CHAPTER 8 Structure and Function of the Shoulder Complex


Coracoclavicular ligament


AC joint


Acromioclavicular ligament


Acromion


Coracoacromial ligament


Humerus


Coracohumeral ligament


Coracoid process


A


Figure 8.11 Anterior view of the right AC joint and associated ligaments. (From Houglum B, Beroto D. Brunnstrom’s Clinical Kinesiology, 6th ed. Philadelphia, PA: F. A. Davis Company, 2012, p. 173, with permission.)


Humerus B


Figure 8.10 The AC joint. (A) Anterior view of the AC joint at rest. (B) Rotation at the AC joint allows the glenoid fossa to maintain its alignment with the humerus as the arm elevates.


stability. The ligament is divided into the trapezoid liga- ment, which runs in a superior and lateral direction from the coracoid process to the clavicle, and the conoid liga- ment, which runs in a more vertical direction from the coracoid process to the clavicle (Fig. 8.11). Fibrocartilage lines the articular surfaces of the AC joint, which is a fi brocartilaginous disc in early childhood. With repeated upper extremity motion, the joint develops a joint space and in a small percentage of the population contains an incomplete articular disc.


Kinematics


AC joint motion occurs around an axis that is oriented to the plane of the scapula. The 3° of AC joint motion are upward and downward rotation, anterior and posterior tilting, and medial and lateral rotation.


Upward and downward rotation occurs around an


anterior-posterior axis that is perpendicular to the plane of the scapula (Fig. 8.12A). During upward rotation, the glenoid fossa rotates superiorly, and the inferior angle of the scapula rotates upward. Downward rotation tilts


the glenoid fossa inferiorly, and the inferior angle of the scapula rotates downward. Upward rotation at the AC joint occurs in conjunction with shoulder fl exion or abduction, and downward rotation is associated with shoulder adduction or extension. Although ranges vary, upward rotation can occur up to 30° during full shoulder elevation and 17° of downward rotation from the resting position (Fig. 8.12B).


Anterior and posterior tilting of the scapula occurs in relationship to the clavicle around an oblique medial- lateral axis. During anterior tilting, the acromion tilts in an anterior direction, and the inferior angle of the scapula tilts posteriorly. The opposite occurs during posterior tilting. The tilting motion allows the scapula to maintain contact with the curvature of the rib cage during shoul- der movement while orienting the glenoid fossa with the humeral head. The range of anterior-posterior tilt is approximately 20° but can extend to 40° during maximal shoulder movement (Fig. 8.12C).


The third degree of motion at the AC joint is medial and lateral rotation. This motion occurs around a verti- cal axis in relationship to the clavicle. During medial rotation, the glenoid fossa moves anteriorly and medi- ally, while the medial scapular border moves away from the thorax. The fossa moves posteriorly and laterally with lateral rotation, while the medial scapular border moves toward the thorax. This plane of motion allows the scapula to maintain contact with the horizontal curvature of the thorax (Fig. 8.12D). Figure 8.12E depicts the 3 degrees of freedom that occur at the AC joint.


Scapulothoracic Joint


The scapula is positioned on the posterolateral thorax but does not form a true anatomical joint with the thorax.


Clavicle


Trapezoid ligament


Conoid ligament


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