Jordy van Sambeeck

Chapter 1 12 trochlea. When this amount of engagement of the patella in the trochlear groove is reached, this is a biomechanically very stable situation due to static stabilization of a congruent patellofemoral joint. Next to this static stabilization, dynamic stabilization is important in maintaining patellofemoral stability when moving the knee. Dynamic stabilization occurs by the activation and relaxation of co-acting muscles and muscle groups that directly or indirectly influence the position of the patella relative to the trochlea. The quadriceps muscle (including the vastus medialis obliquus) and both the hip external rotators and hip abductors are important dynamic stabilizing muscle groups. Activation of the external rotators of the hip, especially the gluteus maximus muscle, may reduce laterally oriented force vector on the patella. By activating these muscles, the femur (including the knee) is externally rotated and the force vector on the patella is directed less laterally due to a change in the functional mechanical alignment. Activation of the vastus medialis obliquus (VMO) resists lateral patellar displacement by carrying out a posterior and medially directed force 3 . Relaxation of the VMO reduces the force required to displace the patella laterally 3 . Pathophysiology of patellofemoral instability Patellofemoral joint disorders can be caused by abnormal bony alignment and abnormal function of surrounding soft-tissue structures and muscles. Patellofemoral joint disorders can roughly be classified as patellofemoral instability and patellofemoral pain. This thesis focuses on patellofemoral instability. Patellofemoral instability includes complaints of giving way, lateral tracking of the patella, lateral subluxation and lateral dislocation of the patella. A primary patellar dislocation often occurs during pivoting activities including sports such as soccer and handball, but can also occur during activities of daily life. The patella usually dislocates in early flexion (0-30 degrees of flexion), often when the knee moves from full extension into early flexion, for example when landing after a jump. It is often a non-contact injury. After a patellar dislocation the medial capsule of the patellofemoral joint is elongated or ruptured, including the MPFL. Patellofemoral instability is a multifactorial problem. Multiple risk factors for primary patellar dislocation and recurrent patellar dislocation have been described in literature and include anatomic and demographic risk factors 4, 5 and lack of dynamic stabilization. Anatomic risk factors for primary and recurrent patellar instability that are in general accepted as evident risk factor are trochlear dysplasia, patella alta and a lateral position of the tibial tubercle in the coronal plane. These risk factors will be outlined below. Contributing anatomic factors are femoral anteversion, tibial exotorsion, knee hyperextension and a valgus knee axis. All of these risk factors directly or indirectly affect bony stabilization of the patella by affecting alignment or local anatomy leading to an increased laterally directed force on the patella or a decreased restraint to lateral translation. These