Chapter 10: The forearm
The forearm is divided into two compartments (a ventromedial or flexor compartment and a dorsolateral or extensor compartment). Muscles of the forearm segregate into these compartments consisting of (1) an anterior group (the flexors of the wrist and fingers and the pronators) and (2) a posterior group (the extensors of the wrist and fingers and the supinator) (fig. 10-1).
Four superficial, one intermediate and three deep muscles occupy the anterior forearm. The superficial group (pronator teres, flexor carpi radialis, plamaris longus and flexor carpi ulnaris) arises mostly from a common flexor tendon that attaches to the anterior part of the medial epicondyle of the humerus, and from adjacent fascia. These muscles are supplied chiefly by the median nerve (fig. 10-2). The intermediate muscle (flexor digitorum superficialis) arises from this common tendon and along the anterior surface of the ulna and radius and is also supplid by the median nerve. The deep group (flexor pollicis longus, flexor digitorum profundus and pronator quadratus) is supplied mostly by the anterior interosseous nerve, a branch of the median. Those muscles in the superficial and deep groups that are not innervated by the median (i.e., the flexor carpi ulnaris and the ulnar part of the flexor digitorum profundus) are supplied by the ulnar nerve. The tendons of the flexor carpi radialis, palmaris longus, and flexor carpi ulnaris are readily palpable. The palmaris longus, however, is often absent. In a finger, as in the palm, the tendons of the flexor digitorum superficialis and profundus are enclosed in a common synovial sheath. Opposite the proximal phalanx, the superficialis tendon splits into two slips that embrace the profundus tendon and then reunite behind it to insert into the middle phalanx. The profundus tendon continues to the distal phalanx. Both tendons are anchored to the phalanges and interphalangeal joints by fibrous bands termed vincula, which carry the blood supply to the tendons.
Increased pressure in the anterior compartment of the forearm, such as can occur due to injury to the brachial artery near the elbow (e.g., from a supracondylar fracture of the humerus) can prevent normal blood flow to the compartment and ischemic damage to the deep flexors (pollicis longus and digitorum profundus). This results in muscle scarring, with flexion deformity of the wrist and fingers (Volkmann's ischemic contracture).
Seven superficial and five deep muscles occupy the posterior forearm. The superficial group arises mostly from the posterior aspect of the lateral epicondyle of the humerus by a common tendon. The muscles are supplied by (1) the radial nerve or (2) its deep branch, which continues as (3) the posterior interosseous nerve.
The brachioradialis can be seen and felt on the lateral side of the elbow and forearm when these are flexed.
The insertion of the extensor carpi radialis brevis into the base of the third metacarpal is a common site for the development of a "ganglion." The term ganglion is used here for a cystic swelling adjacent to, and often communicating with, a synovial sheath or joint. A common site is the dorsum of the wrist.
On the dorsum of the hand, the extensor tendons diverge but are connected by a variable arrangement of bands (see fig. 11-7). A fibrous sheet on the back of each finger is known as the extensor expansion, or dorsal aponeurosis, and it contains a hood of transverse fibers. The expansion is penetrated by the extensor tendon, which then divides into three slips: a central slip to the base of the middle phalanx and two collateral bands, which, fused with expansions from the interossei and lumbricals, unite and proceed to the base of the distal phalanx.
The supinator muscle is arranged in two layers, separated by the deep branch of the radial nerve, which sometimes comes into direct contact with the radius. The nerve meets the posterior interosseous vessels at the lower border of the supinator.
The tendon of the extensor pollicis longus lies in a groove on the medial aspect of the dorsal tubercle of the radius. Inferior to this, digital twigs of the superficial branch of the radial nerve can be felt crossing the tendon. When the thumb is extended, a hollow known as the anatomical snuff-box appears between the tendon of the extensor pollicis longus medially and those of the extensor pollicis brevis and abductor pollicis longus laterally (figs. 10-3 and 10-4). Its floor is formed by the scaphoid and trapezium bones and is crossed by the radial artery. The hollow is limited superiorly by the styloid process of the radius. The narrow tendon sheath of the abductor pollicis longus and extensor pollicis brevis is liable to inflammation, or stenosing tenosynovitis (de Quervain's disease).
The fascia on the distal part of the posterior forearm is thickened to form the extensor retinaculum, which is anchored to the radius and ulna. From the deep aspect of the extensor retinaculum, septa produce half a dozen compartments for the extensor tendons (fig. 10-5). Each compartment contains a synovial sheath.
Radial nerve (see fig. 8-13).
The radial nerve enters the forearm between the brachioradialis and brachialis and, at or inferior to the level of the lateral epicondyle, divides into superficial and deep branches.
The superficial branch is the continuation of the radial nerve, and its distribution is cutaneous and articular. It accompanies the radial artery, lying lateral to it, and then winds dorsally deep to the brachioradialis, to become subcutaneous and supply the lateral part of the dorsum of the hand. Its terminal branches usually supply two and a half fingers but generally reach no further than the proximal phalanges of the index and middle fingers. (The dorsal innervation here is completed by digital branches of the median nerve.) Several cutaneous branches of the radial nerve can be felt by stroking a fingernail down the taut tendon of the extensor pollicis longus.
The deep branch of the radial nerve has a muscular and articular distribution. It winds laterally between the superficial and deep layers of the supinator and often makes direct contact with the radius, being vulnerable in fractures. At the lower border of the supinator, it meets the posterior interosseous vessels. For the rest of its course, the deep branch of the radial nerve is then termed the posterior interosseous nerve, a name that is sometimes applied to the entire course of the deep branch. The posterior interosseous nerve reaches the interosseous membrane distally and ends in an enlargement from which twigs are distributed to adjacent joints.
The brachioradialis and extensor carpi radialis longus (and often brevis) are supplied by the radial nerve. Its deep branch supplies the supinator (and often the extensor carpi radialis brevis), extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris. The posterior interosseous nerve supplies the abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.
Median nerve (see fig. 8-11).
The median nerve usually enters the forearm between the two heads of the pronator teres. It descends in the middle of the forearm to the midpoint between the styloid processes. It is accompanied by the median artery, a branch of the anterior interosseous. The median nerve passes posterior to the tendinous arch connecting the two heads of the flexor digitorum superficialis and remains under cover of that muscle, adherent to its deep surface and lying on the flexor digitorum profundus, until it approaches the wrist, where it becomes more superficial. It may give a cutaneous branch to the palm. The median nerve is found between the flexor digitorum superficialis medially and the flexor carpi radialis laterally, and it may be partly covered by the palmaris longus. It enters the hand by passing through the carpal tunnel, posterior to the flexor retinaculum and anterior to the flexor tendons.
In the cubital fossa, branches are given medially to the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. The anterior interosseous nerve arises from the median nerve in the cubital fossa, and, in company with the anterior interosseous artery, descends along the interosseous membrane between the adjacent margins of the flexor pollicis longus and flexor digitorum profundus, both of which it supplies. It then passes posterior to, and supplies, the pronator quadratus and ends in branches to adjacent joints.
Communications between the median and ulnar nerves in the forearm may be important in that fibers to the small muscles of the hand may, in some people, be transferred from one nerve to the other, resulting in unusual signs in the event of injury.
Ulnar nerve (see fig. 8-12).
The ulnar nerve lies in a groove posterior to the medial epicondyle, where it can be felt against the "funny bone." The ulnar nerve enters the forearm between the two heads of the flexor carpi ulnaris. Shortly thereafter it meets the ulnar artery, which lies on its lateral side. It then becomes superficial. The ulnar nerve lies between the flexor digitorum superficialis laterally and the flexor carpi ulnaris medially. The ulnar nerve and artery enter the hand by passing antrior to the flexor retinaculum, lateral to the pisiform, and between that bone and the hook of the hamate. The course of the ulnar nerve may be represented by a line from the medial epicondyle to the lateral edge of the pisiform.
Branches are given to the two muscles between which the ulnar nerve lies; the flexor digitorum profundus and the flexor carpi ulnaris. In the middle ofthe forearm, a large, cutaneous dorsal branch, which descends between the ulna and the flexor carpi ulnaris, turns dorsally and is distributed to the medial part of the dorsum of the hand. There may also be a variable palmar branch.
Radial artery (see fig. 7-3).
The smaller terminal division of the brachial artery begins in the cubital fossa, opposite the neck of the radius. In the distal forearm, the radial artery lies lateral to the flexor carpi radialis tendon, which serves as a guide to it. Its pulsations can readily be felt by three fingers here, thus supplying information of clinical importance, e.g., pulse rate and rhythm and compressibility and condition of the arterial wall. The vessel leaves the forearm by winding dorsally across the carpus.
A recurrent branch is given to the anastomosis around the elbow joint, and there are also palmar and carpal branches.
Ulnar artery (see fig. 7-3).
The larger terminal division of the brachial artery begins in the cubital fossa, opposite the neck of the radius. It first descends medially and then directly inferiorward. Distally the ulnar artery and nerve emerge laterally from under cover of the flexor carpi ulnaris, and the pulsations can be felt at the wrist. The ulnar artery leaves the forearm by passing anterior to the flexor retinaculum on the lateral side of the pisiform bone. After it gives off its deep palmar branch, it continues as the superficial palmar arch.
A recurrent branch is given to the anastomosis around the elbow joint. The common interosseous artery divides almost immediately into anterior and posterior interosseous arteries, which descend on the anterior and posterior aspect of the interosseous membrane, respectively, and end dorsally on the carpus. The anterior vessel gives off the median artery.
10-1 Which type of contracture may result from injury to the brachial artery?
10-2 What are the two main compartments of the forearm?
10-3 What is the chief origin of the superficial extensors?
10-4 What is a "ganglion" in the clinical sense?
10-5 Which important bones form the floor of the anatomical snuff-box?
10-6 What is the characteristic feature of radial nerve palsy?
10-7 Where is the median nerve situated at the wrist?
10-8 Which muscles are supplied by the anterior interosseous nerve?
10-9 What is the clinical significance of communications between the median and ulnar nerves in the forearm?
10-10 What is the guide to the radial artery?
Figure 10-1 Horizontal section through the middle of the forearm. In A, the nerves and vessels are identified. B shows the flexor muscles (supplied by the ulnar but mainly by the median nerve) anteriorly and the extensors (supplied by the radial and posterior interosseous nerves) posteriorly. The individual muscles are not shown. Note that the flexors and extensors are separated by the subcutaneous posterior border of the ulna.
Figure 10-2 The superficial palmar arch (see also fig. 11-10), the flexor retinaculum, and the structures on the anterior wrist (see also fig. 10-5). The palmar aponeurosis has been removed. Note the median nerve passing deep to the flexor retinaculum (within the carpal tunnel) and, immediately on emerging, giving off its very important recurrent branch to the thenar muscles. (Based partly on Spalteholz.)
Figure 10-3 The "anatomical snuffbox." The tendons of the abductor pollicis longus and extensor pollicis brevis diverge in proceeding distally. The snuff-box is due to the fact that the extensor pollicis longus hooks around the dorsal tubercle of the radius and hence is separated from the extensor pollicis brevis.
Figure 10-4 The "anatomical snuff-box." The radial artery lies on the floor of the snuff-box, which is formed by the scaphoid and trapezium.
Figure 10-5 Horizontal section through the lower ends of the radius and ulna. Three synovial sheaths are shown on the anterior side, but the six sheaths on the posterior aspect (see fig. 11-3) are not included. The radial artery and the styloid process of the ulna indicate the boundaries between the flexor and extensor muscles. The median nerve is overlapped by the palmaris longus (when present) and is at the lateral edge of the flexor superficialis. The radial artery lies lateral to the flexor carpi radialis. Note the relationship of the extensor pollicis longus to the dorsal tubercle of the radius (*). (Modified from Castaing and Soutoul.)