Patellar dislocation
Patellar dislocation occurs when the kneecap (patella) translates out of the femoral trochlear groove-most commonly laterally-after a twisting injury or direct blow. It is a frequent cause of an acutely painful, swollen knee in adolescents and young adults.
Emergency clinicians must rapidly recognise obvious dislocations, provide adequate analgesia and safe reduction when indicated, identify associated fractures or osteochondral fragments, and arrange appropriate imaging and orthopaedic follow‑up.
Most first‑time dislocations are managed conservatively with early rehabilitation focused on quadriceps (vastus medialis) strengthening and proprioception; surgery is reserved for displaced osteochondral injury, irreducible dislocations, extensor‑mechanism rupture or recurrent instability (NICE IPG474; RCEM Learning).
Relevant anatomy and predisposition
- The patella tracks within the trochlear groove and increases quadriceps mechanical advantage.
- The medial patellofemoral ligament (MPFL) is the key static restraint to lateral translation in early flexion.
- Dynamic control is provided by the quadriceps, especially the vastus medialis oblique (VMO).
- Predisposing factors include trochlear dysplasia, patella alta, increased Q‑angle (valgus), torsional abnormalities, generalized ligamentous laxity and weak VMO.
- Typical mechanism: a planted foot with a twisting movement or a direct blow producing lateral displacement.
Epidemiology and risk factors
- Peak incidence in adolescents and young adults, commonly 16-20 years.
- Common in sports with pivoting or jumping (football, gymnastics, martial arts).