Clavicular and Shoulder Joints Anatomy
Three joints contribute to the considerable mobility of the arm; movement occurs between the humerus and the scapula at the shoulder (glenohumeral) joint, and the scapula moves on the chest wall through the joints at each end of the clavicle. Although few muscles attach to the clavicle, the numerous muscles attached to the scapula and upper humerus all contribute to movement at the clavicular joints. Indeed, movement at the shoulder joint is almost always associated with movement at the sternoclavicular and acromioclavicular joints.
The sternoclavicular and acromioclavicular joints are subcutaneous and easily palpable in the living subject. Each has a tubular capsule lined by synovial membrane.
At the sternoclavicular joint (Fig. 3.80), the medial end of the clavicle articulates with the notch on the upper border of the manubrium and with the first costal cartilage. The joint is partitioned by an intracapsular disc of fibrocartilage that attaches superiorly to the clavicle, inferiorly to the first costal cartilage and around its periphery to the capsule. There are two accessory ligaments. Above the capsule is the interclavicular ligament, which joins the medial ends of the clavicles. Just lateral to the joint is the costoclavicular (rhomboid) ligament, which attaches the clavicle firmly to the first costal cartilage. Stability depends on the disc and accessory ligaments, which limit both medial displacement and elevation of the medial end of the clavicle. The sternoclavicular joints are separated from the origins of the brachiocephalic veins and other structures in the root of the neck by the sternohyoid and sternothyroid muscles.
At the acromioclavicular joint (Fig. 3.81) the lateral end of the clavicle articulates with the medial aspect of the acromion of the scapula. The joint capsule attaches to the edges of the articular surfaces that lie obliquely, the clavicular facet facing laterally and inferiorly. Stability of the joint is provided mostly by the strong coracoclavicular ligament linking the coracoid process and the undersurface of the clavicle near its lateral end. This ligament consists of conoid and trapezoid parts.
Both the sternoclavicular and acromioclavicular joints are supplied by branches of the supraclavicular nerves (C3 & C4).
The lateral end of the clavicle may be elevated or depressed and drawn forwards or backwards. The axes of these movements occur at the costoclavicular and coracoclavicular ligaments, rather than through the clavicular joints. Thus, the medial end of the clavicle is elevated during depression of the scapula and moves posteriorly when the scapula is protracted. Full abduction of the upper limb requires rotation of the scapula so that the glenoid fossa tilts upwards. Rotation of the clavicle through 40° at the sternoclavicular joint supplements the 20° of movement available at the acromioclavicular joint, permitting the scapula to rotate through about 60°. The principal muscles of scapular rotation are trapezius and serratus anterior. Protraction is produced by pectoralis minor and serratus anterior and retraction by trapezius and the rhomboids.
The clavicle forms a strut that supports the scapula against the medial pull of muscles such as pectoralis major and latissimus dorsi. The clavicular joints are stabilized by their accessory ligaments, which are so strong that trauma, such as falling onto the outstretched limb, is more likely to fracture the clavicle than rupture the ligaments. The lateral part of a fractured clavicle tends to be displaced inferiorly by the weight of the limb and medially by spasm of pectoralis major and latissimus dorsi muscles, whereas the medial fragment may be elevated by the action of sternocleidomastoid muscle.
The shoulder (glenohumeral) joint is synovial, of the ball-and-socket type, and is capable of a wide range of movement. The hemispherical head of the humerus is directed medially and back-wards and articulates with the much smaller glenoid fossa of the scapula (Fig. 3.82). The fossa faces anterolaterally and is slightly deepened by the glenoid labrum, a cartilaginous lip round its edge.
The joint capsule (Fig. 3.83) forms a loose sleeve attaching medially to the glenoid labrum. Its humeral attachment is around the anatomical neck except inferiorly on the medial side, where it descends to the level of the surgical neck.
Synovial membrane lines the fibrous capsule and covers the intracapsular part of the humeral shaft (Fig. 3.84). The cavity of the joint usually communicates with the subscapular bursa through a deficiency in the anterior part of the capsule. Indistinct thickenings in the capsule form the glenohumeral ligaments. Between the greater and lesser tubercles (Fig. 3.85), the capsule forms the transverse humeral ligament beneath which the tendon of the long head of biceps enters the joint from the intertubercular groove. The tendon is surrounded by a tubular sheath of synovial membrane as it passes over the humeral head to attach to the supraglenoid tubercle (Fig. 3.86).
The joint is intimately related to subscapularis, supraspinatus, infraspinatus and teres minor (Figs 3.86 & 3.87) whose tendons fuse with the capsule to form the rotator cuff (Fig. 3.83). Above the joint is the coracoacromial arch formed by the coracoid process, the acromion and the intervening coracoacromial ligament. The arch is separated from supraspinatus by the subacromial bursa.
Articular nerves are derived from the suprascapular and subscapular nerves and also from the axillary nerve which passes very close to the joint. As this nerve leaves the axilla through the quadrangular space (Fig. 3.64), it lies immediately inferior to the capsule. The vascular supply is provided by branches of the circumflex humeral and suprascapular arteries.
Flexion of the shoulder joint (up to 180°) is produced mainly by the clavicular fibres of pectoralis major and the anterior fibres of deltoid. Extension (limited to about 45°) is produced by latissimus dorsi and the posterior fibres of deltoid. At the shoulder joint itself, about 120° of abduction is possible, produced by supraspinatus and deltoid; simultaneous rotation of the scapula through 60° permits full elevation of the arm above the head.
Adduction, produced by teres major, latissimus dorsi and pectoralis major, is limited by the area of the articular surface of the humerus. Medial rotation is produced by pectoralis major, subscapularis, teres major and the anterior fibres of deltoid, and lateral rotation by infraspinatus, teres minor and the posterior fibres of deltoid.
Although the coracoacromial arch prevents upward displacement of the humerus, stability of the shoulder joint relies principally on the rotator cuff muscles that hold the humeral head firmly in the glenoid fossa. Despite the labrum, the glenoid fossa is a shallow socket. The capsular ligaments are lax in most positions and tighten only near the extremes of movement. Dislocation of the joint, usually with anterior and inferior displacement of the head of the humerus, associated with trauma or weakness of the rotator cuff muscles, is relatively common and may result in damage to the axillary nerve.