Chapter 24 Shoulder Instability and Rotator Cuff Tears

Anatomy Shoulder stability is the end result of the shoulder stabilizing structures working properly together. The constraints on shoulder motion can be divided into static stabilizers and dynamic stabilizers (Table 1). Static stability is chiefly maintained by joint congruency, the labrum, and the shoulder ligaments. The glenohumeral joint is a ball and shallow socket joint with a constant mismatch between the radii of curvatures. The unconstrained bony relationship allows the shoulder to obtain a large excursion and range of motion. The glenoid socket is deepened by the glenoid labrum, which is made of fibrocartilage. The labrum serves as the attachment site for the glenohumeral capsule and biceps tendon, and decreases the glenoid radius of curvature to more closely match the humeral curvature. Discrete thickening of the capsule in consistent locations form the glenohumeral ligaments (Figure 1). The glenohumeral capsule connects to the humerus at the rotator cuff insertion and inferiorly onto the humeral neck.1 The main dynamic stabilizers are the rotator cuff, scapulothoracic rhythm, and the long head of the biceps. The neurologic feedback that connects the dynamic stabilizers to each other and to the static stabilizers is called proprioception. A dysfunction of any of the stabilizers can lead to instability, dysfunction, and pain in the shoulder. The rotator cuff consists of four muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) whose tendons coalesce to dynamically stabilize the humeral head in the center of the glenoid cavity throughout the full range of shoulder motion by generating force couples in the coronal and transverse planes of the glenohumeral joint.2,3 Loss of the coronal plane force couple results in superior head migration, but not necessarily loss of function. Disruption of the transverse force couple can result in pain and loss of function.