Radial Nerve Repair/Graft
The radial nerve innervates the triceps, anconeus, brachioradialis, extensor carpi radialis longus (ECRL), and extensor carpi radialis brevis (ECRB) muscles.
In the forearm, muscles that are supplied by one of the radial nerve′s terminal branches, the posterior interosseus nerve (PIN), include the supinator, the extensor carpi ulnaris (ECU), the ECRL, the extensor digitorum communis (EDC), the extensor indicis (EIP), the extensor digiti minimi (EDM), the extensor pollicis brevis (EPB), the extensor pollicis longus (EPL), and the abductor pollicis longus (APL).
The hallmark of a radial nerve injury in the axilla is triceps weakness, whereas injuries at the distal humerus spare the triceps and lead to weakness of the wrist and forearm extensor muscles, as well as the brachioradialis. The sensory deficit from an injury to the superficial radial nerve branch includes the radial aspect of the distal forearm and the first web space, but this is variable. The radial nerve is injured in 10 to 20% of humeral fractures, and the nerve injury usually occurs at the time of the fracture rather than as a consequence of manipulation.
Nerve injuries associated with fractures and dislocations are classified according to Seddon′s classification (neurapraxia, axonotmesis, and neurotmesis). Radial nerve injuries usually lead to a complete deficit rather than partial deficit.
The usual arm-level injury involving the radial nerve is often associated with a humeral fracture. Other mechanisms of injury to the radial nerve include glass lacerations, knife wounds, gunshot wounds, and blunt trauma. Entrapment at the level of the humeral groove can also result in a radial nerve palsy, albeit less frequently.1
Surgical exploration is indicated for all open injuries with a radial nerve deficit, whereas most closed injuries can usually be observed. Rarely, a neuritis or tumor of the radial nerve at levels as high as the brachial plexus will present with a radial nerve palsy and may require surgical exploration.2
Conservative treatment is indicated for most idiopathic causes of radial nerve paralysis, after treatable causes such as a nerve or soft-tissue tumor have been excluded.
A closed radial nerve injury may result from a fracture of the humerus. In this case, the radial nerve palsy is typically observed for a period of 3 months before surgical exploration is considered.
A neurological assessment should be performed before and after either open or closed manipulation of a humeral fracture to evaluate for a worsening neurological deficit.
If a patient presents with an isolated deltoid and triceps palsy in the presence of normal wrist extension, the site of injury is the axilla at the point where the axillary nerve and the main nerve supply to triceps are closely related to one another.
The triceps is tested with the shoulder partially abducted and the elbow partially flexed to avoid the effects of gravity. If the patient is seated, the patient is asked to extend the elbow so that the forearm is parallel to the floor.
Injury to the radial nerve at a midarm level secondary to a humeral fracture may present with paralysis of the brachioradialis and forearm at wrist extensors with sparing of the triceps.
Radial nerve injury at the distal arm will spare the brachioradialis, which is tested by asking the patient to flex the forearm with the forearm in neutral rotation.
An elbow-level radial nerve injury causes loss of function of the ECRL and ECRB muscles. A PIN injury results in sparing of the ECRL and ECRB function, but there is paralysis of the ECU, which results in radial wrist deviation during attempted extension.
With damage to the PIN, wrist, finger, and thumb extension is absent.
Electrodiagnostic testing is usually done after 3 to 4 weeks from the time of injury to allow time for fibrillation potentials to occur. A nerve conduction study may reveal a complete conduction block across the injury site, but this will also be present in both axonotmesis and neurotmesis.
In a neurapraxic lesion, the distal conduction velocity is normal and the EMG will be normal. The EMG may show evidence of membrane instability, as manifested by increased insertional activity, fibrillation potentials, and positive sharp waves and axonotmesis and neurotmesis after 3 weeks. Nascent motor unit potentials that have a prolonged duration and asynchronous waveforms may appear weeks to months later if there is reinnervation. Polyphasic motor unit potentials can be seen in partial nerve injuries when there is muscle reinnervation through axonal sprouting from adjacent attack motor units.
The radial nerve originates from the C5–C8 spinal roots and continues from the posterior cord of the brachial plexus.
Before exiting the axilla, it gives off branches to the triceps and the posterior cutaneous nerve of the arm.
The radial nerve reaches the extensor compartment of the arm by exiting the axilla through the triangular space, passing obliquely across the back of the humerus laterally into the spiral groove. Here, it gives off the lower lateral cutaneous nerve of the arm to supply the triceps. Then the nerve runs between the triceps heads in the posterior arm and pierces the lateral intermuscular septum, where it enters the anterior compartment of the arm, supplying the brachioradialis, ECRL, and brachialis.
The radial nerve divides to its terminal branches at the level of the lateral epicondyle. The superficial radial nerve (SRN) branch is purely sensory. The PIN, which is the deep branch, supplies the muscles arising from the common extensor origin as well as the deep muscles of the extensor compartment of the forearm.
The radial nerve is subject to compression, injection, penetrating injuries, and blunt trauma in the lower half of the arm, where it is relatively superficial.
Midshaft and oblique distal humeral fractures carry a high risk of radial nerve injury.
Most radial nerve injuries due to humeral fractures are neurapraxic and spontaneously recover.
The brachioradialis muscle is usually the first muscle to recover, indicating successful nerve regeneration.
Lack of neural recovery by 4 to 6 months is an indication for radial nerve exploration.
Consider simultaneous tendon transfer for wrist extension (in late repair of a high palsy).
Avoid transfer of the flexor carpi ulnaris in patients with low palsy, since this will exacerbate the radial wrist deviation.
Never transfer both the flexor carpi radialis and flexor carpi ulnaris, since one major wrist flexor must remain intact.
Upper Arm (Radial Nerve)
At the elbow level, an incision is made between the brachioradialis (BR) and the biceps brachii ( Fig. 10.1 ).
The groove or raphe between the laterally located BR and the more medially located biceps fibers at the level of the flexor crease of the elbow can be bluntly separated to expose the radial nerve.
The nerve can then be followed proximally in the upper arm and distally down the forearm ( Fig. 10.2 ).
Some of the proximal triceps muscle should be split in the direction of its fibers and held apart by retractors to expose the nerve proximally, near the spiral groove, through a proximal incision on the medial arm between the biceps and triceps.
If extension into the axilla is necessary, the anterior border of the pectoralis major is retracted superiorly for exposure of the brachial and subclavian arteries.
The triceps branches may have a very proximal take-off from the radial nerve or from the posterior cord at the junction with the radial nerve.
The medial antebrachial cutaneous nerve branches may overlie the artery, which can be elevated by a vein retractor to expose the radial nerve.
Nerve action potential (NAP) recording using bipolar hook electrodes can be performed across a 4 cm segment of the nerve proximal and distal to a lesion in continuity that may be associated with a midhumeral fracture ( Fig. 10.5 ). At least 4000 axons are necessary for a NAP to be present. If a NAP is present, a neurolysis is performed.
If no NAP is recorded across the humeral groove segment of the nerve, the radial nerve is sectioned as far distally as possible on the medial proximal segment and as far proximally as possible on the distal or lateral arm segment.
By mobilizing the radial nerve distal to the elbow, where the SRN and PIN arise, some length can be gained on the distal stump.
To accomplish this, the nerve is dissected 360 degrees along with its branches, sometimes splitting BR branches back up the nerve to gain length so that they do not tether the distal stump.
Then the nerve is transferred beneath the biceps but anterior to the humerus, and its stump is brought out posterior to the brachial vessels and median nerve. This allows for either an end-to-end repair or nerve grafts, on the medial side of the arm.
To provide a generous passage beneath the biceps, sharp dissection is performed initially using a scalpel and finished with Metzenbaum scissors and a hemostat.