Posterior Compartment of the Forearm Anatomy
The posterior compartment of the forearm lies behind the radius and ulna and the intervening interosseous membrane. It contains the extensor muscles of the wrist and fingers, the extensors and long abductor of the thumb, and also brachioradialis, supinator and anconeus. The muscles are supplied by the posterior interosseous artery and nerve (deep branch of the radial nerve), assisted by the radial nerve itself. The compartment is enclosed by a layer of deep fascia, which attaches to the posterior (subcutaneous) border of the ulna. In the region of the wrist the fascia is thickened to form the extensor retinaculum (Fig. 3.69). The extensor tendons, invested by synovial sheaths, pass deep to the retinaculum and enter the hand.
Four superficial extensors attach proximally to the anterior surface of the lateral humeral epicondyle at the common extensor origin (Fig. 3.70). Extensor carpi radialis brevis (Fig. 3.71) passes distally to the base of the third metacarpal and is an extensor and abductor of the wrist. Extensor digitorum (Figs 3.69 & 3.70) has four tendons which pass to the fingers and form the dorsal expansions or extensor hoods (Fig. 3.51). On the dorsum of the hand these tendons are interconnected by fibrous bands. The muscle is an extensor of the fingers and the wrist joint. Extensor digiti minimi (Fig. 3.70) attaches via two tendons to the dorsal expansion of the little finger and assists extension of this finger. Extensor carpi ulnaris (Fig. 3.70) attaches proximally to both the common extensor origin and the posterior surface of the ulna. Distally, it attaches to the base of the fifth metacarpal bone. The muscle is an extensor and adductor of the wrist. These four superficial extensors span the elbow but, since their attachments are close to the axis of movement, do not act effectively on that joint. They are innervated by the posterior interosseous (deep branch of the radial) nerve.
Two other superficial muscles, brachioradialis and extensor carpi radialis longus, arise from the lateral supracondylar ridge of the humerus and are innervated directly from the main trunk of the radial nerve. Brachioradialis (Figs 3.71 & 3.72) arises from the upper two-thirds of the ridge and attaches distally to the radial styloid process. The muscle is a flexor of the elbow joint and rotates the forearm from full pronation or supination into an intermediate position. Extensor carpi radialis longus (Figs 3.70 & 3.71) arises from the lower third of the supracondylar ridge and attaches distally to the base of the second metacarpal bone. The muscle is an extensor and abductor of the wrist joint.
In addition to their primary roles, the three carpal extensors provide an essential contribution to the power grip by fixing the wrist in an optimum position while the long flexors act on the fingers. Overuse of the muscles attaching to the lateral epicondyle may produce inflammation near the attachment with pain during extension of the wrist and fingers (lateral epicondylitis or ‘tennis elbow’).
The proximal attachments of four of the deep muscles are illustrated in Figure 3.73. The tendon of extensor indicis (Fig. 3.74) passes distally, medial to that of extensor digitorum, and attaches to the extensor expansion of the index finger. The extensor pollicis longus tendon passes distally around the ulnar (medial) side of the dorsal tubercle of the radius (Fig. 3.74), then crosses the radial extensors of the carpus to attach to the base of the distal phalanx of the thumb (Fig. 3.75). The muscle extends the thumb and its carpometacarpal joint. The tendons of extensor pollicis brevis and abductor pollicis longus (Fig. 3.75) lie together as they cross the radial carpal extensors and brachioradialis. Extensor pollicis brevis attaches to the base of the proximal phalanx of the thumb, which it extends. Abductor pollicis longus attaches to the base of the first metacarpal bone, which it extends and abducts.
Although these four deep muscles act primarily on the joints of the hand, they also span the wrist joint; but their actions here are weak. They are all innervated by the posterior interosseous (deep branch of the radial) nerve.
Extension of the thumb creates a hollow on the posterolateral aspect of the wrist called the anatomical snuff box (Fig. 3.75). It is limited anteriorly by the tendons of abductor pollicis longus and extensor pollicis brevis and posteriorly by extensor pollicis longus. Superficial to the snuff box lie the origin of the cephalic vein and branches of the superficial radial nerve supplying the dorsum of the hand. The branches can be palpated where they cross superficial to the tendon of extensor pollicis longus. The tendons of the two radial carpal extensors and the radial artery pass through the snuff box. The bony floor comprises the radial styloid process, scaphoid, trapezium and base of the first metacarpal bone. Fracture of the scaphoid bone often produces pain, swelling and tenderness in the snuff box (p. 122).
Supinator, also a deep muscle, consists of oblique and transverse heads (Fig. 3.76). The oblique head attaches to the lateral epicondyle and collateral ligament, while the transverse head is attached to the supinator crest of the ulna. Both heads wind laterally around the proximal part of the radius and attach to its anterior surface (Fig. 3.73). The muscle supinates the forearm and is supplied by the deep branch of the radial nerve (posterior interosseous nerve), which passes between the two heads of the muscle (Fig. 3.77).
In the cubital fossa, the common interosseous branch of the ulnar artery divides into larger anterior and smaller posterior interosseous arteries, which pass distally into their respective compartments of the forearm. The posterior interosseous artery (Fig. 3.77) lies between and supplies the superficial and deep groups of muscles. Near the wrist, a branch of the anterior interosseous artery penetrates the interosseous membrane to assist in the supply of the distal part of the posterior compartment. In addition, muscles on the lateral aspect of the compartment may receive blood from branches of the radial artery. The arteries of the compartment are accompanied by venae comitantes that drain into veins accompanying the brachial artery.
On the dorsum of the wrist, the radial artery enters the anatomical snuff box from the anterior compartment of the forearm deep to the tendons of abductor pollicis longus and extensor pollicis brevis. The artery crosses the floor of the snuff box and leaves the dorsum of the hand by penetrating the first dorsal interosseous muscle to enter the palm.
The posterior compartment of the forearm is supplied by the radial nerve, which leaves the arm by penetrating the lateral inter-muscular septum. Anterior to the elbow, the nerve lies between brachialis and brachioradialis and divides into superficial and deep branches (Fig. 3.76). The superficial branch continues distally through the forearm covered by brachioradialis, leaving its posterior border near the wrist and crossing the snuff box to terminate as cutaneous branches on the dorsum of the hand (Fig. 3.78). The deep branch, the posterior interosseous nerve, arises from the radial nerve at the level of the neck of the radius and enters the posterior compartment by passing between the two heads of supinator (Fig. 3.77). Initially, it accompanies the posterior interosseous artery but distally lies more deeply on the interosseous membrane. The posterior interosseous nerve gives branches to the elbow, radioulnar and wrist joints, and supplies most of the posterior compartment muscles. Brachioradialis and extensor carpi radialis longus are supplied directly by the radial nerve from branches arising in the arm, and anconeus is supplied by the branch of the radial nerve to the medial head of triceps. Damage to the radial nerve or to its posterior interosseous branch may weaken the extensor muscles of the wrist and fingers, so that they are unable to overcome gravity, causing‘wrist drop’.