The meniscus is cartilage inside the knee joint. Tearing can occur on either side, the medial (inside) or lateral (outside). Meniscus is the name for either of two pieces of tough cartilage inside the knee joint. They are shaped like half moons and are sometimes called semilunar cartilage. One is on the medial side and one is on the lateral side. They act as cushions between the femur, or the thigh bone, and the tibia, which is the major bone of the lower leg, to which they are connected.

Besides acting as shock absorbers, the menisci help to keep the knee joint stable. They also help maintain the shiny, white joint surface on the ends of the bones (the hyaline cartilage).

Tears in the meniscus can vary quite a bit in size and amount of displacement. The pattern of the tear may be lengthwise or radial, or form a flap, or a horizontal split. Another kind is called a bucket-handle tear. In this case, a lengthwise tear flips completely into the joint, instead of staying in its normal place around the edge of the joint. These tears happen suddenly, and are usually caused by a twisting injury. In contrast, degenerative tears are a result of the aging process.