The meniscus is a C-shaped piece of cartilage in the knee joint. It helps the joint move smoothly and acts as a shock absorber. It can also help transfer the load through the joint.
The knee joint has two menisci (plural), the medial (inner portion) and lateral (outer portion). When the outer edge of the cartilage is torn, it looks like a crescent-shaped moon or bucket handle.
At one time it was thought the meniscus didn't have any real function. A tear would result in surgery to remove the whole thing. Later scientists found this treatment led to joint damage and early arthritis. Now most surgeons prefer to repair the tear or just take out the torn section.