There are two menisci in each knee. These are the crescent-shaped cartilages that sit between the thigh bone (femur) and the leg bone (tibia). Their function is to spread the weight across a larger area hence reducing the stress on the joint and to spread fluid evenly around the knee. Twisting the knee whilst weight bearing can tear this cartilage causing immediate pain locally. However, a tear can occur even after fairly normal activity such as squatting, especially deep squatting under load. A torn meniscus often causes pain at the site of the tear that tends to shoot up or down the leg. There may be swelling generally around the knee or a limp.
Taking anti-inflammatory tablets will often help the pain. When there is osteoarthritis in the knee as well, having a cortisone injection can sometimes relieve pain for a little while. However, most meniscal tears are very effectively treated with a simple day procedure called a knee arthroscopy.