Relevant Anatomy Nerve roots from C5 to T1 contribute to the formation of the posterior cord of the brachial plexus, which branches to create the radial (larger) and axillary (smaller) nerves. At its origin, the radial nerve lies posterior to the third part of the axillary artery and anterior to the subscapularis, teres major, and tendinous insertion of the latissimus dorsi. The nerve makes its way toward the humerus, traveling behind the subscapular artery and anterior to the long head of the triceps. It then meets the deep brachial artery. Together, these structures move caudally and posteriorly through the triangular interval into the posterior compartment of the arm. The interval’s borders are marked medially by the long head of the triceps, laterally by the shaft of the humerus, and superiorly by the teres major. Soon after emerging from the triangular interval, the radial nerve and its companion vessel move along the spiral groove, a bony channel that passes inferolaterally around the posterior aspect of the humerus between the origins of the medial and lateral heads of the triceps. Motor branches innervate the three heads of the triceps; these branches form soon after the emergence of the radial nerve from the posterior cord or as the nerve wraps around the posterior humerus. A motor branch to the anconeus muscle leaves the radial nerve while it is still in the spiral groove. The radial nerve is found lateral to the humerus after it exits the spiral groove. Here ~ 53% of the way down the length of the humerus, the radial nerve traverses the lateral intermuscular septum and enters the flexor compartment of the arm between the brachialis and brachioradialis muscles. 1 In the distal arm, the radial nerve innervates a portion of the brachialis and the entire brachioradialis muscles. Sensory branches, derived predominantly from C5 and C6, are given off in the arm and supply the dorsolateral arm and posterior forearm. The radial nerve advances anterior to the lateral epicondyle and then continues caudally between the brachioradialis and the extensor carpi radialis longus. In the elbow region, the radial nerve (or branches) supplies the extensor carpi longus radialis longus and brevis. Bifurcation of the radial nerve often takes place at about the elbow to form the deep and superficial branches of the radial nerve. The superficial branch of the radial nerve, the smaller of the two branches, passes beneath the medial lip of the brachioradialis. Deep to this branch lie the supinator, pronator teres, flexor digitorum superficialis, and flexor pollicis longus muscles. At the level of the wrist, the sensory branch of the radial nerve travels dorsal to the scaphoid bone in the anatomical snuffbox before providing sensation to part of the dorsum of the hand and fingers. The second terminal branch, the deep branch, travels toward the radial tunnel. When it passes between the humeral and ulnar heads of the supinator muscle, it becomes the posterior interosseous nerve (PIN). Composition of the superficial humeral head is notoriously variable—muscular, tendinous, and/or fibrous—and is often described by its eponym, the arcade of Frohse. The PIN provides primarily motor function via the extensor carpi ulnaris, the finger extensors (extensor digitorum, extensor digiti minimi, extensor pollicis longus and brevis, and extensor indicis), and the abductor pollicis longus, as well as some degree of articular sensation to the wrist. 2 Radial innervation of the supinator may occur proximal to the arcade of Frohse or as the nerve travels between the muscle’s two heads. After the PIN emerges from the distal border of the supinator, it is found adjacent to the posterior interosseous artery between the superficial and deep layers of wrist extensors. There the PIN arborizes and typically divides into two major branches, one that serves the superficial extensors and another that serves the deeper extensors. Further subdivision of the nerve creates branches that innervate each of the extensor muscles ( ▶ Table 68.1). Nerve Muscle Nerve Rootsa Radial Triceps C6, C7, C8, T1 Radial Anconeus C7, C8 Radial Brachioradialis C5, C6 Radial Brachialisb C5, C6 Radial and/or Superficial branch and/or Posterior interosseous branch Extensor carpi radialis longus C5, C6, C7, C8 Radial and/or Superficial branch and/or Deep branch Extensor carpi radialis brevis C6, C7, C8 Radial and/or Posterior interosseous nerve Supinator C5, C6, C7 Posterior interosseous nerve Abductor pollicis longus C7, C8 Posterior interosseous nerve Extensor pollicis brevis C7, C8 Posterior interosseous nerve Extensor pollicis longus C7, C8 Posterior interosseous nerve Extensor indicis C6, C7, C8 Posterior interosseous nerve Extensor digitorum C6, C7, C8 Posterior interosseous nerve Extensor digiti minimi C6, C7, C8 Posterior interosseous nerve Extensor carpi ulnaris C6, C7, C8 aBoldface denotes a major contribution. bPrimarily innervated by the musculocutaneous nerve. Numerous conditions, including entrapment, nerve tumors (benign and malignant), trauma, and inflammation may require surgical exposure of the radial nerve or its branches. Trauma and entrapment are the most common mechanisms. In the axilla, traumatic lesions may affect the radial nerve by itself or involve other nerves of the brachial plexus. Spontaneous lesions (especially ones that progress over time) are rare in this localization. The most commonly observed radial nerve injuries occur in the arm. These may consist of traumatic lesions secondary to humeral shaft fractures or deep puncture wounds. Entrapment may also occur at the humeral shaft. “Saturday night palsy” and “honeymooner’s palsy” both describe entrapment-type injuries caused by prolonged pressure applied to the radial nerve against in the humerus in the arm; surgery is often unnecessary in these entrapment cases, as they often tend to resolve spontaneously within days, weeks, or several months. Compression of the nerve has also been described as occurring by the lateral head of the triceps and lateral intermuscular septum. Entrapment of the radial nerve occurs most commonly as the PIN enters the arcade of Frohse. A fibrous edge marks the entrance to the supinator muscle in 30 to 80% of patients. 3, 4 Compression can be exacerbated by repetitive pronation and supination of the forearm. Posterior interosseous nerve syndrome may coexist with lateral epicondylitis, or tennis elbow. Therefore, it is necessary to differentiate the two syndromes by physical examination and electrophysiological studies. Additional sites of radial nerve entrapment in the elbow/proximal forearm region include the fibrous band ventral to the radiocapitellar joint, recurrent radial vessels across the radial neck, proximal border of the extensor carpi radialis brevis, and distal border of the supinator. 5 Mass lesions (e.g., lipomas or extraneural ganglion cysts) may occur in this area. Compression of the superficial branch of the radial nerve at the wrist, also known as Wartenberg disease or cheiralgia paresthetica, or in the distal forearm (between the tendons of the brachioradialis and extensor carpi radialis longus) has also been described. The proximity of the superficial branch of the radial nerve to the carpal bones makes it susceptible to compression by watchbands, tight casts, and handcuffs (i.e., handcuff palsy) or direct injury during trauma. Patients affected by lesions of the radial nerve have varied symptoms, depending on the location and type of their injury. Symptoms of proximal nerve lesions (in the axilla or proximal arm) can manifest as both motor and sensory loss. Weakness in extension of the elbow, wrist, and digits may be coupled with loss of sensation to the dorsum of the arm, forearm, and/or parts of the hand. The most common site of injury of the radial nerve is in the mid or distal arm after either a humeral fracture or compression (Saturday night palsy). Patients with this localization would have a wrist and finger drop with sensory loss in the proximal forearm and dorsal wrist. Triceps strength would be normal. A more distal injury that affects only the PIN may potentially spare those muscles innervated by the radial nerve proper: triceps, anconeus, part of the brachialis, brachioradialis, extensor carpi radialis longus or brevis, and supinator muscles. Cutaneous sensation will remain intact, but wrist and finger extensors will be weakened. The affected patient exhibits radial wrist deviation while attempting wrist extension; this occurs because the extensor carpi radialis longus or brevis muscles are unopposed by the affected extensor carpi ulnaris. Pain may or may not be present. Isolated lesions of the superficial branch of the radial nerve manifest with loss of sensation alone. Patients complain of paresthesias or dysesthesias in the radial nerve distribution of the dorsum of the hand, index and middle fingers, and thumb. A systematic approach including a dedicated history and physical examination supplemented by other studies can help a physician establish the correct diagnosis and localization. Radial nerve lesions should be distinguished from other neurologic conditions including brachial plexus injuries or tumors (including Pancoast tumors), inflammatory disorders (Parsonage-Turner Syndrome) and cervical radiculopathy. Inflammation of the lateral elbow tendons (“tennis elbow”) can mimic symptoms of radial nerve pathology by presenting with tenderness over the radial nerve in the lateral elbow and poor wrist extension secondary to pain. Electromyography (EMG) and nerve conduction studies are potentially helpful in characterizing the location and extent of nerve injury. For example, EMG of the anconeus muscle, which receives innervation from a branch of the radial nerve that originates in the spiral groove, can delineate where pathology has occurred along the nerve’s course. Abnormal sensory and motor latencies across particular nerve segments may also be useful in identifying areas of nerve entrapment. Radiographs are used when diagnosing certain conditions that underlie neural injury, including posttraumatic humeral fracture. Magnetic resonance imaging (MRI) or ultrasound offers superior soft tissue visualization, a beneficial clinical tool when a neoplasm or similar mass is suspected. High-resolution MRI may show signal abnormality in the nerve at a site of injury or entrapment or denervation changes in the affected muscles. Ultrasound can document integrity of the nerve in cases of trauma. Diagnostic nerve blocks can also be considered in patients with pain syndromes. Specifics regarding preoperative preparation depend upon the operation being performed. Surgery is performed under local, regional, or general anesthesia. Tourniquets may be of assistance in operations of the more distal extremity but are not used during proximal nerve exposure. A beanbag may be used to maintain or alter positions intraoperatively. The choice of surgical approach to the radial nerve should be tailored to the anatomical portion of the nerve in which the pathology is thought to reside. In some cases, a combination of approaches may be necessary. Exposure of the nerve proximal to the triangular interval, defined by the medial and long heads of the triceps, can be achieved in the supine position with the arm externally rotated. The standard infraclavicular brachial plexus exposure, using an incision running along the deltopectoral groove, can be extended in the arm along the groove between the biceps and triceps ( ▶ Fig. 68.1). This approach allows visualization of the transition from the posterior cord into the radial nerve. To access the nerve past the triangular interval, an incision is made along a line marked from the acromion to the olecranon process. If the proximal radial nerve must be accessed in the pre- and post-triangular space, the patient is preferably placed in the lateral position with the arm mobile such that it can be rotated medially for access to the posterior arm or rotated laterally for access to the axilla and deltopectoral groove. Fig. 68.1 Surgical exposure of the radial nerve in the axilla and proximal arm. The standard infraclavicular brachial plexus exposure can be used and extended distally. The incision runs along the deltopectoral groove and extends along the groove between the biceps and triceps. Alternatively, the arm may be abducted and flexed to facilitate external rotation, enhancing exposure. (Reproduced with permission from The Cleveland Clinic, Division of Education, Cleveland, Ohio.)
Brachial Plexus and Arm
Elbow, Forearm, and Hand
68.1.2 Pathoanatomy
68.2 Patient Selection
68.2.1 Clinical Presentation
68.2.2 Differential Diagnosis
68.2.3 Useful Studies
68.3 Preoperative Preparation
68.4 Operative Procedure
68.4.1 Exposure of the Radial Nerve in the Arm
Axillary Approach
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