The shoulder is the most mobile joint in the body. It consists of a ball known as the humeral head (the top part of the arm bone) and a socket known as the glenoid (a part of the shoulder blade). Since the shoulder allows motion in all directions, the socket part of the joint needs to be very shallow to allow that motion. To help stabilize the shoulder, our bodies have a cartilage rim around the joint socket called the labrum. The labrum acts as a bumper to prevent the ball from slipping out of the joint. In addition, the shoulder contains a number of ligaments which further stabilize the joint and prevent dislocation. Furthermore, four muscles called the rotator cuff muscles surround the shoulder and contract to keep the shoulder joint reduced in its normal position.
Shoulder dislocation occurs when a forceful movement is applied to the shoulder which overcomes the blocking effect of the cartilage rim and the ligaments and muscles surrounding the shoulder. This can occur during a sporting activity where the shoulder is placed in an extreme throwing position. It can also occur during a motor vehicle accident.
The shoulder can either dislocate to the front (anterior dislocation) or the back (posterior dislocation). As the shoulder dislocates, it may tear a segment of the cartilage rim (labrum) and some of the ligaments surrounding the shoulder. Patients over the age of 40 have a higher chance of developing a rotator cuff tear during a shoulder dislocation.
A shoulder dislocation can be very painful, and depending on the direction of the dislocation, may prevent the patient from moving their arm. In some cases, the nerves surrounding the shoulder may be stretched and can cause a combination of weakness and/or numbness in the area supplied by the stretched nerve(s). Most of these nerve injuries resolve with time.
The initial management of a shoulder dislocation is usually done in an emergency room. After giving appropriate pain medications and numbing injection to the shoulder, the joint is gently put back (reduced) to its normal position. In rare cases, the reduction of a dislocated shoulder may be difficult to perform in the ER, and may have to be done in the operating room under general anesthesia.
After the reduction of the shoulder, a sling is placed to stabilize the shoulder and prevent it from dislocating again. Based on the severity of the injury and the number of previous occurrences, physical therapy is started about 2 weeks after the injury. Physical therapy is used to strengthen the rotator cuff muscles and the shoulder outer stabilizing muscles.
While physical therapy may help some, a good percentage of patients go on to dislocate their shoulder again. Surgery is used to reattach the torn cartilage rim to the socket and to tighten the ligaments around the shoulder. See the section on labral tears for more information.
Further information on this injury can be found in this article on the AAOS OrthoInfo website, an orthopaedic resource center providing expert information.