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).

Aims: description of results obtained with a single radius TKA design. Matherials and methods: In 1999 we started implanting single-radius TKA design (Scorpio–Osteonics). Between January 1999 and April 2002 we have implanted 211 TKA, 15 bilateral, with 24 months average follow-up (range 3–36 months). This type of prosthesis is based on the concept that the normal knee rotates about a single axis – the transepicondylar axis. This reduces mid-flexion instability, improves patello-femoral tracking and decreases the occurrence of anterior knee pain. Results: Knee Society score has improved to 90 points post-op from 39 points pre-op and function evaluation from 44 to 82 points. We had 128 cases excellent (65.6%), 40 good (20.5%), 24 fair (12.4%) and 3 poor (1.5%). X-ray follow-ups have revealed in 83% of cases knee angulation between 2° from neutral, in 11% a varus deviation between 3° and 5° and in 6% a valgus deviation between 3° and 5°. Radiolucencies have been found in 18 patients (9.23%). Conclusions: Scorpio single-radius prosthesis has permitted a full and early recovery of knee function. Bone cuts, especially the tibial one, are reduced. Anterior knee pain is dramatically reduced and is associated with incremented range of motion and improvement in knee stability, especially in the mid flexion. These short term results are really encouraging, but need to be confirmed by future follow-up studies

Physiological kinematics is very difficult to restore after total knee arthroplasty (TKA). A new model of medial stabilized (MS) TKA prosthesis has a high spherical congruence of the internal compartment, which guarantees anteroposterior (AP) stability associated with a flat surface of the insert in the lateral compartment, that allows a greater AP translation of the external condyle during knee flexion. The aim of our study is to evaluate, by dynamic radiostereometric analysis (RSA), the knee in vivo kinematics after the implantation of a MS prosthesis during sit to stand and lunge movements. To describe the in vivo kinematics of the knee after MS Fixed Bearing TKA (GMK Sphere (TM) Medacta International AG, Castel San Pietro, Switzerland) using Model Based dynamic RSA.

A cohort of 18 patients (72.1 ± 7.4 years old) was evaluated by dynamic RSA 9 months after TKA. The kinematic evaluation was carried out using the dynamic RSA tool (BI-STAND DRX 2), developed at our Institute, during the execution of sit to stand and lunge movements. The kinematic data were processed using the Grood and Suntay decomposition and the Low Point method. The patients performed two motor tasks: a sit-to-stand and a lunge. Data were related to the flexion angle versus internal-external, varus-valgus rotations and antero-posterior translations of the femur with respect to the tibia.

During the sit to stand, the kinematic analysis showed the presence of a medial pivot, with a significantly greater (p=0.0216) anterior translation of the lateral condyle (3.9 ± 0.8 mm) than the medial one (1.6 ± 0.8 mm) associated with a femoral internal rotation (4.5 ± 0.9 deg). During the lunge, in the flexion phase, the lateral condyle showed a larger posterior translation than the medial one (6.2 ± 0.8 mm vs 5.3 ± 0.8 mm) associated with a femoral external rotation (3.1 ± 0.9 deg). In the extension phase, there is a larger anterior translation of the lateral condyle than the medial one (5.8 ± 0.8 mm vs 4.6 ± 0.8 mm) associated with femoral internal rotation (6.2 ± 0.9 deg). Analysing individual kinematics, we also found a negative correlation between clinical scores and VV laxity during sit to stand (R= −0.61) and that the higher femoral extra-rotation, the poorer clinical scores (R= 0.65).

The finding of outliers in the VV and IE rotations analysis highlights the importance of a correct soft tissue balancing in order to allow the prosthetic design to manifest its innovative features.

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.

At present, computational modeling has not been utilized as a design tool for total knee replacement (TKR). Also, classifying a new design as successful usually requires many years of long-term clinical follow-up studies. Computational modeling presents an opportunity to contribute to implant design evaluations and prediction of long-term success, during the early stages of the implant design process. The purpose of this study was to construct a computational model that will determine and compare in vivo dynamic forces and torques of the non implanted and implanted knees. It is hypothesized that this model will provide valuable information pertaining to post-implantation boundary conditions during the design phase.

A three-dimensional (3-D), inverse dynamics model of the human lower limb was created. System differential equations were derived for the human lower extremity using Kane’s theory of dynamics.Input kinematics were obtained for five normal knees and five posterior stabilized TKR, determined while subjects performed deep knee bend while under fluoroscopic surveillance. Musculo tendinous units were assumed to act along straight line segments, and ligamentous units were represented by nonlinear elastic elements. Knee kinetics were calculated and compared fo reach group and a comparison was conducted.

Kinetics were much more variable for the TKR group, and tibiofemoral contact forces were on average higher than the normal group: 2.47 times body weight (BW) and 2.21 BW, respectively. Increased posterior femoral rollback lead to lower axial contact forces and lower quadriceps forces in both groups. Force patterns were very sensitive to input patient specific kinematics.

The predicted tibio femoral forces were higher in TKR subjects, which is consistent with current clinical knowledge. Force patterns for the normal subjects were more consistent than those forthe TKR subjects, which was primarily attributed to the greater variance in kinematics for the TKR subjects. This study represents a first step in constructing a design facilitation tool for TKR technology. Successful designs will be determined by producing kinetic patterns most similar to normal knee patterns.

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.

Controversy still remains as to whether patella resurfacing in total knee arthroplasty (TKA) should be conducted as a matter of routine. Some authors recommend routine resurfacing of the patella and advocate this due to the reduction in post-operative anterior knee pain and the reduction in requirement for revision surgery.

A database search was conducted to identify prospective randomized controlled studies only. Eighteen prospective randomized trials were identified and found eligible for inclusion. A cumulative sample of 8006 knees were involved, 3418 undergoing resurfacing and 4588 undergoing no resurfacing. Primary outcome data extracted was incidence of secondary operations, incidence of anterior knee pain and functional outcome scores including subgroup analysis of the type of prosthesis used in each study. Der Simonian Laird metanalysis was conducted if studies were found to be homogenous where statistical significance was defined as an overall alpha error of < 0.05.

No statistically significant differences were found to exist between patients undergoing patella resurfacing as a primary total knee replacement and those who preserved the native patella. No differences could be found between specific prosthetic design when subgroup analysis was conducted.

The study does not offer evidence to suggest any advantage of resurfacing versus non-resurfacing. Furthermore, no significant discrepancy existed when comparing different types of total knee prosthesis. This may be due that all the knees analysed are designed to be patella friendly and allow conformity of articulation of both a native and patella button equally

Aims. The aim of this study was to compare a bicruciate-retaining (BCR) total knee arthroplasty (TKA) with a posterior cruciate-retaining (CR) TKA design in terms of kinematics, measured using fluoroscopy and stability as micromotion using radiostereometric analysis (RSA). Methods. A total of 40 patients with end-stage osteoarthritis were included in this randomized controlled trial. All patients performed a step-up and lunge task in front of a monoplane fluoroscope one year postoperatively. Femorotibial contact point (CP) locations were determined at every flexion angle and compared between the groups. RSA images were taken at baseline, six weeks, three, six, 12, and 24 months postoperatively. Clinical and functional outcomes were compared postoperatively for two years. Results. The BCR-TKA demonstrated a kinematic pattern comparable to the natural knee’s screw-home mechanism in the step-up task. In the lunge task, the medial CP of the BCR-TKA was more anterior in the early flexion phase, while laterally the CP was more posterior during the entire movement cycle. The BCR-TKA group showed higher tibial migration. No differences were found for the clinical and functional outcomes. Conclusion. The BCR-TKA shows a different kinematic pattern in early flexion/late extension compared to the CR-TKA. The difference between both implants is mostly visible in the flexion phase in which the anterior cruciate ligament is effective; however, both designs fail to fully replicate the motion of a natural knee. The higher migration of the BCR-TKA was concerning and highlights the importance of longer follow-up. Cite this article: Bone Joint J 2023;105-B(1):35–46

Introduction. Inability to reproduce 6-degrees of freedom (6DOF) kinematics, abnormal “paradoxical” anterior femoral translation and loss of normal medial pivot rotation are challenges associated with contemporary posterior cruciate retaining and posterior stabilized total knee arthroplasty (TKA). The removal of the anterior and/or both cruciate ligaments in CR/PS TKA, leading to significant kinematic alteration of the knee joint, has been suggested as one of the potential contributory factors in patients remaining dissatisfied after TKA. Bi-cruciate retaining (BCR) TKA designs allow preservation of both anterior and posterior cruciate ligaments with the potential to replicate normal knee joint kinematics. Physically demanding tasks such as sit-to-stand (STS), and deep lunging may be more sensitive tools for investigating preserved kinematic abnormalities following TKA. This study aims to compare in-vivo kinematics between the operated and the contralateral non-operated knee in patients with contemporary BCR TKA design. Methods. Twenty-nine patients (14 male; 15 female, 65.7±7.7 years) unilaterally implanted with a contemporary BCR TKA design featuring an asymmetric femoral component and independently designed medial and lateral bearings were evaluated. Mean follow-up time after BCR TKA was 12.7±5.1 months. All patients received a computer tomography (CT) scan from the pelvis to the ankles for the creation of 3D surface models of both knees (BCR TKA and non-operated). Patients performed single leg deep lunges and sit-to-stand under a validated dual fluoroscopic imaging system (DFIS) surveillance. Each patient's 2D dynamic fluoroscopic images, corresponding 3D surface bone models (for contralateral non-operated knee) and computer aided design (CAD) implant models (for the BCR TKA implanted knee) were imported into a virtual DFIS environment in MATLAB. An optimization procedure was utilized to perform matching between the 3D surface bone models and the 2D fluoroscopic image outlines. In-vivo 6DOF kinematics of the BCR TKA knees and contralateral non-operated side were quantified and analyzed. Results. When performing the high-flexion lunge, BCR TKA knees demonstrated less average femoral posterior translation (13±4mm) during terminal flexion when compared to the contralateral non-operated knees (16.6±3.7mm) (p=0.001). Similarly, during STS, less femoral rollback was observed (11.6±4.5mm vs 14.4±4.6mm, p<0.04) in BCR TKA knees. Overall, BCR TKA knees partially reproduced a normal “screw-home” motion, demonstrating reduced internal rotation during several intervals of the cycles for strenuous flexion activities. BCR TKA knees demonstrated less internal rotation during high-flexion lunge (4±5.6° vs 6.5±6.1°, p=0.05). Similarly, during STS, less internal rotation was observed (4.5±6° vs 6.9±6.3°, p=0.04, p=0.02, p=0.01, p=0.02) in BCR TKA knees. Conclusion. The BCR TKA design demonstrated asymmetries in flexion-extension and internal-external rotation, suggesting that in-vivo tibiofemoral kinematic parameters are not fully restored in BCR patients during functionally strenuous activities such as single leg deep lunges and sit-to-stand. Further studies are required to elucidate the importance of patient factors, surgical component orientation and implant designs in optimizing in vivo kinematics in patients with BCR TKA. For figures, tables, or references, please contact authors directly

Abstract. Background. Oxidized zirconium (OxZr) has been introduced as an alternative bearing for femoral components in Total Knee Arthroplasty (TKA). It has a ceramic-like zirconium oxide outer layer with a low coefficient of friction. Early studies have found OxZr TKA to have a low incidence of early failure in young high demand patients. Currently no study has reported on the outcome of these implants beyond ten years. Objectives. The purpose of our study was to present an in-depth 15-year survival analysis of cemented Profix II OxZr TKA. Study Design & Methods. Data was collected prospectively and survival analysis undertaken with multiple strict end points. Complication rates were recorded and patient reported outcomes were measured. Results. 617 Profix II OxZr TKAs were performed over four years. Forty-nine patients underwent reoperation. Aseptic tibial loosening was the most common cause of failure (32.7%) on average occurring 2.8 years post primary procedure. There was one recorded failure due to loosening of the zirconium femoral component. Revision rate at 15-years was 6.38%. Cumulative survivorship was 91.52% with failure considered to be reoperation for any reason. WOMAC score improved in 86% of patients by year 1. The average score improved by 21.2 points and met the standard for minimum clinically important difference. Conclusions. This study presents the first 15-year survival analysis of cemented Profix II OxZr TKA. Our data supports current literature on the long-term survivorship of oxidised zirconium total knee replacements

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 replacement (TKR) design aims to restore normal kinematics with emphasis on flexion range. The survivorship of a TKR is dependent on the kinematics in six-degrees-of-freedom (6-DoF). Stepping up, such as stair ascent is a kinematically demanding activity after TKR. The debate about design choice has not yet been informed by 6-DoF in vivo kinematics. This prospective randomised controlled trial (RCT) compared kneeling kinematics in three TKR designs. 68 participants were randomised to receive either cruciate retaining (CR-FB), rotating platform (CR-RP) or posterior stabilised (PS-FB) prostheses. Image quality was sufficient for 49 of these patients to be included in the final analysis following a minimum 1-year follow-up. Patients completed a step-up task while being imaged using single-plane fluoroscopy. Femoral and tibial computer-aided design (CAD) models for each of the TKR designs were registered to the fluoroscopic images using bespoke software OrthoVis to generate six-degree-of-freedom kinematics. Differences in kinematics between designs were compared as a function of flexion. There were no differences in terminal extension between the groups. The CR-FB was further posterior and the CR-RP was more externally rotated at terminal extension compared to the other designs. Furthermore, the CR-FB designs was more posteriorly positioned at each flexion angle compared to both other designs. Additionally, the CR-RP design had more external femoral rotation throughout flexion when compared with both fixed bearing designs. However, there were no differences in total rotation for either step-up or down. Visually, it appears there was substantial variability between participants in each group, indicating unique patient-specific movement patterns. While use of a specific implant design does influence some kinematic parameters, the overall patterns are similar. Furthermore, there is high variability indicating patient-specific kinematic patterns. At a group level, none of these designs appear to provide markedly different step-up kinematic patterns. This is important for patient expectations following surgery. Future work should aim to better understand the unique patient variability

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