A hypermobile Wrisberg variant lateral discoid meniscus seen on MRI.

The discoid menisci is a common abnormalitiy of the knee seen by pediatric and general orthopaedists. This aberration of the fibrocartilaginous meniscus can occur medially, but is most common on the lateral side. Because of unusual biomechanical stresses, all discoid menisci are at increased risk for meniscal tears. Types I and II variations have a disc-shaped meniscus, which cover more of the tibial plateau than normal and may be of increased thickness. Both have normal attachment to the tibial plateau and do not require treatment when asymptomatic. The Wrisberg or Type III variant may not be abnormal in shape. It lacks posterior attachment to the tibia resulting in a hypermobile meniscus. The knee may snap or pop when flexed and lock in extension; pain is a common presenting complaint. Radiographs can aid diagnosis, but frequently are not definitive. Identification of discoid menisci and associated meniscal tears can be accomplished with magnetic resonance imaging. Where no tear exists and the meniscus is normal in shape, diagnosis may be more difficult. Presented here is a case of a discoid lateral meniscus not observable on initial magnetic resonance images but visible on magnetic resonance images taken while the affected knee was extended in the locked position.