Arthritis Health Center

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Holding the joint together

Joints function as a way to move two bones with respect to one another. In order for this to work, the bones that meet at the joint must be attached to each other. The attachment needs to be firm enough to hold the joint together, yet flexible enough to allow the bones to move. Damage to these supporting structures of the joint can allow bones to ram into one another, damaging the smooth cartilage that lines the joint.

Bones are attached by strong bands called ligaments. Muscles are attached to the bones by bands called tendons. Ligaments and tendons are made of tissue that is strong enough to hold the joint in place but flexible enough not to tear under normal movement. The placement of tendons and ligaments determines how different joints are able to move. For example, the knee can bend forward but not backward. Abnormal movements can lead to damage to these supporting structures, with long-term consequences for the joint. For example, a common football injury occurs when a player who has his leg extended forward while running is contacted on the outer side of the knee by another player running full tilt at him from the side. The knee, which is supposed to move forward to back, is suddenly wrenched side to side, tearing the supports.

To keep the skin that covers the joint from restricting movement, the movements of the bones within the joint must be isolated from the skin. The structures that separate the joint from the overlying tissues are small, mobile sacs of fluid called bursae. Like other components of the joint, damage to these structures can cause joint pain (bursitis).