (2) an isolated dMCL reconstruction (dMCLR), or (3) a novel single-strand short isometric construct (SIC) would restore translational and rotational stability to a knee with a dMCL and sMCL injury.STUDY DESIGN:Controlled laboratory study.METHODS:Biomechanical testing was performed on 14 fresh-frozen cadaveric knee specimens using a custom multiaxial knee activity simulator. The specimens were divided into 2 groups. The first group was tested in 4 states: intact, after sectioning the sMCL and dMCL, isolated sMCLR, and isolated dMCLR. The second group was tested in 3 states: intact, after sectioning the sMCL and dMCL, and after single-strand SIC reconstruction (SICR). In each state, 4 loading conditions were applied at 0°, 20°, 40°, 60°, and 90° of knee flexion: 8-N·m valgus torque, 5-N·m external rotation torque, 90-N anterior drawer, and combined 90-N anterior drawer plus 5-N·m tibial external rotation torque. Anterior translation, valgus rotation, and external rotation of the knee were measured for each state and loading condition using an optical motion capture system.RESULTS:sMCL and dMCL transection resulted in increased laxity for all loading conditions at all flexion angles. Isolated dMCLR restored external rotation stability to intact levels throughout all degrees of flexion, yet valgus stability was restored only at 0° of flexion. Isolated sMCLR restored valgus and external rotation stability at 0°, 20°, and 40° of flexion but not at 60° or 90° of flexion. Single-strand SICR restored valgus and external rotation stability at all flexion angles. In the combined anterior drawer plus external rotation test, isolated dMCL and single-strand SICR restored stability to the intact level at all flexion angles, while the isolated sMCL restored stability at 20° and 40° of flexion but not at 60° or 90° of flexion.CONCLUSION:In the cadaveric model, single-strand SICR restored valgus and rotational stability throughout the range of motion. dMCLR restored rotational stability to the knee throughout the range of motion but did not restore valgus stability. Isolated sMCLR restored external rotation and valgus stability in early flexion.CLINICAL RELEVANCE:In patients with anteromedial rotatory instability in the knee, neither an sMCLR nor a dMCLR is sufficient to restore stability.

AbstractIntroductionHistoric MCL reconstruction techniques focused on the superficial MCL to restore valgus stability while overlooking tibial external rotation and the deep MCL. This study assessed the ability of a contemporary medial collateral ligament (MCL) reconstruction and a deep MCL (dMCL) reconstruction to restore rotational and valgus knee stability.MethodsSix pairs fresh-frozen cadaveric knee specimens with intact soft tissue were tested in four states: 1) intact 2) after sMCL and dMCL sectioning, 3) contemporary MCL reconstruction (LaPrade et al), and 4) dMCL reconstruction. In each state, four loading conditions were applied at varying flexion angles: 8Nm valgus torque, 5Nm tibial external rotation torque, 90N anterior drawer, and combined 90N anterior drawer plus 5Nm tibial external rotation torque.ResultsTransection of the sMCL and dMCL resulted in increased laxity with valgus torque, external rotation torque, and combined anterior drawer plus external rotation. dMCL reconstruction restored external rotation stability to intact levels throughout all degrees of flexion but did not restore valgus stability at any flexion angle. Contemporary MCL reconstruction restored valgus and external rotation stability at 0° and 20° and valgus stability at 40°. In the combined anterior drawer plus tibial external rotation trial, the dMCL restored stability at 20° and improved stability between 40° and 90° flexion. Conversely, the contemporary MCL reconstruction did not restore stability at any degree of flexion.ConclusionDeep MCL reconstruction restored rotational stability to the knee throughout range of motion but not valgus stability. The contemporary MCL reconstruction restored stability only near full extension.

Background: Due to increasing volume of total hip arthroplasties, periprosthetic femoral fractures have become a common complication with increased revision burden and perioperative morbidity. The objective of this study was to evaluate the fixation stability of Vancouver B2 fractures treated with 2 techniques. Methods: A common B2 fracture was created by reviewing 30 type B2 cases. The fracture was then reproduced in 7 pairs of cadaveric femora. The specimens were divided into 2 groups. In Group I ("reduce-first"), the fragments were reduced first, followed by implantation of a tapered fluted stem. In Group II ("ream-first"), the stem was implanted in the distal femur first, followed by fragment reduction and fixation. Each specimen was loaded in a multiaxial testing frame with 70% of peak load during walking. A motion capture system was used to track the motion of the stem and fragments. Results: The average stem diameter in Group II was 16.1 +/- 0.4 mm, versus 15.4 +/- 0.5 mm in Group I. The fixation stability was not significantly different in the 2 groups. After the testing, the average stem subsidence was 0.36 +/- 0.31 mm and 0.19 +/- 0.14 mm (P = .17) and the average rotation was 1.67 +/- 1.30 degrees and 0.91 +/- 1.11 degrees (P = .16) in Groups I and II, respectively. Compared to the stem, there was less motion of the fragments and there was no difference between the 2 groups (P > .05). Conclusions: When tapered fluted stems were used in combination with cerclage cables for treatment of Vancouver type B2 periprosthetic femoral fractures, both the "reduce-first" and "ream-first" techniques showed adequate stem and fracture stability. (c) 2023 Elsevier Inc. All rights reserved.

Background: The outcome of cementless total knee arthroplasty (TICS) relies on successful bony ingrowth into the implant surfaces. Failures due to aseptic loosening are still reported, especially in younger and more active patients. The objective of this study is to quantify the micromotion of a commercially available design of cementless tibial tray under loading conditions simulating walking and stair descent. Method: A commercially available design of cementless total knee arthroplasty was implanted in 7 cadaveric knees which were preconditioned with 500 cycles of 0 degrees-100 degrees flexion under a vertical load of 1050 N in a custom-built, multiaxial functional activity simulator. This was followed by application of the peak forces and moments occurring during walking and stair descent. During each loading procedure, 3-dimensional motion at the bone-prosthesis interface was measured using digital image correlation. Results: The tray migrated 101 +/- 25 mu m on average during preconditioning, which was dominated by rotation in the sagittal plane (92% of total migration), combined with posterior translation (28%) and minimal rotation in the transverse plane (14%). The migration varied 2.7-fold (61-167 mu m) between the 6 measurement zones. Stair descent produced significantly higher total micromotion than walking in zone #5 (62 +/- 9 vs 51 +/- 10 mu m, P < .05) and zone #6 (68 +/- 17 vs 37 +/- 10 mu m, P < .05). In addition, during stair descent, the tray exhibited significantly more tilting (anterior zones: 31 +/- 17 vs -16 +/- 20 mu m, P < .05; posterior zones: -60 +/- 8 vs -40 +/- 7 mu m, P < .05) and more anteroposterior displacement in the anterior zones (-25 +/- 3 vs -13 +/- 2 mu m, P < .05) when compared to walking. Conclusion: The relative motion at the bone-prosthesis interface varied substantially around the periphery of the cementless tray. Under the loading conditions evaluated, the tray primarily underwent a rocking motion in the sagittal plane. Compared with walking, stair descent produced significantly more micromotion, especially in the posterior zones. (C) 2020 Published by Elsevier Inc.

Introduction:Malrotation of the tibial component is a common error in TKR, and has been frequently cited as the cause of clinical symptoms. Correct rotational orientation of the tibial tray is diff...

IntroductionManifestation of high interface stresses coupled with micromotion at the interface can render the taper lock joint in a modular hip replacement prosthesis at risk for failure. Bending c...

Purpose:Knee pain and instability during high demand activities such as stair descent are reported by patients after TKA. Previous studies theorized that this pain is from increased demand on the q...

IntroductionIncreasing attention to the functional outcome of total knee arthroplasty (TKA) has demonstrated that many patients experience limitations when attempting to perform demanding activities that are normal for age-matched peers, primarily because of knee symptoms. Episodes of instability following TKA are most commonly reported during activities in which significant transverse or torsional forces are supported by the joint with relatively low joint compression forces, including stair-descent and walking on sloped or uneven surfaces. This study was performed to examine the influence of conformity between the femoral and tibial components on the Antero-Posterior (AP) stability of knee during stair descent.MethodsSix cadaveric knees were loaded in a six degree-of-freedom joint simulator, with the application of external forces simulating the action of the quadriceps and hamstring muscles and the external loads and moments occurring during stair descent, including the stages of terminal swing phase, ...

IntroductionA disturbing prevalence of short-term failures of metal-on-metal (MoM) hip resurfacings has been reported by joint registries. These cases have been primarily due to painful inflammatory reactions and, in extreme cases, formation of pseudotumors within periarticular soft-tissues. The likely cause is localized loading of the acetabular shell leading to “edge wear” which is often seen after precise measurement of the bearing surfaces of retrieved components. Factors contributing to edge wear of metal-on-metal arthroplasties are thought to include adverse cup orientation, patient posture, and the direction of hip loading. The purpose of this study was to investigate the role of different functional activities in edge loading of hip resurfacing prostheses as a function of cup inclination and version.MethodsWe developed a computer model of the hip joint through reconstruction of CT scans of a proto-typical pelvis and femur and virtually implanting a hip resurfacing prosthesis in an ideal position. ...

IntroductionMechanically-assisted corrosion of the head-neck junction present a dilemma to surgeons at revision THR whenever the femoral component is rigidly fixed to the femur. Many remove the dam...

Background: Metal wear and corrosion from modular junctions in total hip arthroplasty can lead to further unwanted surgery. Trunnion tribocorrosion is recognized as an important contributor to failure. This study was performed to determine if new metal heads restore mechanical integrity of the original modular junction after impaction on corroded trunnions, and assess which variables affect stability of the new interface created at revision total hip arthroplasty.Methods: Twenty-two trunnions, cobalt-chromium (CoCr) and titanium alloy (TiAIV), (CoCr, n = 12; TiAIV, n = 10) and new metal heads were used, 10 trunnions in pristine condition and 12 with corrosion damage. Test states were performed using an MTS Machine and included the following: 1, Assembly; 2, Disassembly; 3, Assembly; 4, Toggling; and 5, Disassembly. During loading, three-dimensional motion of the head-trunnion junction was measured using a custom jig.Results: There were no statistical differences in the tested mechanical properties between corroded and pristine trunnions implanted with a new metal femoral head. Average micromotion of the head versus trunnion interface was greatest at the start of loading, stabilizing after approximately 50 loading cycles at an average of 30.6 +/- 3.2 mu m.Conclusion: Corrosion at the trunnion does not disrupt mechanical integrity of the junction when a CoCr head is replaced with a CoCr trunnion. However, increased interface motion of a new metal head on a corroded titanium trunnion requires additional study. The evaluation of ball head size on mechanical integrity of trunnions would also be a potential subject of future investigation, as increasing the ball head size at the time of revision is not uncommon in revisions today. (C) 2016 Elsevier Inc. All rights reserved.

INTRODUCTION Stable fixation of cementless tibial trays remains a challenge due bone density variability within the proximal tibia and the spectrum of loads imposed by different activities. This study presents a novel approach to measuring the interface motion of cementless tibial components during functional loading and tests whether interface motion of cementless tibial trays varies around the implant periphery.METHODS We developed a method to measure relative displacement of a tibial tray relative to the underlying bone using 3D digital image correlation (DIC) and multi-camera stereo photogrammetry. A clinically successful design of cementless total knee prosthesis (Zimmer Inc, Warsaw, IN) was implanted in 6 fresh cadaveric knees. A black-on-white stochastic pattern was applied to the outer surface of the tibia and the cementless prosthesis. High resolution digital images were prepared of the interface region and divided into 25 × 25 pixel regions of interest (ROI). Stereo images of the same ROI were generated using two cameras angled at 60 degrees using image correlation techniques. All specimens were mounted in a custom-built functional activity simulator and loaded with the forces and moments recorded during three common functional activities (standing from a seated position, walking, and stair descent), as reported in the Orthoload database, scaled by 50% for application to cadaveric bone. Prior to functional testing, each implant-tibia construct was preconditioned with 500 cycles of flexion from 5–100 degrees under a vertical tibial load of 1050 N at a frequency of 0.2 Hz. During loading, image data was acquired simultaneously (±20 μs) from the entire circumference of the tibial interface forming 4 stereo images using 8 cameras spaced at 90 degree intervals (Allied Vision Technologies, Exton, PA) using custom image acquisition software (Mathworks, Natick, MA) (Figure 1). The multiple stereo images were registered using the surface topography of each specimen as measured by laser scanning (FARO Inc., Montreal) (Figure 2). During post-processing, the circumferential tray/tibia interface was divided into 10 zones for subsequent analysis (Figure 3). Interface displacements were measured on a point-to-point basis at approximately 700 sites on each specimen using commercial DIC software (Dantec Dynamics, Skovlunde, Denmark) (Figure 4).RESULTS The average 3D displacement over 10 circumferential zones of the tray was 83.6±41.5 μm (range: 30.8 to 214.9 μm). The anatomic components of tray migration were 0.4±40.8 μm medially (range: 172 μm lateral to 112 μm medial) and 3.1± 40.6 μm posteriorly (range: 86 μm posteriorly to 61 μm anteriorly). The largest tray displacement was observed in the inferior direction with an average inferior displacement of 37.6±63.8 μm (range: 206 μm inferiorly to 81μm superiorly). The largest displacements were observed posteriorly, with the posteromedial aspect subsiding more the posterolateral aspect.DISCUSSION The stability of tibial trays cannot be accurately assessed by measuring interface motion at a few fixed peripheral sites. If discrete displacement transducers are used for pre-clinical testing, a set of 4–6 transducers should be placed at sites that vary with the pattern of interface motion of each design and the combination of loads and moments applied during testing.

Introduction Numerous factors have been hypothesized as contributing to mechanically-assisted corrosion at the head-neck junction of total hip prostheses. While variables attributable to the implant and the patient are amenable to investigation, parameters describing assembly of the component parts can be difficult to determine. Nonetheless, increasing evidence suggests that the manner of intraoperative assembly of modular components plays a critical role in the fretting and corrosion of modular implants. This study was undertaken to measure the magnitude and direction of the impaction forces applied by surgeons in assembling modular head-neck junctions under operative conditions where both the access and visibility of the prosthesis may potentially compromise component fixation. Methods A surrogate consisting of the lower limb with overlying soft tissue was developed to simulate THR performed via a 10cm incision using the posterior approach. The surrogate was modified to match the resistance of the body to retraction of the incision, mobilization of the femur and hammering of the implanted femoral component. An instrumented femoral stem (SL PLUS) was surgically implanted into the bone after attachment of 3 miniature accelerometers (Dytran Inc) in an orthogonal array to the proximal surface of the prosthesis. A 32mm cobalt chrome femoral head was mounted on the trunnion (12/14 taper, machined) of the femoral stem. 15 Board-certified and trainee surgeons replicated their surgical technique in exposing the femur and impacting the modular head on the tapered trunnion. Impaction was performed using an instrumented hammer (5000 Lbf Dytran impact hammer) that provided measurements of the magnitude and temporal variation of the impact force. The components of force acting along the axis aof the neck and in the AP and ML directions were continuously samples using the accelerometers. Results For all surgeons, the average value of the peak impaction force was 3765±1094N (range: 2358 to 6225N). Head impact was delivered in an average direction of 24.4±7.5 degrees more vertical than the trunnion axis, though this value varies from 14 to 43 degrees between individual surgeons. On average, the off-axis force perpendicular to the trunnion axis was 1586±736N, however, this value ranged from 634 to 2895N with peak loading of both the head and the implant in varus. Almost all of the applied impact was directed within 10 degrees of the mid-plane of the stem (average deviation: 2.5±5.9 degrees of with only a small force directed anteriorly or posteriorly (average force: 140±396N, anterior). The variability in the magnitude and direction of the impaction force was not associated with the level of training or the surgical experience of the participants (pu003e0.05). Conclusions This study shows that large off-axis forces are developed during manual impaction of modular heads onto stem trunnions via the posterior approach. The variation in magnitude and direction of these forces varies between individual surgeons and is not systematically related to the training or experience of each surgeon in joint replacement. This variability in intraoperative assembly of head-neck junctions may contribute to the severity and incidence of mechanically assisted corrosion in total hip replacement.

INTRODUCTION Cementless tibial trays commonly fail through failure of fixation due to excessive interface motion. However, the specific combination of axial and shear forces precipitating implant failure is unknown. This has led to generic loading profiles approximating walking to perform pre-clinical assessment of new designs, even though telemetric data demonstrates that much larger forces and moments are generated during other functional activities. This study was undertaken to test the hypotheses: (i) interface motion of cementless tibial trays varies as a function of specific activities, and (ii) the response of the cementless tibial interface to walking loading is not representative of other functional activities.MATERIALS and METHODS Six fresh-frozen cadaveric tibias were tested using a custom designed functional activity simulator after implantation of a posterior stabilized total knee replacement (NexGen LPS, Zimmer, Warsaw IN). Activity scenarios were selected using force (Fx, Fy, Fz) and moment (Mx, My, Mz) data from patients with instrumented tibial trays (E-tibia) published by Bergmann et al. A pattern of black and white spray paint was applied to the surface of the specimen including the tibial tray and bone. Each specimen was preconditioned through application of a vertical load of 1050N for 500 cycles of flexion-extension from 5–100°. Following preconditioning, each tibia was loaded using e-tibia values of forces and moments for walking, stair-descent, and sit-to-stand activities. The differential motion of the tibial tray and the adjacent bony surface was monitored using digital image correlation (DIC) (resolution: 1–2 microns in plane; 3–4 microns out-of-plane). Four pairs of stereo-images of the tray and tibial bone were prepared at sites around the circumference of the construct in both the loaded and unloaded conditions: (i) before and after pre-conditioning and (ii) before and after the 6 functional loading profiles. The images were processed to provide circumferential measurements of interface motion during loading. Differences in micromotion and migration were evaluated statistically using step-wise multivariate regression.RESULTS The average 3D motion of the tibial tray varied extensively with the loading conditions corresponding to the different activities (Figs 1,2). The largest 3D motion was seen during the first peak of stair descent (86.6±8.0µm) and the first peak of walking (83.1±10.2µm; p=0.5516), both of which were characterized by large adduction moments (18.5 and 19.1Nm respectively). The differences between 3D micromotion of all other pairs of activities were statistically significant (pu003c0.0001 to p=0.0127). Each of the 6 loading scenarios simulated elicited a different combination of components of implant displacement at the cementless interface. The largest differences in interface motion were observed between the first peak of walking and all of the other loading modes with reversal of the direction of the SI (p=0.3828), AP (pu003c0.0001) and ML (pu003c0.0001) components of tray displacement (Figs. 2,3).CONCLUSIONS 1. Magnitude and direction of interface motion between the tibia and a cementless tibial tray vary with specific loading patterns. 2. Interface motion observed during loading conditions representative of walking are not indicative of the stability of cementless implant fixation when exposed to loading conditions generated by other activities.For figures/tables, please contact authors directly.

The objective of rehabilitation following meniscal repair is to promote healing by limiting stresses on repairs, while simultaneously preserving muscle strength and joint motion. Both protective protocols limiting weight bearing and accelerated which do not, have shown clinical success. This study assesses the effects of physiologic gait loading on the kinematic behavior of a repaired medial meniscus.

Introduction Mechanically-assisted corrosion of the head-neck junction present a dilemma to surgeons at revision THR whenever the femoral component is rigidly fixed to the femur. Many remove the damaged femoral head, clean the femoral taper and fix a new head in place to spare the patient the risks associated with extraction and replacement of the well-functioning femoral stem. This study was performed to answer these research questions: Will new metal heads restore the mechanical integrity of the original modular junction after impaction on corroded tapers? Which variables affect the stability of the new interface created at revision THR? Materials and Methods Twenty-two tapers (CoCr, n=12; TiAlV, n=10) were obtained for use in this study. Ten stems were in pristine condition, while 12 stems had been retrieved at revision THR and with corrosion damage to the trunnion (Goldberg scale 4). Twenty-two new metal heads were obtained for use in the study, each matching the taper and manufacturer of the original component. The following test states were performed using a MTS Machine: 1. Assembly, 2. Disassembly, 3. Assembly, 4. Toggling and 5. Disassembly. All head assemblies were performed wet using 50% calf serum in accordance to ISO 7206-10. During toggling, each specimen9s loading axis was aligned 25° to the trunnion axis in the frontal plane and 10° in the sagittal plane (Figure 1). Toggling was performed at 1Hz for 2,000 cycles with a sinusoidal loading function (230N–4300N). During loading, 3D motion of the head-trunnion junction was measured using a custom jig rigidly attached to the head and the neck of each prosthesis. Relative displacement of the head with respect to the neck was continuously monitored using 6 high resolution displacement transducers with an accuracy of ±0.6µm. Displacement data was independently validated using FEA models of selected constructs. Results The average micromotion of the head vs trunnion interface was greatest at the start of loading and stabilized after approximately 50 loading cycles at an average of 30.6±3.2µm (Figure 2). For CoCr couples, interface motion dropped by 17% when a pristine head was mounted on a corroded stem compared to a new stem (25.7±2.7µm (pristine stem), vs. 30.1±4.6µm (corroded stem), p= 0.4023) (Figure 3). However, addition of a new CoCr head with a corroded titanium stem led to an 73% increase in interface motion after assembly with a new CoCr head (Corroded: 43.4±9.8µm, Pristine: 25.2±7.0µm, p=0.1661). The resistance to head-neck disruption was 15% higher in TIALV/CoCr couples compared to CoCr/CoCr (TiAlV: 2558 ±63N, CoCr: 2226±99N, p=0.0111) and was not affected by the presence of corrosion of the trunnion (1% loss of strength in each case). Discussion Corrosion at the trunnion does not disrupt the mechanical integrity of the junction when a CoCr head is replaced on a CoCr taper. We are less sure about the mechanical integrity of a TiAlV taper demonstrated by a trend towards increased micromotion at this junction. Further work is required to better elucidate the role of dissimilar metals in the mechanical integrity of the head-neck junction.

Introduction Manifestation of high interface stresses coupled with micromotion at the interface can render the taper lock joint in a modular hip replacement prosthesis at risk for failure. Bending can lead to crevice formation between the trunnion and the head and can potentially expose the interface to the biological fluids, generating interface corrosion. Additionally, development of high stresses can cause the material to yield, ultimately leading to irreversible damage to the implant. The objective of this study is to elucidate the mechanical response of taper junction in different material combination assemblies, under the maximum loads applied during everyday activities. Methods Computer simulations were executed using a verified FE model. A stable hexahedral mesh (33648 elements) was generated for the trunnion (taper size: 12/14mm) and a tetrahedral mesh (51182 elements) for the head (CoCr, size: 32mm). An assembly load of 4000N was applied along the trunnion axis followed by the application of a load of 230–4300N at 25° and 10° angle to the trunnion axis in the frontal and sagittal planes. A linear static solution was set up using Siemens NX Nastran. Two material combinations were tested - cobalt-chrome head with a titanium alloy trunnion and cobalt chrome head with a cobalt-chrome trunnion. Results Table1 compares the results obtained from the simulation to those observed in experimental simulations performed under similar loading conditions in our lab. Larger vertical interface displacement was observed in the CoCr-CoCr assembly during toggle-inducing loads. The trunnion bending inside the femoral head was higher in the Ti-CoCr assembly (0.056) compared to the CoCr-CoCr assembly (0.027) with the overall bending of the Ti-CoCr assembly also observed to be much higher (Fig.1). Negligible difference between the stress measured in the femoral head and taper was observed (Fig.2). Discussion Bending could potentially lead to the development of higher stresses especially under multiple cycles of loading. Fatigue and plastic deformation could result in irreparable damage to the interface leading to implant failure. Additionally, bending causes a separation of the interfaces at the trunnion-head junction, leading to crevice formation, triggering corrosion by exposure to the surrounding physiological environment. Thus, it is crucial that we understand the mechanics of the trunnion-head junction especially under conditions of functional loading.