- see anatomy and biomechanics of the patella;

- patient characteristics:
   - patellar subluxation or dislocation may occur during childhood but is more frequently seen in adolescence;
   - in some cases, dislocation occurs in knees w/ patellofemoral dysplasia, but this may be less common than previously thought;
   - may occur equally in males and females and often occurs from high level sports;
   - ref: Characteristics of patients with primary acute lateral patellar dislocation and their recovery within the first 6 months of injury.
- pathophysiology:
  
- anatomy of the trochlear groove is the most important factor stabilizing the patellofemoral joint (a dysplastic trochlea is found in more than 90% of patients)
- w/ acute traumatic dislocation, at least one medial structure must fail;
   - most often the medial patellofemoral ligament fails off of its femoral attachment, but in some cases it fails off of the patellar attachment;
   - alternatively, there may be a fracture at the medial chondro-osseous junction;
   - normally the knee is reduced in flexion due to the medializing effect of the lateral trochlear facet;
   - as the knee is extended, however, the patella may subluxate or dislocate if static and dynamic restrainst are deficient; 

- associated injuries:
   - osteochondral fracture of the lateral femoral condyle and/or patella;
   - most common sequelae of lateral patellar instability is damage to articular surface of patella, resulting in chondromalacia patella;
   - medial facet: main site of osteochondral fracture:
   - references:
     - Chondral and osteochondral injuries associated with acute patellar dislocation
     - Second-look arthroscopy of cartilage changes of patellofemoral joint, especially patella, following acute and recurrent patellar dislocation.
     - Cartilage lesions of the patella in recurrent patellar dislocation
     - pediatric patellar avulsion fractures 

- diff dx:
   - subluxation of the patella needs to be distinguished from malalignment of patella:
   - medial synovial plica: 
   - multipartite patella
   - chondromalacia
   - jumper's knee:
   - patellar ligament rupture
   - Sindig-Larsen-Johanssen disease;
   - quadriceps contracture:
   - patients show habitual patellar dislocation in flexion; 

- risk factors:
   - patella alta (most important)
   - generalized ligamentous laxity (perhaps more common with atraumatic instability);
   - genu valgum
   - increased femoral anteversion w/ compensatory external tibial torsion;
   - ref: Internal torsion of the distal femur as a cause of habitual dislocation of the patella: a case report and a review of causes of patellar dislocation.
   - increased Q angle
   - loss of dynamic stability:
   - contribution of distal oblique portion of vastus medialis muscle is critical;
   - loss of static stability:
   - hypoplastic lateral condyle:
   - flat lateral femoral condyle will permit patella to sublux laterally;
   - tear of medial patellofemoral ligament: 

- natural history:
   - need to distinguish between traumatic and atraumatic dislocation;
   - how and when did the patella reduce 

- references:
- The natural history of recurrent dislocation of the patella. Long-term results of conservative and operative treatment. 

- Acute patellar dislocations. The natural history. 

- Patella alta and patella infera. Their etiological role in patellar dislocation, chondromalacia, and apophysitis of the tibial tubercle. 

- Recurrent dislocations and subluxations of the patella. 

- Patella alta and recurrent dislocation of the patella. 

- Recurrent dislocation of the patella. Relation of treatment to osteoarthritis. 

- Acute patellar dislocation in children: incidence and associated osteochondral fractures. 

- Habitual dislocation of the patella in flexion. 

- Epidemiology and Natural History of Acute Patellar Dislocation.