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Lateral patellar dislocation is the most common type of patellofemoral instability, typically occurring when the patella slips laterally out of the trochlear groove, often following trauma or in the setting of predisposing anatomic abnormalities.
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It is especially frequent in adolescents and young adults, with a high recurrence rate in those with underlying trochlear dysplasia or MPFL insufficiency.
| Feature | Description |
|---|---|
| Normal anatomy | Patella glides within the trochlear groove, stabilized by MPFL and lateral trochlear facet |
| Instability | Occurs when lateralizing forces (valgus, external rotation) overpower medial stabilizers |
| Primary stabilizer | Medial patellofemoral ligament (MPFL) – injured in ~90% of first-time dislocations |
| Sign/Symptom | Description |
|---|---|
| Acute lateral displacement | Often during knee flexion with valgus twist |
| Apprehension test | Positive with lateral pressure on patella in 20°–30° flexion |
| Swelling/hemarthrosis | Common after dislocation due to chondral injury |
| Spontaneous reduction | Occurs in most cases |
https://doi.org/10.1148/rg.220177
X-ray
| View | Key Findings |
|---|---|
| AP / lateral | May show joint effusion, osteochondral fragments |
| Axial (Merchant/Sunrise) | Patellar tilt, lateral subluxation, shallow trochlea |
| True lateral | Evaluate for trochlear dysplasia signs: crossing sign, double contour, spur |
MRI
| Feature | MRI Findings |
|---|---|
| MPFL injury | Discontinuity or avulsion at patellar or femoral insertion (medial border) |
| Bone bruises | Lateral femoral condyle and medial patellar facet – pathognomonic |
| Osteochondral injury | May include loose bodies or cartilage flaps (especially at medial patellar facet) |
| Trochlear dysplasia | Flattened or convex groove (Dejour types A–D) |
| TT–TG distance | >20 mm = abnormal, predisposes to lateralization |
| Patella alta | Insall–Salvati >1.2, Caton–Deschamps >1.3 |
CT (Pre-operative planning)
| Use | Details |
|---|---|
| TT–TG measurement | Precise assessment of tibial tubercle alignment |
| 3D anatomy | Visualize trochlear morphology, osteophytes, osteochondral defects |
Radiologic Hallmark Triad (MRI)
| Component | Description |
|---|---|
| 1. MPFL tear | Most often at patellar insertion |
| 2. Bone contusion | Lateral femoral condyle + medial patellar facet |
| 3. Lateral patellar tilt | With subluxation/dislocation |