FRONT OF FOREARM AND HAND

1. Make the following incisions on the anterior aspect of the forearm and hand:

(a)a median incision from the middle of the forearm to the root of the middle finger;

(b) a transverse incision across the wrist;

(c) a curved incision across the roots of all five digits; and

(d) a longitudinal incision along the middle of each digit down to its distal end. Reflect the skin flaps.

2. Clean the portions of the cephalic and basilic veins in the front of the forearm.

3. Clean the deep fascia of the forearm and define the flexor retinaculum at the wrist; this is a thick, quadrangular band of deep fascia bridging the carpal tunnel. This will be examined later.

4. Expose the superficial group of muscles of the forearm by removing the deep fascia which not only covers them but also gives them partial origin. From lateral to medial, these muscles are the superficial head of the pronator teres, flexor carpi radialis, palmaris longus and the flexor carpi ulnaris. All of these arise from the medial epicondyle of the humerus, the common flexor origin, except the superficial part of the pronator teres which has its origin from the medial supracondylar ridge.

5. Trace the flexor carpi ulnaris tendon from the common flexor origin and from the upper part of the posterior border of the ulna to the pisiform bone. The slender palmaris longus passes distally to insert into the flexor retinaculum and palmar aponeurosis. This muscle may be absent. The flexor carpi radialis runs towards the flexor retinaculum where it passes through the retinaculum in a separate compartment to insert into the bases of the second and third metacarpal bones (see later). The pronator teres is a short muscle which runs laterally and downwards to be inserted into the middle of the lateral side of the shaft of the radius.

6. Cut the superficial group of muscles about the middle of their muscle bellies and reflect them. The nerve supply to the pronator teres, flexor carpi radialis and palmaris longus comes from the median nerve. The flexor carpi ulnaris is supplied by the ulnar nerve. Verify that the ulnar nerve runs behind the medial epicondyle and enters the forearm between the two heads of the flexor carpi ulnaris.

7. Identify the flexor digitorum superficialis and the deep head of the pronator teres which lie deep to the superficial group of muscles. Preserve the median nerve as it passes down between the two heads of the pronator teres and deep to the flexor digitorum superficialis. The latter muscle has a broad origin from the medial epicondyle of the humerus, coronoid process of the ulna and anterior border of the radius. Note the fibrous arcade which overlies the median nerve and ulnar artery as they pass deep to this muscle. Observe the tendons of the superficialis as they lie near the wrist and note that they pass to the index, middle, ring and little fingers.

8. Identify the median nerve just proximal to the flexor retinaculum, emerging from beneath the flexor digitorum superficialis and lying between the tendons of palmaris longus and flexor carpi radialis.

9. Cut the flexor digitorum superficialis muscle in its middle and reflect the two parts. Note the innervation from the median nerve. Observe that the neurovascular structures comprising the median nerve, ulnar artery and ulnar nerve lie on the deep group of muscles.

10. Trace the ulnar artery, which runs downwards from the cubital fossa, deep to the deep head of the pronator teres, towards the medial side of the wrist where it lies superficial to the flexor retinaculum. The artery gives off numerous branches, the most important being the common interosseous artery, which arises at the level of the radial tuberosity high up in the forearm and divides into anterior and posterior interosseous branches.

1. Define the central part of the palmar aponeurosis which lies immediately deep to the skin of the palm and trace the four slips passing from it to the roots of the medial four fingers. Trace the digital branches of the median and ulnar nerves as they pass down between these slips accompanied by digital arteries.

2. Examine the flexor retinaculum which is attached laterally to the tubercle of the scaphoid and the tubercule of the trapezium and medially to the pisiform and hook of the hamate. Note the palmaris brevis, a small subcutaneous muscle running transversely from the retinaculum towards the hypothenar eminence and remove it. Identify the ulnar nerve and artery lying superficial to the flexor retinaculum.

3. Carefully reflect the palmar aponeurosis downwards and avoid damaging the deeper structures. Note the fascial septa passing from the palmar aponeurosis to the first and fifth metacarpal bones and separating the flexor tendons from the thenar and hypothenar muscles.

4. Next trace the distal part of the ulnar artery into the palm where it continues as the superficial palmar arch which lies in front of the superficial tendons. This arch is reinforced laterally by the palmar branch of the radial artery. Note that lateral to the pisiform bone the ulnar artery gives off a small deep palmar branch which accompanies the deep branch of the ulnar nerve. The superficial palmar arch gives off four common palmar digital branches to the medial three and a half digits. The radial side of the index and both sides of the thumb are supplied by the radial artery (see later).

5. Trace the ulnar and median nerves from the wrist into the palm where the origin of their digital branches can be seen. Note that the median nerve reaches the palm deep to the flexor retinaculum by passing through the carpal tunnel. The nerve supplies the three thenar muscles.

6. Clean the thenar muscles. Identify the laterally placed abductor pollicis brevis and the more medially situated flexor pollicis brevis. Note the origin of both these muscles from the flexor retinaculum, the scaphoid and the trapezium. Both are inserted into the lateral side of the base of the proximal phalanx of the thumb. Trace their nerve supply from the median nerve. Cut the two muscles in the middle and reflect them and define the more deeply placed opponens pollicis passing from the flexor retinaculum and trapezium to the shaft of the first metacarpal bone.

7. Next turn your attention to the hypothenar muscles. Identify and cut the abductor digiti minimi and flexor digiti minimi brevis in the middle and identify the deeper opponens digiti minimi muscle. They arise from the flexor retinaculum, the pisiform bone and from the hook of the hamate. Identify the insertions of the abductor and flexor into the medial side of the base of the proximal phalanx of the fifth digit and the opponens into the shaft of the fifth metacarpal bone. The nerve supply to these muscles comes from the deep branch of the ulnar nerve which passes between the abductor and flexor digiti minimi to enter the deep aspect of the palm. Note that the branch is accompanied by the deep palmar branch of the ulnar artery. Try to find them.

8. Now clean the following deep structures on the front of the forearm:

(a)laterally flexor pollicis longus muscle arising from the anterior surface of the radius and interosseous membrane;

(b) medially flexor digitorum profundus muscle arising from the interosseous membrane as well as from the anterior and medial surfaces of the ulna;

(c) below pronator quadratus muscle extending between the distal fourth of the ulna and radius and lying deep to the deep flexor tendons. It is the principal pronator; and

(d) the anterior interosseous nerve from the median nerve supplying the above three muscles with the exception of the medial part of flexor digitorum profundus which receives its innervation from the ulnar nerve. The anterior interosseous nerve lies on the interosseous membrane between flexor digitorum profundus and flexor pollicis longus and terminates in the pronator quadratus. The nerve is accompanied by the anterior interosseous artery.

9. Trace the radial artery deep to the brachioradialis. Divide the brachioradialis muscle in its middle and reflect it so as to identify the artery and the accompanying superficial branch of the radial nerve. Follow the radial artery down to the styloid process of the radius. Note the structures on which this artery lies. Its branches in this region are:

(a)the superficial palmar artery arising above the wrist and descending to join the superficial palmer arch;

(b) the arteria princeps pollicis which supplies the thumb;

(c) the arteria radialis indicis which supplies the index finger; and

(d) branches to the neighbouring muscles.

20. Make a vertical incision down the middle of the flexor retinaculum. Observe the arrangement of the tendons of the flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus and flexor carpi radialis as they pass under the retinaculum and the relationship of the median nerve to these tendons. Below the retinaculum identify the four lumbrical muscles arising from the tendons of the flexor digitorum profundus. These muscles are inserted into the radial side of the extensor expansion. Note that the medial two lumbricals receive their innervation from the deep branch of the ulnar nerve and the lateral two lumbricals from the median nerve.

1. Note that the tendons of the flexors digitorum superficialis and profundus are covered by synovial sheaths . Trace these tendons from the palm to one of the digits by incising the fibrous flexor sheath covering them. You will see that the superficialis tendon splits into two bundles which pass around the profundus tendon and insert into the sides of the middle phalanx and that the profundus tendon inserts into the base of the distal phalanx. Then trace the flexor pollicis longus to its insertion into the base of the distal phalanx of the thumb. Next cut the tendons of the flexor digitorum profundus and flexor pollicis longus just above the wrist, and turn them and the superficialis tendons downwards. Also cut through the middle of the superficial palmar arch and the accompanying nerve so as to get a better view of the deep aspect of the palm.

2. Now turn your attention to the deep intrinsic group of muscles which lie deep to the flexor tendons. Clean the adductor pollicis. Its transverse head arises from the shaft of the third metacarpal bone while its oblique head arises from the bases of the second and third metacarpals and capitate. Both heads are inserted into the medial side of the base of the proximal phalanx of the thumb. Note that the potential fascial spaces (palmar spaces) of the hand are situated deep to the flexor tendons and superficial to the adductor pollicis and interossei.

3. Identify the radial artery emerging between the transverse and oblique heads of the adductor pollicis. Reflect the oblique head from its origin. Observe that the deep palmar arch is formed by the continuation of the radial artery and the deep branch of the ulnar artery. Note the palmar metacarpal arteries arise from the deep palmar arch and join the digital branches from the superficial palmar arch.

4. Clean the deep branch of the ulnar nerve which supplies the medial two lumbricals, adductor pollicis and all the interossei muscles. Reflect the transverse head of adductor pollicis from its origin. Note that the four palmar interossei arise from the corresponding metacarpal bones and the four dorsal interossei from the adjacent metacarpal bones. The palmar interossei are inserted into the extensor expansion, and the dorsal interossei are inserted into the bases of the proximal phalanges and the extensor expansion. Primarily, all the interossei and the lumbricals are flexors of the metacarpophalangeal joints, in addition the palmar interossei adduct the digits towards the middle finger and the dorsal interossei abduct the digits and the middle finger. As a consequence of their insertions into the extensor expansions, these three sets of muscles are able to extend the middle and distal phalanges under certain conditions.

Summary

The muscles of the front of the forearm can be subdivided into:

(a)those muscles passing to the digits, i.e. digital flexors; and

(b) those concerned with flexion of the wrist. The tendons of the flexor digitorum superficialis are inserted into the middle phalanges, whereas those of the flexor digitorum profundus and flexor pollicis longus gain insertion into the distal phalanges. It is noteworthy that the presence of separate flexor tendons for the middle and distal phalanges increases the grasping efficiency of the hand. The actions of these slips on the interphalangeal joints are opposed by the slips of insertion of the extensor expansions. However, the action of the flexors is more powerful than that of the extensors. The presence of a separate flexor pollicis longus for the thumb and an early separation of the tendon from the flexor digitorum profundus to the index finger provide a greater degree of freedom of movement to these digits. There is a similar specialisation of the extensor tendons for the thumb and index finger. The flexor carpi radialis and flexor carpi ulnaris are usually flexors of the wrist. But they can also function together with their corresponding antagonistic extensors in producing radial or ulnar deviation of the wrist.

For example, the flexor and the extensor carpi ulnaris act together in producing ulnar deviation of the wrist. The thenar muscles of the hand also exhibit a certain amount of specialisation. The large size of the opponens pollicis and the presence of a special adductor for the thumb are features peculiar to the thumb. Moreover, the thumb is capable of rotation so that its palmar surface can be opposed towards the pulps of the other digits. The muscles of the front of the forearm are innervated by the median nerve or its anterior interosseous branch except the flexor carpi ulnaris and the medial portion of the flexor digitorum profundus which receive their nerve supply from the ulnar nerve. The intrinsic muscles of the hand are supplied by the ulnar nerve except the thenar muscles and the lateral two lumbricals which are innervated by the median nerve. The nerves which supply the intrinsic muscles of the hand are derived from the T1 segment of the brachial plexus. The median nerve may be compressed within the carpal tunnel, giving rise to the carpal tunnel syndrome. Similarly the ulnar nerve may be compressed as it lies behind the medial epicondyle of the humerus. Damage to the median nerve produces a condition known as the simian hand while damage to the ulnar nerve produces a claw hand. The radial and ulnar arteries are the principal vessels of the forearm. In their course, they lie between the radial and ulnar nerves. The ulnar artery continues as the superficial palmar arch while the radial artery continues into the more proximally situated deep palmar arch.