The shoulder is the third-most commonly injured joint during athletic activities, after the knee and the ankle. Sports-related injuries of the shoulder may result from a direct traumatic event or repetitive overuse. Any activity that requires arm motion, particularly overhead arm motion such as throwing, may stress the shoulder, resulting in injury. The glenohumeral joint of the shoulder is mobile partly from minimal containment of the humeral head by the shallow and smaller glenoid fossa. The trade-off for this mobility is less structural restraint to undesirable and potentially damaging movements. Thus, a fine balance must be struck to maintain full range of shoulder motion and normal glenohumeral joint stability.
A. The Bony Articulation of the Glenohumeral Joint
The glenohumeral joint is a modified ball-and-socket joint. The glenoid fossa is a shallow inverted, comma-shaped, articular surface one-fourth the size of the humeral head. The articular surface of the humeral head is retroverted approximately 30 degrees relative to the transverse axis of the elbow. Because the scapula is oriented anterolaterally about 30 degrees on the thorax, relative to the coronal plane of the body, the orientation of the glenoid fossa matches the humeral head retroversion so that with the arm relaxed at the side, the arm is positioned for activities in front of the body. With shoulder motion, the scapula also moves so that the glenoid accommodates changing humeral head positions. As a result, the humeral head is centered in the glenoid throughout most shoulder motions. When this centered position is disturbed, instability may result.
B. The Clavicle and Its Articulations
The clavicle articulates medially with the sternum at the sternoclavicular joint and laterally with the acromion of the scapula at the acromioclavicular joint. The clavicle rotates on its long axis and acts as a strut, serving as the only bone connecting the appendicular upper extremity to the axial skeleton.
C. The Glenohumeral Joint Capsule, Ligaments, and Labrum
The thin redundant joint capsule has almost twice the surface area of the humeral head to allow large range of joint motion. Different regions of the joint capsule provide stability at different joint positions. With the arm at the side, the superior portion of the capsule is taut and the inferior portion is lax. With overhead elevation, this relationship reverses.
The superior capsule forms a thick band called the coracohumeral ligament originating from the base of the coracoid and inserting on the anterior part of the greater tuberosity and superior part of the lesser tuberosity of the humerus. The capsule is uniformly thin and translucent anterior and posterior to the coracohumeral ligament. There are also folds or thickenings visible on the inside of the capsule with the shoulder at the side, which have been termed glenohumeral ligaments. Traditionally, the capsule has ...