Shoulder Joints

Shoulder Joints

The shoulder joints are the most freely movable joints of the body, but they lack stability. They are ball-and-socket joints. The head of the humerus is large and hemispherical. It fits in the glenoid cavity of the scapula, which is shallow and small (FIGURE 8-4). This cavity is made slightly deeper by a fibrocartilage­ rim (the glenoid labrum). Still, it is only about one -third the size of the humeral head, which means the shoulder joint is highly unstable. A shoulder separation is an injury involving ­partial or ­complete dislocation of the acromioclavicular joint.

The joint cavity is enclosed by an extremely loose, thin articular capsule. It runs from the margin of the glenoid cavity to the anatomical neck of the humerus. Only a few ligaments reinforce the shoulder joint, and these are found mostly on its anterior aspect. The supe-rior coracohumeral ligament is the thickest area of the capsule. It helps to support the weight of the arm. The front of the capsule is only slightly strengthened by three glenohumeral ligaments. These are absent in some individuals. Most of the stability of the shoulder joint comes from muscle tendons that cross the joint. The ­primary stabilizing structure is the tendon of the long head of the arm’s biceps brachii muscle. This tendon secures the head of the humerus against the glenoid cavity. It is attached to the glenoid labrum’s superior margin and travels through the shoulder joint cavity. It then continues within the intertubercular sulcus of the humerus.

Associated muscles (the subscapularis, infraspi-natus, supraspinatus, and teres minor) and a total of four other tendons comprise the rotator cuff, which encircles the shoulder joint. The rotator cuff blends with the articular capsule. If the arm is strongly circumducted, the rotator cuff can be stretched severely, which often occurs in athletes who pitch (such as those in baseball or softball). Instability of the shoulder joint also results in frequent disloca-tions. Usually, the humerus dislocates in the forward, downward direction because its reinforcements are weakest anteriorly and inferiorly.