• Clinical science

Patellar dislocation

Abstract

The patella is the largest sesamoid bone in the human body. It is located within the quadriceps femoris tendon and acts as a fulcrum to increase the force exerted on the tibia. In the case of patellar dislocation, the patella slips out of the trochlear groove, often laterally. Patellar dislocation usually occurs spontaneously following torsion of a semiflexed knee and is typically the result of underlying biomechanical abnormalities such as hyperlaxity of the medial patellofemoral ligament, high-riding patella, genu valgum, and/or patellofemoral dysplasia. Less frequently, trauma and congenital defects cause dislocation of the patella. The diagnosis is usually apparent after physical examination. X-ray, MRI, and arthroscopy are used to identify additional injuries. Patellar dislocations can usually be reduced by careful manipulation. In some cases, arthroscopy is also used therapeutically to perform surgeries that stabilize the patella.

Epidemiology

  • Recurrent patellar dislocation
    • Sex: >
    • The first episode of patellar dislocation typically happens before the age of 20
    • 50% of patients will have recurrent patellar dislocations after the first episode
  • Isolated traumatic patellar dislocations and congenital patellar dislocations are quite rare.

Epidemiological data refers to the US, unless otherwise specified.

Etiology

  • Recurrent patellar dislocation: usually results from twisting of a slightly flexed knee joint. The following biomechanical defects predispose an individual to recurrent patellar dislocation:
  • Traumatic patellar dislocation: : usually results from direct sideward impact on the patella; (e.g., during contact sports; , a car accident)
  • Congenital dislocation of the patella: usually associated with other congenital diseases
    • Down's syndrome
    • Arthrogryposis
    • Larson syndrome
    • Ellis-van-Creveld syndrome
    • Nail-patella syndrome

Clinical features

  • General features
    • Severe pain
    • Knee joint effusion
    • Restricted range of motion in the knee; (usually a fixed flexion deformity)
    • The patella is almost always dislocated laterally.
  • Recurrent patellar dislocation
    • The patella often relocates spontaneously
    • Permanent feeling of instability
    • Positive apprehension test
  • Traumatic patellar dislocation: may be associated with ligament injuries and/or fractures
  • Congenital patellar dislocations: present at birth and cannot be corrected by physical manipulation alone

Diagnostics

  • X-ray: to detect the underlying cause (see “Etiology” above) and additional injuries (see “Complications” below)
  • MRI and arthroscopy: to examine the ligaments and cartilaginous structures
  • Knee joint aspiration: : indicated in the case of severe joint effusion
    • Injury to ligamentous structures of the knee (e.g., the medial patellar retinaculum) → hemarthrosis
    • A fracture which extends intraarticularly (e.g., an avulsion fracture of the femoral condyle) → lipohemarthrosis

Lipohemarthrosis in the presence of normal knee x-rays indicates an osteochondral lesion!

Treatment

  • Conservative therapy: indicated if no osteochondral fragment is present
    1. Patellar reduction by gently extending the knee while applying a caudally and medially directed force on the lateral edge of the patella
    2. Reduction should be followed by immobilization of the knee in extension for three weeks.
    3. Physiotherapy to strengthen the quadriceps femoris muscle: to prevent recurrent disclocation
  • Surgical therapy: generally indicated for complicated cases with associated fractures
    • Arthroscopy
    • Recurrent patellar dislocation
      • Different surgical procedures may be used to stabilize the patella: medial patellofemoral ligament repair, release of the lateral retinaculum, medial transfer of the tibial tubercle
      • In the case of severe genu valgum causing patellar dislocation: supracondylar closed wedge osteotomy
    • Congenital patellar dislocation can only be treated by surgical reconstruction.

Complications

  • Additional injuries associated with acute patellar dislocation
    • Tear in the medial patellar retinaculum
    • A small avulsion fracture of the lateral femoral condyle or patella
  • Recurrent patellar dislocationosteoarthritis

We list the most important complications. The selection is not exhaustive.

last updated 11/27/2017
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