Chapter 4: The Shoulder

STRUCTURE AND FUNCTION

Glenohumeral Joint

Anatomy

The glenohumeral 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 tendons of the rotator cuff muscles and by the glenohumeral (superior, middle, inferior) and coracohumeral ligaments (Fig. 4.2).

Osteokinematics

The glenohumeral joint has 3 degrees of freedom. The motions permitted at the joint are flexion–extension, abduction–adduction, and medial–lateral rotation.^1,2 In addition, horizontal abduction and horizontal adduction are functional motions performed at the level of the shoulder and are created by combining abduction and extension, and adduction and flexion, respectively. Full range of motion (ROM) of the shoulder requires humeral, scapular, and clavicular motion at the glenohumeral, sternoclavicular, acromioclavicular, and scapulothoracic joints.

Arthrokinematics

Motion at the glenohumeral joint occurs as a rolling and sliding of the head of the humerus on the glenoid fossa. The convex joint surface of the head of the humerus slides in the opposite direction and rolls in the same direction as the osteokinematic movements of the shaft of the humerus.^2,3 The sliding motions help to maintain contact between the head of the humerus and the glenoid fossa of the scapular during the rolling motions and reduce translational movement of the axis of rotation in the humerus. During abduction the surface of the humeral head slides inferiorly while rolling superiorly.^2,3,4,5 The opposite motions occur during adduction. In medial rotation and flexion, the surface of the humeral head slides posteriorly and rolls anteriorly.^4,5 In lateral rotation and extension, the surface of the humeral head slides anteriorly and rolls posteriorly on the glenoid fossa.^4,5 Arthrokinematic motions during flexion and extension have also been described as a spin.³

Capsular Pattern

The greatest restriction of passive motion is in lateral rotation, followed by some restriction in abduction and less restriction in medial rotation.^5,6

Sternoclavicular Joint