A proof-of-concept investigation combining subject-specific computational modeling with experimental biomechanical testing has identified quadriceps malalignment as the primary mechanical driver of patellar instability and abnormal joint loading following total knee arthroplasty. The research, conducted by an engineering and orthopedic surgery team comprising Florian Michaud, Ánxela Pérez Costa, Daniel Dopico, and Simon Talbot, demonstrates that pre-existing anatomical deviation in the orientation of the quadriceps tendon fundamentally alters post-surgical extensor mechanics. Across simulated active extension-flexion motions, extensor mechanism malalignment produced marked lateralization of the patella, accompanied by peak contact force increases of eleven percent and occurrences of near-dislocation during extension. The dynamic evaluation framework demonstrated that small surgical adjustments to implant placement can either partially mitigate or severely exacerbate patellar maltracking, highlighting how individualized anatomical variations govern joint mechanics after prosthetic reconstruction.
Total knee arthroplasty represents a standard surgical intervention worldwide to relieve persistent joint pain and restore functional mobility in individuals suffering from advanced knee osteoarthritis or extensive joint degeneration. Despite ongoing developments in prosthetic design and operative techniques, patellofemoral complications remain among the most frequent causes of patient dissatisfaction and clinical concern, occurring in up to ten percent of operative cases and frequently necessitating complex revision procedures. The patella fulfills a crucial mechanical role within the knee extensor mechanism, functioning as a sesamoid bone embedded inside the quadriceps tendon that acts as a biomechanical pulley to augment the quadriceps lever arm and support extension strength. Articulation between the patella and the femoral trochlear groove enables smooth joint gliding during cyclic flexion and extension movements. When force imbalances disrupt patellofemoral mechanics, the joint experiences elevated localized contact pressures, abnormal tilt, lateral subluxation, or overt dislocation, undermining surgical recovery.
Standard alignment methodologies in knee arthroplasty, such as kinematic alignment, functional alignment, and gap-balancing techniques, operate under the mechanical premise that the extensor apparatus aligns perpendicularly either to the posterior condylar axis or to an axis defined by balanced collateral ligament tension. These foundational assumptions direct intraoperative cuts, component positioning, and limb alignment decisions. However, clinical evidence increasingly indicates that individual anatomical factors, including quadriceps malalignment, tibial tubercle lateralization, and asymmetrical soft tissue tensions, exert significant influences on patellar tracking independent of coronal tibiofemoral orientation. Restoring the native tibiofemoral joint line or trochlear groove orientation in isolation frequently fails to resolve pre-existing patellar maltracking. To address this clinical challenge, the investigators captured three-dimensional anatomical geometry from computed tomography imaging of a female patient presenting with pronounced quadriceps malalignment, characterized by an axial quadriceps tendon angle of 43.0 degrees, a coronal quadriceps tendon angle of 10.3 degrees, an anterior superior iliac spine Q-angle of 11.8 degrees, and a hip-knee-ankle alignment of one degree valgus.
To investigate post-arthroplasty joint mechanics under controlled conditions, the researchers constructed a subject-specific multibody dynamics model incorporating three rigid segments representing the femur, the patella, and the combined tibia-foot assembly. Three-dimensional bone surfaces were reconstructed from DICOM computed tomography datasets using segmentation software that combined manual refinement, region growing, and intensity thresholding to replicate cortical surfaces accurately. Commercial implant geometries, including a femoral component, a tibial baseplate with an ultra-high-molecular-weight polyethylene insert, and a dome-shaped patellar prosthetic button, were digitally positioned to reproduce post-arthroplasty joint configurations. Anatomical structures were reconstructed through passive soft-tissue elements, connecting the tibia and femur via collateral ligaments, linking the tibia and patella through a two-bundle patellar tendon, and securing the patella to the femur via the medial patellofemoral ligament. All stabilizing soft tissues were represented as tension-only linear extension spring-damper elements exhibiting constant stiffness properties.
The mathematical formulation of the multibody dynamics framework utilized the ALI3-P approach, an Augmented Lagrangian index-3 formulation expressed in mixed coordinates combining natural and relative coordinate sets. Kinematic constraint enforcement occurred via velocity and acceleration projections onto corresponding constraint subspaces, while dynamic equations of motion were integrated through a Newmark integration algorithm operating at a fixed time step of one millisecond. Because lubricating synovial fluid minimizes frictional resistance across joint articulations, tangential shear forces were omitted, and normal contact interactions were modeled using the Flores continuous contact mechanics formulation, which incorporates indentation depth, relative approach velocity, material stiffness, and a coefficient of restitution. Contact detection between the femoral component and the tibial insert utilized triangular mesh-to-mesh algorithms with a maximum element size of 2.5 millimeters. To optimize efficiency, the approximately spherical dome geometry of the patellar prosthetic button was modeled as a geometric primitive utilizing an analytical sphere-to-mesh detection algorithm, calculating normal forces across discrete contact clusters.
Physical validation of the computational predictions was performed using a custom sensorized, three-dimensional-printed knee testing rig driven by a precision linear actuator. Anatomical bone models derived from the patient segmentation were printed using an X1 Carbon additive manufacturing platform to accommodate prosthetic components, while the patellar button was mounted onto a sensorized base equipped with an FX29 miniature piezoresistive load cell to measure normal contact forces along the patellar local z-axis. Optical spatial tracking captured rigid-body kinematics through eight reflective markers registered across eighteen infrared OptiTrack cameras sampling at one hundred hertz. Motion data were filtered using a second-order Butterworth low-pass filter with a 12-hertz cutoff frequency, while force measurements were smoothed using singular spectrum analysis with a window length of twenty. Controlled dynamic extension-flexion cycles between 80 degrees and 10 degrees of joint flexion were tested across multiple implant configurations, providing independent physical benchmarks for evaluating numerical kinematic and kinetic outputs.
Evaluating extensor vector directions revealed that quadriceps force alignment fundamentally dictates patellofemoral stability and stress distribution. When simulations incorporated the patient’s pathological quadriceps tendon alignment oriented at 43 degrees axially, the patellar trajectory displayed pronounced lateralization compared to an idealized reference alignment oriented at zero degrees. The bisect offset index, which quantifies mediolateral displacement relative to the trochlear groove, climbed to 0.90 in the malaligned state, representing a 25 percent increase in lateral displacement and culminating in near-dislocation as the knee extended toward 15 degrees. In contrast, modeling the extensor vector along the anterior superior iliac spine orientation yielded tracking trajectories closely mirroring the idealized zero-degree reference, maintaining a maximum bisect offset index below 0.75. Furthermore, contact force differences between configurations became prominent below 30 degrees of flexion, reaching peak loads near 100 Newtons around 45 degrees of flexion in the malaligned configuration, with lateral contact forces exceeding medial forces by approximately 2.5 times.
Systematic perturbations of coronal component alignment demonstrated that small angular deviations at the femoral or tibial level substantially alter patellofemoral tracking and force equilibrium. Introducing three degrees of valgus alignment to either the femoral or tibial prosthetic component severely degraded joint stability. Tibial valgus produced a transient dislocation during extension followed by re-entry into the trochlear groove during flexion, with the bisect offset index reaching 0.96. Femoral valgus generated complete, persistent patellar dislocation wherein the prosthetic button completed the flexion cycle entirely outside the trochlear groove, reaching a bisect offset index of 0.99. Valgus orientation also exacerbated contact loading, elevating peak normal forces by 3.6 percent at the femur and 2.9 percent at the tibia. Conversely, introducing three degrees of varus alignment medialized patellar tracking, reducing the bisect offset index to 0.89 for tibial varus and 0.81 for femoral varus while reducing peak contact forces by 2.5 percent and 1.4 percent, respectively.
Rotational variations in the transverse plane also influenced joint kinematics and contact forces, though with distinct magnitudes of mechanical sensitivity. Applying four degrees of external rotation to the femoral component produced minimal alterations in patellar kinematics compared to standard kinematic alignment, yielding nearly identical trajectory curves, maximum bisect offset values of 0.92 versus 0.91, and patellar tilt angles of 21.6 degrees versus 21.7 degrees. External rotation decreased peak patellar contact force by 2.2 percent and produced minor improvements in load distribution across medial and lateral compartments. In contrast, applying four degrees of internal femoral rotation noticeably worsened joint stability, promoting increased lateralization, raising the bisect offset index to 0.96, increasing peak contact forces by 2.2 percent, and inducing transient patellar subluxation during terminal extension movements. These numerical results aligned with the physical knee rig measurements, confirming that internal femoral rotation amplifies lateral force concentrations in knees with extensor mechanism deformity.
The multibody dynamics simulation displayed close quantitative concordance with experimental measurements across all tested scenarios, recording an overall contact force root mean square error of 4.5 Newtons, a mean absolute peak force discrepancy of 6.7 Newtons, a mean bisect offset error of 0.01, and an average tilt discrepancy of negative 2.8 degrees. Computationally, the algorithm determined static initial equilibrium in 14 seconds and solved the dynamic extension-flexion cycle in 34 seconds, achieving a total runtime of 48 seconds on standard desktop hardware. The authors noted several experimental limitations, including the representation of tendons using synthetic lines rather than compliant biological tissues, intentional minimization of collateral ligament tension adjustments to isolate component positioning effects, and reliance on an isolated non-weight-bearing extension task from a single patient dataset. Despite these simplifications, the sub-minute computational performance and validated predictive accuracy suggest strong potential for integrating patient-specific multibody dynamics into rapid preoperative surgical planning and intraoperative decision-support workflows.
Subject of Research: Biomechanical simulation of patellofemoral mechanics and quadriceps malalignment after total knee replacement
Article Title: Predicting patellar kinematics and contact forces after TKA: a simulation study on quadriceps malalignment
Article References: Michaud, F., Pérez Costa, Á., Dopico, D., & Talbot, S. (2026). Predicting patellar kinematics and contact forces after TKA: a simulation study on quadriceps malalignment. Arthroplasty, 8(1), Article 50. https://doi.org/10.1186/s42836-026-00407-5
Image Credits: AI Generated
DOI: 10.1186/s42836-026-00407-5
Keywords: Total Knee Arthroplasty, Patellofemoral Biomechanics, Multibody Dynamics, Quadriceps Malalignment, Orthopedic Simulation, Implant Alignment, Contact Mechanics, Knee Kinematics, Predicting, patellar, kinematics, contact
News Source: Ophelia Keating. (November 12, 2026). Knee Replacement Simulation Predicts Patellar Instability Driven by Quadriceps Alignment. Scienmag.
