The menisci are C-shaped cartilage discs that sit on the top of the tibia and act as shock-absorbers. Twisting injuries (typically during sport) can cause tears of the menisci, but tears can also occur without an injury in patients who have some wear and tear of the knee joint (degenerate tears). This is because the slightly roughened surfaces can damage the meniscus when the knee is twisted in certain ways.
The torn flaps of cartilage can irritate or catch in the joint causing pain and swelling. Tears caused by injuries can be repaired or trimmed through keyhole surgery (knee arthroscopy). Only the tear is trimmed, leaving as much normal meniscus behind as possible. Degenerate tears are usually treated by trimming the tear as repair is not usually possible.