One element in the differential diagnosis of knee pathology is a discoid meniscus. Discoid meniscus can manifest itself as an abnormal band, medial and lateral in the same knee, bilateral and medial, or, more commonly, a discoid lateral meniscus. [1, 2, 3]
Discoid lateral menisci were first described in the late 1800s. The normal configuration of a meniscus is that of a matured crescent moon, whereas that of a discoid meniscus generally is a thickened, very early crescent moon. Variations of this general shape occur relatively rarely, and occasionally, the lunar appearance is also found in the medial meniscus. The discoid shape results in a membrane barrier that prevents normal contact between the articular surfaces of the knee and has a high incidence of mechanical deformation.
Pathophysiology and Etiology
Two distinct types of discoid lateral meniscus exist. One is the hypermobile, or Wrisberg, lateral meniscus, and the other is a misshapen or discoid form of an otherwise normal lateral meniscus. Each type presents its own unique pathophysiologic problems.
The Wrisberg type lacks an attachment to stabilize the posterior horn to the tibia.  It may also be of normal shape rather than discoid. The only attachment of the posterior horn is to the Wrisberg or meniscofemoral ligament. The general configuration produces an unstable or hypermobile lateral meniscus.
Discoid lateral menisci have been reported to occur at the rate of 1.5-3% in the general population, whereas discoid medial menisci have been reported to occur at the rate of 0.1-0.3%.  The Asian population has a slightly higher rate of occurrence; Tokyo's Teishin Hospital reported that 16.6% of all knees examined arthroscopically had a discoid lateral meniscus.