Applied Surgical Anatomy of the Anterior Compartment of the Forearm

Applied Surgical Anatomy of the Anterior Compartment of the Forearm

 

 

Overview

Muscles

Two muscle groups form the musculature of the anterior aspect of the forearm: The mobile wad of three (the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis), which is supplied by the radial nerve, forms the lateral border of the supinated forearm; and the flexor-pronator muscles, which are supplied by the median and ulnar nerves, comprise the rest.

The flexor-pronator group is arranged in three layers. In the superficial layer,  muscles arise from the common flexor origin on the medial humeral epicondyle and fan out across the forearm. They are easy to remember by the following simple maneuver. Place the butt of the opposite hand over the medial epicondyle, with the palm on the anterior surface of the forearm: The thumb points in the direction of the pronator teres, the index finger represents the flexor carpi radialis, the middle finger represents the palmaris longus, and the ring finger represents the flexor carpi ulnaris (Figs. 4-11 and 4-12).

The middle layer consists of the flexor digitorum superficialis (Fig. 4-13).

The deep layer comprises three muscles: The flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus. (A th deep muscle, the supinator, is critical to the surgical anatomy of the area, but is not strictly a flexor muscle [Fig. 4-14].)

The keys to the surgical anatomy of the anterior aspect of the forearm are the following three practical internervous planes that are used in operative approaches:

1. Between the radial and median nerves: A dissection between the brachioradialis muscle, the most medial of the three muscles forming the “mobile wad of three” (which is supplied by the radial nerve), and the flexor carpi radialis and pronator teres muscles, the most lateral of the flexor-pronator group (which are supplied by the median nerve; see Fig. 4-3).

    

    

    

   Figure 4-11 Superficial layer of the forearm muscles and vessels.

    

    

   Figure 4-12 The superficial layer of the forearm has been resected, revealing the vessels and nerves. The median nerve pierces the gap between the two heads of the pronator teres. Note the leash of vessels of the radial artery and the recurrent radial artery.

    

    

    

   Figure 4-13 The middle layer of the forearm, with the superficial branch of the radial nerve. In the proximal part of the wound, the median nerve enters the undersurface of the superficialis.

   Flexor Carpi Radialis. Origin. Common flexor origin on medial epicondyle of humerus. Insertion. Bases of second and third metacarpals. Action. Flexor and radial deviator of wrist. Nerve supply. Median nerve.

    

    

    

   Figure 4-14 The deep layer of the forearm. The ulnar nerve and artery and the median nerve lie on the flexor digitorum profundus. Note the position of the anterior interosseous nerve and artery.

    

2. Between the median and ulnar nerves: A dissection between the flexor carpi ulnaris muscle (which is supplied by the ulnar nerve) and the flexor digitorum superficialis muscle, the most medial of the flexor muscles (which is supplied by the median nerve; see Fig. 5-29).

3. Between the ulnar and posterior interosseous nerves: A dissection between the flexor carpi ulnaris muscle (which is supplied by the ulnar nerve) and the extensor carpi ulnaris muscle (which is supplied by the posterior interosseous nerve; see Fig. 4-19).

4. The first of these planes is used in the anterior approach to the radius, the second exposes the ulnar nerve in the forearm, and the third is used for exposure of the ulna.

Nerves and Vessels

The neurovascular architecture of the anterior aspect of the forearm is relatively simple: The forearm is “framed” by its nerves. The superficial

radial nerve runs down the radial aspect of the forearm, with the radial artery lying on its medial side in the distal half of the forearm (see Fig. 4-13). The ulnar nerve runs down the ulnar side of the forearm, with the ulnar artery lying on its lateral side in the distal half of the forearm. The median nerve runs down the middle of the forearm (see Fig. 4-14).

Both the radial and the ulnar arteries are arteries of transit in the forearm; they both are branches of the brachial artery. Because the brachial artery lies in the middle of the anterior aspect of the elbow, with the median nerve lying on its medial side, the ulnar artery and median nerve must cross in the upper forearm, with the nerve superficial to the artery; this crossing occurs at the level of the musculotendinous region of the pronator teres muscle (see Fig. 4-13). The anterior interosseous nerve (which is a branch of the median nerve) and the anterior interosseous artery (which is a branch of the common interosseous artery, which itself is a branch of the ulnar artery) also run down the middle of the forearm, but deeper than the median nerve (see Fig. 4-14).

Incision

Because the incision runs transversely across the lines of cleavage in the forearm, the resultant scar may be broad. Making the incision as a series of gentle curves brings the skin incision closer to the lines of cleavage in the forearm. Such an incision has the effect of reducing tension on the subsequent skin repair.

 

Superficial Surgical Dissection and Its Dangers

Muscles

Superficial surgical dissection opens the plane between the mobile wad of three muscles (the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis) and the pronator teres muscle proximally and flexor carpi radialis muscle distally (see Fig. 4-11).

The mobile wad of three muscles, on the radial side of the forearm, is supplied by the radial nerve. All three muscles take some of their origin from the common extensor origin on the lateral epicondyle of the humerus (see Fig. 4-13).

The brachioradialis pronates the forearm when it is supinated and supinates it when it is pronated. Therefore, it may act as a deforming force in distal radial fractures if the forearm is immobilized in either full pronation or full supination after reduction of the fracture. Its action is one reason for immobilizing distal radial fractures with the forearm in the

neutral position.

The brachioradialis is the only muscle in the body to take origin from the distal end of one bone and insert onto the distal end of another (Fig. 4-15; see Fig. 4-11).

During recovery from high radial nerve palsy, the extensor carpi radialis longus is one of the first muscles to be reinnervated. If the patient recovering from a high radial nerve palsy is asked to extend the wrist, the wrist extends with radial deviation, because the balancing muscle, the extensor carpi ulnaris receives its nerve supply from the posterior interosseous nerve further distally. Reinnervation of the brachioradialis, however, probably is the best way to diagnose both clinically and electrically (by electromyographic studies) a recovering high radial nerve palsy (see Figs. 4-12 and 4-22).

The extensor carpi radialis brevis muscle is a wrist extensor that deviates the wrist neither toward the radius nor toward the ulna. It may be involved in tennis elbow—lateral epicondylitis.

Nerves and Vessels

Palsies of the posterior interosseous nerve caused by compression of the nerve by the tendinous origin of the extensor carpi radialis brevis muscle have been described.

Two structures that lie under the brachioradialis muscle must be preserved during superficial surgical dissection:

1. The radial artery originates from the brachial artery in the cubital fossa. Proximally, it lies just medial to the biceps tendon in a somewhat superficial position. The radial artery angles across the arm as it descends, lying on the supinator, the pronator teres, the origin of the flexor pollicis longus, and the lower part of the anterior surface of the radius, where it can be palpated easily (see Fig. 4-13).

2. The superficial radial nerve is purely sensory in the forearm. It runs along the lateral side, crossing the supinator, the pronator teres, and the flexor digitorum superficialis. Damage to the nerve in the forearm produces an area of diminished sensation on the dorsoradial aspect of the hand. The most important problem associated with such damage is not the sensory loss, however, but the painful neuroma that may result. The nerve runs lateral to the radial artery when the two are together (see Figs. 4-13 and 4-32).

 

Deep Surgical Dissection and Its Dangers

Five muscles must be detached from the radius to expose fully the anterior aspect of the bone. From proximal to distal, they are as follows:

1. The supinator

2. The pronator teres

3. The flexor digitorum superficialis

4. The flexor pollicis longus

5. The pronator quadratus

The nerve supply of the supinator muscle, the posterior interosseous nerve, passes through a fibrous arch known as the arcade of Frohse as it enters the muscle (see Figs. 4-12 and 4-13).6 The arch is formed by the thickened edge of the superficial head of the supinator. Compression of the nerve at that point produces paralysis or dysfunction of all the extensor muscles of the forearm, fingers, and thumb, a lesion that may be incomplete. Compression at the arcade of Frohse is one of the causes of a posterior interosseous nerve entrapment syndrome7 and can be relieved by incising the fibrous arch.8–11 It also is a cause of pain restricted to this area, which may present as a resistant “tennis elbow” (see Fig. 4-13).12

The nerve supply of the pronator teres, the median nerve, enters the forearm between the muscle’s two heads of origin (see Fig. 4-12). The great variations that occur in the site, size, and quality of the ulnar head of the muscle sometimes cause the nerve to become trapped as it traverses the muscle, producing the pronator syndrome, which mimics the carpal tunnel syndrome, but includes pain and paresthesia to the proximal end of the volar aspect of the forearm.13,14 Understandably, the syndrome occurs when the muscle contracts and further compresses the nerve. In this syndrome, the intrinsic muscles of the thumb become weak, but the muscles that are innervated by the anterior interosseous nerve (the flexor pollicis longus, the flexor profundus to the index and middle fingers, and the pronator quadratus) are spared (see Fig. 4-12).

The median nerve passes under the fibrous arch of origin of the flexor digitorum superficialis. It may be compressed by a thickened arch, producing pain or a median nerve palsy (see Fig. 4-13).15 The tendons of the muscle form well above the wrist. Functionally, it is separate muscles; it can flex each finger independently, in contrast to the mass action of the flexor digitorum profundus.

Part of the origin of the flexor digitorum superficialis may have to be detached to expose the anterior part of the shaft of the radius (see Figs. 4-13 and 4-15).

The origin of the flexor pollicis longus, which is the sole long flexor of the thumb, must be stripped off the radius for the bone to be accessible (see Figs. 4-14 and 4-15).

The insertion of the pronator quadratus must be stripped off to expose the distal th of the radius (see Fig. 4-15). Because the muscle is relaxed totally when the forearm is pronated fully, some authors suggest that distal radial fractures should be immobilized in pronation. Clearly, however, the pronator quadratus is not the only possible deforming force on the distal radius; the best position for immobilizing reduced fractures of the distal radius still is a matter of debate and many authors now favor nonoperative treatment of such fractures by reduction followed by immobilization in supination (Fig. 4-16).16

 

 

 

Figure 4-15 The origins and insertions of the muscles of the forearm. Note the anterior interosseous artery lying on the interosseous membrane.

 

Brachioradialis. Origin. Upper two-thirds of lateral supracondylar ridge of humerus. Insertion. Styloid process of radius. Action. Flexor of elbow. Pronator and supinator of forearm. Nerve supply. Radial nerve.

Flexor Digitorum Superficialis. Origin. Medial epicondyle of humerus, medial ligament of elbow, medial border of coronoid process of ulna, fibrous arch connecting coronoid process of ulna with anterior oblique line of radius. Insertion. Volar aspect of middle phalanges of fingers. Action.

Flexor of proximal interphalangeal joints, metacarpophalangeal joints, and wrist joint. Nerve supply. Median nerve.

Flexor Pollicis Longus. Origin. Middle part of anterior surface of radius. Insertion. Distal phalanx of thumb. Action. Main flexor of thumb. Nerve supply. Anterior interosseous nerve.

Pronator Quadratus. Origin. Lower th of volar surface of ulna. Insertion. Lower th of lateral aspect of radius. Action. Weak pronator of forearm. Nerve supply. Anterior interosseous nerve.

Palmaris Longus. Origin. Common flexor origin on humerus. Insertion. Palmar aponeurosis. Action. Weak flexor of wrist. Nerve supply. Median nerve.

Flexor Digitorum Profundus. Origin. Upper three-ths of anterior surface of ulna. Insertion. Distal phalanges of fingers. Action. Flexor of distal interphalangeal joints, proximal interphalangeal joints, metacarpophalangeal joints, and wrist joint. Nerve supply. Dual nerve supply from the anterior interosseous branch of the median nerve and the ulnar nerve.

Flexor Carpi Ulnaris. Origin. From two heads. Humeral head: from common flexor origin on medial epicondyle of humerus. Ulnar head: from medial border of olecranon and upper three-ths of subcutaneous border of ulna. Insertion. Hamate and fifth metacarpal. Action. Flexor and ulnar deviator of wrist. Also weak flexor of elbow. Nerve supply. Ulnar nerve.

 

 

Dang

 

 

Nerves

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

The posterior interosseous nerve is vulnerable during all approaches to the upper third of the radial shaft. Although the nerve can be protected if the insertion of the supinator is detached and the muscle is stripped off the bone subperiosteally, it can be argued that the only certain protection as the upper third of the radius is plated comes from identifying and preserving the nerve via a posterior approach (see Figs. 4-32 and 4-33).

 

 

 

Figure 4-16 The bones of the forearm.

 

Proximally, the median nerve usually passes between the heads of the pronator teres muscle, whereas the ulnar artery passes deep to both the heads. Distal to the pronator teres, the median nerve joins the ulnar artery and passes beneath the fibrous arch of origin of the flexor digitorum superficialis muscle. Then, it runs down the flexor aspect of the forearm, roughly in the midline (see Figs. 4-13 and 4-14).

Because of its proximity to the flexor digitorum superficialis muscle, the median nerve sometimes is mistaken for the superficial tendon to the index finger. To differentiate nerve from tendon, try to find an artery on the structure in question: The median nerve has the median artery running along its surface. The artery, derived from the anterior interosseous artery,

is the original fetal axial artery (see Fig. 4-14).

 

Special Anatomic Points

 

The main surgical use of the palmaris longus muscle is as a graft for tendon repairs. Because it is absent in 10% of the population, it must be identified in the conscious patient before surgery is undertaken. To find it, instruct the patient to touch the thumb and little fingers together while flexing the wrist against resistance. Then, palpate the tendon, which stands out prominently in the forearm (see Fig. 4-11).

Note that the median nerve is immediately below the palmaris longus at the wrist. In the patient with an absent palmaris longus, the nerve actually may be mistaken for the tendon (see Fig. 4-11).

The tendons of the flexor digitorum profundus arise at or below the level of the wrist joint. Therefore, contraction of the muscle produces movement in all tendons, making it a mass action muscle that is used mainly for power grip.

The anterior interosseous nerve arises from the median nerve shortly after the median nerve enters the forearm; the two lie under the tendinous origin of the superficial head of the pronator teres (see Figs. 4-12 and 4-13). The anterior interosseous nerve may be compressed at this point, producing the anterior interosseous nerve syndrome: Paralysis of the flexor pollicis longus and flexor profundus tendons to the index and middle fingers, as well as of the pronator quadratus muscle.17–19