Posterior shoulder instability typically results from a dislocation injury to the shoulder joint when the humeral head (ball) of the humerus (upper arm bone) is displaced from its normal position in the center of the glenoid (socket) and the joint surfaces no longer touch each other. The most common dislocation is anterior (more than 90 percent), where the humeral head is in front and below the glenoid. With falls on an outstretched hand in front of the body, a posterior shoulder dislocation can occur.
X-ray of a posterior shoulder dislocation.
The shoulder has more range of motion than any other large joint in the body and it also is the most commonly dislocated large joint. The shoulder is like a golf ball on a golf tee. Many structures contribute to shoulder stability and include bony contours of the humeral head (ball) and glenoid (socket). The soft tissue bumper of the glenoid is called the labrum. The labrum surrounds the rim of the socket helping keep the head of the humerus in place. Ligaments that attach from the glenoid to the humeral head also assist in keeping the head of the humerus in place. Muscles of the rotator cuff that surround the shoulder also contribute to keeping the shoulder stable. When a shoulder is dislocated from its glenoid, any of these stabilizing structures may be injured including the humeral and glenoid bone, the labrum, the capsule and ligaments, and the rotator cuff. When a younger active patient dislocates his or her shoulder, he or she may injure all these structures, but typically tear the labrum off the glenoid (socket).