A discoid meniscus is a normal but uncommon variant in the shape of the meniscus.
A meniscus is the shock absorber of the knee, that tucks in between the rounded end of the femur bone and the flattened top of the tibia bone. There is one meniscus on each side of the knee - the medial meniscus on the inner side and the lateral meniscus on the outer side.
Usually it is the lateral meniscus that may be discoid, but rarely both menisci may be affected. It is possible to have a discoid meniscus in one knee but not in the other.
About 1% of people are born with a discoid meniscus. Sometimes the finding is incidental and there are no symptoms.
More rounded in shape
The normal meniscus shape is crescentic, but in a discoid meniscus it is either a complete round disc or very nearly so.
Normal meniscus shape Discoid meniscus shape
Less wedge-shaped and tears more common
A discoid meniscus is also not so wedged in profile as a normal meniscus. The blood supply on the inner edge is even less sufficient than in a normal meniscus and the internal structure can break down, so the propensity for a meniscal tear is greater than in a normal meniscus.
Discoid meniscus with vertical tear - A discoid meniscus can be complete - a complete disc - or can be just a bit broader than a normal meniscus, or any grade inbetween. This drawing demonstrates a vertical circumferential tear in a discoid meniscus.
Avulsions more common
The capsular attachments are also weak, so avulsions also occur more frequently. Sometimes the menisco-femoral ligament at the back of the lateral meniscus is damaged or absent.
Symptoms & management
Note that patients with discoid meniscus symptoms often include children.
In terms of symptoms, snapping is common but of only minor concern. Significant symptoms would include feelings of instability - which might indicate degeneration or a tear - and also pain, together with other signs and symptoms of damage to the joint cartilage of the femur and tibia bones where the incompetent meniscus has left them vulnerable.
If a discoid meniscus is identified as an incidental finding, then it should be left alone and the patient followed periodically to monitor any development of symptoms.