Chapter 4: The Shoulder

Structure and Function

Shoulder Complex

The shoulder complex is composed of four joints: the glenohumeral (GH), sternoclavicular (SC), acromioclavicular (AC), and scapulothoracic joints. Full range of motion (ROM) of the shoulder requires coordinated motion at all four of these joints. Although there is some individual and motion-specific variability, about two-thirds of shoulder ROM occurs at the glenohumeral joint and one-third occurs at the remaining joints.^1-3 The distribution of shoulder complex motion across these multiple joints enables greater ROM and stability than if all motion were restricted to a single joint. The congruency between the head of the humerus and glenoid fossa of the scapula is improved, which reduces bone shearing forces. In addition, the muscles acting across the glenohumeral joint are maintained in a more optimal length-tension relationship, which reduces the potential problem of active insufficiency.²

The motions that occur at these joints must be coordinated to enable full, pain-free shoulder motions. For example, to achieve active flexion of the shoulder, the humerus undergoes flexion and some lateral rotation at the GH joint; the clavicle typically undergoes posterior rotation, retraction, and elevation at the SC joint; and the scapula undergoes posterior tilting, upward rotation, and protraction relative to the clavicle at the AC joint.⁴ The combination of retraction of the clavicle and protraction of the scapula results in some retraction of the scapula relative to the thorax. Specific information about the GH, SC, AC, and scapulothoracic joints that comprise the shoulder complex is presented in the following sections.

Glenohumeral Joint

The glenohumeral (GH) joint is a synovial ball-and-socket joint. The ball is the convex head of the humerus, which faces medially, superiorly, and posteriorly with respect to the shaft of the humerus (Fig. 4.1).^1,2 The socket is formed by the concave glenoid fossa of the scapula and faces laterally, superiorly, and anteriorly. The socket is shallow and smaller than the humeral head but is deepened and enlarged by the fibrocartilaginous glenoid labrum. The joint capsule is thin and lax, blends with the glenoid labrum, and is reinforced by the glenohumeral (superior, middle, inferior) and coracohumeral ligaments (Fig. 4.2), as well as the tendons of the rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, and teres minor) and long head of the biceps brachii muscle.

Osteokinematics

The GH joint has 3 degrees of freedom. The motions permitted at the joint are flexion–extension, abduction–adduction, and medial–lateral rotation that lie in the sagittal, frontal, and transverse cardinal planes, respectively.^2,...