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.

Figure 528
Shoulder joint section anterior view - Figure 528
The synovial membrane is reflected from the margin of the glenoid cavity over the glenoid labrum; it is then continued over the inner surface of the capsular ligament, and covers the lower part and sides of the anatomical neck of the humerus as far as the articular cartilage on the head of the bone. The tendon of the long head of the biceps passes through the joint and is enclosed in a tubular sheath of synovial membrane, -which is reflected upon it from the summit of the glenoid cavity and is continued round the tendon into the bicipital groove (intertubercular sulcus) as far as the surgical neck of the humerus (figs. 524, 529).

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.

Figure 529
Synovial cavity of shoulder anterior view - Figure 529
Bursae.—-The bursae in the neighborhood of the shoulder joint are the following : (1) one is constantly found between the tendon of the subscapularis and the joint-capsule (fig. 531) : it communicates with the joint cavity through an opening which is situated between the superior and the middle glenohumeral ligaments; (2) one is ,sometimes found between the tendon of the infraspinatus and the capsule; it occasionally opens into the joint; (3) a large one, named the subacromial bursa (fig, 531), exists between the deltoid and the capsule; it does not communicate with the joint, but is prolonged under the acromion and coraco-acromial ligament, and intervenes between these structures and the supraspinatus, which covers the upper part of the capsule; (4) a large one is situated on the summit of the acromion; (5) one is frequently found between the corticoid process and the capsule : (6) one sometimes exists behind the coracobrachialis; (7) one lies between the teres major and the long head of the triceps; (8) one is placed in front of, and another behind, the tendon of the latissimus dorsi.

Figure 531
Shoulder joint muscles and nerves sagittal section - Figure 531
The muscles in relation with the- joint are, above, the supraspinatus : below, the long head of the triceps; in front; the subscapularis; behind, the infraspinatus and teres minor; within, the tendon of the long bead of the biceps. The deltoid covers the joint in front, behind and laterally (fig. 531).

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.

Figure 530
Shoulder joint interior lateral view - Figure 530
When the movements at the shoulder-joint are being analyzed, the humerus must be considered in its relationship to the scapula and not in its relationship to the sagittal and coronal planes of the trunk. When the arm is by the side in the resting position, the glenoid cavity faces almost equally forwards and laterally, and the position of the humerus corresponds to that of the scapula, although relative to the trunk i.t appears to be rotated medially. As a result flexion carries the arm forwards and medially across the front of the chest, and the movement takes place around an axis which passes through the head of the humerus at right angles to the center (approximately) of the glenoid cavity. Abduction and adduction occur in a plane at right angles to the plane of flexion and extension, and the axis passes through the head of the humerus parallel to the plane of the glenoid cavity. Abduction therefore carries the arm forwards and laterally away from the trunk, and the movement occurs in the plane of the body of the scapula. At the shoulder-joint itself the movements of flexion and abduction are limited to approximately 90�; when. the arm is raised vertically above the head the additional (approximately) 90� are obtained by forward rotation of the scapula at the acromio-clavicular joint. In flexion, however, the humerus moves in a plane at right les to the plane of the body of the scapula; and no amount of rotation of the scapula can increase the degree of elevation (90) obtained in full flexion. If the fully flexed humerus is gradually abducted, the degree of elevation increases until the humerus comes to lie in the plane of the body of the scapula, i.e. when the position of pure abduction is reached; the full 180� of elevation is obtained. In rotation, which may be medial or lateral, the humerus revolves for about one-quarter of a circle about its own long axis. In circumduction, which results from a succession of the foregoing movements, the lower end of the humerus describes the base of a cone, the apex of which is at the head of the bone, but this movement at the shoulder-joint can be increased very substantially by the movements of the scapula, and the combination is well exemplified in the arm movements of a fast bowler in cricket.

Figure 531
Shoulder joint muscles and nerves sagittal section - Figure 531
In. connection with the abduction of the arm and its elevation above the level of the shoulder, Cathcart* has pointed out (1) that when the arm reaches an angle of about 30� with the side of the chest the scapula begins to rotate forwards, and continues to do so throughout the rest of the movement with the exception of a short space at the end; (2) that the humerus troves on the scapula not only while the limb is passing from the hanging position to the level of the shoulder but also while it is travelling upwards, until it approaches the vertical : (3) that the clavicle moves not only during the second half of the movement but in the first as well, though to a less extent. In other words, the scapula and clavicle are concerned in the first stage as well as in the second; and that the hunicrus is partly involved in the, second stage, as well as chiefly in the first.

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.

Figure 532
Shoulder with arteries, nerves, and muscles posterior view - Figure 532
Muscles producing the movements.-The muscles moving the shoulder may be divided into : (a) those acting on the shoulder-girdle and (b) those acting on the shoulder-joint.

(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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