Pectoral girdle and upper limb: overview and surface anatomy

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CHAPTER 45 Pectoral girdle and upper limb: overview and surface anatomy

This chapter is divided into two sections. The first is an overview of the general organization of the upper limb, with particular emphasis on the distribution of the major blood vessels and lymphatic channels, and of the branches of the brachial plexus: it is intended to complement the detailed regional anatomy described in Chapters 46 to 50 Chapter 47 Chapter 48 Chapter 49 Chapter 50. The second section describes the surface anatomy of the upper limb.

BONES AND JOINTS

The bones of the upper limb are the clavicle, scapula, humerus, radius and ulna (connected for a large portion of their length by an interosseous membrane) and the bones of the hand, i.e. the carpals, metacarpals and phalanges (Figs 45.1, 45.2).

The shoulder girdle is extremely mobile because reciprocal movements at the sternoclavicular and glenohumeral joints enable 180° abduction of the upper limb. Movement occurs in all three planes at the glenohumeral joint.

The elbow joint is a hinge joint. It incorporates the superior (proximal) radio-ulnar joint within its capsule. The proximal and distal radio-ulnar joints permit pronation and supination of the forearm – a unique feature of the primate upper limb.

The range of movement at the condyloid wrist joint, between the distal ends of the radius and ulna and the proximal carpal bones, is supplemented by gliding movements between the carpal bones. The saddle-shaped first carpometacarpal joint, between the trapezium and the base of the first metacarpal, is unique to the primate forelimb and permits opposition of the thumb. The hand is clenched by flexion at the metacarpophalangeal joints, supplemented by gliding movements of the fourth and fifth carpometacarpal joints. In grasping, the thumb is of equal value to the remaining four digits: loss of the thumb is almost as disabling as loss of all of the other digits.

SKIN, FASCIA AND SOFT TISSUES

The skin of the anterior aspect of the upper arm and forearm differs from that of the posterior aspect in that it is thinner and hairless. The palmar skin is thick and hairless: firm attachments to the underlying palmar fascia reflect its role in gripping and shock absorption. The dorsal skin of the hand is much thinner, lax and mobile, and this allows the extensor tendons to glide underneath the subcutaneous tissue.

The direction in which skin tension is greatest varies regionally in the upper limb as in other areas of the body. Skin tension lines that follow the furrows formed when the skin is relaxed are known as ‘relaxed skin tension lines’ and can act as a guide in planning elective incisions.

The superficial fascia is a layer of subcutaneous fatty tissue. Its thickness depends on the degree of obesity of the subject: measurement of the thickness of the subcutaneous tissue of the posterior upper arm is used as an indicator of obesity. With the exception of the digital pads, there is less subcutaneous tissue in the palm of the hand than on the dorsum of the hand.

The depth of deep fascia varies according to the stresses to which it is subjected in the different areas of the limb. In the upper arm it is a thin but quite obvious layer. Intermuscular septa pass to the medial and lateral sides of the humerus, separating the upper arm muscles into anterior and posterior groups within their respective compartments. Each muscle also lies within its own delicate fascial shea than arrangement that allows individual muscles to glide upon each other.

The deep fascia condenses anteriorly at the elbow as the tough bicipital aponeurosis; it is otherwise relatively thin in the forearm where it is attached along the subcutaneous border of the ulna. Intermuscular septa divide the forearm into three compartments, namely, anterior (contains the flexor group of muscles), posterior (extensors) and the mobile wad compartment for brachioradialis and extensor carpi radialis longus and brevis.

At the wrist the deep fascia becomes condensed anteriorly and posteriorly as the flexor and extensor retinacula respectively. Further condensation occurs in the palm of the hand, where the palmar aponeurosis is reinforced by the insertion of the tendon of palmaris longus, and in the flexor tendon sheaths and fascial system associated with the digits.

MUSCLES

Posterior and anterior muscle groups connect the pectoral girdle to the axial skeleton; the only bony connection is at the sternoclavicular joint. The posterior muscles are trapezius, levator scapulae and the rhomboid muscles: their actions include raising the shoulder and drawing the scapula medially. The anterior muscles are pectoralis minor, serratus anterior, and subclavius. Serratus anterior and trapezius together rotate the scapula in abduction of the arm.

Latissimus dorsi (posteriorly) and pectoralis major (anteriorly) run from the axial skeleton to the humerus. They are both powerful adductors and medial rotators of the shoulder; latissimus dorsi is also a powerful extensor.

A large group of muscles arise from the shoulder girdle and pass to the humerus. They are the four muscles of the rotator cuff (subscapularis, supraspinatus, infraspinatus and teres minor), teres major, deltoid, the clavicular origin of pectoralis major, biceps brachii, coracobrachialis and the long head of triceps.

The muscles of the upper arm can be divided into an anterior group of elbow flexors (biceps brachii, brachialis and coracobrachialis) and a posterior group of elbow extensors (triceps and anconeus). Biceps brachii is also a powerful supinator of the radio-ulnar joints.

The extensor muscles of the wrist and fingers, together with brachioradialis and a slip of origin of supinator, arise from the lateral epicondyle of the humerus. The principal head of pronator teres, the carpal flexor muscles, palmaris longus and, at a deeper level, the main origin of flexor digitorum superficialis, all arise from the medial epicondyle of the humerus. More deeply, flexor pollicis longus, flexor digitorum profundus and pronator quadratus arise from the anterior aspects of the shafts of the radius and ulna and the intervening interosseous membrane. Abductor pollicis longus, extensors pollicis longus and brevis and extensor indicis all arise from the posterior aspects of these bones and the intervening interosseous membrane.

The small, intrinsic, muscles of the hand consist of a thenar and a hypothenar group, and the muscles of the palm (the anterior and posterior interossei and the lumbricals). They are all concerned with the intricate movements of the digits.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

ARTERIAL SUPPLY

The blood supply to the skin of the upper limb comes from a combination of direct cutaneous, fasciocutaneous and musculocutaneous vessels (Figs 45.3, 45.4; see Fig. 7.19).

image

Fig. 45.4 The anatomical territories served by the cutaneous blood supply to the upper limb.

(From Cormack GC, Lamberty BGH 1994 The Arterial Anatomy of Skin Flaps, 2nd edn. Edinburgh: Churchill Livingstone, with permission.)

The axial artery to the upper limb is the subclavian artery, which becomes the axillary artery after crossing the lateral edge of the first rib. The axillary artery becomes the brachial artery as it crosses the distal edge of the posterior axillary fold, i.e. at the lower border of teres major. At first, the brachial artery lies in the flexor compartment on the medial side of the upper arm, but later it inclines laterally until it lies anterior to the elbow joint. Just distal to the elbow, it divides into the radial and ulnar arteries, both of which remain in, and supply, the flexor compartment. The ulnar artery almost immediately gives rise to the common interosseous artery, and this divides into anterior and posterior interosseous arteries which travel towards the wrist on either side of the interosseous membrane (the anterior interosseous artery lies directly on the membrane and the posterior interosseous artery is separated from it by the deep extensor muscles).

The muscles and other components of the extensor aspect are supplied by the profunda brachii artery in the upper arm, and by the posterior interosseous artery in the forearm. The hand is supplied by rich, and somewhat variable, anastomoses between branches of the radial and ulnar arteries, principally on its palmar aspect.

Each upper limb joint is supplied by an extensive anastomosis of arteries fed by descending vessels that arise proximal to the joint and ascending recurrent branches that arise distal to the joint.

LYMPH NODES AND DRAINAGE

Superficial tissues

Superficial lymphatic vessels begin in cutaneous plexuses. In the hand, the palmar plexus is denser than the dorsal plexus. Digital plexuses drain along the digital borders to their webs, where they join the distal palmar vessels, which pass back to the dorsal aspect of the hand. The proximal palm drains towards the carpus, medially by vessels that run along its ulnar border, and laterally to join vessels draining the thumb. Several vessels from the central palmar plexus form a trunk that winds round the second metacarpal bone to join the dorsal vessels that drain the index finger and thumb.

In the forearm and arm, superficial vessels run with the superficial veins. Collecting vessels from the hand pass into the forearm on all carpal aspects. Dorsal vessels, after running proximally in parallel, curve successively round the borders of the limb to join the ventral vessels. Anterior carpal vessels run through the forearm parallel with the median vein of the forearm to the cubital region, then follow the medial border of biceps brachii before piercing the deep fascia at the anterior axillary fold to end in the lateral axillary lymph nodes (Fig. 45.6).

Lymph vessels that lie laterally in the forearm receive vessels that curve round the lateral border from the dorsal aspect of the limb. They follow the cephalic vein to the level of the deltoid tendon, where most incline medially to reach the lateral axillary nodes; a few continue with the vein and drain into the infraclavicular nodes. Vessels lying medially in the forearm are joined by vessels that curve round the medial border of the limb. They follow the basilic vein. Proximal to the elbow some end in supratrochlear lymph nodes whose efferents, together with the medial vessels that have bypassed them, pierce the deep fascia with the basilic vein and end in the lateral axillary nodes or deep lymphatic vessels.

Collecting vessels from the deltoid region pass round the anterior and posterior axillary folds to end in the axillary nodes. The scapular skin drains either to subscapular axillary nodes or by channels that follow the transverse cervical vessels to the inferior deep cervical nodes.

INNERVATION

OVERVIEW OF THE BRACHIAL PLEXUS

The brachial plexus is formed by the union of the ventral rami of the lower four cervical nerves and the greater part of the first thoracic ventral ramus (Fig. 45.7; see Fig. 46.29).

image

Fig. 45.7 A schematic plan of the left brachial plexus.

(Adapted from Drake, Vogl and Mitchell 2005.)

The fourth ramus usually gives a branch to the fifth, and the first thoracic ramus frequently receives a branch from the second. These ventral rami are the roots of the plexus: they are almost equal in size but variable in their mode of junction. Contributions to the plexus by C4 and T2 vary. When the branch from C4 is large, that from T2 is frequently absent and the branch from T1 is reduced, forming a ‘prefixed’ type of plexus. If the branch from C4 is small or absent, the contribution from C5 is reduced but that from T1 is larger and there is always a contribution from T2: this arrangement constitutes a ‘postfixed’ type of plexus.

Close to their exit from the intervertebral foramina, the fifth and sixth cervical ventral rami receive grey rami communicantes from the middle cervical sympathetic ganglion, and the seventh and eighth rami receive grey rami from the cervicothoracic ganglion. The first thoracic ventral ramus receives a grey ramus from, and contributes a white ramus to, the cervicothoracic ganglion.

The most common arrangement of the brachial plexus is as follows: the fifth and sixth rami unite at the lateral border of scalenus medius as the upper trunk; the eighth cervical and first thoracic rami join behind scalenus anterior as the lower trunk; the seventh cervical ramus becomes the middle trunk. The three trunks incline laterally, and either just above or behind the clavicle each bifurcates into anterior and posterior divisions. The anterior divisions of the upper and middle trunks form a lateral cord that lies lateral to the axillary artery. The anterior division of the lower trunk descends at first behind and then medial to the axillary artery and forms the medial cord; it often receives a branch from the seventh cervical ramus. Posterior divisions of all three trunks form the posterior cord, which is at first above, and then behind, the axillary artery. The posterior division of the lower trunk is much smaller than the others and is frequently derived from the eighth cervical ramus before the trunk is formed; it contains few, if any, fibres from the first thoracic ramus.

Radial nerve (C5–8, T1)

The radial nerve is the continuation of the posterior cord of the brachial plexus (Fig. 45.9). In the upper arm it lies in the spiral groove of the humerus where it is accompanied by the profunda brachii artery and its venal comitantes. It enters the posterior (extensor) compartment and supplies triceps, then re-enters the anterior compartment of the arm by piercing the lateral intermuscular septum. At the level of the lateral epicondyle it gives off the posterior interosseous nerve, which passes between the two heads of supinator and enters the extensor compartment of the forearm. The posterior interosseous nerve supplies these muscles, while the radial nerve proper continues into the forearm in the anterior compartment deep to brachioradialis and terminates by supplying the skin over the posterior aspect of the thumb, index, middle and radial half of the ring finger.

Median nerve (C6–8, T1)

The median nerve is formed by the union of the terminal branch of the lateral and medial cords of the brachial plexus (Fig. 45.11). It has no branches in the upper arm. It enters the forearm between the two heads of pronator teres and gives off the anterior interosseous nerve, which supplies all the flexor muscles of the forearm apart from flexor carpi ulnaris and the ulnar half of flexor digitorum profundus. The median nerve itself passes deep to the flexor retinaculum at the wrist. On entering the palm, it gives off motor branches to the thenar muscles and the radial two lumbricals, and cutaneous branches to the palmar aspect of the thumb, index and middle fingers and the radial half of the ring finger.

Ulnar nerve (C7, C8, T1)

The ulnar nerve is the continuation of the medial cord of the brachial plexus (Fig. 45.12). Like the median nerve, it has no branches in the upper arm. It enters the posterior compartment of the upper arm midway down its length by piercing the medial intermuscular septum, passes behind the medial epicondyle of the humerus to enter the forearm and descends to the wrist deep to flexor carpi ulnaris. The ulnar nerve supplies flexor carpi ulnaris and the ulnar half of flexor digitorum profundus. Just proximal to the wrist it gives off a dorsal cutaneous branch that supplies the skin over the dorsal aspect of the little finger and the ulnar half of the ring finger, and then crosses into the palm superficial to the flexor retinaculum in Guyon’s canal. It divides into a motor branch, which supplies the hypothenar muscles, the intrinsic muscles of the hand (apart from the radial two lumbricals) and adductor pollicis, and cutaneous branches, which supply the skin of the palmar aspect of the little finger and the ulnar half of the ring finger.

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