Identifying alignment parameters affecting implanted patellofemoral mechanics

Kinematic alignment adequately restores trochlear anatomy, patellar kinematics and kinetics in total knee arthroplasty: A systematic review

PURPOSE

Patellofemoral pain, maltracking and instability remain common and challenging complications after total knee arthroplasty. Controversy exists regarding the effect of kinematic alignment on the patellofemoral joint, as it generally leads to more femoral component valgus and internal rotation compared to mechanical alignment. The aim of this systematic review is to thoroughly examine the influence of kinematic alignment on the third space.

METHODS

A systematic search of the Pubmed, Cochrane and Web of Science databases was performed to screen for relevant articles published before 7 April 2024. This led to the final inclusion of 42 articles: 2 cadaveric, 9 radiographic, 12 computer simulation and 19 clinical studies. The risk of bias was evaluated with the risk of bias in non-randomised studies - of interventions tool as the lowest level of evidence of the included clinical studies was IV. The effects of kinematic alignment on patellar kinematics and kinetics, trochlear anatomy reconstruction and patellofemoral complication rate were investigated.

RESULTS

Kinematic alignment closely restores native patellar kinematics and kinetics, better reproduces native trochlear anatomy than mechanical alignment and leads to a 0%-11.4% incidence of patellofemoral complications. A more valgus joint line of the distal femur can cause lateral trochlear undercoverage and a trochlear angle orientation medial to the quadriceps vector when applying kinematic alignment, both of which can be solved by using an adjusted design with a 20.5° valgus trochlea.

CONCLUSION

Kinematic alignment appears to be a safe strategy for the patellofemoral joint in most knees, provided that certain precautions are taken to minimize the risk of complications.

LEVEL OF EVIDENCE

Level IV clinical studies, in vitro research.

Patella height influences patellofemoral contact and kinematics following cruciate-retaining total knee replacement

The role of patella height is discussed controversially in total knee arthroplasty (TKA). Therefore, this computational study aims to systematically analyze the biomechanical effect of different patella heights on patellofemoral (PF) forces and kinematics after cruciate-retaining (CR) TKA. We implemented a CR bicondylar TKA with a dome patellar button in a validated dynamic musculoskeletal multibody model of a male human knee joint. Retropatellar dynamics (contact force [N], shear force [N], patellar shift [mm], tilt [°], and rotation [°]) were evaluated during dual-limb squat motion (flexion from 0° to 90°) with simulated active muscle forces and the effects of different patella heights (Blackburne-Peel [BP] ratio of 0.39, 0.49, 0.65, 0.85, 1.01, and 1.1 were systematically examined). As active knee flexion increased, PF contact force also increased. Patella alta (BP = 1.1) resulted in higher PF contact forces compared to normal patella height (BP = 0.65) by up to 16%. Contrarily, patella baja was associated with decreased PF forces by 7%. Compared to patella baja (BP = 0.39), patella alta (BP = 1.1) considerably increased the contact force by up to 25%. Different patellar heights mainly affected PF shear forces during early knee flexion. Concerning PF kinematics, patella alta (BP = 1.1) yielded a greater lateral tilt of more than 4° and higher patellar rotation by up to 3° during deep knee flexion, compared to normal patella height (BP = 0.65). Our computational study indicates that patella alta is associated with the highest PF contact and shear force after the implantation of a CR bicondylar TKA. This should be considered in PF disorders following TKA.

Computer-based analysis of different component positions and insert thicknesses on tibio-femoral and patello-femoral joint dynamics after cruciate-retaining total knee replacement

BACKGROUND

Positioning of the implant components and tibial insert thickness constitute critical aspects of total knee replacement (TKR) that influence the postoperative knee joint dynamics. This study aimed to investigate the impact of implant component positioning (anterior-posterior and medio-lateral shift) and varying tibial insert thickness on the tibio-femoral (TF) and patello-femoral (PF) joint kinematics and contact forces after cruciate-retaining (CR)-TKR.

METHOD

A validated musculoskeletal multibody simulation (MMBS) model with a fixed-bearing CR-TKR during a squat motion up to 90° knee flexion was deployed to calculate PF and TF joint dynamics for varied implant component positions and tibial insert thicknesses. Evaluation was performed consecutively by comparing the respective knee joint parameters (e.g. contact force, quadriceps muscle force, joint kinematics) to a reference implant position.

RESULTS

The PF contact forces were mostly affected by the anterior-posterior as well as medio-lateral positioning of the femoral component (by 3 mm anterior up to 31 % and by 6 mm lateral up to 14 %). TF contact forces were considerably altered by tibial insert thickness (24 % in case of + 4 mm increase) and by the anterior-posterior position of the femoral component (by 3 mm posterior up to 16 %). Concerning PF kinematics, a medialised femoral component by 6 mm increased the lateral patellar tilt by more than 5°.

CONCLUSIONS

Our results indicate that regarding PF kinematics and contact forces the positioning of the femoral component was more critical than the tibial component. The positioning of the femoral component in anterior-posterior direction on and PF contact force was evident. Orthopaedic surgeons should strictly monitor the anterior-posterior as well as the medio-lateral position of the femoral component and the insert thickness.

The Design of the Patellar Component Does Not Affect the Patient-Reported Outcome Measures in Primary Posterior-Stabilized Total Knee Arthroplasty: A Randomized Prospective Study

This randomized comparative study was conducted to investigate the outcomes of patellar resurfacing with a medialized dome or an anatomical type in patients receiving primary unilateral posterior-stabilized TKA. Between March 2019 and January 2021, 98 knees were randomly assigned to receive patellar resurfacing by a medialized dome type (group D, 49 knees) or an anatomic type (group A, 49 knees). The primary outcome was the Knee Injury and Osteoarthritis Outcome Score. The secondary outcomes were the Western Ontario and McMaster Universities Osteoarthritis Index, Feller’s patella score, the Kujala anterior knee pain score, knee joint range of motion (ROM), and postoperative complications, including periprosthetic patellar fracture, patellar tilt angle, and lateral patellar shift. Patient-reported outcomes were not significantly different between the two groups. The ROM of the knee joint was significantly better in group A at six months after surgery (p = 0.021). No complications such as patellar fractures were observed. The anatomic type of patellar component showed a significant improvement of the patellar tilt angle after surgery compared with the medialized dome type of component. However, there were no significant differences in patient-reported clinical outcomes between the two groups during the follow-up period of 12 months.

Difference in patient-reported outcomes of various patellar component designs in total knee arthroplasty: A randomized clinical study

PURPOSE

This study investigated the clinical effects of different patellar components without being affected by the femoral component design in total knee arthritis (TKA) for patients with knee osteoarthritis (OA).

METHODS

In total, 48 patients with OA who met the criteria of the American College of Rheumatology for OA were enrolled and randomly assigned in a 1:1 ratio to two groups according to the usage of patellar component design for TKA (medialized dome type [dome group] or medialized anatomic type [anatomic group]). To evaluate the clinical outcomes for TKA, knee range of motion (ROM), pain intensity of 0-100 mm visual analog scale (pain VAS), and the Japanese Knee Osteoarthritis Measure (JKOM) score were obtained at baseline and year 1.

RESULTS

The difference in knee ROM, pain VAS, or total JKOM score at year 1 was not significant between the dome and anatomic groups (p = 0.398, 0.733 and 0.536, respectively). Moreover, similar results were obtained for changes in knee ROM, pain VAS, or total JKOM scores from baseline. In both groups, the pain VAS and total JKOM scores were significantly improved at year 1.

CONCLUSION

Both dome and anatomic groups in TKA are significantly effective for pain and function using the JKOM score. However, their efficacy did not differ, according to the JKOM score. Results of this study are rare information focusing on the patellar component design and provide one of the insights into the TKA clinical management.

Computational framework for population-based evaluation of TKR-implanted patellofemoral joint mechanics

Differences in patient anatomy are known to influence joint mechanics. Accordingly, intersubject anatomical variation is an important consideration when assessing the design of joint replacement implants. The objective of this study was to develop a computational workflow to perform population-based evaluations of total knee replacement implant mechanics considering variation in patient anatomy and to assess the potential for an efficient sampling strategy to support design phase screening analyses. The approach generated virtual subject anatomies using a statistical shape model of the knee and performed virtual implantation to size and align the implants. A finite-element analysis simulated a deep knee bend activity and predicted patellofemoral (PF) mechanics. The study predicted bounds of performance for kinematics and contact mechanics and investigated relationships between patient factors and outputs. For example, the patella was less flexed throughout the deep knee bend activity for patients with an alta patellar alignment. The results also showed the PF range of motions in AP and ML were generally larger with increasing femoral component size. Comparison of the 10-90% bounds between sampling strategies agreed reasonably, suggesting that Latin Hypercube sampling can be used for initial screening evaluations and followed up by more intensive Monte Carlo simulation for refined designs. The platform demonstrated a functional workflow to consider variation in joint anatomy to support robust implant design.

Musculoskeletal Multibody Simulation Analysis on the Impact of Patellar Component Design and Positioning on Joint Dynamics after Unconstrained Total Knee Arthroplasty

Patellofemoral (PF) disorders are considered a major clinical complication after total knee replacement (TKR). Malpositioning and design of the patellar component impacts knee joint dynamics, implant fixation and wear propagation. However, only a limited number of studies have addressed the biomechanical impact of the patellar component on PF dynamics and their results have been discussed controversially. To address these issues, we implemented a musculoskeletal multibody simulation (MMBS) study for the systematical analysis of the patellar component’s thickness and positioning on PF contact forces and kinematics during dynamic squat motion with virtually implanted unconstrained cruciate-retaining (CR)-TKR. The patellar button thickness clearly increased the contact forces in the PF joint (up to 27%). Similarly, the PF contact forces were affected by superior–inferior positioning (up to 16%) and mediolateral positioning (up to 8%) of the patellar button. PF kinematics was mostly affected by the mediolateral positioning and the thickness of the patellar component. A medialization of 3 mm caused a lateral patellar shift by up to 2.7 mm and lateral patellar tilt by up to 1.6°. However, deviations in the rotational positioning of the patellar button had minor effects on PF dynamics. Aiming at an optimal intraoperative patellar component alignment, the orthopedic surgeon should pay close attention to the patellar component thickness in combination with its mediolateral and superior–inferior positioning on the retropatellar surface. Our generated MMBS model provides systematic and reproducible insight into the effects of patellar component positioning and design on PF dynamics and has the potential to serve as a preoperative analysis tool.

Influence of Component Geometry on Patellar Mechanics in Posterior-Stabilized Rotating Platform Total Knee Arthroplasty

BACKGROUND

Patellofemoral complications may cause pain and discomfort, sometimes leading to revision surgery for total knee arthroplasty patients, and patellar implant design has an impact on function of the reconstructed knee. The purpose of this in vivo biomechanics study was to understand the kinematic, functional, strength, and patient-reported outcome data of patients with anatomic and dome patellar implants.

METHODS

Satisfactory age-matched, gender-matched, and body mass index-matched patients who underwent rotating-platform total knee arthroplasty from one joint replacement system with either dome (n = 16) or anatomic (n = 16) patellar components were tested in a human motion laboratory using high-speed stereoradiography during an unweighted seated knee extension and a weight-bearing lunge activity. Patellar kinematics, range of motion, strength, and patient-reported outcomes were compared between subjects with anatomic or dome component geometry.

RESULTS

Both groups of patients achieved similar functional knee range of motion and reported similar outcomes and satisfaction. On average, patients with the anatomic component had 36% greater extensor strength compared with dome subjects. Patients with anatomic patellar components demonstrated significantly greater flexion of the patella relative to the femur and lower external rotation during the weighted lunge activity.

CONCLUSIONS

Relative to the modified dome geometry, patients with anatomic patellar geometry achieved greater patellar flexion which may better replicate normal patellar motion. Patients with anatomic implants may regain more extensor strength compared to patients with dome implants due to geometric differences in the patellar component designs.

Superior-inferior position of patellar component affects patellofemoral kinematics and contact forces in computer simulation

BACKGROUND

Anterior knee pain has been reported as a major postoperative complication after total knee arthroplasty, which may lead to patient dissatisfaction. Rotational alignment and the medial-lateral position correlate with patellar maltracking, which can cause knee pain postoperatively. However, the superior-inferior position of the patellar component has not been investigated. The purpose of the current study was to investigate the effects of the patellar superior-inferior position on patellofemoral kinematics and kinetics.

METHODS

Superior, central, and inferior models with a dome patellar component were constructed. In the superior and inferior models, the position of the patellar component translated superiorly and inferiorly, respectively, by 3mm, relative to the center model. Kinematics of the patellar component, quadriceps force, and patellofemoral contact force were calculated using a computer simulation during a squatting activity in a weight-bearing deep knee bend.

FINDINGS

In the inferior model, the flexion angle, relative to the tibial component, was the greatest among all models. The inferior model showed an 18.0%, 36.5%, and 22.7% increase in the maximum quadriceps force, the maximum medial patellofemoral force, and the maximum lateral patellofemoral force, respectively, compared with the superior model.

INTERPRETATION

Superior-inferior positions affected patellofemoral kinematic and kinetics. Surgeons should avoid the inferior position of the patellar component, because the inferior positioned model showed greater quadriceps and patellofemoral force, resulting in a potential risk for anterior knee pain and component loosening.

Patellar component design influences size selection and coverage

BACKGROUND

Patellofemoral (PF) complications following total knee arthroplasty continue to occur. Outcomes are influenced by implant design, size and alignment in addition to patient factors. The objective of this study was to assess the effect of implant design, specifically round versus oval dome patellar components, on size selected and bony coverage in a population of 100 patients.

METHODS

Intraoperative assessments of patella component size were performed using surgical guides for round and oval designs. Digital images of the resected patellae with and without guides were calibrated and analyzed to measure bony coverage. Lastly, the medial-lateral location of the median ridge was assessed in the native patella and compared to the positioning of the apex of the patellar implants.

RESULTS

In 82% of subjects, a larger oval implant was selected compared to a round. Modest, but statistically significant, differences were observed in selected component coverage of the resected patella: 82.7% for oval versus 80.9% for round. Further, positioning of the apex of oval patellar components reproduced the median ridge of the native patella more consistently than for round components.

CONCLUSIONS

These findings characterized how implant design influenced size selection and coverage in a population of patients. The ability to "upsize" with oval dome components led to increases in bony coverage and better replication of the median ridge compared to round components. Quantifying the interactions between implant design, sizing and coverage for a current implant system in a population of patients supports surgical decision-making and informs the design of future implants.

The effect of axial rotation of the anterior resection plane in patellofemoral arthroplasty

BACKGROUND

Patellofemoral arthroplasty (PFA) has a small but definite place in replacement surgery of the knee, especially in young patients. The main surgical considerations in PFA are the patient's anatomy, the type of prosthesis and the surgical technique. The surgical technique and PFA success rely heavily on the anterior resection. In this study we investigate the effect of axial rotation of the anterior resection plane.

METHODS

We tested the outcome of PFA fit based on resection footprint measurements, axial and coronal groove angles, and lateral trochlear inclination (LTI) angle in a virtual PFA model. The range of anterior resection plane axial rotations was from five degree internal to five degree external with an increment of one degree.

RESULTS

Axial rotation of anterior resection plane changes the resection footprint dimension, which leads to coronal rotation of the femoral component. External rotation of the resection plane results in valgus rotation of the trochlear groove and decreased LTI after PFA and the opposite was observed for internal rotation.

CONCLUSION

Our study showed that by changing the axial rotation of the anterior cut, the coronal groove of the prosthesis can be altered to lie more closely with the native groove line without compromising the prosthesis-cartilage transition.

The linea aspera as a rotational landmark: an anatomical MRI-based study

PURPOSE

The linea aspera can be used as a landmark to assess the rotation of the distal femoral epiphysis when performing an endoprostheses. However, no study has assessed the reliability of this landmark. We therefore asked whether the linea aspera could be used as a rotational landmark for positioning distal femoral knee megaprostheses.

MATERIALS

This is an anatomic MRI-based study of 50 femurs (27 subjects). For each femur, multiple axial sections were obtained from the intercondylar line at the knee joint to the lesser trochanter; each axial section was superposed with that where the posterior condyles were seen and the R angle was measured. The R angle is the angle measured medially where the line passing through the linea aspera and the line tangent to the posterior condyles intersects.

RESULTS

There were considerable differences between and within subjects with a maximum R angle ranging from 100° to 120°. Regression models showed that the R angle was significantly associated with distance from knee joint and subjects.

CONCLUSION

Surgeons should have the R angle measured before performing a distal femoral reconstruction.

MRI-based analysis of patellofemoral cartilage contact, thickness, and alignment in extension, and during moderate and deep flexion

BACKGROUND

Several factors are believed to contribute to patellofemoral joint function throughout knee flexion including patellofemoral (PF) kinematics, contact, and bone morphology. However, data evaluating the PF joint in this highly flexed state have been limited. Therefore, the purpose of this study was to evaluate patellofemoral contact and alignment in low (0°), moderate (60°), and deep (140°) knee flexion, and then correlate these parameters to each other, as well as to femoral morphology.

METHODS

Sagittal magnetic resonance images were acquired on 14 healthy female adult knees (RSRB approved) using a 1.5 T scanner with the knee in full extension, mid-flexion, and deep flexion. The patellofemoral cartilage contact area, lateral contact displacement (LCD), cartilage thickness, and lateral patellar displacement (LPD) throughout flexion were defined. Intra- and inter-rater repeatability measures were determined. Correlations between patellofemoral contact parameters, alignment, and sulcus morphology were calculated.

RESULTS

Measurement repeatability ICCs ranged from 0.94 to 0.99. Patellofemoral cartilage contact area and thickness, LCD, and LPD were statistically different throughout all levels of flexion (p<0.001). The cartilage contact area was correlated to LPD, cartilage thickness, sulcus angle, and epicondylar width (r=0.47-0.72, p<0.05).

DISCUSSION

This study provides a comprehensive analysis of the patellofemoral joint throughout its range of motion.

CONCLUSIONS

This study agrees with past studies that investigated patellofemoral measures at a single flexion angle, and provides new insights into the relationship between patellofemoral contact and alignment at multiple flexion angles.

CLINICAL RELEVANCE

The study provides a detailed analysis of the patellofemoral joint in vivo, and demonstrates the feasibility of using standard clinical magnetic resonance imaging scanners to image the knee joint in deep flexion.

Surrogate modeling of deformable joint contact using artificial neural networks

Deformable joint contact models can be used to estimate loading conditions for cartilage-cartilage, implant-implant, human-orthotic, and foot-ground interactions. However, contact evaluations are often so expensive computationally that they can be prohibitive for simulations or optimizations requiring thousands or even millions of contact evaluations. To overcome this limitation, we developed a novel surrogate contact modeling method based on artificial neural networks (ANNs). The method uses special sampling techniques to gather input-output data points from an original (slow) contact model in multiple domains of input space, where each domain represents a different physical situation likely to be encountered. For each contact force and torque output by the original contact model, a multi-layer feed-forward ANN is defined, trained, and incorporated into a surrogate contact model. As an evaluation problem, we created an ANN-based surrogate contact model of an artificial tibiofemoral joint using over 75,000 evaluations of a fine-grid elastic foundation (EF) contact model. The surrogate contact model computed contact forces and torques about 1000 times faster than a less accurate coarse grid EF contact model. Furthermore, the surrogate contact model was seven times more accurate than the coarse grid EF contact model within the input domain of a walking motion. For larger input domains, the surrogate contact model showed the expected trend of increasing error with increasing domain size. In addition, the surrogate contact model was able to identify out-of-contact situations with high accuracy. Computational contact models created using our proposed ANN approach may remove an important computational bottleneck from musculoskeletal simulations or optimizations incorporating deformable joint contact models.

Contribution of geometric design parameters to knee implant performance: Conflicting impact of conformity on kinematics and contact mechanics

BACKGROUND

Articular geometry of knee implant has a competing impact on kinematics and contact mechanics of total knee arthroplasty (TKA) such that geometry with lower contact pressure will impose more constraints on knee kinematics. The geometric parameters that may cause this competing effect have not been well understood. This study aimed to quantify the underlying relationships between implant geometry as input and its performance metrics as output.

METHODS

Parametric dimensions of a fixed-bearing cruciate retaining implant were randomized to generate a number of perturbed implant geometries. Performance metrics (i.e., maximum contact pressure, anterior-posterior range of motion [A-P ROM] and internal-external range of motion [I-E ROM]) of each randomized design were calculated using finite element analysis. The relative contributions of individual geometric variables to the performance metrics were then determined in terms of sensitivity indices (SI).

RESULTS

The femoral and tibial distal or posterior radii and femoral frontal radius are the key parameters. In the sagittal plane, distal curvature of the femoral and tibial influenced both contact pressure, i.e., SI=0.57; SI=0.65, and A-P ROM, i.e., SI=0.58; SI=0.6, respectively. However, posterior curvature of the femoral and tibial implants had a smaller impact on the contact pressure, i.e., SI=0.31; SI=0.23 and a higher impact on the I-E ROM, i.e., SI=0.72; SI=0.58. It is noteworthy that in the frontal plane, frontal radius of the femoral implant impacted both contact pressure (SI=0.38) and I-E ROM (SI=0.35).

CONCLUSION

Findings of this study highlighted how changes in the conformity of the femoral and tibial can impact the performance metrics.

Posterior stabilized versus cruciate retaining total knee arthroplasty designs: conformity affects the performance reliability of the design over the patient population

Commercially available fixed bearing knee prostheses are mainly divided into two groups: posterior stabilized (PS) versus cruciate retaining (CR). Despite the widespread comparative studies, the debate continues regarding the superiority of one type over the other. This study used a combined finite element (FE) simulation and principal component analysis (PCA) to evaluate "reliability" and "sensitivity" of two PS designs versus two CR designs over a patient population. Four fixed bearing implants were chosen: PFC (DePuy), PFC Sigma (DePuy), NexGen (Zimmer) and Genesis II (Smith & Nephew). Using PCA, a large probabilistic knee joint motion and loading database was generated based on the available experimental data from literature. The probabilistic knee joint data were applied to each implant in a FE simulation to calculate the potential envelopes of kinematics (i.e. anterior-posterior [AP] displacement and internal-external [IE] rotation) and contact mechanics. The performance envelopes were considered as an indicator of performance reliability. For each implant, PCA was used to highlight how much the implant performance was influenced by changes in each input parameter (sensitivity). Results showed that (1) conformity directly affected the reliability of the knee implant over a patient population such that lesser conformity designs (PS or CR), had higher kinematic variability and were more influenced by AP force and IE torque, (2) contact reliability did not differ noticeably among different designs and (3) CR or PS designs affected the relative rank of critical factors that influenced the reliability of each design. Such investigations enlighten the underlying biomechanics of various implant designs and can be utilized to estimate the potential performance of an implant design over a patient population.

In vitro method for assessing the biomechanics of the patellofemoral joint following total knee arthroplasty

The patellofemoral joint is a common site of pain and failure following total knee arthroplasty. A contributory factor may be adverse patellofemoral biomechanics. Cadaveric investigations are commonly used to assess the biomechanics of the joint, but are associated with high inter-specimen variability and often cannot be carried out at physiological levels of loading. This study aimed to evaluate the suitability of a novel knee simulator for investigating patellofemoral joint biomechanics. This simulator specifically facilitated the extended assessment of patellofemoral joint biomechanics under physiological levels of loading. The simulator allowed the knee to move in 6 degrees of freedom under quadriceps actuation and included a simulation of the action of the hamstrings. Prostheses were implanted on synthetic bones and key soft tissues were modelled with a synthetic analogue. In order to evaluate the physiological relevance and repeatability of the simulator, measurements were made of the quadriceps force and the force, contact area and pressure within the patellofemoral joint using load cells, pressure-sensitive film, and a flexible pressure sensor. The results were in agreement with those previously reported in the literature, confirming that the simulator is able to provide a realistic physiological loading situation. Under physiological loading, average standard deviations of force and area measurements were substantially lower and comparable to those reported in previous cadaveric studies, respectively. The simulator replicates the physiological environment and has been demonstrated to allow the initial investigation of factors affecting patellofemoral biomechanics following total knee arthroplasty.

Patellofemoral joint replacement, an evolving concept

Isolated patellofemoral arthritis is a rare disease, whose management is challenging and controversial. Patellofemoral joint replacement can be an effective treatment for this condition. The very concept of a patellofemoral implant has evolved throughout the years, resulting in more anatomic designs and reproducible surgical techniques. The clinical outcomes of this procedure are strictly related to surgical indications, implant design and appropriate surgical technique.

The role of patient, surgical, and implant design variation in total knee replacement performance

Clinical studies demonstrate substantial variation in kinematic and functional performance within the total knee replacement (TKR) patient population. Some of this variation is due to differences in implant design, surgical technique and component alignment, while some is due to subject-specific differences in joint loading and anatomy that are inherently present within the population. Combined finite element and probabilistic methods were employed to assess the relative contributions of implant design, surgical, and subject-specific factors to overall tibiofemoral (TF) and patellofemoral (PF) joint mechanics, including kinematics, contact mechanics, joint loads, and ligament and quadriceps force during simulated squat, stance-phase gait and stepdown activities. The most influential design, surgical and subject-specific factors were femoral condyle sagittal plane radii, tibial insert superior-inferior (joint line) position and coronal plane alignment, and vertical hip load, respectively. Design factors were the primary contributors to condylar contact mechanics and TF anterior-posterior kinematics; TF ligament forces were dependent on surgical factors; and joint loads and quadriceps force were dependent on subject-specific factors. Understanding which design and surgical factors are most influential to TKR mechanics during activities of daily living, and how robust implant designs and surgical techniques must be in order to adequately accommodate subject-specific variation, will aid in directing design and surgical decisions towards optimal TKR mechanics for the population as a whole.