Discoid Meniscus 

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. See the images below.

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.

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]}

Frequency

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%.^[4] The Asian population has a slightly higher rate of occurrence; Tokyo's Teishin Hospital reported 16.6% of all knees examined arthroscopically had a discoid lateral meniscus.^[5]

A discoid lateral meniscus results from a developmental anomaly before birth.^[6] After birth, no sudden change occurs in meniscal development.^[7]

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. Both types present unique pathophysiologic problems.

The Wrisberg type lacks an attachment to stabilize the posterior horn to the tibia.^[8] 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.

Patients present with any combination of pain, giving way, effusion, and clicking or snapping knee.^[9]

Children with discoid meniscus usually present with a snapping knee joint, especially those around 7 years old. The snap can be seen and heard. Translation of the femoral condyle over a thickened posterior rim of lateral meniscus occurs. If the child remains otherwise asymptomatic, only observation is necessary; however, snapping greatly increases the chance of tearing the lateral meniscus, either by continued microtrauma or by trauma that would not cause tearing otherwise.^{[10, 11]}

In a retrospective study of 40 children with symptomatic lateral discoid meniscus, mini-arthrotomy and arthroscopy were compared. Mini-arthrotomy was found to provide slightly better results than arthroscopic resection regarding functional outcome. The authors recommended mini-arthrotomy for the resection of lateral discoid meniscus particularly in young children with narrow joint spaces and for surgeons who are not familiar with arthroscopy in small joints.^[12]

Abnormalities of knee function, pain, and effusion are indications for surgical treatment.

Arthroscopic removal of a torn, normally configured lateral meniscus, in its entirety, is accomplished by first releasing the anterior horn, then releasing the attachment to the popliteal tendon, and then partially releasing the posterior horn. Finally, the meniscus is displaced into the intercondylar notch to complete the posterior release and remove the entire meniscus.^[13]

A discoid lateral meniscus often has a continuous attachment from the popliteal tendon to the posterior horn. Removal of the anterior horn is necessary; the remainder of the discoid meniscus is then removed in a piecemeal fashion. An arthroscopic Bovie or other type of coagulation system should be available to stop possible bleeding from a branch of the lateral geniculate artery.

An otherwise asymptomatic knee with the incidental finding of discoid meniscus is a contraindication for surgical treatment.