The shoulder joint is a ball and socket joint. The end of the upper arm bone (humerus) is shaped like a round ball. This fits into the socket of the scapula (wing bone). Tendons, ligaments, and cartilage on all sides hold the joint steady and in place.
Even with almost 360 degrees of shoulder motion, the head of the humerus stays firmly in the socket. In fact, it stays in the center during all motions. The four rotator cuff tendons around the shoulder are the key to keeping this central position.
A solid and stable shoulder joint is important for all athletes. When the shoulder joint is hurt, it can become unstable. The head of the humerus can move out of the socket (a dislocation). Sometimes, this happens because of a known injury to the tendons or joint. This is called traumatic instability. In other cases, the cause of the dislocation is unknown. This is called atraumatic instability, meaning without trauma.
New computer technology is helping doctors understand these injuries better. A three-dimensional (3-D) view of the shoulder joint is now possible. To get this view, virtual reality software is used on a multiprocessing computer.
Studies have been done using this new equipment. In the normal shoulder, muscle activity is able to keep the head of the humerus in the center of the socket. This is not always true for the injured shoulder joint. It is also clear that shoulder motion is different between traumatic and atraumatic injuries.
When there is traumatic injury, the joint gets off center. This occurs when the arm is raised overhead with the elbow out to the side. By comparison, atraumatic instability causes the humeral head to move off center with all movements.
These new facts about shoulder instabilities will help doctors. In cases of unclear or unknown history, 3-D studies of the shoulder can show the type of instability. Treatment to help the muscles center the joint may be the key to getting back joint stability.