In everyday usage, "ligament" means "a tie" or "something that binds things together." In biology, we use the term ligament to mean the tough tissue that ties, or holds, bones together. Ligaments are made mostly of a protein called collagen. Collagen is found in many places in the human body--skin, hair, and fingernails, to name a few. Collagen is flexible and strong, which makes it a perfect material for ligaments. What do ligaments do? Ligaments allow bones to move around a joint by adding stability. For example, your knee is made mostly of a hinge joint that allows the femur and tibia to move back and forth. The ligaments around this joint prevent the bones from moving side to side. Ligaments also help to prevent joints from going in the wrong direction. Unfortunately, one thing that ligaments do not do is heal very well. If a joint is dislocated, which means that the ligament is stretched so that the bones no longer meet at the proper point, it must be put back into place as quickly as possible so that the ligament does not stretch out of shape. If a ligament is totally torn, as is very common in sports injuries, surgery is often needed to repair it. Ligaments play an important role in the skeletal system.