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. Level of evidence In vitro biomechanical tests.

The development of an instrumented patellar prosthesis, able to measure the contact forces at the patellofemoral joint, can significantly aid in investigating the causes of total knee arthroplasty failures due to patellar complications. This study focuses on developing and validating an instrumented patellar prosthesis to measure contact forces in the patellofemoral joint. A piezoresistive force sensor was characterized and integrated into a conditioning circuit, with the aim of its implementation in the prosthesis. To measure medial and lateral forces independently, the sensors were trimmed in half. Compression tests (up to 2000 N) assessed sensor performance in terms of linearity (R² = 0.998 intact vs. 0.989 trimmed), repeatability (0.9% intact vs. 0.8% trimmed), and accuracy (1.7% intact vs. 2.3% trimmed) for forces up to 250 N. Higher force levels resulted in increased errors, but at a rate still comparable to that of existing sensors in the literature. Key considerations for the design of the instrumented prosthesis, such as minimizing point and shear loads, were identified. A prototype prosthesis capable of housing the sensor was proposed. The integrated system shows potential for improving the understanding of Total knee arthroplasty (TKA) failures through in vitro studies and could serve as an intraoperative tool for the evaluation of bone resections.

Purpose: Patellofemoral (PF) complications are a common cause of dissatisfaction and revision following total knee arthroplasty (TKA), often linked to altered kinematics and implant design. “Patella-friendly” femoral components with a wider, funnel-shaped trochlear groove may better restore native patellar motion. This study evaluated PF kinematics both before and after TKA performed using kinematic alignment, investigating the role of implant design and quadriceps loading. Methods: In total, 12 paired fresh-frozen cadaveric lower limbs were tested before and after TKA. Within each pair, one limb received a traditional medial pivot femoral component, while the contralateral limb received a “patella-friendly” medial pivot femoral component. Native and implanted knees were tested by flexing the knee under the action of an external load applied through the quadriceps tendon, varying its magnitudes (20, 160, 280N) and directions in the frontal (neutral,±6°, ±12°) and sagittal plane (neutral, +5° anterior). Motion was captured using an eight-camera optoelectronic system. Results: In the reference condition (20N, neutral direction), neither design showed statistical differences versus native ( p > 0.05). However, the patella excursion in varus–valgus rotation was much higher in the specimens implanted with the traditional femoral component design (35.1° versus 14.6° native) than with the patella-friendly (20.5°). Differences between the designs emerged mainly with quadriceps load variations, especially frontal direction changes, which significantly affected patellar motion in both native and implanted knees ( p < 0.05). Overall, the patella-friendly design better reproduced native kinematics under most conditions. However, with extreme medial loading (12°), three out of six specimens implanted with the patella-friendly femoral component were untestable owing to instability, and others exhibited high lateral displacement and trochlear dysplasia. In contrast, all traditional design implants remained stable, though with greater deviation from native kinematics. Conclusions: This study provides foundational insights into PF biomechanics before and after TKA with kinematic alignment. By analyzing the interplay between implant geometry and quadriceps loading direction, it emphasizes the importance of selecting femoral components on the basis of individual patient anatomy. Our findings suggest that patella-friendly femoral components—although capable of better reproducing native motion in some cases—may not be suitable for patients with medially directed quadriceps forces or severely varus morphotypes.