Patellar Instability & Dislocation

Blausen Medical · Wikimedia Commons · CC BY-SA 4.0

Patellar instability describes a spectrum from subtle subluxation (partial slip) to complete dislocation of the patella from the trochlear groove. First-time dislocations typically occur in adolescents and young adults during a pivoting movement or direct blow. A dislocation tears the medial patellofemoral ligament (MPFL) — the primary soft-tissue restraint preventing lateral patellar displacement — and damages the medial facet of the patella and the lateral trochlear wall.

Recurrence rates after a first dislocation are 15–40% in adults and higher in adolescents. Risk factors for recurrence include trochlear dysplasia, high patella (patella alta), and increased TT-TG distance.

Acute dislocation presents with lateral displacement of the patella (usually reduces spontaneously), hemarthrosis, and medial retinacular tenderness. The patellar apprehension test (lateral pressure on the patella with knee extended) is positive. X-rays include a merchant/sunrise view and lateral view. MRI evaluates MPFL integrity, osteochondral injury, and trochlear anatomy.

First-time dislocation with a significant osteochondral fracture requires early surgery. Recurrent instability — especially with anatomic risk factors — is the primary indication for reconstruction.

External patient-education references and related OSI pages for additional background:

• Britannica — Dislocation
• Britannica — Knee (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language