Surgical Exposure of Peripheral Nerves of the Upper Extremity. I: Median Nerve

Surgical exposure of the median nerve in the arm and forearm is used for decompression of nerve entrapments, resection of nerve sheath tumors, as well as repair of injured nerves with nerve grafts or transfers. For pathology involving long nerve segments, a combination of two or more approaches may be necessary. In this chapter, we present the relevant surgical anatomy of the median nerve in the arm and forearm and then will illustrate its clinical relevance with a discussion of the various median nerve entrapments (carpal tunnel syndrome is presented in Chapter 53).

Entrapment of the median nerve in the arm and forearm occurs much less frequently than carpal tunnel syndrome. Nevertheless, patients afflicted by these entrapment neuropathies require accurate diagnosis and proper treatment. A thorough history and physical examination can differentiate these entrapments from more commonly encountered cervical radiculopathies. Nerve conduction studies and electromyography may also assist in the diagnosis. Conservative treatment often is sufficient, but surgical intervention may be warranted in persistent or progressive cases.

We describe the relevant anatomy of the median nerve, followed by a discussion of the diagnosis and management of entrapment neuropathies of the median nerve in the arm and forearm.

67.2 Surgical Anatomy of the Median Nerve

The median nerve is formed by the medial and lateral cords of the brachial plexus and receives contributions from nerve roots at C6, C7, C8, and T1 levels; upper roots generally serve sensory functions of the nerve; lower roots contribute the motor portions. The origin of the median nerve occurs at the medial aspect of the coracobrachialis muscle just anterior to the third part of the axillary artery. As it enters the arm, the median nerve is situated in a lateral course with the brachial artery just medial to it; both structures travel anterior to the coracobrachialis muscle and medial to the biceps brachii muscle. There are usually no motor branches arising from the median nerve in the arm. In the midarm, the nerve then passes over the brachial artery to establish a course medial to the artery. After this point, the median nerve and brachial artery enter the antecubital fossa in a medial position relative to biceps brachii tendon and in a superficial position relative to the brachialis muscle. To summarize, from medial to lateral, the order of structures can be remembered as Nerve, Artery, Tendon in this area.

Continuing on in the cubital fossa, the median nerve and brachial artery dive beneath the bicipital aponeurosis, also known as the lacertus fibrosus, which occurs as a strong fibrous band between the biceps brachii tendon and antebrachial fascia. During forearm pronation, the bicipital aponeurosis normally tightens as a result of rotation of both the bicipital tuberosity of the radius and biceps tendon. As it exits the cubital fossa toward the forearm, the median nerve runs between the two heads of the pronator teres. It is just after this point or while in the cubital fossa that the anterior interosseous nerve (AIN) arises from the median nerve, approximately 5 to 8 cm distal to the lateral epicondyle. The AIN innervates the radial half of the flexor digitorum profundus (FDP), the flexor pollicis longus (FPL), and the pronator quadratus muscles. The anterior interosseous nerve may send a branch to the ulnar nerve, also known as the Martin–Gruber anastomosis. Next, approximately 2 cm distal to the ulnar head of the pronator teres, the median nerve passes beneath the fibrous tissue of the heads of the flexor digitorum superficialis (FDS) muscle, after which it runs between the FDP and FDS muscles toward the wrist. The last major branch of the forearm is the palmar cutaneous branch, which is entirely sensory; this branch arises approximately 5.5 cm proximal to the radial styloid.

At the wrist, the median nerve bends around the FDS tendons laterally into a superficial position, situated between the palmaris longus and flexor carpi radialis muscle tendons. Once it reaches the wrist crease, the median nerve enters the carpal tunnel, covered by the flexor retinaculum, or transverse carpal ligament (TCL), superficially. The remainder of the carpal tunnel is formed by the hook of hamate and pisiform medially, the scaphoid and trapezium laterally, and the palmar carpal ligament deeply. The distal aspect of the TCL is thicker and under more tension than the proximal part, in general. Beyond the carpal tunnel, the median nerve splits into its terminal branches: the recurrent motor branch to the thenar muscles, a palmar digital nerve to the radial side of the index finger, and three common palmar digital nerves to the first four fingers. The recurrent motor branch reaches the thenar muscles laterally by passing distal to the flexor retinaculum through an oblique fascia and inserting on the underside of the palmar aponeurosis. It may also reach the thenar muscles by running distal to the flexor retinaculum but not through the oblique fascia or, rarely, passing through the flexor retinaculum itself. ¹

67.3 Entrapment Neuropathies of the Median Nerve: Patient Selection and Surgical Approaches

67.3.1 Ligament of Struthers

Patient Selection

The median nerve is rarely entrapped in the arm; the ligament of Struthers is the most common site. The ligament of Struthers is connected to a supracondylar spur, located ~ 5 cm proximal to the medial epicondyle and present in ~ 1% of people. ²^, ³^, ⁴ This ligament may also be present without the bony spur. ⁵ This is a site of compression of the median nerve as it travels deep to the ligament. ⁶ Patients have deep, aching pain in the elbow region and proximal forearm, hand, and pronation weakness and numbness along the distribution of the median nerve. The symptoms may be worsened by repetitive supination and pronation of the forearm. ⁷ Palpation along the distal medial border of the humerus may identify the spur, and there may be an associated Tinel’s sign over the entrapped median nerve. Radiographs with an internally rotated elbow in the oblique view may demonstrate the supracondylar spur. ⁸

Electrophysiological studies that demonstrate evidence of pronator teres muscle involvement may further assist with the localization process. Surgical treatment of this entrapment neuropathy involves removal of the bony process and cutting the ligament. ⁹ To prevent recurrence, it is suggested that the bony spur’s periosteum should also be removed at the time of surgery. ¹⁰^, ¹¹^, ¹²

Operative Procedure

The patient is positioned supine on the operating table with the arm abducted and supinated on a hand table or double arm board. A linear incision in the medial arm along the intermuscular septum between the biceps and triceps muscles is recommended ( ▶ Fig. 67.1). The biceps and triceps muscles are gently retracted, revealing the median nerve and the brachial artery in the proximal aspect of the incision ( ▶ Fig. 67.2). As the nerve is followed distally, it passes under the ligament of Struthers together with the brachial artery. Decompression is achieved by dividing this ligament, removal of the supracondylar spur if applicable, and verifying that there is no proximal or distal compression.

Fig. 67.1 Surgical exposure of the median nerve in the arm. A linear incision is made in the medial arm along the intermuscular septum between the biceps and triceps muscles.

Fig. 67.2 The median nerve is identified in the rostral aspect of the exposure after gentle retraction of the biceps and the triceps muscles. The nerve is followed distally as it passes under the ligament of Struthers together with the brachial artery. (Reproduced with permission from The Cleveland Clinic, Division of Education, Cleveland, Ohio.)

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