## 4 Discussion  
The simulations considered the effects of the hammstrings, gastrocnemius, quadriceps, and medial retinaccumulation during knee flexion and how an imbalance of these structures could cause patellar instability and displacement, leading to pain, reduced function, and potentially osteoarthritis. Clinical studies have shown that restoring normal knee function in cases of patellar instability is difficult and that a multidisciplinary approach is required to better understand knee biomechanics to determine the most appropriate strategy treatment [15; 35; 54]. FE analysis, supported by clinical studies, can lead to a better disease-management approach in terms of time, results, and improved healthcare costs [55]. Effective treatment options are important to improve patient outcomes and prevent further deterioration; thus, conservative treatment through gradual muscle-strengthening and improvement of joint proprioception plays a fundamental role in the management of the pathology [7]. Prevention is also essential in patients with anatomical and biomechanical predispositions, and in those with a history of dislocation [25]; however, the best treatment for patellar instability remains controversial, with differing opinions regarding surgical versus non-surgical approaches and the most effective surgical techniques [27]. The results showed that the combined effect of quadriceps intensity and lateral retinaculum weakness produced a patellar displacement of 14 mm and increased stress in the contact area from a knee-flexion angle of 25-30\({}^{\circ}\). These are in line with other studies. Figure 11: Contact-force curves versus lateral displacements and knee-flexion angle for a quadriceps force equal to 90%. Figure 12: Histogram showing correlation between the effects of quadriceps decreased force in addition with an increased unbalancing of the left medial retinaculum. [PERSON] et al. and [PERSON] et al. simulated muscular forces using wires to induce patellar instability [56; 57; 58]. The stresses acting in the FE model were analysed in terms of the equivalent von Mises stress and the results confirm maximum values of approximately 80 MPa. According to [PERSON] et al., the von Mises stress criterion is a very useful instrument to predict material failure, based on the consideration that when the material reaches the yield stress during numerical simulation, it loses its elastic properties and cannot return to its original shape, causing complications related to dislocation initiation or interactions with the contact interface, among others [59]. Furthermore, [PERSON] et al. developed a patellofemoral loading system to study the effects of the quadriceps and medial retinaculum by studying it in vitro and obtained a patellar-restraint force of approximately 50 N, which correlates with a 5 mm lateral displacement of the patella in a healthy knee [60]. Quadriceps forces had important clinical implications for patellar stability, so that when the distribution of quadriceps forces changed, so did the holding force [33]. The study highlights the important role of both active (quadriceps muscles) and passive (medial retinaculum) restraints in the assessment and treatment of patellar instability. Clinical findings and the literature report similar results, including that the vastus medialis contributes to patellar subluxation and dislocation [40; 42; 61]. At maximum knee extensions, the soft tissues, muscles and retinacula perform a restraining effect on the patella lateral displacement [11] and during knee flexion, the femur and patella are engaged, resulting in an increased joint compression force vector [47]. This results in an almost constant holding force from 0 to 60\({}^{\circ}\) flexion, despite the diminishing effect of the retinacular structures [29]. The patella makes contact with the femur at approximately 10\({}^{\circ}\) of flexion, while the role of the retinaculum decreases with flexion, becoming almost zero at 35-45\({}^{\circ}\) of flexion [32].