The medial and lateral menisci (plural of meniscus) of the knee are two crescent moon-shaped discs of tough tissue that lie between the ends of the upper leg bone and lower leg bone that form the knee joint. Meniscus tears commonly occur during sports when the knee is twisted while the foot is planted firmly on the ground. In people over the age of 40 whose menisci are worn down, a tear might occur with normal movement, minimal activity, or minor injury.