Median Nerve Repair/Graft
All nerve repairs in the upper extremity are performed for the purposes of restoring motor and sensory function to the arm and hand. Hand function is obviously predicated on appropriate nerve function, and loss of this function may be due to injury by compression or open trauma, tumor resection, or metabolic problems. This chapter will deal primarily with loss of median nerve function secondary to open traumatic injury to the nerve. The median nerve is usually thought of as the “primary” nerve of hand function, as it provides important motor and sensory function of the hand.
All injured nerves undergo Wallerian degeneration of the axons distal to the site of injury, and thus all patients undergoing nerve repairs will have a variable period of time before function returns, obviously depending on the distance from the injury to the target organ. Although the sensory end-organs typically do not undergo degeneration over time (and thus are available for reinnervation for some time after the injury), the motor end plates supplying nerve function to muscle do suffer from fallout over time. Thus the timing of nerve repair is important, and in most instances, nerve repair should be performed as soon as possible after the initial injury. This approach obviously lessens the time for functional return and potentially gives better motor recovery.
Nearly every patient with an injury to the median nerve should at least be considered a candidate for repair. The ideal candidate would be a younger patient (< 60 years) with a sharp laceration of the median nerve in the distal forearm. Early repair would provide the best chance for return of full function in the hand in this individual.
Again, even more elderly patients should be considered for repair with distal injuries, as loss of sensation in the radial side of the hand is a devastating functional problem. Even if these individuals may not regain full motor function of the thenar muscles, repair should probably be considered an option if there are no overriding systemic medical problems that would preclude operative intervention.
The timing of repair of the median nerve should almost always be immediate, but in those patients with severe soft-tissue injury or severe contamination, repair and grafting should be delayed, at least until a stable soft-tissue envelope has been achieved. This would also apply in patients with a severely unstable bony injury, in whom repair of the nerve should not be undertaken until some form of bone stability is provided (via internal or external fixation).
Patients who suffer from very severe proximal injuries to the median nerve may not be good candidates if they are elderly (owing to the length of time necessary for nerve regeneration to the hand).
Patients with very large gaps (> 6 cm) in the median nerve may or may not be candidates for repair, depending on their general status, the status of the wound, and the availability of adequate material for nerve grafting of the gap in the median nerve. As noted above, repair or grafting should be delayed until adequate bone stabilization and stable soft-tissue coverage have been achieved. It should be noted, however, that delay in repair of a sharply lacerated nerve (for whatever reason) should be avoided if at all possible.
Delay over several months will lead to neuroma formation in the proximal stump, which will necessitate excision of a portion of the nerve, possibly necessitating nerve grafting, which will likely adversely affect the eventual functional recovery (compared with early primary repair).
Examination of the patient with a suspected median nerve injury should focus on the median-innervated sensory area of the hand and the median-innervated muscles (primarily the flexor digitorum superficialis, flexor carpi radialis, and thenar muscles, as discussed below).
The median or ulnar nerve may send motor fibers to the other nerve in the proximal forearm, the so-called Martin-Gruber anastomosis. More commonly this will be the median or anterior interosseous nerve sending fibers to the ulnar nerve, but rarely the ulnar will send fibers to the median. This may lead to sparing of the function of the thenar muscles in the face of a complete median nerve laceration above this point. One should be aware of these potential anomalies when examining a patient with a suspected median or ulnar nerve injury.
The median nerve is composed of fibers from the sixth, seventh, and eighth cervical nerves. It forms from a coalescence of the lateral and medial cord just above the axilla and runs in the medial aspect of the upper arm ( Fig. 11.1 ).
The median nerve provides no innervation in the upper arm, but in the forearm it typically supplies motor fibers to the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum profundus as it passes beyond the antecubital fossa.
The anterior interosseous nerve arises from the median nerve at the level of the tuberosity of the radius and runs along the interosseous membrane, eventually innervating the pronator quadratus and providing some sensory fibers to the wrist joint.
After giving off the anterior interosseous, the median nerve passes between the heads of the pronator teres, then along the forearm between the flexores digitorum superficialis and profundus and gives innervation to the flexor pollicis longus and radial portions of the flexor digitorum profundus. As it reaches the wrist, the palmar cutaneous nerve branches off between the flexor carpi radialis tendon and tendon of palmaris longus.
The median nerve then continues through the carpal tunnel, with the motor (or “recurrent”) branch supplying the abductor pollicis, opponens pollicis, and superficial head of the flexor pollicis brevis muscles ( Fig. 11.2 ).
The terminal branches of the median nerve supply sensation to the thumb, index finger, middle finger, and usually the radial half of the ring finger. ( Fig. 11.3 )
As noted above, Martin-Gruber anastomoses may exist in the proximal forearm between the median and ulnar nerves; however, most of these provide motor fibers from the median nerve to the ulnar-innervated nerves in the hand. Likewise, there may be a communicating branch between the median and ulnar nerves in the palm (the Riche-Cannieu anastomosis). In this setting, the ulnar nerve usually provides motor fibers to the recurrent branch of the median nerve to innervate the thenar muscles.