The forearm is enclosed in deep fascia (antebrachial fascia) that blends medially with the ulna and laterally sends an intermuscular septum to the radius.  The intermuscular septum, radius, interosseous membrane, and ulna divide the forearm into an anterior (flexor) compartment and a posterior (extensor) compartment. 

Most of the forearm muscles have proximal muscle bellies and slender distal tendons that enter the wrist and hand.  The slender distal tendons plus the numerous small bones in the wrist and hand combine to allow relatively precise movements of the hand, especially the fingers.  The muscles in the anterior compartment of the forearm flex the wrist and fingers and pronate the forearm. They are arranged in superficial, intermediate, and deep groups.  The superficial group of forearm flexor muscles share a common proximal attachment (common flexor tendon) from the medial epicondyle of the humerus (Fig. 1A; G11 6.60, Table 6.8 [p. 533]; G12 6.10A & D, 6.55A; N434).  From lateral to medial in the superficial group are the pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris.

The pronator teres has a superficial head from the medial epicondyle, which is easily identified, and a small deep head that is attached to the coronoid process of the ulna (G11 6.49; G12 6.45D; N428A-B, 435).  The muscle descends laterally to insert into the middle of the lateral surface of the radius.  The pronator teres assists the pronator quadratus in pronating the forearm when additional force or speed is needed.  The median nerve enters the forearm between the superficial and deep heads of pronator teres (Fig. 1A; G11 6.49D; G12 6.45D; N435, 461), where it is in danger of becoming entrapped (pronator teres syndrome). 

The flexor carpi radialis is a long fusiform muscle located just medial to the pronator teres.  The muscle belly ends in the middle of the forearm in a long tendon that pierces the lateral part of the flexor retinaculum to attach into the base of the second metacarpal bone, and often the third (G11 6.60, 6.64C, 6.73, 6.74; G12 6.57B, 6.61C, 6.70, 6.71; N430, 434, 435).  The flexor carpi radialis flexes and abducts the wrist.

The palmaris longus muscle has a small belly and a long, very slender tendon that is continuous with the palmar aponeurosis of the hand (Fig. 1A; G11 6.60; G12 6.57B; N434).  This muscle is absent from one or both upper extremities in about 13% of the population (but, yes, there is still a palmar aponeurosis present if the palmaris longus is absent).  When the palmaris longus is present, its tendon provides some protection against traumatic injury of the median nerve at the wrist (G11 6.64C; G126.61C; N434, 448A-B).  The muscle tenses the palmar aponeurosis and helps to flex the wrist.  The palmaris longus tendon sometimes is surgically removed for use in transplants.

The flexor carpi ulnaris is the most medial of the superficial flexor muscles (G11 6.60-6.63, 6.64C; G12 6.57B, 6.58B, 6.59B, 6.60B, 6.61C; N430, 434).  It has an ulnar head of proximal attachment from the olecranon and posterior border of the ulna.  The distal attachment is the hook of the hamate and the fifth metacarpal with the pisiform bone of the wrist classified as a sesamoid bone in the tendon of the flexor carpi ulnaris muscle.  The flexor carpi ulnaris flexes and adducts the hand at the wrist.  The ulnar nerve enters the forearm between the humeral and ulnar heads of flexor carpi ulnaris (cubital tunnel) (G11 6.51B-C; G12 6.47B, 6.48; N432, 433) and may become entrapped there in cubital tunnel syndrome (e.g., see http://www.emedicine.com/orthoped/TOPIC479.HTM ).  The patient may experience weakened wrist flexion and adduction, weakened grip, and tingling (paresthesia) and numbness along the medial side of the hand and medial 1½ fingers.

An intermediate layer of forearm flexor muscles is formed by a single muscle, the flexor digitorum superficialis (Fig. 1B; G11 6.61, Table 6.8 [p. 533]; G12 6.58B, Table 6.10 [p.553]; N431, 435).  It shares an attachment from the medial epicondyle with the superficial group (humeroulnar head) and also has a radial head of attachment along the superior half of the anterior border of the radius.  The muscle’s four tendons, one each to digits 2-5, traverse a confined space at the wrist, the carpal tunnel, to enter the palm of the hand (G11 6.71B-C, 6.94C-D; G12 6.68B-C, 6.92C-D; N449A-B, 450A-B).  It will be seen later that each tendon splits over the proximal phalanx of its digit to allow passage of a flexor digitorum profundus tendon and then inserts into the sides of the middle phalanx (G11 6.62, 6.72B, 6.74; G12 6.59B, 6.69B, 6.71; N431, 452 [middle figure]).  The flexor digitorum superficialis flexes the proximal interphalangeal (PIP) joints of digits 2-5 and can help to flex the metacarpophalangeal (MP) joints.  

The deep layer of forearm flexor muscles comprises three muscles, none of which attach to the medial epicondyle of the humerus—the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus (Fig. 2; G11 6.62, 6.63, 6.74; Table 6.8 [p. 533]; G12 6.59B, 6.60B, 6.71, Table 6.10 [p. 553]; N428A-B, 431, 436).  All three receive innervation from the anterior interosseous branch of the median nerve, but the medial (ulnar) half of the flexor digitorum profundus, the part moving the fourth and fifth digits, is supplied by the ulnar nerve. 

The flexor digitorum profundus attaches proximally to the ulna and interosseous membrane.  Its four tendons descend from the muscle belly to enter the carpal tunnel deep to the tendons of flexor digitorum superficialis.  In the palm the flexor profundus tendons give rise to lumbrical muscles, which pass to the dorsal surface of their respective digits (G11 6.70, 6.71C; G12 6.67B, 6.68C; N451A-B-C, 452A-B-C).  In fingers 2-5 each flexor digitorum profundus tendon passes through a split in the corresponding flexor digitorum superficialis tendon to attach to the distal phalanx of its digit (G11 6.72B, 6.74; Table 6.8 [p. 533]; G12 6.69B, 6.71, Table 6.10 [p.553]; N431, 451A-B-C and 452A-B-C [middle figure]).  The flexor digitorum profundus is the only muscle that can flex the distal interphalangeal (DIP) joints of fingers 2-5.  Note that the long forearm muscles can help move more proximal joints as well as their primary joint of action (e.g., flexor digitorum profundus can flex not only the DIP joint, but also help with the PIP and MP joints of digits 2-5, although it cannot flex all of those joints through their full range of motion at the same time).

As the four tendons of flexor digitorum profundus traverse the carpal tunnel to enter the hand (G11 6.63, 6.94C-D; G12 6.60B, 6.92C-D; N449A-B, 450A-B), they share the common flexor synovial sheath (ulnar bursa) with the tendons of flexor digitorum superficialis muscle.  Synovial sheaths, like bursae, are connective tissue sacs with smooth internal facing surfaces lubricated by a small amount of synovial fluid (G11 6.71, 6.72D; G12 6.68A-B-C, 6.69D; N448A-B, 452A-B-C).  Synovial sheaths enclose tendons, allowing them to glide through confined spaces or over bone with minimum friction. 

A synovial sheath may become inflamed (tenosynovitis), with edema and pain, and irritate or compress adjacent structures (e.g., nerves).  There is a loss of range of motion.  Chronic inflammation involving the tendon and tendon sheath may result in tendon rupture.  For example, see http://www.merck.com/mmpe/sec04/ch040/ch040c.html#sec04-ch040-ch040b-806  Tendon rupture also may occur following a course of some antibiotics (e.g., fluoroquinolones, such as Levaquin) or steroid treatment.  If inflammation results from infection, the infection may spread along a synovial sheath (e.g., from the little finger to the carpal tunnel at the wrist [G11 6.71B-C; G126.68B-C; N448A-B, 449A-B, 450A-B]). 

The flexor pollicis longus is the second of the deep forearm flexor muscles.  It lies lateral to the flexor digitorum profundus, passing from the anterior surface of the radius and interosseous membrane to the base of the distal phalanx of the thumb (Fig. 2; G11 6.62, 6.63, 6.71C; G12 6.59B, 6.60B, 6.68C; N431A-B, 435).  As the tendon traverses the carpal tunnel, it has its own synovial sheath (radial bursa) lateral to the tendons of the flexor digitorum muscles (G11 6.71C, 6.94C-D; G126.68C, 6.92C-D; N449A-B, 450A-B).  The flexor pollicis longus flexes the interphalangeal (IP) joint of the thumb and can help flex its MP joint. 

The median nerve descends through the forearm in the fascial interval between flexor digitorum superficialis and flexor digitorum profundus (G11 6.61, 6.62; G12 6.58B, 6.59B; N435, 463).  The median nerve or its branches supply all of the muscles in the anterior compartment of the forearm except the flexor carpi ulnaris and the medial (ulnar) half of flexor digitorum profundus, which are innervated by the ulnar nerve.  The anterior interosseous nerve branches from the median nerve in the proximal forearm to supply the three deep forearm flexor muscles (G11 6.62; G12 6.59B; N436).  The anterior interosseous nerve may become entrapped (anterior interosseous syndrome), resulting in weakened pronation (pronator quadratus) with inability to flex the DIP joints of fingers two and three (flexor digitorum profundus) and the interphalangeal joint of the thumb (flexor pollicis longus). 

The ulnar nerve continues distally in the forearm from the cubital tunnel, which is located between the two head of the flexor carpi ulnaris.  The nerve descends between the flexor carpi ulnaris and flexor digitorum profundus until just above the wrist, where it lies under the lateral edge of the flexor carpi ulnaris with the ulnar artery (Fig. 1; N435, 464). 

After branching from the brachial artery, the ulnar artery inclines medially to descend with the ulnar nerve (G11 6.62, Table 6.7 [p. 530]; G12 6.59B, 6.55A; N422, 436).  Near its beginning the ulnar artery gives off the common interosseous artery, which quickly branches into an anterior interosseous artery and a posterior interosseous artery.  The posterior interosseous artery passes into the posterior compartment above the interosseous membrane and gives rise to the interosseous recurrent artery (G11 6.58, 6.62, 6.63, Table 6.7 [p. 530]; G12 6.55A-B-C, 6.59B, 6.60B, 6.55A; N422, 433).  The interosseous recurrent anastomoses with the middle collateral artery from the profunda brachii artery as part of the collateral circulation around the elbow (G11 6.11A & C, Table 6.7 [p. 530]; G12 6.7A & C, 6.55A; N422, 433). 

The anterior interosseous artery descends along the anterior surface of the interosseous membrane with the anterior interosseous nerve.  The artery pierces the interosseous membrane near the proximal edge of pronator quadratus to enter the posterior compartment of the forearm.  

This collateral circulation (four sets of collateral and recurrent arteries) around the elbow is important when elbow flexion, traumatic injury, or surgical ligation impedes flow through the terminal part of the brachial artery.  The branching points for arteries that participate in collateral circulation around a joint, such as the elbow, determine where the artery can be ligated without risking loss of the extremity due to ischemic necrosis of tissues.  Loss of the arterial supply to the flexor compartment of the forearm is one cause of a compartment syndrome with Volkmann’s ischemic contracture (e.g., see http://www.emedicine.com/Orthoped/topic578.htm )  Would it be safe to ligate the brachial artery just proximal to the branching point of the profunda brachii (G11 6.11; G12 6.7A & C; N422)?

Study the relationships on the anterior aspect of the distal forearm and the wrist (G11 6.64; G12 6.61A-B-C; N434).  These relationships are clinically important in lacerations (e.g., suicide attempts) and other injuries of the wrist.  From lateral to medial are the tendon of abductor pollicis longus, radial artery, tendon of flexor carpi radialis, median nerve, tendon of palmaris longus (when present), tendons of flexor digitorum superficialis, ulnar artery, ulnar nerve, and tendon of flexor carpi ulnaris.

The palm of the hand is subdivided into a central compartment covered superficially by the thick palmar aponeurosis, a medial hypothenar compartment covered by thin hypothenar fascia, and a lateral thenar compartment covered by thin thenar fascia (G11 6.66; G12 6.63; N447).  The hypothenar compartment contains intrinsic hand muscles that move the little finger, and the thenar compartment contains intrinsic hand muscles that move the thumb.  In the small fat pad superficial to the hypothenar fascia a small group of transversely oriented muscle fibers courses laterally from the palmar aponeurosis as the palmaris brevis muscle (G11 6.69; G12 6.67A; N447).

Deep to the central compartment of the palm is the interosseous-adductor compartment, which contains the adductor pollicis muscle and the palmar and dorsal interosseous muscles (G11 6.67A, 6.95; G12 6.64A, 6.93A-B; N450 [lower figure]).  The deep palmar arch and deep branch of the ulnar nerve also are present. These will be studied after the more superficial structures of the palm.

The palm of the hand receives a rich blood supply via two palmar arterial arches (G11 6.69, 6.73, 6.75; Table 6.10 [p. 553]; G12 6.67A, 6.70, 6.72A-B, Table 6.12 [p. 573]; N448, 453, 454A-B).  The superficial palmar arch is the direct continuation of the ulnar artery.  The ulnar artery passes anterior to the flexor retinaculum and through the depression between the pisiform bone and hook of the hamate (Guyon’s canal) with the ulnar nerve.  The artery divides into superficial and deep branches, and the larger superficial branch becomes the superficial palmar arch. 

The superficial palmar arch passes radially across the palm deep to the palmar aponeurosis but superficial to the long flexor tendons (G11 6.69; G12 6.67A; N448).  It is located distally about midlength of the metacarpal bones.  The superficial arch may be completed laterally by a small superficial palmar branch of the radial artery.  The superficial palmar arch gives off common palmar digital arteries, which in turn divide into proper palmar digital arteries to the adjacent sides of fingers (G11 6.75A; Table 6.10 [p. 553]; G12 6.72A, Table 6.12 [p. 573] N454A-B). 

The deep palmar arch crosses the palm deep to the long flexor tendons and will be studied later.

The flexor retinaculum (transverse carpal ligament) is a strong band of fibrous connective tissue spanning the ventral concavity created by the articulated carpal bones to form the carpal tunnel (Fig. 6-2, G11 6.62, 6.70, 6.74, 6.94C-D; G12 6.59B, 6.67B, 6.71, 6.92C-D; N435, 442A-B, 449A-B).  The retinaculum prevents bowstringing of the long forearm flexor tendons during wrist flexion.  Note that the ulnar nerve and artery and the tendon of palmaris longus pass superficial to the flexor retinaculum and that the median nerve and most long flexor tendons pass deep to the retinaculum through the carpal tunnel (G11 6.94C-D; G12 6.92C-D; N449A-B). 

The carpal tunnel is a confined space with rigid walls, and any condition that significantly reduces the volume of the carpal tunnel (e.g., inflammation of the synovial sheaths due to repetitive motion or infection) may compress the median nerve.  This carpal tunnel syndrome is a relatively frequent condition.  It is characterized by pain and/or paresthesia, often worse at night, along the palmar surface of the lateral 3½ digits, sometimes radiating proximally into the forearm.  In severe cases, there is loss of strength (paresis) and atrophy of the thenar compartment muscles with loss of precise thumb movements.  See   http://www.emedicine.com/EMERG/topic83.htm

The thenar and hypothenar compartments both contain a short abductor and a short flexor muscle that attach on the proximal phalanx of its digit (thumb and little finger, respectively) and an opponens muscle that inserts on the metacarpal bone.  Start with the thenar compartment.  The two superficial thenar muscles are the flexor pollicis brevis and abductor pollicis brevis (G11 6.69, Table 6.9 [pp. 544-545]; G12 6.67A, 6.66D, 6.76D; N453A-B-C, 454A-B, 463).  The flexor takes origin from the flexor retinaculum and the tubercle of the trapezium.  The abductor takes origin from the flexor retinaculum and the tubercle of the scaphoid.  The flexor pollicis brevis flexes the carpometacarpal (CM) and the metacarpophalangeal (MP) joints, and the abductor pollicis brevis abducts the carpometacarpal (CM) joint of the thumb. 

Deep to the abductor pollicis brevis is the opponens pollicis (G11 6.70; G12 6.67B; N453A-B-C), which performs the important movement of opposition of the thumb.  During opposition the pad of the thumb is brought into contact with the pad of another finger.  Opposition is a component of many activities and is estimated to be responsible for as much as 80% of the usefulness of the hand.  The thumb is so important that the great toe may be transplanted or the index finger may be mobilized and re-positioned in an attempt to partially replace the function of an amputated thumb.      

The recurrent branch of the median nerve innervates the muscles of the thenar compartment.  An inconstant deep head of the flexor pollicis brevis usually is innervated by the deep branch of the ulnar nerve when the deep head is present, but for testing purposes, the innervation of thenar compartment muscles is the recurrent branch of the median nerve.  

Turn your attention to the hypothenar compartment.  The abductor digiti minimi passes to the little finger from the pisiform bone, and the flexor digiti minimi brevis reaches it from the flexor retinaculum and hook of the hamate (G11 6.70, 6.71C; Table 6.9 [p. 545]; G12 6.67B, 6.68C, 6.66D, 6.76D; N453A-B-C [upper figure], 464).  The two muscles insert together on the medial side of the proximal phalanx of the little finger at its base.  The flexor is variable in size and may be absent.  The deep branches of the ulnar nerve and artery enter the hypothenar compartment between the abductor digiti minimi and flexor digiti minimi brevis.

The third muscle in the hypothenar compartment is the opponens digiti minimi.  It lies deep to the abductor and flexor, passing from the flexor retinaculum and hook of the hamate to the fifth metacarpal bone. 

The ulnar nerve may be compressed by any space-occupying lesion (e.g., a ganglion cyst) in Guyon’s canal (for example see http://www.emedicine.com/Orthoped/topic574.htm  ). 

The adductor pollicis muscle is located deep to the long flexor tendons in the adductor-interosseous compartment of the palm.  The adductor pollicis is formed by oblique and transverse heads (G11 6.73; Table 6.9 [p. 545];G12 6.70, 6.66D, 6.76D; N450, 453).  The former arises from the capitate and the bases of the second and third metacarpal bones, and the transverse head passes laterally from the shaft of the third metacarpal to join it.  The adductor pollicis inserts on the medial side of the proximal phalanx of the thumb. 

The deep palmar arch is formed mainly by the radial artery with usually a contribution medially from the deep branch of the ulnar artery (G11 6.73, 6.75; Table 6.10 [p. 553]; G12 6.70, 6.72A-B, Table 6.12 [p. 573]; N453A-B-C, 454A-B).  The radial artery enters the palm by piercing the first dorsal interosseous muscle and gives off radialis indicis and princeps pollicis branches.  The radialis indicis artery passes along the lateral side of the index finger to supply it.  The princeps pollicis artery divides at the base of the proximal phalanx of the thumb into palmar digital branches to supply the sides of the thumb.  The radial artery continues between the two heads of the adductor pollicis as the deep palmar arch.  Palmar metacarpal arteries branch from the deep palmar arch.  They give perforating branches to dorsal metacarpal arteries before ending in the common palmar digital branches of the superficial arch. 

The deep branch of the ulnar nerve branches from the ulnar nerve in Guyon’s canal (G11 6.70, 6.73, 6.74; G126.67B, 6.70, 6.71; N448A-B, 453A-B-C, 464).  In addition to innervating the hypothenar muscles, the deep branch supplies the medial two lumbrical muscles, all of the interosseous muscles, and the adductor pollicis.  The deep branch of the ulnar nerve also supplies the deep head of the flexor pollicis brevis muscle when it is present, while the superficial head of the flexor is typically innervated by the recurrent branch of the median nerve. 

The dorsal and palmar interosseous muscles are located in the spaces between the metacarpal bones (G11 6.74, 6.79; Table 6.9 [p. 545]; G12 6.71, 6.76, 6.66D, 6.76D; N452A-B-C, 453A-B-C, 458A-B).  There are four dorsal interossei but only three palmar interossei because the palmar interosseous muscle between the first and second metacarpals fuses with the adductor pollicis during development.  Dorsal interossei attach into both the proximal phalanges and extensor expansions on the lateral sides of digits two and three and the medial side of digits three and four.  The palmar interossei insert on the medial side of digit two and the lateral sides of digits four and five.  These attachments make it easy to remember the functions of the interossei:  the dorsal interossei abduct digits (easily remembered as “dab”) from the reference digit of the hand (middle finger), and the palmar interossei adduct digits (pad).  Both groups of interosseous muscles also assist with MP flexion when combined with PIP and DIP extension. 

The muscles of the posterior compartment of the forearm are divided into superficial and deep layers.  Four of the superficial group of forearm extensor muscles share a proximal attachment (common extensor tendon) from the lateral epicondyle of the humerus, and the other two superficial muscles attach to the lateral supracondylar ridge.  The superficial group consists of, from lateral to medial, the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris (G11 6.76A; G12 6.73A; N432). 

The brachioradialis muscle passes from the upper 2/3 of the lateral supraconylar ridge of the humerus to the lateral surface of the radius just proximal to the styloid process (G11 6.60, 6.62, 6.76, 6.77, Table 6.11 [p. 555]; G12 6.57B, 6.59B, 6.73A-B-C-D, 6.74A-B, Table 6.13 (1-2), (3-4) [p. 575]; N432, 433, 334, 435, 438).  It is the only muscle in the superficial group of forearm extensors that doesn’t cross the wrist joint.  The brachioradialis is an unusual muscle in that it is innervated by the radial nerve but helps to flex the forearm when additional speed or force is required.  It may perform weak elbow flexion in a patient who has a lesion of the musculocutaneous nerve, which would paralyze the brachialis and biceps brachii.

The extensor carpi radialis longus takes origin from the lower 1/3 of the lateral supracondylar ridge of the humerus and attaches distally to the base of the second metacarpal bone (G11 6.76A & C, 6.77A; G12 6.73A & C, 6.74A; N429A-B, 432, 433).

The extensor carpi radialis brevis passes from the common extensor tendon on the lateral epicondyle to the base of the third metacarpal.  The humeral attachment of this muscle becomes inflamed due to repeated forceful wrist extension in lateral epicondylitis (tennis elbow).  The two extensor carpi radialis muscles extend the hand at the wrist joint and abduct (radially deviate) it. 

The extensor digitorum divides into tendons for digits 2-5 proximal to the extensor retinaculum.  Note that the extensor digitorum tendon for the little finger may not separate from that of the ring finger until near the heads of the metacarpal bones.  The tendons will be followed into the hand later, where they form an extensor expansion over the proximal phalanx of each digit (G11 6.79B & D, 6.80; G12 6.76B & E, 6.77; N429A-B, 452A-B-C, 458A-B).  The extensor digitorum extends digits 2-5 during gross opening and closing movements of the hand and can help to extend the wrist.

The extensor digiti minimi lies medial to the extensor digitorum.  It goes to the little finger (digit 5), usually receiving a tendinous band from the extensor digitorum tendon of the ring finger just proximal to the MP joint (G11 6.79B & D; G12 6.76B & E; N429A-B, 432, 458A-B).  The tendon of extensor digiti minimi itself frequently divides into two tendons before entering the little finger (G11 6.79B & D; G12 6.76B & E; N458A-B).  The extensor digiti minimi provides independent extension of the little finger. 

The most medial muscle in the superficial group is the extensor carpi ulnaris muscle, which passes from the common extensor tendon to the base of the fifth metacarpal bone.  It extends and adducts (ulnarly deviates) the hand at the wrist.  During routine extension of the hand at the wrist, the extensor carpi ulnaris and the two extensor carpi radialis muscles are the prime movers.

The deep group of forearm extensor muscles consists of the supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis muscles.  The supinator muscle passes from the lateral epicondyle of the humerus, radial collateral and anular ligaments, and supinator crest and fossa of the ulna to the lateral and posterior surfaces of the proximal 1/3 of the radius (G11 6.61A-B-C, 6.63, 6.77; G12 6.58B, 6.60B, 6.74A-B; N428A-B, 438).  The deep branch of the radial nerve pierces the supinator muscle and emerges into the posterior compartment of the forearm with a new name, the posterior interosseous nerve (G11 6.77; G12 6.74A-B; N433).  The supinator is always active in supination of the forearm but is assisted by the biceps brachii during rapid supination or against resistance. 

The deep branch of the radial nerve may become entrapped within the supinator muscle or damaged by a fracture of the neck of the radius.  Since the branches to the extensor carpi radialis muscles arise more proximally, wrist extension is spared but there is loss of thumb extension and MP extension of fingers 2-5.  In this lesion, how is interphalangeal extension of digits 2-5 still possible when it is combined with MP flexion?

The abductor pollicis longus attaches proximally to the radius, ulna, and interosseous membrane.  From this origin it passes distally to attach on the base of the first metacarpal bone (G11 6.76A, 6.77; G12 6.73A, 6.74A-B; N429A-B, 433, 439).  The abductor pollicis longus abducts and extends the carpometacarpal (CM) joint of the thumb.  Traveling with the abductor pollicis longus, and sometimes partially fused with it, is the extensor pollicis brevis.  This muscle traverses the forearm from a proximal attachment on the radius and interosseous membrane to the base of the proximal phalanx of the thumb.  The extensor pollicis brevis extends the metacarpophalangeal (MP) joint of the thumb. Observe the relationship of these two muscles in your cadaver. 

The tendons of abductor pollicis longus and extensor pollicis brevis share a tendon sheath and compartment deep to the extensor retinaculum (G11 6.79B-C; G12 6.76B-C; N433, 458A-B).  They may become inflamed deep to the retinaculum following repetitive movements (de Quervain’s stenosing tenosynovitis, de Quervain’s syndrome), causing tenderness and lateral wrist pain that may radiate both proximally and distally.  The differential diagnosis includes arthritis of the first carpometacarpal joint and scaphoid fracture.  

Identify the extensor pollicis longus muscle, which passes from the middle 1/3 of the ulna and interosseous membrane to the distal phalanx of the thumb.  It extends the distal phalanx of the thumb at the interphalangeal (IP) joint and can help extend the MP joint. 

The tendon of extensor pollicis longus forms the posterior boundary of a depression visible when the thumb is fully extended, the anatomical snuffbox (G11 6.76A & D, 6.77A, 6.81B-D; G12 6.73A & D, 6.74A, 6.78B-D; N432, 455, 457).  The anterior boundary is the tendons of abductor pollicis longus and extensor pollicis brevis.  The pulse of the radial artery can be felt in the anatomical snuffbox, which it traverses.  The floor of the snuffbox is formed by the radial styloid process, scaphoid, and trapezium bones. 

The scaphoid is the most frequently fractured carpal bone.  Pain with hand movement and tenderness on deep palpation in the anatomical snuffbox following a fall on an outstretched hand are characteristic of a scaphoid fracture.  The fracture itself may not be apparent on x-rays for 10-14 days, until bone resorption has occurred at the fracture site.  A scaphoid fracture may result in interruption of the blood supply with avascular necrosis of the proximal fragment. 

The extensor indicis has a proximal attachment from the distal 1/3 of the ulna and interosseous membrane (G11 6.77; G12 6.74A-B; N429A-B, 433).  Look for the tendon of this muscle, which will attach into the extensor expansion of the index finger.  The extensor indicis provides extension of the index finger independent of the extensor digitorum muscle.

The radial nerve divides into superficial and deep branches anterior to the lateral epicondyle in the fascial interval between the brachioradialis and brachialis muscles (G11 6.49D, 6.62; G12 6.45D, 6.59B; N435).  The superficial branch of the radial nerve (superficial radial nerve), a cutaneous nerve that descends under cover of the brachioradialis to the distal forearm, passes laterally superficial to the anatomical snuffbox to reach the dorsum of the hand (G11 6.60, 6.78, 6.81A; G12 6.57B, 6.75A-B-C, 6.78A; N404A-B, 435, 455).  It provides cutaneous innervation to the dorsum of the hand and lateral 3½ digits, except over the distal phalanges, where the skin is supplied by branches of the median nerve. 

After passing above the upper border of the interosseous membrane to enter the posterior compartment of the forearm, the posterior interosseous artery gives off the interosseous recurrent artery and descends with the posterior interosseous nerve between the superficial and deep layers of forearm extensor muscles (G11 6.77A; G12 6.74A; N422, 433).  Just above the wrist the anterior interosseous artery pierces the interosseous membrane to enter the posterior compartment en route to the dorsal carpal arch (G11 6.11D, 6.58B; Table 6.10 [p. 553]; G12 6.7D, 6.55C, Table 6.12 [p. 573]; N433, 457).  Therefore, the anterior interosseous artery substitutes distally for the short posterior interosseous artery. 

The extensor retinaculum is a transverse thickening of antebrachial fascia.  It has an attachment to the distal radius laterally and to the distal ulna and triquetrum medially (G11 6.76A, 6.77A; G12 6.73A, 6.74A; N432, 458A-B).  The space deep to the extensor retinaculum is subdivided into six osseofibrous compartments by connective tissue septa (G11 6.79B-C; G12 6.76B-C; N458A-B).  Note the contents of each compartment.  The most lateral compartment contains the tendons of abductor pollicis longus and extensor pollicis brevis.  The second compartment, which includes the anatomical snuffbox, contains the tendons of extensor carpi radialis longus and brevis.  The third compartment has only one tendon traversing it, the extensor pollicis longus tendon.  The broad fourth compartment over the middle of the dorsum of the distal forearm contains the tendons of extensor digitorum and the deeper extensor indicis.  The extensor digiti minimi tendon occupies the fifth compartment.  The final, most medial, compartment contains the tendon of extensor carpi ulnaris.