## 1 Introduction  
The knee joint is essential for movement, standing, and walking, and is the focus of much research into therapeutic treatments and biomechanical modelling [1]. Data on the biomechanics of the knee, the response to external forces, combined with the mechanical properties of the individual tissues involved, can be used to better understand joint disease [2]. The knee joint is a highly mobile joint made up of the tibial-femoral and patellofemoral joints [3], the first consists of two geometrically incongruent bone segments, the femur, and the tibia, which are joined by the condyles of the femur and the tibial plateau, between which the menisci are located [4]. These two bony ends are covered by a strong fibrous capsule which holds them together in synergy with the action of the ligaments, which, combined with the geometry, restrict movement [5; 6]. On the other hand, the patellofemoral joint is composed of the patella and the trochlear surface, which are morphologically related, i.e., the patella has an optimised surface for sliding over the femur, which in turn forms a groove that acts as a track for the patella [7]. Bone surfaces are covered by a superficial layer of cartilage that promotes gliding and contributes to cushioning [4]. The ability of the knee to move in different planes varies according to the angle of flexion-extension and the patellofemoral joint holds a prominent place in these movements [8]. In fact, the patella is a fundamental contributor to knee dynamics because it is the point where the quadriceps muscle bundles converge [9]. The connection between the patella and the femur is provided by the quadriceps muscle tendon and the patellofemoral joint is functionally dependent on the patellar tendon insertion because it improves biomechanics during knee extension [5]. The patella is stabilized by several structures, such as the patellar tendon, retinacula (lateral and medial) and quadriceps tendon distal portion [10]; the retinacular ligaments, also called the patellofemoral and patellotibial ligaments, connect the patella to the femoral condyles and the patella to the front of the tibia [11]. Thus, at maximum knee extension in the frontal plane, the patella is in a medial position between the femoral condyles, in a relaxed and slightly mobile state [12]. In this condition, it is possible to identify the direction of quadriceps traction, the Q angle, which is the angle between the quadriceps traction line and the patella centre and the line joining the patella to tibial tuberosity [13]. In addition, the patella plays an important biomechanical role during flexion-extension movement, adapting extension quadriceps force and moving the point of application to a more favourable position [10]. The most significant patellar movement occurs in the sagittal plane, but significant horizontal sliding is possible in the frontal plane [7]. In order to provide an anatomical description, as has been done previously by other authors [14], Figure 1 shows a 3D volume rendering of the knee from the TC images. During the flexion-extension movement, the patella is guided by the posterior action of the quadriceps muscle, which forces it to slide along the geometric coupling with the femoral groove [15]. Patellar instability occurs when the patella does not slide correctly over the femur, causing a dislocation, and accounts for 2-3% of knee accidents with 6/100,000 patients' yearly incidence, rising to 29/100,000 in teenagers [16; 17; 18]. This condition can be caused by injury to the patellar ligaments, muscle imbalances, anatomical deformity, or changes in biomechanics, often leading to pain and disability [19; 20; 21]. The medial patellofemoral ligament (MPFL) is the main resistance ligament during lateral dislocation and has the highest frequency of accidents [16; 22]. Clinical signs of patellar dislocation may include knee effusion, bone bruising and damage to the MPFL and medial retinaculum [5; 23], making approximately one third of patients susceptible to repeated dislocation [24].