Aim
Quadriceps malalignment — a mismatch between the line of pull of the quadriceps and the flexion axis of the knee — produces a chronically unbalanced lateral force on the patella. This has been shown to be clinically important in the development of lateral patella osteoarthritis, recurrent patella instability, and poor outcomes following total knee replacement (TKR), and these associations will be discussed. The aim of this study was to determine whether computer simulation can accurately reproduce the abnormal patellofemoral joint (PFJ) mechanics caused by quadriceps malalignment after TKR, and whether it can be used to identify improved implant positioning for affected patients.
Methods
A patient-specific computational model of the post-TKR knee was constructed using three-dimensional bone geometry and tendon attachment sites derived from medical imaging of a patient with severe quadriceps malalignment (quadriceps tendon axis of 43°). The model incorporated the femur, tibia, and patella with their corresponding implant components, connected by the collateral ligaments, patellar tendon, and medial patellofemoral ligament. Tendons were modelled as multi-segment springs capable of wrapping around bony surfaces, replicating realistic force transmission. Starting from 80° of flexion, a full extension–flexion cycle was simulated by replicating quadriceps contraction. Contact forces across the patellar implant were calculated throughout the cycle. Simulation outputs were validated against a sensorised 3D-printed knee rig replicating the same patient anatomy.
Results
The simulation accurately reproduced patellar tracking and contact force distribution, with strong agreement between predicted and experimentally measured kinematics and forces. In the malaligned knee, the model demonstrated marked mediolateral imbalance of contact forces across the patellar button throughout the flexion–extension cycle, consistent with the abnormal lateral loading expected from a lateralised quadriceps force vector. The computational efficiency of the model allowed rapid testing of multiple implant orientations, enabling identification of femoral component positioning that restored a more balanced PFJ contact force distribution.
Conclusion
Computer simulation can accurately model the unbalanced PFJ mechanics produced by quadriceps malalignment after TKR and has the potential to guide personalised implant positioning in affected patients. Quantifying the force imbalance across the patella in this way provides a biomechanical rationale for the clinically observed association between quadriceps malalignment and poor TKR outcomes, and supports the use of increased femoral component external rotation as a corrective strategy. This technology may offer a practical tool for pre-operative planning in the growing number of patients identified with this condition.
