Pathophysiology of Lateral Patellar Dislocation

Patellofemoral disorders represent 20–40% of all knee problems and can be one of the most common complaints in sports related injuries. These disorders are a major cause of disability, particularly in females, and in extreme cases may contribute to termination of athlete’s career and could lead to degenerative arthritic changes of the knee joint. For these reasons, disorders and in particular patellar instability often pose a diagnostic and therapeutic dilemma for the orthopedic surgeon. This dilemma implies that usually no single pathophysiology or therapeutic approach can fully explain and solve patellofemoral instability. In fact the patellofemoral joint is biomechanically one of the most complex human articulations with different anatomical components like bone shape, capsuloligament structures, and muscle that could alone or in combination be responsible for patellar instability. These factors are often present in combination in one patient, but the severity of each pathology can be different resulting in variable patterns of instability and pain that determine that each patient is almost unique; thus the characterization in a classification is a simplification of a very complex issue. Moreover the multifactoriality and variability of pathogenesis has determined in the past numerous misunderstanding. These misconceptions have been responsible for the high variety of surgical procedures proposed to treat patellofemoral instability, leading to less than completely satisfactory clinical results also related to iatrogenic cause. Central to the development of a rational therapy for these patients is a complete and deep knowledge of the various anatomical abnormalities that can be responsible for patellofemoral instability. For a true comprehension of the influence on patellar instability by each risk factor it is fundamental to clearly understand the biomechanical rule on which the normal physiology of the patellofemoral joint is based. The “valgus law” underlines the prevalence of the lateral structures with respect to the medial ones [23]. The lateral knee compartment of the patellofemoral joint is normally wider than the medial one. In fact the lateral condyle is larger than the medial one with an external part of the patellar groove higher, wider, and forward with respect to the medial compartment. The external patellar facet is larger in respect to the medial facet. At the capsular level is present a prevalence of the lateral retinaculum that is stronger and wider with respect to the medial one (Fig. 2.1). The patella is the largest sesamoid bone in the body, and resides within biarticular muscles (the quadriceps and patellar tendons). The patella functions both as a lever and a pulley. As a lever, the patella magnifies the forces exerted by the quadriceps on knee extension. As a pulley, the patella redirects the quadriceps force as it undergoes normal lateral tracking during flexion. Pathophysiology of Lateral Patellar Dislocation