Patello-femoral joint biomechanics during knee flexion: an in-silico investigation

The objective of this study was to develop a Finite Element (FE) model of the knee joint with different configurations of patella height to analyse the biomechanics of the patellofemoral joint (PF) during knee flexion. The PF joint is of key importance in the biomechanics of the knee. The primary role of the patella is to evenly distribute the load of the quadriceps and facilitate efficient knee extension. When the patella deviates from its normal tracking, it causes elevated strains on the PF ligaments, potential damage to soft tissues, and knee pain. Furthermore, this misalignment can result in excessive joint reaction forces and elevated stress on the articular cartilage, increasing the probability of cartilage wear and the formation of bone abnormalities that contribute to the development of osteoarthritis. An FE model of the knee joint was developed with 3D geometry reconstructed from patient-specific medical images and considering the mechanical behavior of bones (considering cortical and cancellous bone), cartilage, menisci, ligaments, and tendons in a healthy native condition. After the validation of the model under physiological conditions, the position of the native patella was modified to simulate the high-riding patella syndrome (patella alta) and the low-riding patella syndrome (patella baja). In the literature, the patellar height is considered a factor that could impair patellofemoral contact force, contact area and contact pressure. Patella alta can occur as a consequence of sports-related trauma; however, it appears to be a predominantly congenital/developmental condition, not related to traumatic events. The exact pathophysiology remains unclear, but it is hypothesized that abnormally elongated patellar tendons represent one of the etiologic factors contributing to the development of high patella. On the contrary, patella baja can be caused by a variety of factors, including surgical interventions, traumatic events, or congenital abnormalities. From a biomechanical perspective, it decreases the lever arm of the quadriceps tendon, requiring increased quadriceps force to achieve complete knee extension. This inefficiency in muscle function can result in modified joint loading and increased stress on the patellofemoral joint. Different FE models are developed, based on the anatomy of a subject with physiological patellar height and modifying the anatomical structure of the knee joint by increasing or decreasing the patella height with respect to the reference case along a superior (alta) or an inferior (baja) axis to achieve a different Blackburne-Peel index, which measures various anatomical relationships between the patella and the proximal tibia and is one of the most widely used methods of evaluating patella height. The flexion motion was performed for all three models within a knee flexion range of 0° to 90°. The comparison between numerical results under different conditions, namely physiological and modified patellar heights, allows one to quantify the differences in contact pressure and areas in a healthy and pathological state. Specifically, it can be observed that the patella alta exhibits the highest overall contact area and the lowest force attributable to contact pressure. This phenomenon arises because the point of contact with the femur is located more distally than in the native and patella baja configurations, and because the patella engages with the trochlear groove at deeper degrees of flexion (beyond 90°). The results contribute to our understanding of the underlying mechanisms of patellofemoral disorders and can guide the development of more effective diagnostic and treatment approaches. Furthermore, the FE models developed in this study can serve as a valuable tool for future research in patellofemoral biomechanics and related research areas.