Introduction. Total knee arthroplasty (TKA) has achieved excellent clinical outcomes and functional performances. However, there is a need for greater implant longevity and higher flexion by younger and Asian patients. We determined the relationship between mobility and stability of TKA product because they are essential for much further functional upgrading. This research evaluated the geometry characteristics of femorotibial surfaces quantitatively by measuring their force of constraint by computer simulation and mechanical test. Methods. We measured the force of constraint of femorotibial surfaces in order to evaluate the property of femorotibial surfaces. A total knee system was used for this evaluation, and has an asymmetrical joint surface, which restores the anatomical jointline in both sagittal and coronal planes, and is expected to permit normal kinematics, with cruciate-retaining fixed type. We performed computer simulation using finite element analyses (FEA) and mechanical tests using knee simulator to measure the force of constraint regarding anterior-posterior (AP) and internal-external (IE) rotational direction in extension position, 90-degree flexion and a maximum flexion of 140-degree. In the FEA, Young's modulus and Poisson's ratio were set to 213 GPa and 0.3 for Co-Cr-Mo alloy as the femoral component, and 1 GPa and 0.3 for UHMWPe as the tibial insert, respectively. The force load to AP direction of tibial tray was measured when the femoral component moved plus or minus 10 millimeters. The moment load to IE rotational direction of tibial tray was measured when the femoral component moved plus or minus 20 degrees. The vertical load of 710 N was loaded on the femoral component during these measurements. Results. Regarding AP direction, the results of FEA showed 506 N (0-degree), 421 N (90-degree), and 389 N (140-degree) as the maximum load for anterior direction, and 152 N (0-degree), 166 N (90-degree), and 174 N (140-degree) for posterior direction. The results of mechanical tests showed 463 N (0-degree), 387 N (90-degree), and 332 N (140-degree) as the maximum load for anterior direction, and 108 N (0-degree), 121 N (90-degree), and 197 N (140-degree) for posterior direction [Fig. 1]. As the maximum moment load to IE rotational direction, the results of FEA showed 7.0 N-m (0-degree), 6.6 N-m (90-degree), and 5.5 N-m (140-degree) to tibial internal rotation of femoral component, and 9.5 N-m (0-degree), 8.1 N-m (90-degree), and 5.5 N-m (140-degree) to tibial external rotation of femoral component. The results of mechanical tests showed 4.5 N-m to tibial internal rotation of femoral component in all position, 8.6 N-m (0-degree), 6.5 N-m (90-degree), and 5.2 N-m (140-degree) to tibial external rotation of femoral component [Fig. 2]. Discussion. The force to AP direction of constraint for posterior was obviously lower than one for anterior. The torque to IE rotation for tibial internal rotation was lower or equal than tibial external rotation. These results suggest that this total knee system permits femoral rollback and tibial internal rotation with medial pivot pattern, which is required to achieve high functional performance. Furthermore, computer simulation can be a good method in this evaluation for their consistency

Introduction. Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. A well-functioning TKA prosthesis should be designed with a good balance between stability and mobility, meaning the femorotibial constraint of the artificial joint should be appropriate for the device's function. To assess the constraint behavior of a TKA prosthesis, physical testing is typically required, and an industrial testing standard has been developed for this purpose [1]. Computer simulation has become increasingly useful in many industries, including medical device research and development where finite element analysis (FEA) has been extensively used in stress analysis and structural evaluation. This study presents an FEA-based simulation to evaluate the femorotibial constraint behavior of TKA prosthesis, and demonstrated the effectiveness of the method by validating through physical testing. Methods. A Cruciate Retaining (CR) TKA prosthesis design (Optetrak Logic CR, Exactech, USA) was used in this study. CAD models of the implants assembled at 0° of flexion were used for the simulation. Finite element models were generated using with all materials assumed linear elastic. Boundary conditions were set up according to the ASTM F1223 standard (Figure 1). The tibial baseplate was fixed distally. A constant compressive force (710 N) was applied on the femoral component. Nonlinear Surface-Surface-Contact was defined at the femorotibial articulating surfaces. Coefficient of friction was determined from physical test. The femoral component was driven under a displacement-controlled scheme to slide along the anterior-posterior (AP) direction on the tibial insert. At each time step, constraint force occurring at the articulating surface was derived from the reaction force at the distal fixation of the tibial baseplate. A nonlinear FEA solver (NX Nastran SOL601, Siemens, USA) was used to solve the simulation. In addition, five samples of the prostheses were physically tested, and the results were compared with the simulation. Results. The simulation successfully captured the movement of contact location and pressure along the movement of the femoral component (Figure 2). The force-displacement curve predicted by the simulation exhibited a very close hysteresis loop profile as the results of physical testing (Figure 3). Using the curve slope from 0 to 5 mm to characterize the linear constraint, the simulation predicted 45.7 N/mm anteriorly and 36.4 N/mm posteriorly, which are less than 10% different from the physical testing results (46.4 N/mm anteriorly and 39.6 N/mm posteriorly). Discussion/Conclusion. This study demonstrated that the simulation was able to closely predict the femorotibial constraint behavior of the TKA prosthesis under ASTM F1223 testing. The simulation results resembled the physical testing results not only in the general curve profile but also in the magnitude of slope values. The increased difference at the far anterior region could be related to the fact that no material nonlinearity was currently considered, which could be improved in future studies. A validated simulation method could be very useful in TKA prosthesis design. Since no physical prototypes are required, design evaluation and optimization can be achieved in a much easier and faster manner

«Purpose». High tibial osteotomy (HTO) is a useful treatment option for osteoarthritis of the knee. Closing-wedge HTO (CW-HTO) had been mostly performed previously, but the difficulties of surgical procedure when total knee arthroplasty (TKA) conversion is needed are sometimes pointed out because of the severe deformity in proximal tibia. Recently, opening-wedge HTO (OW-HTO) is becoming more popular, but the difference of the two surgical techniques about the influence on proximal tibia deformity and difficulties in TKA conversion are not fully understood. The purpose of this study was to compare the influence of two surgical techniques with CW-HTO and OW-HTO on the tibial bone deformity using computer simulation and to assess the difficulties when TKA conversion should be required in the future. «Methods». In forty knees with medial osteoarthritis, the 3D bone models were created from the series of 1 mm slices two-dimensional contours using the 3D reconstruction algorithm. The 3-D imaging software (Mimics, materialize NV, Leuven, Belgium) was applied and simulated surgical procedure of each CW-HTO and OW-HTO were performed on the same knee models. In CWHTO, insertion level was set 2cm below the medial joint line [Fig.1]. While in OW-HTO, that was set 3.5cm below the medial joint line and passed obliquely towards the tip of the fibular head [Fig.2]. The correction angle was determined so that the postoperative tibiofemoral angle would be 170 degrees. The distance between the center of resection surface and anatomical axis, and the angle of anatomical axis and mechanical axis were measured in each procedure. Secondly, a simulated TKA conversion was operated on the each tibial bone models after HTO [Fig.3]. The distance between the nearest points of tibial implant and lateral cortical bone was assessed as the index of the bone-implant interference. «Results». The distance between the center of resection surface and anatomical axis was significantly shifted to the lateral side in CW group (0.62 ±2.95 mm lateral shift) than in OW group (0.93 ± 3.68 mm medial shift) (P<0.01). The angle of anatomical axis and mechanical axis was significantly increased in the CW group (CW: 0.77 ± 0.79 degree, OW: 0.49 ± 0.83 degree, P<0.01). In the simulation of TKA conversion, if thickness of the lateral cortical bone was 3mm, it was showed that the tibial implant was more interfered with the lateral cortical bone in CW group (2.77 ± 1.38 mm) than in OW group (4.32 ± 1.61 mm) (P<0.01). «Conclusions». The results suggested that bone deformity in proximal tibia after HTO might affect the difficulty of TKA conversion, particularly in the case of CWHTO

Introduction

The midcortical line, the midline between the anterior and the posterior cortical walls has been reported as an intraoperative reference guide for reproducing the true femoral anteversion in cross-sectional computed tomography (CT) image study but we suspected that the version of the midcortical line on the cutting surface is different from that on the axial image. The three-dimensional (3D) CT-based preoperative planning software for THA enabled us to evaluate the cut surface of the femoral neck osteotomy. When we planned the straight non-anatomic stem placement in 20° of anteversion, we noticed that the line connecting the trochanteric fossa and the middle of the medial cortex of the femoral neck (T line) was coincident with the component torsion in almost all cases except those involving secondary osteoarthritis of the hip. Therefore we hypothesised that the T-line would provide an accurate reference guide for anteversion of the femoral component in THA. We performed this study to answer the question: which is the better intraoperative reference guide for reproducing the true femoral anteversion, the midcortical line or the T line?

Introduction. Using the tibial extramedullary guide needs meticulous attention to accurately align the tray in total knee arthroplasty (TKA). We previously reported the risk for varus tray alignment if the anteroposterior (AP) axis of the ankle was used for the rotational direction of the guide. The purpose of our study was to determine whether aligning the rotational direction of the guide to the AP axis of the proximal tibia reduced the incidence of varus tray alignment when compared to aligning the rotational direction of the guide to the AP axis of the ankle. Materials and Methods. Clinical Study. A total of 80 osteoarthritis (OA) knees after posterior stabilized TKA were recruited in this study. From 2002 to 2004, the rotational alignment of the guide was adjusted to the AP axis of the ankle (Method A: Figure 1, N = 40 knees). After 2005, the rotational alignment of the guide was adjusted to the AP axis of the proximal tibia (Method B: Figure 1, N = 40 knees). The AP axis of the proximal tibia was defined as the line connecting the middle of the attachment of the PCL and the medial third border of the attachment of the patellar tendon. The guide was set at a level of 10 mm distal to the lateral articular surface. Postoperative alignment was compared between the two groups using full-lengthanteroposterior radiograph. Computer simulation. Computer simulation was performed to determine the effect of ankle rotation on tibial tray alignment, using three-dimensional bone and skin model reconstructed from CT images of 75 OA knees (Figure 2). The position of the distal end of the guide in Method B was evaluated on the coronal plane perpendicular to the AP axis of the proximal tibia and of the ankle respectively. <Displacement> was the distance from the distal end of the guide to the midpoint-malleolar points (+: medial position). <Distance ratio> was the ratio of <Displacement> dividing by the entire width of the malleolar. Results. The results of the postoperative alignment for both methods from the clinical study are shown in Table 1. The number of the knees with more than 3 degrees of varus aligned tibial component significantly decreased with the Method B from the Method A. The computer simulation showed that the position of the guide varied great among individuals in the direction of the AP axis of the ankle joint. Discussion. When an extramedullary alignment guide is used in TKA, a rotational mismatch between the proximal part of the tibia and the ankle joint can induce a varus alignment of the tibial component. Computer simulation also supported our conclusion that the surgeon should not evaluate the distal end of the guide in the direction of the ankle joint to minimize the effects of anatomic variation for proper coronal alignment

Introduction:. Acetabular revision Jumbo cups are used in revision hip surgeries to allow for large bone to implant contact and stability. However, jumbo cups may also result in hip center elevation and instability. They may also protrude through anterior wall leading to ilopsoas tendinitis. Methods:. The study was conducted using two methods:. Computer simulation study. 265 pelvic CT scans consisting of 158 males and 107 females were converted to virtual 3-dimensional bones. The average native acetabular diameter was 52.0 mm, SD = 4.0 mm (males in 52.4 mm, SD = 2.8 mm and 46.4 mm, SD = 2.6 mm in females). Images were analyzed by custom CT analytical software (SOMA™ V.3.2). 1. and over-sized reaming was simulated. Four distinct points, located in and around the acetabular margins, were used to determine the reamer sphere. Points 1, 2, 3 were located at the inferior and inferior-medial acetabular margins, and Point 4 was located superiorly and posteriorly in the acetabulum to simulate a bony defect in this location, Point 4 was placed at 10%, 20%, 30%, 40%, 50% and 60% of the distance from the superior – posterior margin of the acetabular rim to the sciatic notch to simulate bony defects of increasing size. (Figure 1). Radiographical study. Retrospective chart review of patient records for all cementless acetabular revisions utilizing jumbo cups between January 1, 1998 and March 30, 2012 at UCFS (98 patients with 57 men, 41 women). Jumbo cups: ≥66 mm in males; <62 mm in females. Reaming was directed inferiorly to the level of the obturator foramen to place the inferior edge of the jumbo cup at the inferior acetabulum. To determine the vertical position of the hip center, a circle was first made around both the jumbo and the contralateral acetabular surfaces using Phillips iSite PACS software. The center of this circle was assumed to correspond to the “hip center”. The height of the hip center was estimated by measuring the height of a perpendicular line arising from the interteardrop line (TL) and ending at the hip center. Results:. The computer simulation and radiographic analysis deomonstrated similar results. The computer simulation predicted that the hip center shifted superiorly and anteriorly as the reamer size increased. The hip center shifted 0.27 mm superiorly and 0.02 mm anteriorly for every millimeter in diameter increased for the reaming. (Figure 2) Anterior column bone removal was increased 0.86 mm for every 1 mm of reamer size increase. (Figure 3). Results of radiographical study is shown in Table bellow:. Discussion:. Use of a jumbo cup in revision THA results in elevation of the hip center. Therefore a longer femoral head may be needed to compensate for hip center elevation when a jumbo cup is used. Reaming for a jumbo cup can also result in loss of anterior bone stock and protrusion of the cup anteriorly which may cause iliopsoas tendonitis

Introduction. Sagittal pelvic tilt (SPT) can change with spinal pathologies and fusion. Change in the SPT can result in impingement and hip instability. Our aim was to determine the magnitude of the SPT change for hip instability to test the hypothesis that the magnitude of SPT change for hip instability is less than 10° and it is not similar for different hip motions. Methods. Hip implant motions were simulated in standing, sitting, sit-to-stand, bending forward, squatting and pivoting in Matlab software. When prosthetic head and liner are parallel, femoral head dome (FHD) faces the center of the liner. FHD moves toward the edge of the liner with hip motions. The maximum distance between the FHD and the center in each motion was calculated and analyzed. To make the results more reliable and to consider the possibility of bony impingement, when the FHD approached 90% of the distance between the liner-center and liner-edge, we considered the hip “in danger for dislocation”. The implant orientations and SPT were modified by 1-degree increments and we used linear regression with receiver operating characteristic (ROC) curve and area under the curve (AUC) to determine the magnitude of SPT change that could cause instability. Results. SPT modification as low as 7° could result in dislocation during pivoting (AUC: 87.5; sensitivity: 87.9; specificity 79.8; p=0.0001). This was as low as 10° for squatting (AUC: 91.5; sensitivity: 100; specificity 75.9; p=0.0001) and as low as 13° for sit-to-stand (AUC: 94.6; sensitivity: 98; specificity 83; p=0.0001). SPT modification affects hip stability more in pivoting than sit-to-stand and squatting. Discussion. Our results show the importance of close collaboration between the hip and spine surgeons in treating patients who undergo THA and spinal fusion. The postoperative SPT modification should be considered for preoperative computer simulation for determining the implant safe zone

Introduction. Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. Femorotibial constraint is a key property of a TKA prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually used for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses may be used. A CRC tibial insert usually has a more conforming sagittal profile especially in the anterior aspect to provide increased constraint to prevent paradoxical femoral translation during knee flexion. A quantitative understanding of the constraint behavior of a prosthesis design is critical to ensure its functional outcome. Using a validated computer simulation, this study evaluated the anterior-posterior (AP) constraint of two types of tibial inserts (CR and CRC) from a same TKA product family. Methods. Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, USA). Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed at 0° flexion per ASTM F1223 standard [1] (Figure 1). The simulation has been validated with physical testing (more details submitted in a separate abstract to ISTA 2013). Briefly, FEA models were created with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation. A nonlinear FEA solver was used to solve the simulations. Results. The force-displacement curves predicted by the simulation exhibited the hysteresis loop appearance for both CR and CRC inserts (Figure 2). The anterior aspect of the CRC curves showed a steeper raise than the CR curves, and the trend was consistent across sizes. Taking the slope from 0 to 5 mm range, the anterior constraint of the CRC insert was significantly greater than the CR insert, while the posterior constraint of the CRC insert was also slightly greater (Figure 3). Discussion/Conclusion. The increased AP constraint of the CRC insert revealed in the study is consistent with the design geometry and functional intent of the device. With a much increased anterior lip, the CRC insert is expected to provide substantially greater anterior constraint than the CR insert to prevent paradoxical femoral translation for patients without a fully functioning PCL. The CRC insert is also expected to provide slightly increased posterior constraint due to its gently elevated posterior lip. This study quantitatively demonstrated the effect of design geometry on the outcome constraint function of different TKA prostheses

Introduction. To control anteversion of the acetabular cup and femoral stem within an appropriate angle range is extremely important in total hip arthroplasty. The sum of these angles is called the “combined anteversion” (CA), and a navigation system is necessary for its accurate intraoperative evaluation. However, navigation is too expensive and time-consuming to be commonly used. Therefore, a cheaper and easier tool for intraoperative CA evaluation is desired in the clinical field. I had an idea of marking ruler-like scales on a trial femoral head ball for this purpose. The purpose of this study was to introduce the idea in a computer simulation. Materials and Methods. An acetabular cup, a femoral head, and a femoral stem were designed virtually using three- dimensional computer graphics software (FreeCAD). The head was assembled with the femoral stem, and the axis of the stem was tilted 7 degrees to the vertical axis, referring the angle between mechanical and anatomical axes of the femur. Ruler-like scales and a horizontal line were marked on the surface of the head. The cup inclination angle was fixed at 40 degrees and paired with the head and stem assembly. The cup axis was on the stem–neck plane, which meant that CA was zero before rotating the cup and the stem. The scale at an intersecting point of the inner edge of the cup and the horizontal line was read before and after rotating the cup and the stem. I confirmed if the sum of the rotated angles of the cup and stem and the angle indicated by the scales were consistent when they were rotated at an arbitrary angle. Results. CA was successfully evaluated by the difference in angle indicated by the scales before and after rotation. Discussion. There are several definitions for cup and stem anteversion. The CA evaluated in this study was the sum of anatomical anteversion of the cup and the angle between the neck axis and epicondylar or posterior-condylar axes of the knee projected on the horizontal plane. There are several factors that make the CA evaluation by this method inaccurate. For example, when the cup inclination angle is not 40 degrees, or the pelvis or the femur are not held at the intended position, the CA indicated by the scales is not accurate. It is my future work to assess whether this method is accurate enough to be used in the clinical situation. Conclusion. Marking ruler-like scales on the femoral head would be a low-cost and effective method for rough intraoperative evaluation of CA

Introduction. Wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee prostheses is one of the major restriction factors on the longevity of these implants. In retrieved hip prostheses with screw holes in the metal acetabular cup for fixation to the pelvis, the generation of cold flow into the screw holes is frequently observed on the backside of the UHMWPE acetabular cup liner. In most retrieved cases, the protruded areas of cold flow on the backside were located on the reverse side of the severely worn and deformed surface of the polyethylene liner. It would appear that the cold flow into screw holes contributes to increase of wear and damages of the polyethylene liner in hip prosthesis. Methods. In a previous study (Cho et al., 2016), we pointed out the generation of cold flow into the screw holes on the backside of the retrieved UHMWPE acetabular cup liner as shown in Figure 1. The primary purpose of this study was to investigate the influence of the cold flow into the screw holes on the wear of the polyethylene liner in hip prosthesis. In this study, computer simulations of the generation of cold flow were performed using the finite element method (FEM) in order to propose the design criteria about the cold flow of the hip prosthesis for improving the wear resistance of the polyethylene liner. We especially focused on the influence of polyethylene thickness and contact surface conformity on the generation of cold flow into the screw hole. Results. An example of the results of a series of the FEM simulations performed in this study is shown in Figure 2. This figure shows the distributions of the contact stress in the polyethylene liners. The graphs shown in Figure 3 are the summary of results of a series of the FEM simulations performed in this study. The graph in Figure 3(a) shows the changes in the maximum contact stress in the polyethylene liner with the thickness of polyethylene liner. The graph in Figure 3(b) shows the changes in the maximum contact stress in the polyethylene liner with the radial clearance between the femoral head and the polyethylene liner. Discussion and Conclusions. It was found that the magnitudes of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with decreasing the thickness of polyethylene liner. It was also found that the magnitude of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with increasing the radial clearance between the femoral head and the polyethylene liner. The results of this study suggest that polyethylene thickness and contact surface conformity have a significant influence on the generation of cold flow into the screw holes and wear of the polyethylene liner. For any figures or tables, please contact authors directly

Introduction. Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE in knee and hip prostheses after total joint replacement is one of the major restriction factors on the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear of UHMWPE. A number of studies have investigated the factors influencing the wear of UHMWPE acetabular cup liner in hip prosthesis. Most of these studies, however, have focused on the main articulating surfaces between the femoral head and the polyethylene liner. Materials and Methods. In a previous study (Cho et al., 2016), the generations of cold flow into the screw holes in the metal acetabular cup were observed on the backside of the retrieved UHMWPE acetabular cup liners as shown in Figure 1. We focused on the screw holes in the metal acetabular cup (Figure 2) as a factor influencing the wear behavior of polyethylene liner in hip prosthesis. In this study, computer simulations of the generation of cold flow into the screw holes were performed using the finite element method (FEM) in order to investigate the influence of the screw holes in the metal acetabular cup on the mechanical state and wear behavior of polyethylene liner in hip prosthesis. Results. An example of the results of the FEM simulations performed in this study is shown in Figure 3. In the region which the cold flow into the screw holes occurred, it was found that locally high contact stresses which exceed the yield stress of UHMWPE and considerable plastic strains were generated throughout the overall thickness between the backside and top surface of the polyethylene liners. On the contrary, in the case of the polyethylene liner combined with the metal acetabular cup without screw hole, although the regions of high contact stress and high plastic strain had a tendency to be limited around contact surface compared with those of the combination with screw holes, the values of contact stress and plastic strain were lower than the combination with screw holes. Discussion and Conclusions. The results of this study suggest that the cold flow generated by the existence of the screw holes in the metal acetabular cup of hip prosthesis reduces the wear resistance of the UHMWPE acetabular cup liner. It would appear that the cold flow into the screw holes contributes to structural weakening of the UHMWPE and reduction of the polyethylene thickness, thus increase of internal stresses and plastic strains in and around the regions of cold flow. Therefore, it is required that improvement of the screw holes in the metal acetabular cup and/or improvement of fixation method of the metal acetabular cup to a pelvis in order to enhance the wear resistance of the polyethylene liner. For any figures or tables, please contact authors directly

Objective. Kinematically aligned total knee arthroplasty (TKA) is of increasing interest because this method may improve patient satisfaction. However, the biomechanics of kinematically aligned TKA remain largely unknown. Therefore, we analyzed whether the kinematic alignment method cause to increase the contact force on patellofemoral and tibiofemoral joints. Methods. A musculoskeletal computer simulation was used to determine the effects of kinematically or mechanically aligned TKA. Patellofemoral and tibiofemoral contact forces were examined for a mechanically aligned model and a kinematically aligned model using finite element analysis. Results. The peak contact stress on the patellofemoral joint in the kinematically aligned model was greater than that in the mechanically aligned model at 30° and 60°. Maximum peak contact stress was found at 30° flexion in the kinematically aligned model (73 MPa) and this was 221% higher than the stress in the mechanically aligned model (33 MPa). Similarly, peak contact stress of 33.0 MPa at 60° flexion occurred in the kinematically aligned model and this was 114% higher than that in the mechanically aligned model (29 MPa). The peak contact stress on the tibiofemoral joint in the kinematically aligned model was greater than that in the mechanically aligned model at 30°, 60° and 90° flexion. Maximum peak contact stress was found at 30° flexion in the kinematically aligned model (22 MPa) and this was 200% higher than the stress in the mechanically aligned model (11 MPa). Conclusions. Kinematically aligned TKA may have increased risks for implant longevity. Therefore, a strict surgical indication, including age and implant design, is needed to achieve excellent longevity after kinematically aligned TKA

Assessing glenoid version is important for a successful total shoulder arthroplasty. Glenoid version is defined as the orientation of the glenoid cavity in relation to a plane perpendicular to the scapula body. Glenoid revision averages between 1 to 2 degrees of retroversion and varies between race and sex. In general glenoid retroversion is overestimated by 6.5 degrees on plain radiographs. Furthermore standard axial 2D CT is aligned to the patient's body and not aligned to the scapula. Therefore 3D reconstructions generated from standard CT allows for analysis of the scapula as a free body and correct version measurements can be made unaffected by positioning. If you add a computer modeling coordinate system in which implants can be added, then computer simulation surgery can be performed. This is important because implanting a glenoid component in excessive retroversion leads to increased stress at the glenoid component and cement mantle and decreased contact with the humeral component. Also excessive reaming of the glenoid surface to neutral retroversion can lead to excessive bone loss and penetration of the glenoid vault by either the pegs or the keel of the glenoid component

A design modification to the DJO Linear hip stem was performed to facilitate use of the stem with the minimally invasive direct anterior approach. While the main design consideration was to reduce the overall stem length, it was also important to increase congruency of the implant and proximal cortical bone to ensure initial stability. An initial design attempt produced a geometry that was difficult to insert into the femur; therefore, reconstructed digital models of the femur (ADaMs by Materialise) were obtained and used to delineate the best fit implant cross section. The ADaMs models were constructed from 74 CT scans taken from northern Europeans undergoing investigations for cardio-vascular conditions. Using equivalency points, models representing the bone mean, ±1σ, and ±2σ were constructed. The ADaMs models are pictured in Figure 1. After importing the ADaMs models in the Solidworks CAD environment, the existing Linear stem was ideally positioned in the femur model and equally spaced planes parallel to the resection plane were defined as shown in Figure 2. At each plane, the shape of the cortical bone was determined and then used to define an implant cross section that was congruent to the bone, at least as large as the Linear hip stem, and symmetric about its midline. After using the base ADaMs models to drive the design's geometry, the final design fit was validated for very small patients using a hypothetical size −4σ extrapolation of the ADaMs models. The digital reconstructions improved the design process by providing accurate, tangible models of the actual femur geometry. From these models, the design team was able to visualize how implant geometry should be constructed to optimize congruency, symmetry, and favorable insertion characteristics. Additionally, the ADaMs models served to validate the design for a challenging condition and as a starting point for computer simulations that were able to predict the insertion difficulty encountered in the initial, pre ADaMs model design. The final redesign was launched in the US in 2014 as the TaperFill hip stem

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

Introduction. Postoperative dislocation remains a vexing problem for patients and surgeons following total hip arthroplasty (THA). It is the commonest reason for revision THA in the US. Dual mobility (DM) THA implants markedly decrease the risk of THA instability. However, DM implants are more expensive than those used for conventional THA. The purpose of this study was to perform a cost-effectiveness analysis of DM implants compared to conventional bearing couples for unilateral primary THA using a computer model-based evaluation. Methods. A state-transition Markov computer simulation model was developed to compare the cost-utility of dual mobility versus conventional THA for hip osteoarthritis from a societal perspective (Figure 1). The model was populated with health outcomes and probabilities from registry and published data. Health outcomes were expressed as quality-adjusted life years (QALYs). Direct costs were derived from the literature and from administrative claims data, and indirect costs reflected estimated lost wages. All costs were expressed in 2013 US dollars. Health and cost outcomes were discounted by 3% annually. The base case modeled a 65-year-old patient undergoing THA for unilateral hip osteoarthritis. A lifetime time horizon was analyzed. The primary outcome was the incremental cost-effectiveness ratio (ICER). The willingness-to-pay threshold was set at $100,000/QALY. Threshold, one-way, two-way, and probabilistic sensitivity analyses were performed to assess model uncertainty. Results. DM THA exhibited absolute dominance over conventional THA with lower accrued costs (US$45,960 versus $47,103) and higher accrued utility (12.08 QALY versus 11.84 QALY). The ICER was -$4,771/QALY, suggesting that DM THA is cost-saving compared to conventional THA. The cost threshold at which dual mobility implants were cost-ineffective was $25,000 (Figure 2), and the threshold at which DM implants ceased being cost-saving was $12,845. Sensitivity analyses demonstrated that the probability of intraprosthetic dislocation, primary THA utility, and age at index THA most influenced model results. In the probabilistic sensitivity analysis, 90% of model iterations resulted in cost savings for DM THA (Figure 3). Discussion. Dual mobility components showed clear cost-utility advantages over conventional THA components, and DM implants are cost-saving for primary unilateral THA from a societal perspecitve. Model results suggest that DM THA need not be limited to only high-risk patients, although patient age and risk of dislocation are important determinants of its cost-utility

Patella resection has been the least controlled element of total knee arthroplasty (TKA). We have developed an intraoperative guide system involving a custom-made surgical template designed on the basis of a three-dimensional computer simulation incorporating computed tomography (CT) data for several years. This time we have applied this intraoperative guide system for the patella resection in TKA. We investigated the accuracy of CT-based patient-specific templating (PST) for patella resection using cadaveric knee joints in vitro. To plan the corrective patella resection, we attempted to simulate a three-dimensional patella resection with the use of computer models of the patella. From CT images of the patella we obtained three-dimensional surface models of the patella by performing a three-dimensional surface generation of the bone cortex. After the patella resection using CT-based custom-made surgical templating instrumentation, CT scan was performed again and we compared the patella shape in three-dimensional patella bone model reconstructed from pre and after cut from CT data. We compared the accuracy of patella cut using three-dimensional patella bone model reconstructed from pre and after cut from CT data. Statistical analysis was performed using paired t test. The difference between patella cut with CT-based custom-made surgical templating instrumentation and pre-operative planning were 0.8±1.2mm (medial side) and 0.1±1.4mm (lateral side). More than 60% resulted within 2mm from the pre-operative planning. There were significant differences both in flexion/extension, external/internal rotation and bone cut depth between CT-based custom-made surgical templating instrumentation and conventional instrument. The results in this study demonstrated the usefulness of CT-based custom-made surgical templating instrumentation for patella resection in TKA

Introduction. Malrotation of the tibial component would lead to various complications after total knee arthroplasty (TKA) such as improper joint kinematics, patellofemoral instability, or excessive wear of polyethylene. However, despite reports of internal rotation of the tibial component being associated with more severe pain or stiffness than external rotation, the biomechanical reasons remain largely unknown. In this study, we used a musculoskeletal computer model to simulate a squat (0°–130°–0° flexion) and analyzed the effects of malrotated tibial component on lateral and medial collateral ligament (LCL and MCL) tensions, tibiofemoral and patellofemoral contact stresses, during the weight-bearing deep knee flexion. Materials and Methods. A musculoskeletal model, replicating the dynamic quadriceps-driven weight-bearing knee flexion in previous cadaver studies, was simulated with a posterior cruciate-retaining TKA. The model included tibiofemoral and patellofemoral contact, passive soft tissue and active muscle elements. The soft tissues were modeled as nonlinear springs using previously reported stiffness parameters, and the bony attachments were also scaled to some cadaver reports. The neutral rotational alignment of the femoral and tibial components was aligned according to the femoral epicondylar axis and the tibial anteroposterior axis, respectively. Knee kinematics and ligament tensions were computed during a squat for malrotated conditions of the tibial component. The tibial rotational alignments were changed from 15° external rotation to 15° internal rotation in 5° increments. The MCL and LCL tensions, the tibiofemoral and patellofemoral contact stresses were compared among the knees with different rotational alignment. Results. For the MCL, the neutral rotated tibial components caused a maximum tension of 67.3 N. However, the 15° internally rotated tibial components increased tensions to 285.2N as a maximum tension [Fig.1]. By contrast, with external rotation of the tibial component, the MCL tensions increased only a small amount. The LCL tension also increased but up to less than half of the MCL value [Fig.2]. The tibiofemoral and patellofemoral contact stresses increased because of a decreased contact area [Fig.3]. Discussion and Conclusion: In this computer simulation, excessive internal rotation in the tibial component increased MCL tensions and patellofemoral and tibiofemoral contact stresses. The current study suggests that increased MCL tensions and patellofemoral and tibiofemoral contact stresses caused by a malrotated tibial component could be one cause of patient complaints and polyethylene problems after TKA

We developed a custom-made template for corrective femoral osteotomy during THA in a patient with a previous Schanz osteotomy. A seventy-year-old woman presented to our clinic with a chief complaint of right hip, left knee and left ankle pain with marked limp. She had undergone Schanz osteotomy of the left femur because of high dislocation of the left hip when she was 20 years old. After right THA was performed, we decided to perform left THA with corrective femoral osteotomy. A custom-made osteotomy template was designed and manufactured with use of CT data. During surgery, we placed the template on the bone surface, cut the bone through a slit on the template, and corrected the deformity as preoperatively simulated. Two years after surgery, she had no pain in any joints, could walk more than one hour without limp. Japanese Orthopedic Association hip score were 100 points for both hips. THA in patients with previous Schanz osteotomy was reported to be technically demanding and the rate of complications was high. In 2008, Murase T et al. developed a system, including a 3D computer simulation program and a custom-made template to corrective osteotomy of malunited fractures of the upper extremity. We applied the system to corrective femoral osteotomy during THA in a patient with a previous Schanz osteotomy. The surgical procedure was technically easy and accurate osteotomy brought the patient to acquire good alignment of lower extremities with good clinical results

INTRODUCTION. The timely identification of outliers (implants, surgeons or patients) using prospectively collected registry data is confounded by many factors, including the assumption that the sampled population is representative of the entire cohort of patients. In this study we utilized a computer simulation of a joint registry to address the question: How does incomplete enrollment of patients in registries affect the reliability of identification of outliers, and what percent capture of the target population is sufficient?. MATERIALS AND METHODS. A synthetic registry was created consisting of 10,000 patients (100 surgeons), of whom, 1000 underwent joint replacement using a new implant. A predictive model for the risk of revision was created from data published by the Swedish TKR Registry and the AOANJRR. The pairing of patients, surgeons and implants was randomized and for each assignment, the probability of revision was computed. We then chose random samples of all patients in 10% increments from 10% to 100%, simulating incomplete capture of all potential cases by the registry. For each sample we calculated the number of cases of the new implant predicted to end in revision. The assignments were repeated 2000 times using implants with revision rates of 1.5%, 2.0% and 3.0% per annum vs. 1.0% for all other implants of the same class. RESULTS. The observed failure rate of the new implant averaged 2.0%, but varied from 0.7–3.8% over the 2000 trials, with 100% enrollment. With only 10% enrollment, the spread of failure rates increased to 0.0–7.8%, corresponding to a 152% increase in the variability of the observed revision rate. When enrollment was increased from 80% to 100%, the variability of the failure rate changed by only 9% from a range of 1.63% (1.23–2.86%) to 1.50% (1.30–2.80%) (90% CI). The reliability of detection of poorly performing implants improved dramatically with enrollment. With 70% enrollment, an implant with a 2.0% failure rate could be detected with 95% confidence, while a 3.0% implant became apparent with only 21% enrollment. Conversely, with even 100% enrollment it was not possible to identify implants with a 1.5% annual failure rate with 95% confidence. CONCLUSIONS. If registries collect a truly representative sample of only 50–80% of the total patient population, there will be only a slight increase in the risk of overlooking an inferior outlier, including poorly-performing implants, compared to 100% patient capture. Our results suggest that enrollment of every patient receiving a given treatment is not nearly as important as randomization of the sample subjected to analysis