Discoid meniscus is a rare human anatomic variant that usually affects the lateral meniscus of the knee. Usually a person with this anomaly has no complaints; however, it may present as pain, swelling, or a snapping sound heard from the affected knee. Strong suggestive findings on magnetic resonance imaging includes a thickened meniscal body seen on more than two contiguous sagittal slices. Description
The Watanabe classification of discoid lateral meniscus is: (A) Incomplete, (B) Complete, and C) Wrisberg-ligament variant   Normally, the meniscus is a thin crescent-shaped piece of cartilage that lies between the weight bearing joint surfaces of the femur and the tibia. It is attached to the lining of the knee joint along its periphery and serves to absorb about a third of the impact load that the joint cartilage surface sees and also provides some degree of stabilization for the knee. There are two menisci in the knee joint, with one on the outside (away from midline) being the lateral meniscus and the inner one (towards midline) the medial meniscus. A discoid meniscus is a congenital anomaly of the knee found in 3% of the population (up to 15% in Asia). It typically affects the lateral meniscus and may be found bilaterally (20%). Instead of the narrow crescent shape, as seen in a normal meniscus above, a discoid meniscus is thickened, and has a fuller crescent shape. In addition, a discoid meniscus does not taper as much towards the center of the joint and is shaped like a disc.
Rarely, X-ray may show lateral joint space widening, squaring of the lateral condyle, cupping of the lateral tibial plateau and hypoplasia of the lateral tibial spine that suggest discoid meniscus. Treatment/Management
In a patient complaining of acute onset knee pain, an X-ray study would be done to rule out any bony pathology such as a fracture. Since it is difficult to diagnose meniscal anomalies with X-ray, an MRI would be necessary to visualize the discoid meniscus. If the patient is asymptomatic and does not complain of significant locking sensation in the knee or pain, treatment is conservative and consists of stretching and strengthening exercises for the quadriceps and hamstring muscles. “In cases where there is significant disability, surgical excision may be needed. When the discoid meniscus is severely damaged, total excision of the meniscus may be necessary. In most cases where the meniscus is still generally intact, a partial excision to preserve the cushioning function may be sufficient.”

See also
Knee pain
Knee osteoarthritis
Meniscal cyst

References
External links
Discoid Meniscus in eMedicine