**3. Patellofemoral instability: recurrent dislocation of the patella**

Patellar instability by definition is a disorder where the patella pathologically subluxates or dislocates out of the trochlear groove. This most often involves multiple factors, such as acute trauma, chronic ligamentous laxity, connective tissue disorder, anatomical abnormality, or osseous malalignment [50]. Over a period of time, patients with patellar instability can have debilitating pain, limitations in knee function, loss of time from work and/or sports, and long-term arthritis.

Although medial, superior, and intra-articular dislocations of the patellae have been reported, most patellar dislocations are lateral. In clinical practice, lateral patellar subluxations or dislocations are far more common than medial subluxations or dislocations. Medial subluxation of the patella is usually iatrogenic. Medial subluxation may occur as a complication of an extensive lateral release, a lateral release

performed for an incorrect indication, overtightening of the medial structures, or blunt or surgical trauma resulting in scarring and inferomedial tethering of the patella [50].

Two mechanisms of acute lateral patellar dislocation have been described: an indirect (noncontact) injury and a direct blow (contact injury) (**Figure 2**). The indirect mechanism is more common and involves the combination of a strong quadriceps contraction, a flexed and valgus knee position, and an internally rotated femur on an externally rotated tibia with the foot planted to the ground [50]. Patients with dislocations due to an indirect mechanism frequently have one or more predisposing anatomical risk factors. These risk factors include genu valgum, increased Q-angle, increased femoral anterversion (internal femoral torsion), external tibial torsion, patella alta, generalized ligamentous laxity, weakening or hypoplasia of the VMO, and trochlear dysplasia or hypoplasia (**Table 1**). Generalized ligamentous laxity is seen in various orthopedic disorders, such as Down syndrome, Ehlers Danlos syndrome, Marfan syndrome, osteogenesis imperfecta, and Morquio-Brailsford syndrome.
