SHOULDER INSTABILITY – RELEVANT ANATOMY & FUNCTION
The main “ball and socket” joint of the shoulder (the glenohumeral joint) is formed between the relatively small, shallow socket of the shoulder blade (the scapula) and the large ball of the upper arm bone (the humerus), much like a golf ball balancing on a golf tee. This relationship allows for a highly mobile joint, but one that is, unfortunately, not well-stabilized by the bones alone. Therefore, the surrounding soft tissues of the shoulder play a pivotal role in keeping the ball from falling off the edge of the socket, or dislocating.
The main soft tissues involved in stabilizing the shoulder, include the labrum, the joint capsule, and the surrounding shoulder muscles. The labrum is a round, rubber-like bumper that attaches along the perimeter of the shallow socket, effectively raising and widening the socket’s edge. This provides more stability to the shoulder in the same way that using a larger golf tee would stabilize a golf ball.
The joint capsule is like sleeve of tissue surrounding the joint with thickened areas that form capsular ligaments. Based on one’s genetics, the capsule can vary tremendously in terms of its thickness and elasticity, which directly impacts its role as a stabilizing structure. A thicker, less elastic capsule will do a better job of keeping the ball on the socket (more stability), but may limit some shoulder motion or even cause stiffness. Conversely, a thinner, more elastic shoulder capsule will allow for greater motion, but may also lead to a loose, unstable joint.
The surrounding shoulder muscles, and in particular, the muscles of the rotator cuff, provide another layer of stability. These muscles work together in a well-orchestrated manner to keep the ball centered on the socket during shoulder motions, much like a seal balancing a beach ball on its nose. When the muscles are well-coordinated and strong, they are better able to quickly and effectively re-balance the ball on the socket in the face of sudden or high intensity forces, such as when a pitcher throws a baseball or when a gymnast performs a back handspring.
Nels Sampatacos, MD
Orthopedic Surgeon and Sports Medicine Specialist
