Quadriceps force direction affects patellofemoral kinematics without impacting tibiofemoral stability: a cadaveric study

Background Surgical interventions to correct abnormal quadriceps direction are performed in cases of patellofemoral joint disorders, to medialize the patella and reduce lateral stress. However, excessive medialization can lead to increased contact forces and joint instability, underscoring the need for a more detailed understanding of the effects of quadriceps alterations on joint biomechanics. The purpose of this study was to evaluate the impact of variations of the magnitude and direction of the quadriceps force on the kinematics of the patellofemoral joint. Methods A total of 12 cadaveric knees were evaluated in flexion–extension applying different loads to the quadriceps tendon. Specifically, we evaluated five different directions of the quadriceps line of action in the frontal plane: neutral, ±6° and ±12°; and two directions in the sagittal plane: neutral and 5° anterior. Three load magnitudes were simulated: 20 N, 160 N, and 280 N. Relative motion between the patella, femur, and tibia was measured using an optoelectronic system. Results The comparison under reference loading conditions (neutral direction, 20 N) across all specimens demonstrated consistent patellofemoral motion. Similarly, tibiofemoral kinematics was comparable between specimens and with the literature. Variations of the direction of the quadriceps force in the frontal plane exerted a significant impact on all components of motion in the patellofemoral joint. Compared with the reference condition, at full extension, 12° medialization increased patellar varus rotation (−6.2° ± 3.3°), while at high flexion it increased valgus rotation (4.8° ± 4.8°). Lateralization reversed this pattern, causing valgus at extension (7.7° ± 3.6°) and varus in flexion (−2.8° ± 1.8°). Medial–lateral patellar translation exceeded ±6 mm under 12° deviations. Sagittal-plane changes had minimal impact, mostly in extension when the patella is not yet in the trochlea. Tibiofemoral kinematics was more sensitive to load magnitude, although frontal-plane direction also affected joint rotation. Conclusions This study provides essential insights into the biomechanical interplay between quadriceps alignment and patellofemoral kinematics. These findings may inform surgical strategies for optimizing patellar tracking.