Patellar dislocation

Lateral

MPFL tear · osteochondral fracture

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The patella slips out of the trochlear groove, almost always laterally, tearing the medial patellofemoral ligament (MPFL). Typical in teenagers and young adults, especially with a shallow trochlea or a high patella.

Lateral patellar dislocation, AP knee. Arrow: the patella lies lateral to the femoral condyle, out of the trochlear groove. — James Heilman, MD, CC BY-SA 4.0, via Wikimedia Commons.

Mechanism

Twisting with the foot planted and the knee slightly flexed and in valgus (sport, dance). Less often a direct blow.

Clinical findings

The patella sits on the lateral side of the knee, knee held flexed; it often reduces as the knee is straightened.
Haemarthrosis suggests an osteochondral fracture. Medial tenderness at the adductor tubercle (MPFL). Apprehension test later.

Imaging

AP, lateral and skyline (Merchant) views: look for an osteochondral fragment. MRI after a first dislocation with haemarthrosis, and to assess trochlear dysplasia and patellar height.

Classification

First-time vs recurrent
Risk factors: Trochlear dysplasia (Dejour), patella alta (Caton–Deschamps), TT–TG distance > 20 mm, valgus, femoral anteversion, laxity.

Types shown in 3D

Lateral dislocation: The patella almost always goes laterally, with the knee slightly flexed and the leg twisting inwards over a planted foot. The medial patellofemoral ligament (MPFL) tears. Often reduces when the knee is straightened.
With osteochondral fracture: As the patella relocates it strikes the lateral femoral condyle: an osteochondral fragment from the medial facet (or the condyle) can float free in the joint. A large fragment is fixed back, a small one removed.

Treatment

First dislocation: Reduce by extending the knee, brace for comfort, physiotherapy (quadriceps and hip control).
Osteochondral fracture: Arthroscopic or open fixation of a large fragment, removal of a small one, usually with MPFL repair.
Recurrent: MPFL reconstruction, plus tibial tubercle osteotomy or trochleoplasty for marked anatomical risk factors.

Aftercare

Return to sport after 3–6 months when strength is restored.

Complications

Recurrence (about 1 in 3 after a first dislocation)
Patellofemoral pain and arthritis
Loose bodies