Locked knee caused by meniscal subluxation: magnetic resonance imaging and arthroscopic verification.

Subluxation or dislocation of an intact lateral meniscus is a controversial and rarely reported cause of knee pain and locking. We report a case of knee locking caused by lateral meniscal subluxation in the absence of a meniscal tear or true discoid meniscus, with both magnetic resonance imaging (MRI) and arthroscopic verification. A 9.5-year-old child experienced multiple episodes of locking in full flexion of the knee. After 6 months of symptoms, arthroscopy was performed and showed no meniscal tear or a discoid meniscus. The patient's knee locking recurred after arthroscopy. MRI was performed when the patient presented acutely with the knee locked. MRI showed anterior dislocation of the posterior horn of the lateral meniscus with the knee in the locked position. The MRI was immediately repeated after the author reduced (manipulated) the locked knee into extension. On the repeat MRI, the lateral meniscus had returned to a normal position. On repeat arthroscopy, the posterior horn of the lateral meniscus was hypermobile and could be displaced into the notch and did not show a frank tear. The meniscus was repaired to the capsule with sutures. At the 2-year follow-up evaluation, the patient had no complaints and no clinical signs of locking.