Emergent care / stabilization: For the reduction maneuver for a dislocated patella, see Management Pearls. Before performing a reduction, radiographs should be evaluated to ensure there is not an associated fracture.
Causes / typical injury mechanism: The mechanism of injury is usually a noncontact, twisting injury, with an extended and valgus knee and an externally rotated foot. Less commonly, a patient may suffer a direct blow to the knee (the patella is particularly vulnerable to lateral instability with the knee in extension). The resulting patellar dislocations are predominantly lateral since the quadriceps pull is somewhat lateral to the mechanical axis of the lower extremity. Medial dislocations usually stem from iatrogenic causes, such as lateral release surgery.
Classic history and presentation: Patients experience anterior knee pain and knee instability. In these acute cases, knee hemarthrosis typically develops and may be accompanied by injury to medial soft tissues and cartilage, especially as the patella relocates. Occasionally, osteochondral loose bodies in the knee joint can also occur after patellofemoral dislocation. Medial patellofemoral ligament (MPFL) injury occurs 90% of the time. Later issues that develop include pain, impaired activity, patellofemoral arthritis, and recurrent instability. About 44% of patients experience recurrent dislocations.
Prevalence: Patellar dislocations account for approximately 3% of all knee injuries. The annual incidence is estimated to be between 5.8 and 7.0 per 100 000 person-years in the general population.
- Age – The incidence is higher in military service members. In both the military and civilian populations, it is most common in people younger than 20 years.
- Sex / gender – Acute traumatic patellar dislocations occur equally among both sexes, while chronic dislocation occurs more commonly in females, likely due to increased ligamentous laxity.
Risk factors for recurrence include:
- Trochlear dysplasia has been identified as the greatest risk factor for recurrent instability in patients of any age.
- Younger age, skeletally immature patient
- Sports-related injuries
- Patella alta
- Patellar translation as measured by increased tibial tubercle to trochlear groove (TT-TG) distance
- "Miserable malalignment syndrome" – increased Q angle caused by femoral anteversion, genu valgum, and external tibial torsion / pronated feet
- Vastus medialis oblique (VMO) muscle hypoplasia
- Lateral femoral condyle hypoplasia
- Tightness of the vastus lateralis and/or the iliotibial band results in the habitual form where painless dislocation occurs repeatedly with knee flexion.