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Patellar dislocation in Adult
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Patellar dislocation in Adult

Contributors: Brianna Caraet MD, Sandeep Mannava MD, PhD
Other Resources UpToDate PubMed

Synopsis

Patellar dislocation occurs when the patella is displaced from the femoral groove.

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: Laxity due to connective tissue disorders, such as Ehlers-Danlos syndrome or Marfan syndrome, also predisposes patients to patellar dislocation. About half of all patellar dislocations occur during athletic activity, particularly basketball, soccer, and football.

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.
Pathophysiology: The mechanism of injury is usually a noncontact, twisting injury, with an extended and valgus knee and an externally rotated foot, and laterally directed, direct blow to the knee, when the knee is in extension (less common).

Codes

ICD10CM:
S83.006A – Unspecified dislocation of unspecified patella, initial encounter

SNOMEDCT:
263029007 – Dislocation of patellofemoral joint

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Last Reviewed: 09/13/2020
Last Updated: 09/30/2020
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Patellar dislocation in Adult
Copyright © 2020 VisualDx®. All rights reserved.