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’).
Deep muscles
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’.
