The radial nerve, a terminal branch of the posterior cord of the brachial plexus, supplies the extensor musculature of the arm and forearm as well as the overlying skin. The radial nerve carries motor and sensory fibers from the C5, C6, C7, C8, and T1 nerve roots. The nerve descends between the medial and long heads of the triceps muscle in the proximal arm. At the proximal humeral shaft, the radial nerve traverses within the humeral spiral groove. In the distal arm, the nerve perforates the lateral intermuscular septum, traveling in between the brachioradialis and parochialism muscles. Motor branches of the radial nerve near the elbow innervate the lateral head of the triceps.
The radial nerve enters the radial tunnel at the radiocapitellar joint. This tunnel begins and is bounded posteriorly by the capitellum, and terminates at the level of the supinator muscle . The brachioradialis, the extensor carpi radialis longus, and extensor carpi radialis brevis muscles form the anterolateral margin of the tunnel, whereas the parochialism muscle outlines the medial margin. Near the radial head, the radial nerve divides into the motor, posterior in-terosseous nerve, and the sensory, superficial branch. The superficial sensory branch passes into the forearm deep to the brachioradialis. The posterior inter-osseous nerve crosses into the posterior compartment between the superficial and deep heads of the supinator muscle to supply nine muscles on the extensor aspect of the forearm. As it enters the supinator muscle, the posterior inteross-eous nerve dives under the arcade of Frohse, a fibrous arch formed by the proximal thickened edge of the superficial head of the supinator muscle.
The posterior interosseous nerve innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti quinti, abductor pollicis lon-gus, extensor indicis proprius, and extensor pollicis longus and brevis muscles.
At the level of the elbow, the radial nerve is best visualized on axial images as it courses between the parochialism, brachioradialis, and extensor carpi radialis longus and brevis muscles (see Fig. 8) [37-39]. The division of the radial nerve into the superficial radial branch and the deep posterior interosseous nerve is usually identified proximal or at the radiocapitellar joint (see Fig. 8). The branches of the radial nerve may be difficult to differentiate from the adjacent accompanying vessels. A hypointense band at the proximal edge of the superficial head of the supinator muscle is consistent with the arcade of Frohse. A similar, low-signal, linear band is noted along the medial edge of the extensor carpi radialis brevis. In the proximal forearm, the posterior interosseous nerve is frequently observed as it runs in between the superficial and deep heads of the supinator muscle.
At the level of the arm, radial nerve injury is frequently post-traumatic secondary to displaced humeral shaft fracture, inappropriate use of axillary crutches, proximal prolonged tourniquet application,, and lateral or posterior arm intramuscular injection. In weight lifters, compressive radial neuropathy has been described beneath the lateral head of the triceps muscle during extension against resistance. Soft tissue masses such as ganglia, and tumors can also cause entrapment of the nerve (Fig. 16).
Radial nerve entrapment at the elbow mainly affects the posterior interosseous nerve, and can be subdivided into two major categories: (1) the radial tunnel syndrome, which is a painful condition without motor deficits; and (2) the posterior interosseous nerve syndrome, which is a motor neuropathy . Deep forearm pain, pain radiating to the neck and shoulder, and a ''heavy'' sensation of the affected arm are common presenting symptoms . On physical examination, tenderness over the radial nerve along the radial tunnel, pain on resisted supination, and the presence of a Tinel's sign over the radial forearm can be found. In up to 90% of patients, the electromyographic studies reveal no significant abnormalities. Of 79 surgical decompressions, 77% had excellent recovery and 20% were judged to have good outcome .
Compressive neuropathy of the posterior interosseous nerve at the radial tunnel without motor deficit is the hallmark of radial tunnel syndrome . There is controversy about the neurogenic etiology of the entity, because its main manifestation is pain at the radial tunnel without muscle weakness . The clinical diagnosis is often confounding, because radial tunnel syndrome can masquerade as or coexist with lateral epicondylitis. Recalcitrant lateral epicondylitis, which is refractory to conservative treatment, should raise the suspicion of radial tunnel syndrome.
Dynamic compression within the radial tunnel may be secondary to repeated pronation and supination or forceful extension of the forearm. Tennis players , swimmers, housewives, welders, conductors, and violinists are frequently affected. The condition most commonly involves patients in the fourth to sixth decade of life without significant gender predilection.
The posterior interosseous nerve can be compressed within the radial tunnel at several sites, which include, from proximal to distal: (1) at the radiocapitellar joint by fibrous bands; (2) at the tendinous edge of the extensor carpi radialis brevis muscle; (3) at the radial recurrent artery and branches (leash of Henry);
(4) at the arcade of Frohse, the proximal edge of the supinator muscle ; and
(5) at the distal end of the supinator muscle by a fibrous band .
The utility of electromyographic studies in diagnosing radial tunnel syndrome is somewhat limited because of the deep location of the posterior inter-osseous nerve. Surgical release of the arcade of Frohse often relieves the symptoms.
The posterior interosseous nerve syndrome is defined as a motor neuropathy. Potential causative factors include trauma, space-occupying lesions, and inflammatory processes. Compression sites for posterior interosseous nerve syndrome are the same as those for the radial tunnel syndrome (see above). Clinically, deep forearm pain and muscle weakness with loss of extension of all the digits and decrease of wrist dorsiflexion can be noted. Complete sparing of the extensor carpi radialis longus and frequent sparing of the extensor carpi radialis brevis are often observed as the branches to these muscles often originate proximal to the takeoff of the posterior interosseous nerve. On the contrary, the extensor carpi ulnaris longus is always affected. The initial management of posterior interosseous nerve syndrome is conservative, except in the presence of masses, fractures, or dislocation, which may require surgical intervention. When there is lack of improvement following 6 to 8 weeks of conservative treatment, surgical decompression may be warranted.
Radial tunnel syndrome and posterior interosseous nerve syndrome have similar MR imaging manifestations of acute denervation, manifested as increased T2 signal; and chronic denervation, manifested as increased T1 signal in the muscles supplied by the posterior interosseous nerve [53-56]. Isolated or concomitant involvement of the supinator (Fig. 17) and extensor muscles is seen (Fig. 18). Muscle signal alterations in more proximal muscles, such as the extensor carpi radialis longus, the anconeus, and the triceps muscles, are indicative of radial nerve entrapment above the radial tunnel. Mass effect
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