The shoulder-joint (figs. 527 to 532) is a ball-and-socket joint. The bones entering into its formation are the hemispherical head of the humerus and the shallow glenoid cavity of the scapula, a construction which permits of very considerable movement but seriously affects the stability of the joint. Structurally the shoulder-joint is weak, since, for such strength as it possesses, it is dependent on the support given by the muscles which surround it and not on its bony conformation or the presence of any strong ligaments. It is, however, protected above by an arch, formed by the corticoid process, the acromion and the coraco-acromial ligament. The articular cartilage on the head of the humerus is thicker at the center than at the circumference, the reverse being the case with the articular cartilage of the glenoid cavity. The ligaments of the articulation are: Capsular, The glenoid labrum, Coracohumeral, & Transverse humeral.
The capsular ligament (figs. 527, 528) envelops the joint, and is attached, medially, to the circumference of the glenoid cavity beyond the glenoid labrum; above, it encroaches on to the root of the corticoid process so as to include the origin of the long head of the biceps within the joint. Laterally it is attached to the anatomical neck of the humerus, except on the medial side where it descends for rather more than 1 cm on to the shaft of the bone. It is so remarkably loose and lax that the bones may be separated from each other for a distance of 2 or 3 cm, an evident provision for the great freedom of movement which is permitted at this articulation. It should be noted, however, that this separation can be effected only after the superior part of the ligament has been relaxed by the movement of abduction. The capsular ligament is strengthened, above, by the supraspinatus; below, by the long head of the triceps; behind, by the tendons of the infraspinatus and teres minor; and in front, by the tendon of the subscapularis. The tendons of the subscapularis, supraspinatus, infraspinatus, and teres minor are all more or less completely blended with the capsular ligament, and this arrangement increases the value of the support which they supply, The relationship of the long head of the triceps is not so intimate, for it is separated from the inferior part of the capsule by the circumflex (axillary) nerve and the posterior circumflex humeral vessels as they pass backwards on leaving the axilla (fig. :531). It is the inferior part of the capsule, therefore, which is least supported, and it is just this part which is subjected to the greatest strain, because it is stretched tightly across the rounded head of the humerus when the arm is abducted.
There are usually three openings in the capsule. One anteriorly, below the coracoid process; establishes a communication between the joint and a bursa behind the tendon of the subscapularis; another. between the tuberosities of the humerus, gives passage to the long tendon of the biceps and its synovial sheath; the third, which is not constant, is at the posterior part, between the joint and a, bursal sac under the tendon of the infraspinatus.
Three supplemental bands (fig. 530), which are named the glenohumeral ligaments strengthen the capsule. These are best seen by opening the posterior part of the capsule of the joint and removing the head of the humerus. At their scapular ends they are all attached to the upper part of the medial margin of the glenoid cavity and are intimately connected with the glenoid labrum. The superior band passes along the medial edge of the tendon of biceps and is attached to a small depression above the lesser tuberosity of the humerus, the middle band reaches to the lower part of the lesser tuberosity; the inferior band extends to the lower part of the anatomical neck of the humerus. In addition to these, the capsule is strengthened in front by two bands, one derived from the tendon of the pectoralis major, the other from the tendon of the teres major.
The coracohumeral ligament (fig. 527) is a broad band which strengthens the upper part of the capsule. It arises from the lateral border of the root of the corticoid process, and passes obliquely downwards and laterally to the front of the greater tuberosity of the humerus, blending with the tendon of the supraspinatus. The hinder and lower border of the ligament is united to the capsular ligament; its anterior and upper border is free, and overlaps the capsular ligament.
The transverse humeral ligament (fig. 529) is a broad band passing from the lesser to the greater tuberosity of the humerus; it converts the bicipital groove (intertubercular sulcus) into a canal, and its attachment is always limited to that portion of the bone which lies above the epiphyseal line.
The glenoid labrum (figs. 530, 531) is a fibrocartilaginous rim attached round the margin of the glenoid cavity. It is triangular on section, the base being fixed to the circumference of the cavity, while the free edge is thin and sharp. It is continuous above with the tendon of the long head of the biceps, which gives off two fasciculi to blend with the fibrous tissue of the labrum. It deepens the articular cavity, and protects the edges of the bone. Its attachment to the margin of the glenoid cavity is sometimes deficient in parts; the deficiency occurs most commonly at the notch on the upper part of the anteromedial margin, and a small fringe of the synovial membrane occasionally protrudes through the gap.
The arteries supplying the joint are derived from the anterior and posterior circumflex humera, and suprascapular (transverse scapular) arteries : the nerves, from the circumflex (axillary) and suprascapular nerves.
Movements.–The shoulder is a ball-and-socket joint, and therefore is capable of flexion, extension, abduction, adduction, circumduction and rotation. The laxity of its capsular ligament and the large size of the head of the humerus compared with that of the shallow glenoid cavity give to the shoulder a wider range of movement than is possible at any other joint.
The peculiar relation of the tendon of the long head of the biceps to the shoulder-joint appears to subserve various purposes. By its connection with both the shoulder and elbow the muscle harmonises the action of the two joints, and acts as an elastic ligament during all the movements which occur at these articulations. It strengthens the upper part of the shoulder-joint, and prevents the head of the humerus from being pressed up against the acromion when the deltoid contracts; it thus fixes the head of the humerus as the center of motion in the g.lenoid cavity. By its passage along the bicipital groove (intertubercular sulcus) it assists in steadying the head of the humerus in the various movements of the arm.
(a) Muscles acting on, the shoulder-girdle.-The chief effect of these muscles is to displace the point; of the shoulder, either by pulling directly on the shoulder-girdle or by rotating the scapula. The scapula may be rotated forwards or backwards, but it can also be elevated or depressed, drawn forwards round the chest wall or backwards towards the median plane, without undergoing any rotation at all.
- Forward rotation.-Serratus anterior, Trapezius.
- Backward rotation.-Levator scapulae, Rhomboidei, Pectoralis minor.
- Elevation.-Levator scapula, Trapezius (upper fibers).
- Depression.-Serratus anterior (lower fibers). Pectoralis minor, Subclavius (acting through the acromioclavicular joint). Trapezius (lower fibers).
- Forward movement-Serratus anterior. Pectoralis minor.
- Backward movement.-Trapezius, Rhomboidei.
It will be observed that muscles which are antagonists in the execution of one movement are associated -with one another in the execution of another movement. The serratus anterior is associated with the trapezius in the production of forward rotation of the scapula, but the two muscles are opposed to each other in the pure backward and forward movement of the bone round the chest wall.
(b) Muscles acting on the shoulder-joint.
- Flexion.-Subscapularis, Deltoid (anterior part), Pectoralis major (clavicular head), Coracobrachialis, Biceps.
- Extension.-Infraspinatus, Teres minor, Teres major, Latissimus dorsi, Triceps (long head), Pectoralis major (sternocostal head)
- Abduction.-Supraspinatus, Deltoid.
- Adduction.-Subscapularis. Infraspinatus. Teres minor, Pectoralis major, Latissimus dorsi, Teres major, Coracobrachialis, Biceps, Triceps.
- Medial rotation.-Subscapularis, Pectoralis major, Latissimus dorsi, Teres major.
- Lateral rotation.-Infraspinatus; Teres minor, Deltoid (posterior fibers).
Applied Anatomy.-Owing to the construction of the shoulder-joint and the wide range of movement which it enjoys, as well as in consequence of its exposed situation, it is more frequently dislocated than any other joint. Dislocation occurs when the arm is abducted. In that position the head of the humerus presses against the lower and front part of the capsule, which is the thinnest and least supported part of the ligament. The rent in the capsule almost invariably takes place in this situation, and through it the head of the bone escapes, so that the dislocation in most instances is primarily subglenoid. If, after the dislocation has been reduced, abduction of the arm is prevented, the dislocation cannot recur.
When the shoulder-joint is ankylosed, the loss of movement in the joint is partly compensated for by increased mobility of the scapula. In treating conditions of the shoulder joint likely to lead to ankylosis, the humerus should be kept in the position it assumes when the palm of the hand is placed on the back of the neck, i.e. abducted, slightly rotated laterally, and flexed anteriorly, so as to make full use of this compensating mobility of the scapula.
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