The shoulder is often referred to as the glenohumeral joint because this articulation is the only moveable part of the shoulder anatomy. This joint is a ball-and-socket joint similar to the hip, but much less stable (the tradeoff being more freedom of motion). The glenohumeral joint is the interaction of the head of the humerus and the glenoid cavity. The joint is stabilized a ring of fibrous cartilage called the labrum. There are four main articulating ligaments that sustain stability in this section of the shoulder. First, the coracohumeral ligament connects the humerus to the coracoid process of the scapula, and three ligaments that lock the humerus to the glenoid cavity of the scapula called the superior, middle and inferior glenohumeral ligaments (SGHL, MGHL, IGHL). Four muscles collectively called the rotator cuff also assist stability; these muscles include the subscapularis, supraspinatus, infraspinatus and teres minor (1).
How the Injury Occurs
A shoulder dislocation is an injury that occurs when the top
of the arm bone (humerus) loses contact with the shoulder blade (scapula). This
injury is often confused with a shoulder separation (involving the A-C joint),
but these are two very different injuries. It is important to distinguish between
these two problems because the issues with management and treatment are different.
The shoulder joint is the most frequently dislocated major joint of the body. In most cases of a dislocated shoulder, a strong and sudden traumatic force that pulls the shoulder outward (abduction) or extreme rotation of the joint pops the ball of the humerus out of the glenoid socket of the scapula. Dislocation commonly occurs when there is a backward pull on the arm that either catches the muscles unprepared to resist or overwhelms the muscles. This results in tearing or stretching of the glenohumeral ligaments and debilitating instability of the shoulder. The majority of the time, the inferior glenohumoral ligament is the damaged structure (6). When a shoulder dislocates frequently, the condition is referred to as shoulder instability. A partial dislocation where the upper arm bone is partially in and partially out of the socket is called a subluxation. About 95% of thetime, when the shoulder dislocates, the top of the humerus is sitting in front of the shoulder blade - an anterior dislocation (9).
Following diagnosis of the shoulder dislocation, the shoulder must be "reduced," or put back in place. Once the joint is in place, repeat x-rays are performed to ensure it is indeed in the correct position, and to evaluate for other injuries such as fractures. One of the most common problems following a shoulder dislocation is called chronic shoulder instability. Many times physicians will opt for rehabilitation and strengthening exercises first to help with shoulder instability. Specific physical therapy exercises are prescribed with the aim of helping to hold the joint in correct position and to strengthen the rotator cuff muscles (17). However, sometimes surgery is necessary when a patient's shoulder instability is too great. An orthopedic surgeon can go into the joint using either an arthroscopic approach or with an open surgery procedure to stabilize the joint by securing the glenohumeral ligaments and by tightening the joint capsule using suturing and anchoring devices (17). This type of intervention will prevent a patient from normal activity for three to six months.
Patients who have sustained a prior shoulder dislocation often develop chronic instability. In these cases the shoulder ligaments heal too loosely, and the shoulder will be prone to repeat dislocation and episodes of instability. When younger patients (less than about 35 years old) sustain a traumatic dislocation, shoulder instability will follow in about 80% of patients (17). While later on they may not completely dislocate the joint, the apprehension, or feeling of being about to dislocate, may limit their ability to play certain sports. It is also very possible for a patient to have repeated complete shoulder dislocations when force is applied to the joint (17). For repeated dislocations surgery will be necessary to prevent further episodes of instability.