Introduction. Total knee arthroplasty (TKA) designs evolve as evidence accumulates on natural and prosthetic knee function. TKA designs based upon a medially conforming tibiofemoral articulation seek to reproduce essential aspects of normal knee stability and have enjoyed good clinical success and high patient satisfaction for over two decades. Fluoroscopic kinematic studies on several medially conforming knee designs show extremely stable knee function, but very small ranges of tibial axial rotation compared to healthy knees. The GMK Sphere TKA is a recent evolution in medially-conforming TKA designs that adopts a sagittally unconstrained lateral tibiofemoral articulation to allow more natural tibial rotation. This study was conducted to quantify motions in knees with this prosthesis to address two questions:. Does the medially conforming GMK Sphere design provide an AP-stable articulation that provides for tibiofemoral translations that are comparable to, but not larger than, translations measured in natural knees?. Does the medially conforming GMK Sphere design provide sufficient rotatory laxity to allow tibiofemoral rotations comparable to, but not larger than, rotations measured in natural knees?. Materials and Methods. Fifteen patients (9 females), mean age 65 years and mean BMI of 30 ±3, consented to participate. Sixteen knees received the GMK Sphere TKA. Mean Oxford Knee Score (OKS) improved significantly from 19±7 to 40±3 six months post surgery (P< 0.0001). On the day of the study, the mean OKS, Knee Society Score, EQ5D and Heath status scores were 40, 87, 0.83 and 85 respectively. Mean ROM from active maximum extension till maximum supine flexion was 108°±8°. Motions in 16 knees were observed using pulsed-fluoroscopy during a range of activities. Subjects were observed in maximum flexion kneeling and lunging positions, and in stepping up/down on a 22cm step. Model-image registration methods were used to quantify three-dimensional knee motions from digitized fluoroscopic images. Results. Tibial internal rotation averaged 8° during lunge and kneeling activities. During lunging, the medial and lateral condyles were an average of 2mm and 8mm posterior to the tibial sulcus, respectively, and 2mm and 9mm posterior to the tibial sulcus during kneeling. During the stair-stepping activity, the medial condyle did not translate significantly, while the lateral condyle moved 5mm posteriorly with flexion, accompanying 5° tibial internal rotation. Discussion. The GMK Sphere TKA was designed to provide intrinsic stability through a medially conforming articulation, and provide for more natural tibial rotation with an unconstrained lateral articulation. Fluoroscopic observation of these knees during lunge, kneel and stair-stepping activities showed a stable medial articulation with little translation, and a lateral articulation translating in direct relation to tibial rotation. Tibial rotation during kneeling (8° average) was approximately twice that observed in knees with an earlier medially conforming TKA design (Moonot et al., Knee Surg Sports Traumatol Arthrosc, 2009) and similar to that observed in natural knees with medial osteoarthritis (Hamai et al., J Orthop Res, 2009). At only six months follow-up, knees with the GMK Sphere arthroplasty show functional kinematics that are AP stable and have more natural tibial rotation, consistent with the implant design intent

BACKGROUND. The need for post-operative manipulation under anesthesia (MUA) for stiffness after primary total knee arthroplasty is a frustrating complication that can lead to suboptimal outcomes if range-of-motion to a functional level is not regained. Implant morphology and kinematics, PCL imbalance, and soft-tissue balancing can all contribute to post-operative stiffness. Utilization of total knee arthroplasty components that replicate the native knee's medial ball and socket kinematics may lead to easier maintenance of flexion post-operatively compared to conventional components. PURPOSE. To determine if a medial pivot total knee arthroplasty design can reduce the need for post-operative MUA after primary total knee arthroplasty. METHODS. A retrospective chart review of primary total knee arthroplasties performed between 2013 and 2016 by a single fellowship-trained joint replacement surgeon was performed. Cases that met criteria for inclusion were: primary total knee arthroplasty, identifiable implant based on operative report and/or post-operative radiographs, immediate post-operative passive flexion against gravity of at least 110 degrees, and availability of post-operative follow-up notes documenting range-of-motion that was either satisfactory or necessitating need for MUA. The need for a MUA was deemed necessary if post-operative flexion was not beyond 90 degrees within six weeks of surgery. The percentage of patients requiring MUA for a group implanted with the EVOLUTION Medial Pivot System was compared to a group implanted with all other designs (Stryker Triathlon CR, PS, TS). RESULTS. One hundred fifty-six cases met criteria for inclusion and were reviewed. The Triathlon system was used predominantly in the first half of the study period and accounted for 65 (42%) of the cases performed. Six patients in this group underwent MUA and two patients required repeated MUA. An additional patient in the Triathlon group met the criteria for MUA but had other conditions which prevented the investigators from performing it. The percentage of patients who met the indication for MUA in the Triathlon group was 10.8%. The EVOLUTION system was used predominantly in the second half of the study period and accounted for 91 (58%) of the cases performed. There were two patients (2.2%) who met criteria for MUA and both patients subsequently underwent MUA. There was a statistically significant reduction in the number of patients meeting criteria for MUA in the EVOLUTION group compared with the Triathlon group (p=0.024). CONCLUSION. Utilization of a medial ball and socket design for primary total knee arthroplasty allows the polyethylene implant to control the position of the femur on the tibia. This design possibly allows for improved early maintenance of post-operative flexion, which may minimize the need for post-operative MUA. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Introduction:

Following total knee arthroplasty, patients often complain of an unnatural feeling in their knee joint, which in turn limits their activities [Noble et al, CORR 2006]. To develop an implant design that recreates the motion of the natural knee, both the functional kinematics as well as the laxity of the joint need to be understood. In vitro testing that accurately quantifies the functional kinematics and laxity of the knee joint can facilitate development of implant designs that are more likely to result in a natural feeling, reconstructed knee. The objective of this study is to demonstrate that robotic in vitro testing can produce clinically relevant functional kinematics and joint laxities.

Introduction

The SAIPH™ (MatOrtho, UK) total knee replacement is a new fixed-bearing prosthesis design having attributes of a mobile bearing and the posterior stabilised categories for knee arthroplasties. The implant design goal is an articulation that provides definitive anteroposterior stability to beneficially control tibiofemoral translation, the ability for the tibia to axially rotate to accommodate various lifestyle activities, and to maintain a relatively posterior femoral position on the tibia to facilitate range of motion. This study aims to analyze knee kinematics of the SAIPH™ total knee arthroplasty (TKA) by videofluroscopy during four different weightbearing activities.

Introduction:

While survivorship of total knee arthroplasty (TKA) is excellent, up to 25% of patients remain dissatisfied with their outcome [1, 2]. Knee instability, which is common during high demand activities, contributes to patient dissatisfaction [3]. As younger patients undergo TKA, longevity requirements and functional demands will rise [4]. Design factors influence the functional outcome of the procedure [5, 6], although in clinical studies it can be difficult to distinguish joint mechanics differences between designs due to confounding variability in patient-related factors. The objective of the current study was to assess the stability and mechanics of several current TKA designs during high-demand dynamic activities using a computational model of the lower limb.

Introduction. Total-knee-arthroplasty (TKA) is a well-established method to restore the joint function of the human knee. Different types of TKA designs are clinically available which can be divided in two main groups, the posterior-cruciate- ligament (PCL) sacrificing and retaining group. However, pre-operatively it is often difficult to plan for one or the other. Therefore, the research question was: Is it possible to develop a TKA bearing design which works for both the cruciate sacrificing and retaining technique? A medial-congruent (MC) bearing design was developed, characterized by a high medial sagittal conformity and lower lateral sagittal conformity, which can be used for both cruciate ligament states. This study compares the laxity and kinematics of this MC design to a contemporary PS design for the cruciate sacrificing technique and to a contemporary CR design for the cruciate retaining technique. Methods. Four specimen-specific computer models of the human knee, consisting of a femur, tibia and fibula bone as well as the contribution of the ligaments and capsule, were virtually implanted with three TKA designs in four constellations: 1) MC without PCL, 2) MC with PCL, 3) contemporary PS without PCL and 4) contemporary CR with PCL following the design specific surgical technique and tibia slopes. Laxity tests in internal-external rotation (moment ± 4 Nm) were performed with the implanted models for a weight bearing case (500N compression). In addition, a high demanding activity (lunge) was simulated. The resulting averaged laxities and kinematics were analysed and compared to each other. Results. When sacrificing the PCL, MC showed lower medial laxity throughout flexion and higher lateral laxity above 60° flexion compared to the PS design. When retaining the PCL, the MC resulted in lower medial laxity throughout flexion, lower lateral laxity in extension and similar lateral laxity in flexion compared to the CR design. When sacrificing the PCL in the lunge activity, the MC design had a more posterior position throughout flexion on both condyles until deep flexion when the engagement of the cam/spine occurred for the PS design and posterior motion of the medial condyle during mid-flexion as opposed to anterior motion for the PS design. When retaining the PCL in the lunge-activity, the MC design had a more posterior position throughout the activity, and similar medial and lateral condyle motion throughout flexion compared to the CR design. Conclusion. When sacrificing the PCL, MC behaved similar to a contemporary PS design with more medial stability, more lateral laxity in deep flexion, and a posterior position during a lunge activity that did not depend on a cam/spine mechanism. When retaining the PCL, MC behaved similar to a contemporary CR design with more medial stability, similar lateral laxity in deep flexion, and a posterior position during a lunge activity demonstrating that the increased medial conformity did not cause a kinematic conflict with the retained PCL. These findings illustrate the concept that the MC design can be used for both the PCL sacrificing and retaining technique

Introduction

Conventional implant designs in total knee arthroplasty (TKA) are based on metal on UHMWPE bearing couples. Although this procedure is quite successful, early loosening is still a matter of concern. One of the causes for early failure is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The introduction of a polyetheretherketone (PEEK) on UHMWPE bearing couple could address this problem. With mechanical properties more similar to distal (cortical) bone it could allow stresses to be distributed more naturally in the distal femur. A potential adverse effect, however, is that the femoral component and the underlying cement mantle may be at risk of fracturing. Therefore, we analyzed the effect of a PEEK-Optima® femoral component on stress shielding and the integrity of the component and cement mantle, compared to a conventional Cobalt-Chromium (CoCr) alloy implant.

Introduction

Many prosthetic design changes have been introduced in attempt to improve outcomes following TKA; however there is no consensus as to whether these changes confer benefits to patients. This study aimed to assess whether patients treated with a modern implant design had an enhanced patient outcome compared to a traditional model in a double blind randomised controlled trial.

Introduction. The mobile-bearing (MB) total knee arthroplasty (TKA) design was introduced with the aim of reducing polyethylene wear and component loosening seen in the fixed-bearing (FB) design. A recent joint registry study has revealed increased risk for all-cause revision, but not revision for infection, in MB-TKA. We used the New Zealand Joint Registry (NZJR) to compare all-cause revision rates, and revision rates for aseptic loosening of MB-TKA compared with fixed bearing (FB) TKA. Methods. All patients who underwent a primary TKA registered in the NZJR between the 1st January 1999 to 31st December 2021 were identified. Analysis compared MB to FB designs, with sub analysis of implants from a single company. We identified 135,707 primary TKAs, with 104,074 (76.7%) FB-TKAs and 31,633 (23.3%) MB-TKAs recorded. We examined all-cause revision rates, reasons for revision and performed survival analyses. Results. For all-comers, MB-TKA had an all-cause revision rate of 0.43/100-component-years (OCY) compared with 0.42/OCY for FB-TKA (p=0.09). The all-cause revision rate was higher for those age < 65 years (MB TKA 0.60/OCY vs. FB-TKA 0.59/OCY) compared to those > 65 years at time of primary TKA (MB-TKA 0.29/OCY vs. FB-TKA 0.32/OCY), however there was no statistically significant difference between implant design in either age group (p=0.16 and p=0.64; respectively). Similarly, there was no difference in revision rates for aseptic loosening between implant designs. Kaplan-Meier survival analysis demonstrates no statistically significant difference in revision-free survival of implants, with both MB-TKA and FB-TKA demonstrating ∼93% revision free survival at 23 years. Conclusions. Both FB- and MB-TKA demonstrated excellent survivorship, with no significant difference in all-cause revision rates or revision for aseptic loosening between implant designs

Total Knee Arthroplasty (TKA), has now become a reliable, successful, and widely used treatment for osteoarthritis. Numerous reports indicate that for the majority of patients, the TKA lasts a lifetime with pain relief and the ability to perform most everyday activities. However there are a number of ways in which the procedure can be further improved, the focus here being on function. One of the problems in evaluating function is that it depends upon the inherent ability, motivation, and expectation of the patients. There are several well-used questionnaire systems which capture functional ability objectively. In the effort to simplify evaluation, a ‘forgotten knee’ evaluation has been introduced, the concept being that ‘the ideal TKA design’ would feel and function like a normal knee. Such a measure would include factors such as surgical technique, alignment, and rehabilitation, as well as the TKA design itself. Another approach to evaluation is to measure biomechanical parameters, such as in gait analysis and fluoroscopy, which evaluate kinematic or kinematic parameters, using normal controls for comparison. Nevertheless, such evaluations still include factors other than the TKA design itself, and do not apply to new designs. The approach taken here for the evaluation of a new TKA design independent of other factors, is to measure the neutral path of motion and the laxity boundaries of the loaded knee on the application of shear and torque over a full range of flexion. The benchmark is the same kinematic data from the normal intact knee. The rationale has some analogy to the ‘forgotten knee’ in that if the laxity response of a design of TKA is the same as that of the anatomic knee itself, the behavior of that implanted knee in any functional condition will be indistinguishable from that of the anatomic knee itself. Such a testing concept has some similarities to the constraint test described in the ASTM standard. In this paper, a novel design algorithm is proposed for creating different design concepts. First, a general morphological form is formulated for each design concept, a Cam-Post PS, a Saddle-Ramp, and a Converging Condyle, all with overall anatomic-like surfaces. Each femoral component is then designed, which is then moved through the normal neutral path and laxity paths, which creates the tibial surface. The concepts are evaluated using a Desktop Knee Machine configured to move the knee dynamically through full flexion while applying combinations of compression, shear and torque; kinematic data being captured optically and plotted using custom software. The normal benchmark was obtained from 10 normal knee specimens, which showed the restraint of the medial femoral condyle to anterior displacement and the overall rollback and laxity laterally. Compared with standard CR and PS designs, the Guided Motion designs were seen to more closely resemble normal. It is proposed that this approach can result in designs which will more likely reproduce a ‘forgotten knee’ and achieve the optimal function for a given patient

Background. Although early TKA designs were symmetrical, during the past two decades TKA have been designed to include asymmetry, pertaining to either the trochlear groove, femoral condylar shapes or the tibial component. More recently, a new TKA was designed to include symmetry in all areas of the design, in the hopes of reducing design and inventory costs. Objective. The objective of this study was to determine the in vivo kinematics for subjects implanted with this symmetrical TKA during a weight-bearing deep knee bend activity. Methods. In vivo deep knee bend (DKB) kinematics for 21 subjects implanted with symmetrical posterior cruciate sacrificing (PCS) fixed bearing TKA were obtained using fluoroscopy. A 3D-to-2D registration technique was used to determine each subjects anteroposterior translation of lateral (LAP) and medial (MAP) femoral condyles and tibiofemoral axial rotation and their weight-bearing knee flexion. Results. During the DKB, the average maximum weight-bearing flexion was 111.7° ± 13.3°. On average, from full extension to maximum knee flexion, subjects experienced 2.5 mm ± 2.0 mm femoral rollback on lateral condyle −2.5 mm ± 2.2 mm of medial condyle motion in the anterior direction (Figure 1). This medial condyle motion was consistent for the majority of the subjects with the lateral condyle exhibiting rollback from 0° to 60° of flexion and then an average anterior slide of 0.3 mm from 60° to 90° of flexion. On average, the subjects in this study experienced 6.6° ± 3.3° of axial rotation, with most of rotation occurring in early flexion, averaging 4.9° (Figure 2). Discussion. Although subjects in this study were implanted with a symmetrical TKA, they did experience femoral rollback of the lateral condyle and positive axial rotation. Both of these kinematic parameters were normal-like in pattern, compared to the normal knee in early flexion, but in deeper flexion the pattern of motion varied from the normal knee. Also, the magnitude of posterior femoral rollback and axial rotation revealed similarities to previous fluoroscopy studies on subjects implanted with an asymmetrical TKA design. This was only a single surgeon study, so it is unclear if the results are TKA or surgeon influenced. Therefore, it is proposed that more patients be analyzed having this TKA implanted by other surgeons. For any figures or tables, please contact the authors directly

Introduction. A common goal of total knee arthroplasty (TKA) is to restore normal knee kinematics. While substantial data is available on TKA kinematics, information regarding non-implanted knee kinematics is less well studied especially in larger patient populations. The objectives of this study were to determine normal femorotibial kinematics in a large number of non-implanted knees and to investigate parameters that yield higher knee flexion with weight-bearing activities. Methods. Femorotibial kinematics of 104 non-implanted healthy subjects performing a deep knee bend (DKB) activity were analyzed using 3D to 2D fluoroscopy. The average age and BMI were 38.1±18.2 years and 25.2±4.6, respectively. Pearson correlation analysis was used to determine statistical correlations. Results. On average, subjects experienced 21.5±7.2 mm, 13.8±8.9 mm, and 27.1°±12.1° of lateral rollback, medial rollback, and external femorotibial axial rotation, respectively (Figure 1). Most rollback occurred in early flexion, with 10.2±6.4 mm and 5.3±6.3 mm of rollback for the lateral and medial condyles, respectively. While the lateral condyle consistently moved posteriorly, the medial condyle experienced 1.8±4.8 mm of anterior sliding between 90° to 120° of flexion. There was a positive correlation between higher weight-bearing flexion and lateral condylar rollback (r=0.5480, p<.0001) (Figure 2), medial condylar rollback (r=0.3188, p=0.001) (Figure 3), and external axial rotation (r=0.5505, p<.0001) (Figure 4). There was an inverse correlation between advancing age and knee flexion (r=-0.7358, p<.0001) as well as higher BMI and flexion (r=-0.3332, p=0.0007), indicating that multiple factors contribute to postoperative range-of-motion. Conclusion. This represents one of the largest studies on normal knee femorotibial kinematics in non-implanted healthy subjects. These results indicate that increased condylar rollback and external axial rotation correlate with increased weight-bearing knee flexion, while increased age and BMI yield decreased flexion. Therefore, in order to achieve higher weight-bearing flexion following TKA, normal-like kinematics such as high rollback and external axial rotation should be incorporated into TKA design. For any figures or tables, please contact the authors directly

Introduction. Varus alignment in total knee replacement (TKR) results in a larger portion of the joint load carried by the medial compartment. [1]. Increased burden on the medial compartment could negatively impact the implant fixation, especially for cementless TKR that requires bone ingrowth. Our aim was to quantify the effect varus alignment on the bone-implant interaction of cementless tibial baseplates. To this end, we evaluated the bone-implant micromotion and the amount of bone at risk of failure. [2,3]. Methods. Finite element models (Fig.1) were developed from pre-operative CT scans of the tibiae of 11 female patients with osteoarthritis (age: 58–77 years). We sought to compare two loading conditions from Smith et al.;. [1]. these corresponded to a mechanically aligned knee and a knee with 4° of varus. Consequently, we virtually implanted each model with a two-peg cementless baseplate following two tibial alignment strategies: mechanical alignment (i.e., perpendicular to the tibial mechanical axis) and 2° tibial varus alignment (the femoral resection accounts for additional 2° varus). The baseplate was modeled as solid titanium (E=114.3 GPa; v=0.33). The pegs and a 1.2 mm layer on the bone-contact surface were modeled as 3D-printed porous titanium (E=1.1 GPa; v=0.3). Bone material properties were non-homogeneous, determined from the CT scans using relationships specific to the proximal tibia. [2,4]. The bone-implant interface was modelled as frictional with friction coefficients for solid and porous titanium of 0.6 and 1.1, respectively. The tibia was fixed 77 mm distal to the resection. For mechanical alignment, instrumented TKR loads previously measured in vivo. [5]. were applied to the top of the baseplate throughout level gait in 2% intervals (Fig.1a). For varus alignment, the varus/valgus moment was modified to match the ratio of medial-lateral force distribution from Smith et al. [1]. (Fig.1b). Results. For both alignments and all bones, the largest micromotion and amount of bone at risk of failure occurred during mid stance, at 16% of gait (Figs.2,3). Peak micromotion, located at the antero-lateral edge of the baseplate, was 153±32 µm and 273±48 µm for mechanical and varus alignment, respectively. The area of the baseplate with micromotion above 40 µm (the threshold for bone ingrowth. [3]. ) was 28±5% and 41±4% for mechanical and varus alignment, respectively. The amount of bone at risk of failure at the bone-implant interface was 0.5±0.3% and 0.8±0.3% for the mechanical and varus alignment, respectively. Discussion. The peak micromotion and the baseplate area with micromotion above 40 µm increased with varus alignment compared to mechanical alignment. Furthermore, the amount of bone at risk of failure, although small for both alignments, was greater for varus alignment. These results suggest that varus alignment, consisting of a combination of femoral and tibial alignment, may negatively impact bone ingrowth and increase the risk of bone failure for cementless tibial baseplates of this TKR design

Introduction. In addition to traditional posterior-stabilized (PS) designs with cam-post articulations, there are two new design concepts used in total knee replacement (TKR) to “substitute” for cruciate ligament function and restore anterior-posterior stability. These include i) guided-motion PS designs with a modified cam-post that is less restrictive to axial rotation; and ii) non-PS designs that incorporate progressive articular congruency to substitute the function of the resected anterior cruciate ligament (ACL-substituting). Early post-marketing surveillance of such new TKR designs is valuable because instability, loosening, and high complication rates within the initial 5 year follow-up interval have proven problematic for some design. This study reports the early clinical performance of sequential patients implanted with a new ACL-substituting TKR design at a German Center of Excellence for Arthroplasty (EPZ-Max) hospital. Methods. This is a single-site, multi-surgeon retrospective study with Institutional Review Board approval. The nine surgeons involved all used uniform surgical techniques, including a mid-vastus approach, PCL preservation with a bone block, tibial component alignment with the natural tibial slope, no patellar resurfacing, and cement fixation. All patients meeting the following inclusion criteria were contacted by phone: a) primary TKR from July 2008-June 2009; b) implanted with an ACL-substituting design (3D Knee™, DJO Surgical); c) no prior knee arthroplasty; and d) willing to consent to participate. Recorded outcomes at the 5 year follow-up interval included range of motion, Knee Society knee/function scores (KSS), and radiographic results (alignment, radiolucent lines, osteolysis). Additional surgery was classified as “revision” (metal components removed) or “reoperation” (metal components not removed). Results. Out of 166 sequential patients, a study cohort of 69 patients (84 TKR) consented to participate. Average follow-up was 6.2+0.6 (4.7–7.3) years. Maximum flexion averaged 115°+9° (85°–145°), including 39% at >120°. The TKR had stable function and average KSS scores of 94 (knee) and 94 (function). There were four patients (5 TKR, 6.3%) with function scores of <60 points who had considerable pre-operative extension lags (>10°) that lingered (5°–10°) at follow-up. No TKR had evidence of osteolysis or loosening. Non-progressive radiolucent lines were evident post-operatively in 10 TKR and in one additional TKR at last follow-up. Seven (8.3%) TKR required reoperation or revision. Five TKR in the 0–5 year interval required reoperation to treat acute infection, progressive patellar arthritis, and traumatic patellar fracture, and revision to treat pain of unknown etiology (2 TKR revised at another hospital). Two TKR in the 5–6 year interval required revision arthroplasty to treat pain associated with a loose tibial insert screw and unknown symptoms (1 TKR revised at another hospital). Conclusion. This new ACL-substituting design incorporates progressive congruency in the lateral compartment. These data, combined with previous studies (Table 1), provides evidence that this ACL-substituting TKR design restores stability while being robust to surgical and patient variations. These results for a fixed-bearing, non-PS TKR design are supported by international registry data from more than 370,000 TKR in six countries, which demonstrate that survivorship of fixed-bearing cruciate-retaining TKR designs is significantly higher than posterior-stabilized TKR designs, both with and without patella resurfacing

Background. Aseptic loosening is the leading cause of total knee arthroplasty (TKA) failure in the long term, of which osteolysis from polyethylene wear debris remains a problem that can limit the lifetime of TKA past the second decade. To help speed up design innovations, our goal was to develop a computational framework that could efficiently predict the effect of many sources of variability on TKA wear—including design, surgical, and patient variability. Methods. We developed a computational framework for predicting TKA contact mechanics and wear. The framework accepts multiple forms of input data: patient-specific, population-specific, or standardized motions and forces. CAD models are used to create the FEA mesh. An analytical wear model, calibrated from materials testing (wheel-on-flat) experiments, is fully integrated into the FEA process. Isight execution engine runs a design of experiments (DOE) analysis with an outcome variable, such as volumetric wear, to guide statistical model output. We report two DOE applications to test the utility of the computational framework for performing large variable studies in an efficient manner: one to test the sensitivity of TKA wear to the femoral center of rotation, and the second to test the sensitivity of TKA wear to gait input perturbations. Results. Using this method, we demonstrated that choice of femoral center of rotation matters, and that although volumetric wear was most sensitive to variation in flexion/extension peaks, no one kinematic factor dominates TKA volumetric wear variability. Conclusion. The two DOE applications represent initial first attempts to study variability in component alignment and input waveforms across large solution spaces. The computational framework will be most useful if it can be used in a TKA design setting, where new innovations can be tested as soon as they are developed to see if they are worthy of further mechanical testing

Aims. Recently, total knee arthroplasty (TKA) has been generalized as an operation that achieves excellent clinical results. However, younger and Asian patients require even greater implant longevity and functional performance. We hypothesized a novel posterior cruciate-retaining TKA design that restores the anatomical jointline in both sagittal and coronal planes, maintains the femoral posterior condylar offset, and provides low contact stress would provide enhanced patient function with the potential for greater implant longevity. Methods. The novel TKA design was created based on geometry determined from anatomic specimens, 3-degree step of femorotibial jointline was incorporated in the TKA design for Asian. The novel TKA has an asymmetrical design between the medial and lateral femoral condyle, the medial femoral condyle designed to be 3 degrees larger than the lateral femoral condyle. It refined using finite element analyses (FEA) to minimize peak contact stresses. The alignment evaluation after TKA was performed using using bidirectional CR and CT images. Femorotibial-angle (FTA), the position of the femoral component relative to the 3D mechanical axis, and the rotational alignment of the femoral component relative to the PC line were evaluated before and after TKA to identify changes in the femoral condylar shape. The kinematic evaluation after TKA was performed using a 3D-to-2D model registration technique. Single-plane fluoroscopic imaging was used to record and quantify the motions of knees during a stair-step activity. The contact points between the tibiofemoral motions and the tibial rotational angle were evaluated. Results. FEA showed peak von Mises stress of 10.5 MPa under 2000 N load, well below the plastic limit of UHMWPe. Changes in the femoral condylar shape after TKA were small based on alignment analysis of the medial and lateral condyles. Kinematic analysis revealed an average 16.9 degrees tibial internal rotation with flexion to 90 degrees. Condylar translations exhibited a medial pivot pattern from 0 to 90 degrees of flexion, with an average of 0.9 mm anterior translation of the medial condyle and 9.0 mm posterior translation of the lateral condyle. As a result, tibial internal rotation with medial pivot pattern was accepted. Conclusions. The novel TKA design based on restoration of articular surface orientation and bony geometry exhibits in vivo kinematics and alignment, contact mechanics that are compatible with excellent functional performance and implant longevity for Asian

Introduction. We introduce the concept of total knee arthroplasty (TKA) fingerprinting as a tool to characterize and graphically convey the sensitivity of a TKA design to surgical variability in implant component position and patient-related anatomic factors. Identifying sensitive directions preoperatively which would cause undesirable effects may decrease revision surgery by informing surgical decisions and planning. To provide several examples of TKA fingerprinting, we estimated and compared the contact forces in a single TKA type for several configurations, simulating surgical variability and patient-related anatomical factors during a loaded deep squat. The purpose of this study is not to analyze the behavior of this specific TKA design but rather to illustrate a tool that could be used to show, in general, how surgical errors or anatomical factors can alter patello-femoral (PF) and tibio-femoral (TF) contact forces compared to its own reference configuration. Materials and methods. Computed tomography images of one full cadaveric leg were used to generate 3D models of the bones and to obtain a physiological knee model assuming standard positions of the main soft tissue insertions. A fixed bearing posterior stabilized knee TKA design was considered in this study. The prosthesis was a medium size, replaced both cruciate ligaments and resurfaced the patella. Following standard surgical procedure, the TKA was virtually implanted, thus defining its reference configuration. Each derivative replaced knee model was then obtained by changing the values of one parameter, or a combination of two, in a range based on literature and surgical experience (Table 1). A 10 s loaded squat to 120° was performed for each configuration, with a constant vertical hip load of 200 N. These settings match the experimental tests performed in a previous in-vitro analysis on cadaver legs. Each replaced model was developed and analyzed using a validated musculoskeletal modeling software. The model of the knee included TF contacts and PF contacts of the TKA components, passive soft tissues and active muscle elements. The external forces (ground reaction and weights), the muscle forces (quadriceps and hamstrings) and the frictional forces are applied to the knee joint through the machine. The mechanical properties of the tissues were obtained from literature. With these settings, for each model, both the maximum PF and TF contact forces have been evaluated. Results. Examples of fingerprint graphs are shown presenting the main results (Figures 1 and 2). Figure 1 displays a fixed rotation femoral component and a variable rotation tibial component. FIgure 2 displays a variable rotation patella component. Discussion and Conclusion. In general a TKA should be implanted without surgical errors to obtain the biomechanical behavior for which a TKA was designed because surgical errors can alter the functionality of a TKA. A fingerprinting tool for TKAs was developed and used to show the sensitivity of PF and TF contact forces in surgical variability. The graphs show that PF contact forces are altered mostly by errors in positioning of the patellar component, while TF contact forces are mostly affected internal and external femoral component rotation and ligament release

A unique, laterally stabilized design concept (3D Knee-DJO Surgical, Inc) for total knee arthroplasty (TKA) without traditional post and cam construct was developed to allow surgeons to resurface the arthritic knee while choosing to maintain or sacrifice the posterior cruciate ligament (PCL). Reported complications with current ‘post and cam’ designs of PCL-substituting TKRs include higher polyethylene wear associated with cam-post impingement, increased bone interface shear stresses, and more distal femoral bone resection making revisions more complex and problematic. The effectiveness of this laterally stabilized TKA design has been extensively studied biomechanically using both in-vitro and in-vivo methods. It was hypothesized that for this total knee arthroplasty design; the mid-term clinical, radiographic and functional results would be the same for patients having two different surgical techniques in which the posterior cruciate ligament was either completely retained or completely resected. This study reports on eight year clinical results as well as in-vivo fluoroscopic results and retrieval data. Reported are 159 patients with 116 knees done by a surgeon who preserved the PCL with a bone block technique and 43 knees by a second surgeon who completely resected the PCL. Clinical results did not statistically differ between the two groups and found Knee Society Scores of 96 for Pain and 91 for Function. Average ROM was measured at 124 degrees. Comparative fluoroscopic imaging analysis of in-vivo dynamic flexion activities of thirty-three (20 PCL-preserved and 13 PCL resected) knees was performed demonstrating stable performance and only small (non-significant) mechanical differences. Analysis of two unrelated groups of tibial polyethylene inserts, the first retrieved from patients after 1–4 years in-vivo function (n = 14) and the second after in-vitro knee wear simulation (n = 4) showed low wear rates with no delamination. There was only one failure for mechanical loosening in the cruciate resected group and radiolucent lines of greater than 2 mm were only seen in 4% with none being progressive. Kaplan-Meier Survivorship, using mechanical loosening as the end point, was 99.2% at an average of 8.8 years. In summary, this laterally stabilized TKR design offers a very good alternative to standard ‘post and cam’ PCL sacrificing TKRs while still giving surgeons the ability to maintain the PCL if desired

Background. Previous in vivo fluoroscopic studies have documented that subjects having a PS TKA experience a more posterior condylar contact position at full extension, a high incidence of reverse axial rotation and mid flexion instability. More recently, a PS TKA was designed with a Gradually Reducing Radius (Gradius) curved condylar geometry to offer patients greater mid flexion stability while reducing the incidence of reverse axial rotation and maintaining posterior condylar rollback. Therefore, the objective of this study was to assess the in vivo kinematics for subjects implanted with a Gradius curved condylar geometry to determine if these subjects experience an advantage over previously designed TKA. Methods. In vivo kinematics for 30 clinically successful patients all having a Gradius designed PS fixed bearing TKA with a symmetric tibia were assessed using mobile fluoroscopy. All of the subjects were scored to be clinically successful. In vivo kinematics were determined using a 3D-2D registration during three weight-bearing activities: deep-knee-bend (DKB), gait, and ramp down (RD). Flexion measurements were recorded using a digital goniometer while ground reaction forces were collected using a force plate as well. The subjects then assessed for range of motion, condyle translation and axial rotation and ground reaction forces. Results. During a DKB, subjects implanted a Gradius designed, PS fixed bearing TKA design exhibited an average of 3.35 mm of posterior femoral rollback of the lateral condyle and 2.73 mm of the medial condyle with an average axial rotation of 4.90° in the first 90° of flexion. The average max flexion was 111.4°. From full extension to maximum flexion, the average axial rotation was 4.73°, while the subjects experienced 5.34 and 1.97 mm on the lateral and medial condyle rollback, respectively. During mid flexion from 30 to 60 degrees of flexion, the subjects experienced 1.34° of axial rotation, −1.13 and −0.11 mm of lateral and medial condyle motion. Conclusions. Subjects in this study did experience good weight-bearing flexion and magnitudes of axial rotation and posterior femoral rollback similar to previous PS TKA designs. During mid flexion, subjects in this study did experience less mid flexion paradoxical sliding than other PS TKA, leading to greater mid flexion stability for the patients

INTRODUCTION. There are many factors which contribute to function after TKA. In this study we focus on the effect of varus-valgus (VV) balancing measured externally. A loose knee can show instability (Sharkey 2014) while too tight, flexion can be limited. Equal lateral-medial balancing at surgery leads to a better result (Unitt 2008; Gustke 2014), which is generally the surgical goal. Indeed similar varus and valgus laxity angles have been found in most studies in vitro (Markolf 2015; Boguszewski 2015) and in vivo (Schultz 2007; Clarke 2016; Heesterbeek 2008). The angular ranges have been 3–5 degrees at 10–15 Nm of knee moment, females having the higher angles. The goal of this study was to measure the varus and valgus laxity, as well as the functional outcome scores, of two cohorts; well-functioning total knees after at least one year follow-up, and subjects with healthy knees in a similar age group to the TKR's. Our hypothesis was that the results will be equal in the two groups. METHODS & MATERIALS. 50 normal subjects average age 66 (27 male, 23 female) and 50 TKA at 1 year follow-up minimum average age 68 years (16 male, 34 female) were recruited in this IRB study. The TKA's were performed by one surgeon (PAM) of one TKA design, balancing by gap equalization. Subjects completed a KSS evaluation form to determine functional, objective, and satisfaction scores. Varus and valgus measurements were made using the Smart Knee Fixture (Figure 1)(Borukhov 2016) at 20 deg flexion with a moment of 10 Nm. RESULTS. The statistical results are summarized in table 1. There was no significant difference in either varus or valgus laxity between the two groups (p= 0.9, 0.3 respectively). Pearson's correlation coefficient between varus and valgus laxity of the healthy group was 0.42, while for the TKA group was 0.55. In both cohorts varus laxity was significant higher than valgus laxity (p= 0.001 for healthy subjects and p=0.0001 for TKA). The healthy group had higher functional and objective KSS scores (p= 0.005, and p=0.004 respectively), but the same satisfaction scores as the TKA (p=0.3) (Table 2). No correlation was found between the total laxity of the TKA group and the KSS scores (functional, objective and satisfaction). Total laxity in females was significantly higher than in males in the healthy group, but no differences was found in the TKA group. DISCUSSION. The hypothesis of equal varus and valgus angles in the 2 groups was supported. The larger varus angle implied a less stiff lateral collateral compared with the medial collateral. If the TKA's were balanced equally at surgery, it is possible there was ligament remodeling over time. However the functional scores were inferior for the TKA compared with normal. This finding has not been highlighted in the literature so far. The causes could include weak musculature (Yoshida 2013), non-physiologic kinematics due to the TKA design, or the use of rigid materials in the TKA. The result presents a challenge to improve outcomes after TKA. For figures/tables, please contact authors directly.