## 1 Introduction
In addition, the main complications can be pain and functional limitations in the short term and, over time, cartilage damage and osteoarthritis, leading to a deterioration in the quality of life [25]. The risk of recurrent dislocation may be increased by anatomical problems, i.e., MPFL incompetence, trochlear anatomical changes, and tibial tubercle lateralisation [26; 27; 28]. Figure 1: Representation of the knee in different planes by 3D volume rendering of TC images. Therefore, a better understanding of the anatomical stability factors of the patella is required to guide effective treatment [29; 30; 31]. The study of patellofemoral joint stability has been the subject of much research, focusing on the interaction of various factors, including quadriceps muscle forces, retinacular structures, tibial rotation, and articular geometry [32; 31]. Some authors have extended the analysis of Q-angle [33; 34], quadriceps tension imbalances [35] and retinacular releases on patellar kinematics and contact patterns [36]. Others have examined the role of tibial rotation and the correlation between patellar tracking and femoral and patellar topography [37; 38; 39]. Bone abnormalities associated with patellar malalignment include conditions such as high or low patella, high tibial tuberosity, trochlear dysplasia, shallow or convex intercondylar fossa, patellar dysplasia, external tibial rotation, or internal femoral rotation [40; 41]. Many studies have investigated the weakened vastus medialis obliquus (VMO) muscle as a cause of patellar malalignment, as it is unable to adequately limit patellar displacement and loses its stabilising effect [42; 43]. Other soft-tissue abnormalities associated with patellar malalignment include medial retinaculum damages, augmented Q-angle, lateral retinaculum resistance, hamstrings,iliotibial band, and vastus lateralis (Figure 2) [44]. Some authors support the thesis that changes in patellar alignment are due to a general malfunction in the synergies of the joints of the lower limb, particularly in imbalances of forces between the extrarotator and intrarotator hip muscles [45; 46]. However, less attention has been paid to the patellar lateral ligaments; in fact, only a few studies have investigated the role of the different medial retinaculum portions [47; 48]. Previous studies have analysed the patellar lateral displacement restraint-force variations during knee flexion and quadriceps loading [49]. However, although many studies have only looked at specific anatomical areas, others have attempted to take a broader view by extending the analysis to all the joints of the lower limb [50]. The main objective of this study was to provide greater knowledge of the quadriceps muscles and the role of the medial retinaculum structures in providing stability to the patellofemoral joint, using numerical finite element (FE) modelling. The model was developed to assess the specific contribution of the quadriceps tendon, by reducing its intensity by 10%, and the retinaculum structure to the imbalance of the left aging forces on the patella. This approach can provide interesting information on the forces involved in patella displacement at different knee-flexion angles and can offer possible new insights on stabilising contributions of the structures involved. Figure 2: (**a**) Anatomical representation of a normal knee; (**b**) radiological image in antero-posterior projection of patellar dislocation; and (**c**) anatomical representation of a patellar dislocation. A better understanding of the causes of patellar dislocation and the muscular implications can help to guide treatment strategies for patellofemoral instability disorders.