Two flat, C-shaped pieces of cartilage in each knee serve as cushions between the thigh and shin bones. These constitute the meniscus. A torn meniscus is a common knee injury. Anyone can tear a meniscus, but athletes are especially at risk. One cause is an abrupt change in direction while running or walking. This can tear the meniscus by twisting the knee. Signs of a torn meniscus include pain, swelling, and stiffness.