Axillary Nerve (C5, C6)

The axillary nerve is a branch of the posterior cord of the brachial plexus. It winds posteriorly around the neck of the humerus together with the circumflex humeral vessels and supplies the deltoid and teres minor and an area of skin over the deltoid region (Fig. 18.3).

Fig. 18.3 Motor and sensory branches of the axillary nerve.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Radial Nerve (C5–8, T1)

The radial nerve is the continuation of the posterior cord of the brachial plexus (Fig. 18.4). In the upper arm it lies in the spiral groove of the humerus, where it is accompanied by the profunda brachii artery and its venae comitantes. It enters the posterior (extensor) compartment and supplies triceps, then reenters 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 the supinator and enters the extensor compartment of the forearm. The posterior interosseous nerve supplies these muscles. The radial nerve itself continues into the forearm in the anterior compartment deep to the brachioradialis. It terminates by supplying the skin over the posterior aspect of the thumb, index, middle fingers and radial half of the ring finger.

Fig. 18.4 Motor and sensory branches of the radial nerve. Variation exists in the cutaneous innervation of the dorsal aspects of the digits. Here, the radial nerve is shown supplying all five digits. The dorsum of the ring and little fingers is frequently innervated by the dorsal branch of the ulnar nerve.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Musculocutaneous Nerve (C5–7)

The musculocutaneous nerve is formed from the continuation of the lateral cord of the brachial plexus. It pierces the coracobrachialis; supplies it, biceps and brachialis; then continues into the forearm as the lateral cutaneous nerve of the forearm (Fig. 18.5).

Fig. 18.5 Motor and sensory branches of the musculocutaneous nerve.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

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. 18.6). 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 except for 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.

Fig. 18.6 Motor and sensory branches of the median nerve. Flexor digitorum profundus to the ring and little fingers is supplied by the ulnar nerve. Flexor pollicis brevis may be supplied by both the median nerve and the ulnar nerve.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Ulnar Nerve (C7, C8, T1)

The ulnar nerve is the continuation of the medial cord of the brachial plexus (Fig. 18.7). 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 and passes behind the medial epicondyle of the humerus to enter the forearm. It passes to the wrist deep to flexor carpi ulnaris, giving branches to this muscle and to 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. The ulnar nerve 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 intrinsics (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 ulnar half of the ring finger.

Fig. 18.7 Motor and sensory branches of the ulnar nerve together with the medial cutaneous nerves of the arm and forearm. Flexor digitorum profundus to index and middle fingers is supplied by the median nerve. Flexor pollicis brevis may be supplied by both the median nerve and the ulnar nerve.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Dermatomes

Our knowledge of the extent of individual dermatomes, especially in the limbs, is based largely on clinical evidence (Fig. 18.8). The dermatomes of the upper limb arise from spinal nerves C5–8 and T1. C7 supplies the central part of the hand. Considerable overlap exists between adjacent dermatomes innervated by nerves derived from consecutive spinal cord segments.

Fig. 18.8 Arrangement of dermatomes and cutaneous nerves of the left upper limb. A, Viewed from the anterior aspect. The heavy black line represents the ventral axial line; overlap across it is minimal. In marked contrast, overlap is considerable across the interrupted lines. B, Viewed from the posterior aspect. The heavy black line represents the dorsal axial line; overlap across this line is minimal. Overlap is considerable across the interrupted lines.

Myotomes

Each spinal nerve originally supplies the musculature derived from its own myotome. Where myotomal derivatives remain entities, they retain their original segmental supply. When derivatives from adjoining myotomes fuse, the resulting muscles do not always retain a nerve supply from each corresponding spinal nerve. Because muscles develop in situ, in the mesodermal cores of the developing limbs, it is impossible to identify their original segments by a developmental study. Most limb muscles are innervated by neurones from more than one segment of the spinal cord. Tables 18.1 to 18.4 summarize the predominant segmental origins of the nerve supply for each of the upper limb muscles and for movements taking place at the joints of the upper limb; damage to these segments or to their motor roots results in maximal paralysis.

Table 18.1 Movements, muscles and segmental innervation in the upper limb

Table 18.2 Segmental innervation of muscles of the upper limb

Table 18.3 Segmental innervation of joint movements of the upper limb

Reflexes

Supraclavicular Branches

Supraclavicular branches arise from roots or from trunks:

Branches to the scaleni and longus colli arise from the lower cervical ventral rami near their exit from the intervertebral foramina. The phrenic nerve is joined by a branch from the fifth cervical ramus anterior to scalenus anterior.

Long Thoracic Nerve

The long thoracic nerve is usually formed by roots from the fifth to seventh cervical rami, although the last ramus may be absent (Fig. 18.9). The upper two roots pierce scalenus medius obliquely, uniting in or lateral to it. The nerve descends dorsal to the brachial plexus and the first part of the axillary artery and crosses the superior border of serratus anterior to reach its lateral surface. It may be joined by the root from C7, which emerges between scalenus anterior and scalenus medius, and descends on the lateral surface of medius. The nerve continues downward to the lower border of serratus anterior and supplies branches to each of its digitations.

Fig. 18.9 Nerves of the left upper limb, dissected from the anterior aspect.

The long thoracic nerve is the most common nerve to be affected by neuralgic amyotrophy. Winging of the scapula may be the only clinical manifestation; it is best demonstrated by asking the patient to push against resistance with the arm extended at the elbow and flexed to 90° at the shoulder (Fig. 18.10; see Case 1).

Fig. 18.10 Winging of the right scapula. The patient is pushing against the wall at a 90° angle.

Suprascapular Nerve

The suprascapular nerve is a large branch of the superior trunk (Fig. 18.11). It runs laterally, deep to trapezius and omohyoid, and enters the supraspinous fossa through the suprascapular notch inferior to the superior transverse scapular ligament. It runs deep to supraspinatus and curves around the lateral border of the spine of the scapula with the suprascapular artery to reach the infraspinous fossa, where it gives two branches to supraspinatus and articular rami to the shoulder and acromioclavicular joints. The suprascapular nerve rarely has a cutaneous branch. When present, it pierces the deltoid close to the tip of the acromion and supplies the skin of the proximal third of the arm within the territory of the axillary nerve.

Fig. 18.11 Lower part of the posterior triangle showing the relations of the third part of the right subclavian artery. The clavicle has been removed, but its outline is indicated by a dashed line. In this dissection, the middle trunk of the brachial plexus gives an unusual contribution to the medial cord.

Lesions of the Suprascapular Nerve

The most common cause of lesions involving the suprascapular nerve is neuralgic amyotrophy, as described earlier. An entrapment neuropathy may occur in the scapular notch, or the nerve may be damaged by trauma to the scapula and shoulder. There is pain in the shoulder and wasting and weakness of supraspinatus and infraspinatus.

Infraclavicular Branches

Infraclavicular branches come from the cords, but their axons may be traced back to the spinal nerves detailed below:

Axillary Nerve

The axillary nerve arises from the posterior cord (C5, C6). It is initially lateral to the radial nerve, posterior to the axillary artery and anterior to subscapularis (Fig. 18.12). At the lower border of subscapularis it curves back inferior to the humeroscapular articular capsule and, with the posterior circumflex humeral vessels, traverses a quadrangular space bounded above by subscapularis (anterior) and teres minor (posterior), below by teres major, medially by the long head of triceps and laterally by the surgical neck of the humerus. In the space it divides into anterior and posterior branches. The anterior branch curves around the neck of the humerus with the posterior circumflex humeral vessels, deep to deltoid. It reaches the anterior border of the muscle, supplies it and gives off a few small cutaneous branches that pierce deltoid and ramify in the skin over its lower part. The posterior branch courses medially and posteriorly along the attachment of the lateral head of triceps, inferior to the glenoid rim. It usually lies medial to the anterior branch in the quadrangular space. It gives off the nerve to teres minor and the upper lateral cutaneous nerve of the arm at the lateral edge of the origin of the long head of triceps. The nerve to teres minor enters the muscle on its inferior surface. The posterior branch frequently supplies the posterior aspect of deltoid, usually via a separate branch from the main stem, or occasionally from the superior lateral cutaneous nerve of the arm. However, the posterior part of deltoid has a more consistent supply from the anterior branch of the axillary nerve, which should be remembered when performing a posterior deltoid-splitting approach to the shoulder. The upper lateral cutaneous nerve of the arm pierces the deep fascia at the medial border of the posterior aspect of deltoid and supplies the skin over the lower part of deltoid and upper part of the long head of triceps. The posterior branch is intimately related to the inferior aspects of the glenoid and shoulder joint capsule, which may place it at particular risk during capsular plication or thermal shrinkage procedures (Ball et al 2003). There is often an enlargement or pseudoganglion on the branch to teres minor. The axillary trunk supplies a branch to the shoulder joint below subscapularis.

Fig. 18.12 Dorsal scapular muscles and triceps on the left side. The spine of the scapula has been divided near its lateral end, and the acromion has been removed along with a large part of deltoid. The humerus is laterally rotated, and the forearm is pronated.

Lesions of the Axillary Nerve

The most common causes of axillary nerve lesions are trauma (dislocation of the shoulder, fracture of the surgical neck of the humerus) and neuralgic amyotrophy. There is deltoid wasting and weakness, which is usually clinically evident, and a patch of sensory loss on the outer aspect of the arm. This can be differentiated from a C5 root lesion by the finding of normal function in the distribution of the suprascapular nerve.

Brachial Plexus Lesions

Lesions of the brachial plexus commonly affect either the upper part of the plexus (i.e. C5 and C6 roots and the upper trunk) or the lower part of the plexus (i.e. C8 and T1 roots and the lower trunk). Lesions affecting the upper part are usually traumatic, whereas those affecting the lower part may be caused by trauma, malignant infiltration or thoracic outlet syndrome. Severe trauma may affect the whole plexus.

Lower Plexus Palsies

Upward traction on the arm, such as in a forcible breech delivery, may tear the lowest root, T1, which provides the segmental supply to the intrinsic muscles of the hand. The hand assumes a clawed appearance, reflecting the unopposed action of the long flexors and extensors of the fingers (Klumpke’s paralysis). There is sensory loss along the medial aspect of the forearm and often an associated Horner’s syndrome (ptosis and constriction of the pupil), which occurs as a result of traction on the cervical sympathetic chain.

Malignant infiltration of the brachial plexus may result from extension of an apical lung carcinoma (Pancoast tumour) or from metastatic spread, often from carcinoma of the breast. There is slowly progressive weakness that usually starts in the small muscles of the hand (T1) and spreads to involve the finger flexors (C8). This is usually a painful condition, and the pain may be severe. There is sensory loss on the medial aspect of the forearm (T1), extending into the medial side of the hand and to the little finger (C8). Horner’s syndrome may occur if there is involvement of the cervical sympathetic ganglia. A similar syndrome may occur following radiotherapy for breast carcinoma, but this is usually painless. Thoracic surgery involving a sternal split may cause traction on the brachial plexus and usually affects the lower part of the plexus.

The lower trunk of the brachial plexus (C8, T1), together with the subclavian artery, may be angulated over a cervical rib (thoracic outlet syndrome). Patients may present with vascular symptoms as a result of kinking of the subclavian artery (this is more likely to occur with large bony ribs), or they may present with neurological deficits (this is more likely in patients with small rudimentary ribs that extend into a fibrous band that joins the first rib anteriorly). Cervical ribs are quite common and are rarely associated with symptoms. There is a slow, insidious onset of wasting of the small muscles of the hand, which often starts on the lateral side with involvement of the thenar eminence and first dorsal interosseous. There is pain and paraesthesia in the medial aspect of the forearm, extending to the little finger; this is often aggravated by carrying shopping bags or suitcases. A bruit may be heard over the subclavian artery, and the radial pulse may be easily obliterated by movements of the arm, particularly with the arm extended and abducted at the shoulder.

CASE 2 Pancoast Tumour

A 59-year-old man, a heavy smoker for many years, develops posterior left shoulder pain radiating down the medial aspect of the left arm into the fourth and fifth digits of the hand, with weakness that ultimately involves the entire left hand. He has lost 35 pounds in the past 3 months. Examination demonstrates wasting and weakness of all intrinsic hand muscles on the left, as well as weakness of wrist flexion. There is decreased sensation in the left medial upper arm, forearm and hand, involving especially the fifth digit (Fig. 18.13). He has a mild left Horner’s syndrome, with noticeable ptosis and miosis.

Fig. 18.13 Brachial plexopathy (brachial neuritis). Atrophy of the intrinsic hand muscles due to denervation.

Discussion: Progressive lesions of the lower trunk of the brachial plexus associated with pain in the involved hand and accompanied by a history of weight loss and smoking are most suggestive of a Pancoast tumour, a tumour of the apex of the lung (Fig. 18.14). An enlarging tumour in the apex may erode bone locally and compress the lower trunk of the brachial plexus. Because the C8 and T1 nerve roots form the lower trunk of the brachial plexus, all median- and ulnar-innervated muscles are affected, as is the pectoralis muscle to some extent. Sensory loss appears in the distribution of C8 and T1 dermatomes. Extension of the mass superiorly into the stellate ganglion is responsible for an associated Horner’s syndrome.

Fig. 18.14 Pancoast tumour. Carcinoma of the lung at the superior apex (arrow) extending into the overlying brachial plexus.

Median Nerve

The median nerve enters the arm lateral to the brachial artery (see Fig. 18.9; Figs 18.15, 18.16). Near the insertion of coracobrachialis it crosses in front of (rarely behind) the artery, descending medial to it to the cubital fossa, where it is posterior to the bicipital aponeurosis and anterior to brachialis, separated by the latter from the elbow joint.

Fig. 18.15 Muscles, vessels and nerves of the left upper arm viewed from the medial aspect.

Fig. 18.16 Anterior aspect of the left elbow showing deep structures.

It gives off vascular branches to the brachial artery and usually a branch to pronator teres, a variable distance proximal to the elbow joint.

Musculocutaneous Nerve

The musculocutaneous nerve is the nerve of the anterior compartment of the arm (see Fig. 18.9). It gives a branch to the shoulder joint and then passes through coracobrachialis, which it supplies, emerging to pass between biceps and brachialis. It sends branches to both these muscles. In the cubital fossa it lies at the lateral margin of the biceps tendon, where it continues as the lateral cutaneous nerve of the forearm.

The musculocutaneous nerve has frequent variations. It may run behind coracobrachialis or adhere for some distance to the median nerve and pass behind biceps. Some fibres of the median nerve may run in the musculocutaneous nerve, leaving it to join their proper trunk; less frequently, the reverse occurs, and the median nerve sends a branch to the musculocutaneous. Occasionally it supplies pronator teres and may replace radial branches to the dorsal surface of the thumb.

Ulnar Nerve

The ulnar nerve has no branches in the arm (see Figs 18.9, 18.15). It runs distally through the axilla medial to the axillary artery and between it and the vein, continuing distally medial to the brachial artery as far as the midarm. There it pierces the medial intermuscular septum, inclining medially as it descends anterior to the medial head of triceps to the interval between the medial epicondyle and the olecranon, along with the superior ulnar collateral artery. At the elbow, the ulnar nerve is in a groove on the dorsum of the epicondyle. It enters the forearm between the two heads of flexor carpi ulnaris superficial to the posterior and oblique parts of the ulnar collateral ligament (Figs 18.16, 18.17, 18.18).

Fig. 18.17 Posterior aspect of the left elbow showing superficial structures.

Fig. 18.18 Posterior aspect of the left elbow region showing deep structures.

Radial Nerve

The branches of the radial nerve in the upper arm are as follows: muscular, cutaneous, articular and superficial terminal and posterior interosseous.

The radial nerve descends behind the third part of the axillary artery and the upper part of the brachial artery, anterior to subscapularis and the tendons of latissimus dorsi and teres major (see Fig. 18.9; Fig. 18.19). With the profunda brachii artery it inclines dorsally, passing through the triangular space below the lower border of teres major, between the long head of triceps and the humerus. There it supplies the long head of triceps and gives rise to the posterior cutaneous nerve of the arm, which supplies the skin along the posterior surface of the upper arm. It then spirals obliquely across the back of the humerus, lying posterior to the uppermost fibres of the medial head of triceps, which separate the nerve from the bone in the first part of the spiral groove. There it gives off a muscular branch to the lateral head of triceps and a branch that passes through the medial head of triceps to anconeus. On reaching the lateral side of the humerus it pierces the lateral intermuscular septum to enter the anterior compartment; it then descends deep in a furrow between brachialis and, proximally, brachioradialis, then, more distally, extensor carpi radialis longus. The radial nerve divides into the superficial terminal branch and the posterior interosseous nerve just anterior to the lateral epicondyle (see Fig. 18.16).

Fig. 18.19 Muscles, vessels and nerves of the left upper arm viewed from the posterior aspect.

Medial Cutaneous Nerve of the Arm

The medial cutaneous nerve of the arm supplies the skin of the medial aspect of the arm (see Figs 18.8, 18.9). It is the smallest branch of the brachial plexus, arises from the medial cord and contains fibres from the eighth cervical and first thoracic ventral rami. It traverses the axilla, crossing anterior or posterior to the axillary vein, to which it is then medial, and communicates with the intercostobrachial nerve; it descends medial to the brachial artery and basilic vein (see Fig. 18.9) to a point midway in the upper arm, where it pierces the deep fascia to supply a medial area in the distal third of the arm, extending to its anterior and posterior aspects. Rami reach the skin anterior to the medial epicondyle and over the olecranon. It connects with the posterior branch of the medial cutaneous nerve of the forearm. Sometimes the medial cutaneous nerve of the arm and the intercostobrachial nerve are connected in a plexiform manner in the axilla. The intercostobrachial nerve may be large and reinforced by part of the lateral cutaneous branch of the third intercostal nerve. It then replaces the medial cutaneous nerve of the arm and receives a connection representing the latter from the brachial plexus (occasionally, this connection is absent).

Medial Cutaneous Nerve of the Forearm

The medial cutaneous nerve of forearm comes from the medial cord (see Fig. 18.9; Fig. 18.20). It is derived from the eighth cervical and first thoracic ventral rami. At first it is between the axillary artery and vein and gives off a ramus that pierces the deep fascia to supply the skin over biceps, almost to the elbow. The nerve descends medial to the brachial artery, pierces the deep fascia with the basilic vein midway in the arm and divides into anterior and posterior branches. The larger, anterior branch usually passes in front of, or occasionally behind, the median cubital vein, descending anteromedially in the forearm to supply the skin as far as the wrist and connecting with the palmar cutaneous branch of the ulnar nerve. The posterior branch descends obliquely medial to the basilic vein, anterior to the medial epicondyle, and curves around to the back of the forearm, descending on its medial border to the wrist, supplying the skin. It connects with the medial cutaneous nerve of the arm, posterior cutaneous nerve of the forearm and dorsal branch of the ulnar.

Fig. 18.20 Anterior aspect of the left elbow, showing superficial structures.

Median Nerve

The median nerve usually enters the forearm between the heads of pronator teres (Figs 18.21–18.25). (Occasionally, the nerve passes posterior to both heads of pronator teres, or it may pass through the humeral head.) It crosses to the lateral side of the ulnar artery, from which it is separated by the deep head of pronator teres. It passes behind a tendinous bridge between the humero-ulnar and radial heads of flexor digitorum superficialis, and descends through the forearm posterior and adherent to flexor digitorum superficialis and anterior to flexor digitorum profundus. About 5 cm proximal to the flexor retinaculum it emerges from behind the lateral edge of flexor digitorum superficialis and becomes superficial just proximal to the wrist. There it lies between the tendons of flexor digitorum superficialis and flexor carpi radialis, projecting laterally from beneath the tendon of palmaris longus (see Fig. 18.23). It then passes deep to the flexor retinaculum into the palm. In the forearm the median nerve is accompanied by the median branch of the anterior interosseous artery.

Fig. 18.21 Superficial flexor muscles of the left forearm.

Fig. 18.22 Deep flexor muscles of the left forearm.

Fig. 18.23 Transverse section through the left forearm, passing through the distal end of the radius and the styloid process of the ulna, with the hand and forearm in full supination (distal [inferior] aspect).

Fig. 18.24 Transverse section through the left forearm at the level of the radial tuberosity (proximal aspect).

Fig. 18.25 Transverse section through the middle of the left forearm (distal aspect).

The course and distribution of the median nerve in the wrist and hand are described below.

Ulnar Nerve

The ulnar nerve descends on the medial side of the forearm, lying on flexor digitorum profundus (see Figs 18.22–18.25). Proximally, it is covered by flexor carpi ulnaris; its distal half lies lateral to the muscle and is covered only by skin and fasciae. In the upper third of the forearm, the nerve is distant from the ulnar artery, but more distally, it comes to lie close to the medial side of the artery. About 5 cm proximal to the wrist it gives off a dorsal branch that continues distally into the hand, anterior to the flexor retinaculum on the lateral side of the pisiform and posteromedial to the ulnar artery. It passes deep to the superficial part of the retinaculum (in Guyon’s canal) with the artery and divides into superficial and deep terminal branches.

The course and distribution of the ulnar nerve in the hand are described below.

Radial Nerve

There is some variation in the level at which branches of the radial nerve arise from the main trunk in different subjects (Fig. 18.26; see also Figs 18.23, 18.25). Branches to extensor carpi radialis brevis and supinator may arise from the main trunk of the radial nerve or from the proximal part of the posterior interosseous nerve, but almost invariably above the arcade of Frohse.

Fig. 18.26 Left supinator muscle (posterolateral aspect).

Medial Cutaneous Nerve of the Forearm

The medial cutaneous nerve of the forearm has already divided into anterior and posterior branches before it enters the forearm (see Fig. 18.20). The larger anterior branch usually passes in front of, or occasionally behind, the median cubital vein and descends anteromedially in the forearm to supply the skin as far as the wrist. It curves around to the back of the forearm, descending on its medial border to the wrist, supplying the skin. It connects with the medial cutaneous nerve of the arm, posterior cutaneous nerve of the forearm and dorsal branch of the ulnar nerve.

Lateral Cutaneous Nerve of the Forearm

The lateral cutaneous nerve of the forearm is a direct continuation of the musculocutaneous nerve as it lies lateral to the biceps tendon in the antecubital fossa (see Figs 18.5, 18.8). It passes deep to the cephalic vein, descending along the radial border of the forearm to the wrist. It supplies the skin of the anterolateral surface of the forearm and connects with the posterior cutaneous nerve of the forearm and the terminal branch of the radial nerve by branches that pass around its radial border. Its trunk gives rise to a slender recurrent branch that extends along the cephalic vein as far as the middle third of the upper arm, distributing filaments to the skin over the distal third of the anterolateral surface of the upper arm close to the vein. At the wrist joint the lateral cutaneous nerve of the forearm is anterior to the radial artery. Some filaments pierce the deep fascia and accompany the artery to the dorsum of the carpus. The nerve then passes to the base of the thenar eminence, where it ends in cutaneous rami. It has branches that connect with the terminal branch of the radial nerve and the palmar cutaneous branch of the median nerve.

Posterior Cutaneous Nerve of the Forearm

The posterior cutaneous nerve of the forearm passes along the dorsum of the forearm to the wrist. It supplies the skin along its course and near its end joins the dorsal branches of the lateral cutaneous nerve of the forearm.

Median Nerve

The median nerve proximal to the flexor retinaculum is lateral to the tendons of flexor digitorum superficialis and lies between the tendons of flexor carpi radialis and palmaris longus. It passes under the retinaculum in the ‘carpal tunnel’ (see below), where its compression may lead to carpal tunnel syndrome. Distal to the retinaculum the nerve enlarges and flattens and usually divides into five or six branches; the mode and level of division are variable.

Palmar Digital Branches (Figs 18.27, 18.28)

The median nerve usually divides into four or five digital branches. It often divides first into a lateral ramus, which provides digital branches to the thumb and radial side of the index finger, and a medial ramus, which supplies digital branches to adjacent sides of the index, middle and ring fingers. Other modes of termination can occur.

Fig. 18.27 Cutaneous nerves of the hand. A, Palmar aspect. B, Dorsal aspect. The anular and cruciate pulleys are shown schematically in the ring finger.

Fig. 18.28 Palmar aponeurosis and distal fascial complex. A, Schematic diagram of the palmar fascia. B, More detailed view of structures at the web space. C, Fate of the distal longitudinal fibres. D and E, Normal digital fascia.

Digital branches are commonly arranged as follows. They pass distally, deep to the superficial palmar arch and its digital vessels, at first anterior to the long flexor tendons. Two proper palmar digital nerves, sometimes from a common stem, pass to the sides of the thumb; the nerve supplying its radial side crosses in front of the flexor pollicis longus tendon. The proper palmar digital nerve to the lateral side of the index also supplies the first lumbrical. Two common palmar digital nerves pass distally between the long flexor tendons. The lateral one divides in the distal palm into two proper palmar digital nerves that traverse adjacent sides of the index and middle fingers. The medial one divides into two proper palmar digital nerves that supply adjacent sides of the middle and ring fingers. The lateral common digital nerve supplies the second lumbrical, and the medial receives a communicating twig from the common palmar digital branch of the ulnar nerve and may supply the third lumbrical. In the distal part of the palm the digital arteries pass deeply between the divisions of the digital nerves; the nerves lie anterior to the arteries on the sides of the digits. The median nerve usually supplies palmar cutaneous digital branches to the radial three and a half digits (thumb, index, middle and lateral sides of the ring finger); sometimes the radial side of the ring finger is supplied by the ulnar nerve. Occasionally, there is a communicating branch between the common digital nerve to the middle and ring fingers (derived from the median nerve) and the common digital nerve to the ring and little fingers (derived from the ulnar nerve). This can explain variations in sensory patterns that do not conform to the classic pattern.

The proper palmar digital nerves pass along the medial side of the index finger, both sides of the middle finger and the lateral side of the ring finger. They enter these digits in fat between slips of the palmar aponeurosis. Together with the lumbricals and palmar digital arteries, they pass dorsal to the superficial transverse metacarpal ligament and ventral to the deep transverse metacarpal ligament. In the digits, the nerves run distally beside the long flexor tendons (outside their fibrous sheaths), level with the anterior phalangeal surfaces and anterior to the digital arteries, between Grayson’s and Cleland’s ligaments . Each nerve gives off several branches to the skin on the front and sides of the digit, where many end in Pacinian corpuscles. It also sends branches to the metacarpophalangeal and interphalangeal joints.

The digital nerves supply the fibrous sheaths of the long flexor tendons, digital arteries (vasomotor) and sweat glands (secretomotor). Distal to the base of the distal phalanx, each digital nerve gives off a branch that passes dorsally to the nail bed. The main nerve frequently trifurcates to supply the pulp and skin of the terminal part of the digit. Distal to the base of the proximal phalanx, each proper digital nerve also gives off a dorsal branch to supply the skin over the back of the middle and distal phalanges. The proper palmar digital nerves to the thumb and lateral side of the index finger emerge with the long flexor tendons from under the lateral edge of the palmar aponeurosis. They are arranged in the digits as described earlier, but in the thumb, small distal branches supply the skin on the back of the distal phalanx only.

Other Branches

In addition to the branches of the median nerve already described, variable vasomotor branches supply the radial and ulnar arteries and their branches. Some of the intercarpal, carpometacarpal and intermetacarpal joints are thought to be supplied by the median nerve or its anterior interosseous branch; the precise details are uncertain.

CASE 6 Carpal Tunnel Syndrome

A 41-year-old right-handed man runs heavy machinery at work. In the last year he has developed progressive numbness and tingling in the thumb, index and middle fingers, plus half of the ring finger of the right hand. It is most prominent on arising in the morning and sometimes wakes him from sleep. He has mild difficulty twisting the tops off bottles. In the past 2 months, he has developed similar symptoms in the left hand.

On examination, there is decreased sensation over the thumb, index and middle fingers and half of the ring finger, with sparing of the remainder of the hand, including the thenar eminence. There is mild weakness of abductor pollicis brevis, but strength is otherwise normal (Fig. 18.29). Tapping distal to the proximal (dominant) wrist crease between the tendons of palmaris longus and flexor carpi radialis elicits an electric shock–like sensation in the hand (Tinel’s sign). Reflexes are normal.

Fig. 18.29 Carpal tunnel syndrome. There is marked wasting of the thenar eminence bilaterally (arrow) in an advanced case of carpal tunnel syndrome.

Discussion: Carpal tunnel syndrome, or median neuropathy at the wrist, reflects the close anatomical relationship between the flexor retinaculum and the deep branch of the median nerve. With wrist flexion and extension, the median nerve must slide up and down a fibro-osseous tunnel beneath the fibrous flexor retinaculum. The nerve can be compressed between these structures, especially if there are bony changes from arthritis or soft tissue thickening due to repetitive injury, with or without inflammation, at that site. The disorder appears in patients with diabetes, as a form of mononeuropathy, in myxoedema associated with acromegaly, in association with generalized oedema from obesity or pregnancy or in patients with arthritis of the wrist. Symptoms usually predominate in the dominant hand, most likely owing to greater use and potential repeat compression. Numbness typically appears, as in this man. The sensory innervation of the ring finger is variable and is often spared by virtue of input from the ulnar nerve. Because the palmar cutaneous branch of the median nerve branches off before the median nerve dips below the flexor retinaculum and into the carpal tunnel, sensation over the thenar eminence is spared. Abductor pollicis brevis is innervated by the median nerve, so weakness of this muscle may be found. This indicates a distal median nerve injury; median-innervated muscles proximal to the flexor retinaculum are unaffected in carpal tunnel syndrome.

Ulnar Nerve

At the wrist, the ulnar nerve passes under the superficial part of the retinaculum (in Guyon’s canal) with the ulnar artery and divides into superficial and deep terminal branches.

Deep Terminal Branch

The deep terminal branch accompanies the deep branch of the ulnar artery as it passes between abductor digiti minimi and flexor digiti minimi and then perforates the opponens digiti minimi to follow the deep palmar arch dorsal to the flexor tendons (Fig. 18.30). At its origin, it supplies the three short muscles of the little finger. As it crosses the hand, it supplies the interossei and the third and fourth lumbricals. It ends by supplying adductor pollicis, first palmar interosseous and usually flexor pollicis brevis. It sends articular filaments to the wrist joint.

Fig. 18.30 Deep structures in the left palm and wrist.

The medial part of flexor digitorum profundus is supplied by the ulnar nerve, as are the third and fourth lumbricals, which are connected with the tendons of this part of the muscle. Similarly, the lateral part of flexor digitorum profundus and the first and second lumbricals are supplied by the median nerve. The third lumbrical is often supplied by both nerves. The deep terminal branch is thought to give branches to some intercarpal, carpometacarpal and intermetacarpal joints; precise details are uncertain. Vasomotor branches, arising in the forearm and hand, supply the ulnar and palmar arteries.

Radial Nerve

Branches of the superficial branch of the radial nerve reach the hand by curving around the wrist deep to the brachioradialis tendon. They divide into dorsal digital nerves. On the dorsum of the hand they usually communicate with the posterior and lateral cutaneous nerves of the forearm.

Closing the Hand

It is clear that the fingers and palm of the hand flex in gripping, grasping or making a fist, but there are subtle differences in hand posture in these various activities. The basic mechanisms of hand closure are described before special grips are considered.

As the digits flex, the wrist usually extends (dorsiflexes) at the same time. The involvement of the long digital flexors in this movement is considered first, followed by an analysis of the role of the wrist.

Making a Tight Fist

It is possible to observe and palpate the muscle groups that are active in making a tight fist. The flexor compartment of the forearm is contracted tightly, and electromyographic evidence confirms that flexor digitorum profundus and flexor digitorum superficialis are active. Flexor carpi ulnaris may be seen and felt to contract strongly. The extensor compartment is tightly contracted, and the wrist extensors would certainly be expected to be active. Palpation of the long digital extensors on the back of the wrist shows that these are contracting as well. It seems that when the fingers are held tightly closed, the long digital extensors are unable to move the extensor apparatus; they have acquired a new fixed point on which to act—namely, the proximal limit of the extensor apparatus over the metacarpophalangeal joint. They therefore perform the only task available to them and act together as an additional wrist extensor.

In the thumb web, palpation confirms that the first dorsal interosseous is contracting, as are all the other interossei and the thenar and hypothenar muscles. As the firm fist is swung forward in anger, the brachioradialis stands out, and at the moment of impact, virtually every muscle in the limb is in a state of contraction, with the exception of the lumbricals.

Opening the Hand

The hand is opened from its relaxed balanced posture, such as when stretching out to reach an object. This motion is made up of extension of the distal interphalangeal, proximal interphalangeal and metacarpophalangeal joints. The hand is provided with an ingenious mechanism that allows this to happen. The laws of mechanics suggests that one motor would be required for every joint in a chain, together with some sort of controlling mechanism to ensure that the chain of joints moves together in a coordinated fashion. In the hand, this is achieved through an extensor apparatus that minimizes the number of motors required for movement by allowing the muscles to act on more than one joint and by linking different levels in the mechanism so that the arc of motion is controlled.

The tendons of extensor digitorum run distally over the metacarpal heads, forming the major component of the extensor apparatus. Extensor digitorum has no insertion into the proximal phalanx and therefore exerts its extensor action on the metacarpophalangeal joint indirectly through more distal insertions. The first point of insertion is at the base of the middle phalanx (in clinical practice, the term ‘central slip’ has been adopted). Acting at this insertion alone, extensor digitorum can extend both metacarpophalangeal and proximal interphalangeal joints together. The interossei are also active in hand opening because they tend to increase extension of the proximal interphalangeal joint. There is therefore a range of possibilities. At one extreme, with no interosseous contribution, the long extensor exerts all its action at the metacarpophalangeal joint; this leads to full extension or even hyperextension, while the proximal interphalangeal joint remains flexed (the typical claw hand of ulnar nerve paralysis, or ‘intrinsic-minus’ hand). At the other extreme, when the intrinsics act strongly together with extensor digitorum, the proximal interphalangeal joint extends completely while the metacarpophalangeal joint remains flexed (‘intrinsic-plus’ hand). Thus, in the proximal part of the extensor apparatus, the hand possesses a variable mechanism that allows different amounts of relative metacarpophalangeal or proximal interphalangeal joint motion.

In contrast, the more distal part of the extensor apparatus acts as an automatic or fixed mechanism, whereby the two interphalangeal joints, proximal and distal, move together. The lateral slips of the extensor apparatus arise from extensor digitorum and pass distally on either side of the central slip and thus over the proximal interphalangeal joint. Being farther lateral, they are nearer the joint axis, because the dorsal surface curves away on each side. A helpful analogy is to consider this arrangement as consisting of two pulleys of different sizes on one axle. The central slip can be regarded as a cord that passes over the larger wheel, and each lateral slip as a cord that passes over the smaller wheel. Because these latter pulleys are smaller, there is less longitudinal excursion for a given rotation of the wheel, and this allows some of the excursion to be used for another function—namely, extension at the distal joint. There is an additional mechanism by which the lateral slips move laterally during flexion of the proximal interphalangeal joint. The effect of this lateral movement is to further reduce the distance between the lateral slips and the joint axis, thereby reducing the amount of excursion at the proximal interphalangeal joint even more and allowing more excursion at the distal joint. When the hand flexes, this mechanical linkage system allows both interphalangeal joints to flex together in a coordinated way.

The extensor expansion also receives contributions from the interossei and lumbricals, which approach the digits from the webs and join the corresponding expansion in the proximal segment of the digit. These small muscles can therefore act on the extensor apparatus at two levels: they can extend the proximal interphalangeal joint through fibres that radiate toward the central slip, and they can act on the distal interphalangeal joint through fibres that join the lateral slip.

Apart from the components of the extensor expansion concerned with joint function, the whole structure requires additional anchorage. This must be arranged in such a way that it is not displaced from the underlying skeleton, yet it must not restrict longitudinal movement. These difficult requirements are met by transverse retinacular ligaments at the level of the joints, the transverse ligaments running to relatively fixed attachment points in the region of the joint axis. As the expansion glides backward and forward, the transverse fibres move like bucket handles. Smooth gliding layers are required under the expansion and retinacular ligaments to allow motion to occur without friction.

One final component of the extensor apparatus provides an additional automatic function. This is a fibrous anchorage system, Landsmeer’s oblique retinacular ligament, which anchors the distal expansion to the middle phalanx. The role of the oblique retinacular ligament is controversial (reviewed by Bendz 1985). Some argue that it may act in a dynamic tenodesis effect to synchronize the movements of the interphalangeal joints; that is, it may initiate extension of the distal interphalangeal joint as the proximal interphalangeal joint is extended from a fully flexed position, and it may relax with proximal interphalangeal joint flexion to allow full distal interphalangeal joint flexion. Others argue that it becomes taut only when the proximal interphalangeal joint is fully extended and the distal interphalangeal joint is flexed, so that it functions as a restraining force to stabilize the fingertip when it is flexed against resistance (e.g. in the hook grip). Another possibility is that the ligament is merely a secondary lateral stabilizer of the proximal interphalangeal joint and that it acts to centralize the extensor components over the dorsum of the middle phalanx.

Movements of the Thumb

An opposable thumb requires a different system of control from the other digits. Because the metacarpal is much more mobile than in the digits, muscles are needed to control the extra freedom of movement.

The thumb does not easily assume the classic anatomical position. Therefore, the normal descriptive terms—anterior, posterior, medial and lateral—do not readily apply. The terms palmar, dorsal, ulnar and radial have been adopted in clinical practice.

The basic active movements are flexion–extension, abduction–adduction, rotation and circumduction. In the resting position of the first metacarpal, flexion and extension are parallel with the palmar plane, and abduction and adduction occur at right angles to this.

Flexion and extension should be confined to motion at the interphalangeal or metacarpophalangeal joints (Fig. 18.31A–C). Palmar abduction (Fig. 18.31D, E), in which the first metacarpal moves away from the second at right angles to the plane of the palm, and radial abduction (Fig. 18.31D, F), in which the first metacarpal moves away from the second with the thumb in the plane of the palm, occur at the carpometacarpal joint. The opposite of radial abduction is ulnar adduction, or transpalmar adduction, in which the thumb crosses the palm toward its ulnar border. In clinical practice, the term adduction is generally used without qualification. Circumduction describes the angular motion of the first metacarpal, solely at the carpometacarpal joint, from a position of maximal radial abduction in the plane of the palm toward the ulnar border of the hand, maintaining the widest possible angle between the first and second metacarpals (Fig. 18.31G). Lateral inclinations of the first phalanx maximize the extent of excursion of the circumduction arc. Opposition is a composite position of the thumb achieved by circumduction of the first metacarpal, internal rotation of the thumb ray and maximal extension of the metacarpophalangeal and interphalangeal joints (Fig. 18.31H). Retroposition is the opposite of opposition (Fig. 18.31I). Flexion adduction is the position of maximal transpalmar adduction of the first metacarpal: the metacarpophalangeal and interphalangeal joints are flexed, and the thumb is in contact with the palm (Fig. 18.31J).

Fig. 18.31 A–J, Movements of the thumb.

Rotary movements occur during circumduction. The simple angular movements described earlier combine with rotation about the long axis of the metacarpal shaft. In opposition, the shaft must rotate medially into pronation. In retroposition, the thumb must rotate laterally into supination. Axial rotation of the thumb metacarpal is produced by muscle activity (which moves the thumb through its arc of circumduction), the geometry of the articular surfaces of the trapeziometacarpal joint and tensile forces in the ligaments (which combine with forces exerted by the muscles of opposition and retroposition to produce axial rotation). The stability of the first metacarpal is greatest after complete pronation in the position of full opposition, when ligament tension, muscle contraction and joint congruence combine to maximal effect.

Position of Rest

The hand has a well-recognized position of rest, with the wrist in extension and the digits in some degree of flexion. The precise position of the thumb in the position of rest is variable. Typically, it is considered to be the midpoint between maximal palmar abduction and maximal retroposition. In this position, the carpometacarpal joint lies within 20° of radial abduction and 30° of palmar abduction. Based on clinical observations, it seems that the metacarpophalangeal joint lies within approximately 40° of flexion and the interphalangeal joint between extension and 10° of flexion.

From the position of rest, the tip of the thumb can approach the radial aspect of the fingers without incurring axial rotation because the palmar and dorsal trapeziometacarpal ligaments remain relaxed (see later).

Grips

From different positions of the arc of circumduction, numerous types of pinch grip are possible (Fig. 18.32). In clinical practice, these have been classified into two main types: tip pinch and lateral (or key) pinch. Many forces contribute to these configurations.

Fig. 18.32 Some of the many functional postures that can be adopted by the human hand. A, In the power grip, the fingers are flexed around an object, with counterpressure from the thumb. Any skill in wielding the object derives from the limb, including the wrist; relative movements of the thumb and fingers are not involved. B, The precision grip, which varies considerably with the task, stabilizes the object between the tips of one or more fingers and the thumb. The gross position of the object can be adjusted by movements at the wrist, elbow or even shoulder, but the most skilled manipulations are carried out by the digits themselves, such as when advancing a thread through the eye of a needle. C, The hook grip is used to suspend or to pull open objects. The fingers are flexed around the object; the thumb may or may not be involved. It is a grip for the transmission of forces, not for skillful manipulation. D, Powerful opposition of the thumb to the radial side of the index finger produces a lateral pinch grip, such as to hold a door key; here the object is larger than a key, and all the fingers are involved. E, Many activities involve a combination of grips. Here a fountain pen is stabilized in a power grip by flexion of digits 4 and 5 against the palm, while the index finger and thumb, used in a precision grip, unscrew the cap. F, Complex manipulation.

The thumb is a triarticular system, unlike the finger, which is a biarticular system. The thumb is activated by monoarticular muscles (abductor pollicis longus and opponens pollicis), biarticular muscles (extensor pollicis brevis, adductor pollicis, abductor pollicis brevis and flexor pollicis brevis), and triarticular muscles (extensor pollicis longus and flexor pollicis longus). It appears, however, that even a monoarticular muscle can change posture in all three joints by altering the overall balance of forces, making it very difficult to attribute function to the individual intrinsic muscles. However, the thumb muscles seem to provide two broad functions: they control metacarpal positioning (the guy-rope function), an activity that is automatically accompanied by rotation, and they control the axial stability of the skeleton of the thumb.

The thumb muscles can be classified into those used for retroposition, opposition and pinch grip.