A Case of Bilateral Permanent Subluxation of the Lateral Meniscus

Abstract

We report a case of bilateral, permanent subluxation of the lateral meniscus. To our knowledge, the present case is the first reported description of bilateral irreducible anterior dislocation of the posterior segment of the lateral meniscus. This disorder is characterized by a flipped meniscus sign of the lateral meniscus on sagittal magnetic resonance images of the knee joint, with no history of trauma or locking symptoms. A detailed examination of serial magnetic resonance images of the lateral meniscus can help differentiate this condition from malformation of the lateral meniscus, that is, a double-layered meniscus. We recommend two-stage treatment for this disorder. First, the knee joint is kept in straight position for 3 weeks after the lateral meniscus is reduced to the normal position. Second, if subluxation of the lateral meniscus recurs, meniscocapsular suture is then performed. Although subluxation of the lateral meniscus without locking symptoms is rare, it is important to be familiar with this condition to diagnose and treat it correctly.

Figure 1

Magnetic resonance imaging of the right knee joint. (a) T1-weighted coronal image showing the posterior segment of the lateral meniscus dislocated into the intercondylar space. The arrow indicates the dislocated posterior segment, which is adjacent to the anterior cruciate ligament (ACL). (b) T2-weighted sagittal image of the lateral compartment depicting the middle segment of the lateral meniscus flipped onto the anterior segment, producing the flipped meniscus sign (black arrow). The popliteus tendon is located on the tibial plateau (asterisk) and displays a similar appearance to the posterior segment of the lateral meniscus. The white arrow indicates an osteochondral defect on the lateral femoral condyle.

Figure 2

Arthroscopic findings of the right knee joint. (a) The anteriorly dislocated middle segment of the lateral meniscus (double asterisks). The asterisk indicates the anterior horn of the lateral meniscus, which is located in the normal position. (b) An osteochondral defect (asterisk) on the lateral femoral condyle (LFC). The black arrowheads indicate the anterior margin of the defect, and the white arrowheads indicate the posterior margin. The double asterisks indicate the anteriorly dislocated middle segment of the lateral meniscus. (c) The popliteus tendon (white asterisk) originating from the LFC, running on the articular surface of the lateral tibial plateau (LTP), and continuing to the popliteus muscle (black asterisk). A fibrous band (double asterisks) connects the popliteus tendon to the dislocated posterior horn of the lateral meniscus.