Applied Surgical Anatomy of the Posterior Approach to the Radius

Overview

Twelve muscles appear on the dorsal aspect of the forearm. They are divided into three groups, as follows:

1. The mobile wad of three (the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis) runs along the lateral side of the forearm. These three muscles arise from a continuous line on the lateral supracondylar ridge and lateral epicondyle of the humerus.

2. The  superficial extensor muscles fan out from the lateral epicondyle of the humerus. From the ulnar to the radial side of the forearm, they consist of the anconeus, the extensor carpi ulnaris, the extensor digiti minimi, and the extensor digitorum communis (Figs. 4-31 and 4-32). One internervous plane exists in this layer of musculature: Between the extensor carpi ulnaris muscle (which is supplied by the posterior interosseous nerve) and the flexor carpi ulnaris muscle (which is supplied by the ulnar nerve) on the ulnar side (see Fig. 4-19). The intermuscular plane between the extensor carpi radialis brevis muscle27 (which is supplied by the posterior interosseous nerve proximal to the supinator muscle) and the extensor digitorum communis muscle (which is supplied by the posterior interosseous nerve in the forearm) is utilized in the posterior approach to the radius. The plane is safe to use in the forearm because the extensor carpi radialis brevis receives its nerve supply well proximal to the dissection (see Fig. 4-25).

3. Of the five deep muscles, three (the abductor pollicis longus, the extensor pollicis brevis, and the extensor pollicis longus) supply the thumb. The three cross the forearm obliquely from the ulnar to the radial side, and two of them (the abductor pollicis longus and the extensor pollicis brevis) wind around the dorsal and lateral aspects of the radius. The remaining two muscles of the deep group are the supinator and the extensor indicis (Fig. 4-33).

The critical nerve in the area, the posterior interosseous nerve, innervates the muscles of the extensor compartment; it is the key anatomic structure from an operative point of view. The only major arterial supply of the compartment is the posterior interosseous artery.

Figure 4-31 Superficial muscles of the posterior aspect of the forearm.

Figure 4-32 The superficial muscles have been removed to reveal the course of the posterior interosseous nerve as it enters the supinator muscle through the arcade of Frohse and the course of the superficial radial nerve, which is sensory and supplies no muscles of the forearm.

Figure 4-33 The course of the posterior interosseous nerve through the supinator muscle, as it runs to supply muscles in the forearm.

Extensor Carpi Ulnaris. Origin. Common extensor origin on lateral epicondyle of humerus and subcutaneous border of ulna. (Shared origin with flexor carpi ulnaris.) Insertion. Base of fifth metacarpal. Action. Extensor and ulnar deviator of wrist. Nerve supply. Posterior interosseous nerve.

Extensor Digitorum Communis. Origin. Common extensor origin on lateral epicondyle of humerus. Insertion. Into extensor apparatus of fingers. Action. Extensor of wrist and fingers. Nerve supply. Posterior interosseous nerve.

Landmarks and Incision

Landmarks

The lateral epicondyle of the humerus, located just lateral to the olecranon process, is smaller than the medial epicondyle, but its lateral supracondylar line, which runs superiorly, is longer than the medial supracondylar line, extending almost to the deltoid tuberosity. The lateral epicondyle is the site of the common origin of the superficial muscles of the extensor compartment of the forearm. The extensor carpi radialis brevis, extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris all originate from fused tendons that attach just anterior to the epicondyle. The brachioradialis and extensor carpi radialis longus arise from the lateral

supracondylar ridge.

Tenderness over the common extensor origin occurs in lateral epicondylitis; the pain that is characteristic of this condition can be reproduced by providing resistance to extension of the wrist.

Compression of the posterior interosseous nerve at the arcade of Frohse may produce a syndrome similar to lateral epicondylitis. In these cases, the tenderness usually is elicited more distally in the course of the posterior interosseous nerve or anteriorly over the arcade of Frohse. Both conditions can exist in a single patient.28

Incision

A longitudinal incision on the posterior aspect of the forearm crosses the lines of cleavage of the skin at right angles and often leaves a broad scar.

Superficial Surgical Dissection

In the proximal half of the wound, dissection opens the plane between the extensor carpi radialis brevis and the extensor communis. In the distal third of the wound, the intermuscular plane lies between the extensor carpi radialis brevis and the extensor pollicis longus (see Fig. 4-31).

Deep Surgical Dissection

For the proximal third of the radius, dissection consists of detaching the insertion of the supinator muscle from the radius while preserving the posterior interosseous nerve (see Fig. 4-30).

For the middle third of the bone, dissection involves mobilizing and retracting two muscles, the abductor pollicis longus and the extensor pollicis brevis (see Fig. 4-29).

In the distal third of the radius, dissection opens the intermuscular plane between the extensor pollicis longus and the extensor carpi radialis brevis.

The proximal third of the radius is covered by the supinator muscle, through which the posterior interosseous nerve passes on its way to the posterior compartment of the forearm (see Figs. 4-32 and 4-33). For additional information on the supinator muscle, see the section regarding the anterior approach to the radius.

Dang

Nerves

The posterior interosseous nerve is the motor nerve of the posterior compartment of the forearm. A branch of the radial nerve, it passes between the two heads of the supinator muscle and actually may come in direct contact with the neck of the radius or the extreme proximal end of the radial diaphysis. At that point, it may be trapped beneath incorrectly positioned plates. After emerging from the supinator muscle, the nerve passes distally over the origin of the abductor pollicis longus muscle to reach the interosseous membrane. It continues distally on the interosseous membrane to the level of the wrist joint, which it supplies with some sensory branches. The nerve supplies those muscles that arise from the common extensor origin and the deep muscles of the extensor compartment of the forearm (see Fig. 4-33).

When performing deep surgical dissection, it is important to remember that the posterior interosseous nerve is vulnerable during all approaches to the proximal third of the radial shaft. Although the nerve can be protected by detaching the insertion of the supinator and stripping the muscle off the bone subperiosteally, the only certain protection of the posterior interosseous nerve during plating of the upper third of the radius may come with full dissection via a posterior approach.

Vessels

The posterior interosseous artery accompanies the posterior interosseous nerve as it runs along the interosseous membrane in the proximal two-thirds of the forearm. The posterior interosseous artery enters the extensor compartment of the forearm by passing between the radius and the ulna through the interosseous membrane (Fig. 4-34). The artery then joins the posterior interosseous nerve distal to the distal edge of the deep head of the supinator muscle.

The posterior interosseous artery is too small to be dissected easily down to the level of the wrist. Most of the blood supply for the posterior area comes from an anterior interosseous artery via branches that perforate the interosseous membrane. The tendons running in this area may have a marginal blood supply.

Although the artery may be damaged during the posterior approach to the radius, good collateral circulation appears to protect the extremity from any functional deficits (Fig. 4-35).

Figure 4-34 The origins and insertions of the muscles of the posterior aspect of the forearm.

Extensor Carpi Radialis Longus. Origin. Lower third of lateral supracondylar ridge of humerus, lateral intermuscular septum of arm. Insertion. Base of second metacarpal. Action. Extensor and radial deviator of wrist. Nerve supply. Radial nerve.

Extensor Carpi Radialis Brevis. Origin. Common extensor origin on lateral epicondyle of humerus and radial collateral ligament of elbow. Insertion. Base of third metacarpal. Action. Extensor and radial deviator of wrist. Nerve supply. Posterior interosseous nerve.

Supinator. Origin. From two heads. Superficial head: from lateral epicondyle of humerus, lateral collateral ligament of elbow, and supinator crest of ulna. Deep head: from supinator crest and fossa of ulna. Insertion. Anterior aspect of radius. Action. Supinator of forearm. Weak flexor of elbow. Nerve supply. Posterior interosseous nerve.

Extensor Pollicis Longus. Origin. Posterior surface of ulna in its middle third and from interosseous membrane. Insertion. Distal phalanx of thumb. Action. Extensor of thumb and wrist. Nerve supply. Posterior interosseous nerve.

Abductor Pollicis Longus. Origin. Posterior surface of ulna, posterior interosseous membrane, and middle third of posterior surface of radius. Insertion. Base of thumb metacarpal. Action. Abductor and extensor of

thumb. Nerve supply. Posterior interosseous nerve.

Extensor Pollicis Brevis. Origin. Posterior surface of radius and interosseous membrane. Insertion. Base of proximal phalanx of thumb. Action. Extensor of proximal phalanx of thumb. Nerve supply. Posterior interosseous nerve.

Extensor Indicis. Origin. Posterior surface of ulnar shaft and interosseous membrane. Insertion. Extensor apparatus of index finger via ulnar side of tendon of extensor digitorum that runs to index finger. Action. Extensor of index finger. Nerve supply. Posterior interosseous nerve.

Extensor Digiti Minimi. Origin. Common extensor origin on lateral epicondyle of humerus. Insertion. Extensor apparatus of little finger. Action. Extensor of little finger. Nerve supply. Posterior interosseous nerve.

Figure 4-35 The bones of the posterior aspect of the forearm.