Shoulder joint - part II

 PART I

Blood and nerve supply

The arterial supply is from numerous sources as there is an important anastomosis around the scapula involving vessels from the subclavian and axillary arteries and the descending aorta. The supply to the shoulder joint is by branches from the suprascapular branch of the subclavian artery, the acromial branch of the thoracoacromial artery, and branches from the anterior and posterior circumflex humeral arteries. The latter three are all branches of the axillary artery. The venous drainage is by similarly named veins which drain into the external jugular and axillary veins.

Lymphatic drainage of the joint is to the lymph nodes within the axilla, eventually passing by the apical group of nodes into the subclavian lymph trunk.

The nerve supply to the shoulder joint is by twigs from several nerves which pass close to the joint. The twigs come from the suprascapular, axillary, subscapular, lateral pectoral and musculocutaneous nerves, and have a root value of C5, 6 and 7.

Relations

The shoulder joint is almost completely surrounded by muscles passing between the pectoral girdle and the humerus. They serve to protect the joint by helping to suspend the upper limb from the pectoral girdle and so convey some degree of stability to the joint. Some muscles are more important in this respect than others. The anterior, superior and posterior parts of the joint are directly related to the tendons of subscapularis, supraspinatus, and infraspinatus and teres minor respectively. These tendons all blend with the humeral part of the joint capsule, and because of their action on the shoulder joint, have become known as rotator cuff. Covering the superolateral part of the joint is deltoid, giving the shoulder its rounded appearance. The greater and lesser tubercles of the humerus can be palpated through deltoid, as can the long head of biceps as it emerges from within the capsule. The tendon of the long head of biceps passes directly above the joint within the capsule. Superiorly, and separated from the joint by the tendon of supraspinatus, is the coracoacromial arch. Inferiorly, coming from the infraglenoid tubercle, is the long head of triceps as it passes into the arm almost parallel to the humerus.

Immediately below the shoulder joint is the quadrilateral space, bounded superiorly by teres minor, inferiorly by teres major, medially by the long head of triceps and laterally by the shaft of humerus. Passing through this space from anterior to posterior are the axillary nerve and posterior circumflex humeral artery. Downward dislocation of the head of the humerus or prolonged upwardly applied pressure, for example as when falling asleep with the arm hanging over the back of a chair, may cause temporary or permanent damage to the nerve and consequent loss of function of deltoid and teres minor. Immediately medial to the quadrilateral space is a triangular space, bounded by teres minor and major and the long head of triceps through which pass the circumflex scapular vessels. A further triangular space is found below the quadrilateral space. This has the long head of the triceps as its medial border, the shaft of the humerus laterally and the lower border of teres major as its base. It is an important space as the radial nerve and the profunda brachii vessels pass through it to the posterior compartment of the arm. Fractures of the shaft of the humerus, or pressure from the axillary pad of an incorrectly used crutch, may involve the radial nerve, resulting in radial nerve palsy, that is wrist drop, which will affect the functional use of the hand.

The axilla

Inferomedial to the shoulder joint is the pyramidal-shaped axilla – the space between the arm and the thorax which enables vessels and nerves to pass between the neck and the upper limb(a). The apex of the axilla is formed by the clavicle anteriorly, scapula posteriorly and outer border of the first rib medially. Its concave base or floor is formed by deep fascia extending from the fascia over serratus anterior to the deep fascia of the arm, attached in front and behind to the margins of the axillary folds and supported by the suspensory ligament of the axilla, which is a downward extension of the clavipectoral fascia below pectoralis minor(b). The anterior axillary wall is formed by pectoralis major, its lower rounded fold formed by the twisting nature of the muscle fibres as they pass from the chest wall to the humerus. The posterior wall, which extends lower than the anterior, is formed by subscapularis and teres major, with the tendon of latissimus dorsi twisting around teres major(b). The medial wall is formed by serratus anterior, and the lateral wall by the floor of the intertubercular groove(c). Both the anterior and posterior axillary folds can be readily palpated. A vertical line midway between them, running down the thoracic wall, is the midaxillary line.

When the arm is fully abducted the axillary folds virtually disappear as the muscles forming them run almost parallel to the humerus. Indeed, the axillary hollow may be replaced by a bulge. 

The principal contents of the axilla are the blood vessels and nerves which pass between the neck and the upper limb(b,c). These are the axillary artery and its branches, the corresponding vein and its tributaries, and the branches of the brachial plexus. Together, with the various groups of lymph nodes, they are surrounded by fat and loose areolar tissue. The tendon of long head of biceps runs in the intertubercular groove, and so is just within the axilla. Also within the axilla are the short head of biceps and coracobrachialis. The major vessels and nerve trunks are enclosed within the axillary sheath, a fascial extension of the prevertebral layer of cervical fascia. The axillary sheath is adherent to the clavipectoral fascia behind pectoralis minor, and just beyond the second part of the axillary artery blends with the tunica adventitia of the vessels.

The axillary artery runs through the axilla, being posterior and superior to the vein, and can be indicated on the surface of the abducted arm by a straight line running from the middle of the clavicle to the medial prominence of coracobrachialis. With the arm hanging by the side, the artery describes a gentle curve with the concavity facing inferomedially.

The artery is crossed anteriorly by the tendon of pectoralis minor, which serves to divide it into three parts. Above the first part of the artery lie the lateral and posterior cords of the brachial plexus, and behind it is the medial cord – it is crossed by a communicating loop between the medial and lateral pectoral nerves. The second part of the axillary artery, behind pectoralis minor, has the various cords of the brachial plexus in their named positions. The third part of the artery, lying laterally against coracobrachialis, has the musculocutaneous nerve laterally, the median nerve anteriorly, the ulnar and medial cutaneous nerves of the arm and forearm medially, and the axillary and radial nerves posteriorly.

The axillary lymph nodes are widely distributed within the axillary fat, but may be conveniently divided into five groups(b,c). The lateral nodes lie along and above the axillary vein and receive the majority of the lymphatic drainage of the upper limb. The subscapular(posterior) nodes lie along the subscapular artery and receive lymph from the scapular region and back above the level of the umbilicus. The pectoral nodes, along-side the lateral thoracic artery, receive lymph from the anterior chest wall including the breast. These groups of nodes draing to a central group, which lie above the axillary floor. From here efferents pass to the apical group of nodes(the only group lying above the tendon of pectoralis minor) and thence to the subclavian lymph trunk.

Because of the involvement of the axillary nodes in the lymphatic drainage of the breast, they may be subjected to radiotherapy treatment in an attempt to limit the secondary spread of cancer from the breast. It is important to remember that the lateral group of nodes lie above the axillary vein so that they can be excluded from the treatment programmes, otherwise severe problems with the lymphatic drainage of the upper limb may result.

Stability

The incongruity of the articular surfaces, together with the laxness of the joint capsule, suggest that the shoulder joint is not very stable. Although dislocation of the shoulder is rather common it is by no means an everyday occurrence. What factors are responsible for conferring stability on the joint? The glenoid labrum, as well as deepening the fossa, also makes the joint surfaces more congruent, and thus becomes a significant stabilizing factor. Fracture of the glenoid or tearing of the labrum often results in dislocation.

Perhaps the most important factor, however, is the tone in the short scapular(rotator cuff) muscles. Not only do these muscles(supraspinatus, infraspinatus, teres minor and subscapularis) attach very close to the joint, but they fuse with the lateral part of the capsule. In this way they act as ligaments of variable length and tension, and also prevent the lax capsule and its synovial lining from being trapped between the articulating bones. The inferior part of the capsule is the weakest, being relatively unsupported by muscles. However, as the arm is gradually abducted, the long head of triceps and teres major become increasingly applied to this aspect of the joint.

In addition to the rotator cuff muscles, all the muscles passing between the pectoral girdle and the humerus assist in maintaining the stability of the joint. Particularly important are the long heads of triceps and biceps. The tendon of the long head of biceps, being partly intracapsular, acts as a strong support over the superior part of the joint. The long head of triceps gives support below the joint when the arm is abducted.

An upward displacement of the head of the humerus is resisted by the overhanging coracoacromial arch. Although not part of the joint, this arch, separated from the joint by the subacromial bursa, functions mechanically as an articular surface. The arch is so strong that an upward thrust on the humerus will fracture either the clavicle or the humerus first before compromising the arch.

Dislocation of the shoulder is more common than for many joints, being favoured by the need to have the joint as mobile as possible. In addition, the long humerus has great leverage in dislocating forces. In anterior dislocation, which is more common, the head of the humerus comes to lie under the coracoid process producing a bulge in the region of the clavipectoral groove. At the same time the roundness of the shoulder is lost. In such dislocations, the humeral head usually comes through the joint capsule between the long head of triceps and the inferior glenohumeral ligament.

Because the glenoid fossa faces anterolaterally, it is better situated to resist posteriorly directed forces. The presence of infraspinatus and teres minor also reinforces the capsule posteriorly. Posterior dislocation may result when a large force is applied to the long axis of the humerus when the arm is medially rotated and abducted. The joint capsule tears in the region of teres minor with the head of the humerus coming to lie below the spinous process of the scapula.