Nerve Transfers to Restore Wrist and Finger Extension
A loss of wrist and finger extension can result from myriad causes, including brachial plexus injury, radial nerve injury due to trauma, nerve tumors, and compression or idiopathic neuritis, as well as posterior interosseous nerve (PIN) compression. In the latter case, some radial wrist extension may be preserved. Until recently, tendon transfers have been the mainstay of treatment. Nerve transfers have long been used for brachial plexus reconstruction. The principles of nerve-to-nerve transfer, or neurotization, have recently been applied to peripheral nerve injuries, with encouraging results. The following discussion will center on neurotization to restore wrist and finger extension.
The radial nerve arises from the posterior cord of the brachial plexus. It receives contributions from C5–C8 spinal roots. The nerve contains ~ 16,000 myelinated fibers.
It runs medial to the axillary artery; then, at the level of the coracobrachialis muscle, it courses posteriorly to lie in the spiral groove of the humerus. In the lower arm it pierces the lateral intermuscular septum to run between the brachialis and brachioradialis.
Opposite the head of the radius, there are some fibrous bands from the joint′s capsule, and immediately distal to this, the nerve is regularly crossed by several prominent veins, the “leash of Henry.” It divides 2 cm distal to the elbow into a superficial radial sensory branch (SRN) and a deep motor branch, the PIN. It gives off branches to the extensor carpi radialis longus (ECRL) and brevis (ECRB), brachioradialis (BR), and anconeus before giving off the PIN branch ( Fig. 17.1 ).
The PIN continues between the superficial and deep heads of the supinator muscle, to exit on the dorsal forearm. After it emerges from the distal border of the supinator, the PIN sends branches to the extensor digitorum communis (EDC), extensor carpi ulnaris (ECU), extensor digiti minimi, extensor pollicis longus (EPL) and brevis, and the extensor indicis proprius (EIP) in descending order, although there may be considerable variation.
In an anatomic study by Ukrit et al on 10 cadaver arms, the PIN arose from the radial nerve. Its origin was 2.9 ± 0.7 cm below the interepicondylar line.1 It contained 7,447 ± 1,325 myelinated axons.
The ECRB branch arose from the radial nerve in six specimens and from the PIN in four. The ECRB branch arose at 2.3 ± 1.5 cm below the interepicondylar line, and the average length was 3.3 ± 1.6 cm.
The average number of myelinated nerve fibers of the ECRB branch was 2,798 ± 464.
Just distal to the cubital fossa, the motor branches of the median nerve consistently collect into 3 fascicular groups ( Fig. 17.2 ). There is an anterior group to the pronator teres (PT) and flexor carpi radialis (FCR); a middle group consisting of motor fascicles to the flexor digitorum sublimus (FDS) and the hand intrinsics; sensory fascicles to the thumb, index, and middle fingers; and a posterior group to the anterior interosseous nerve (AIN). These branch groups can be traced proximally without harm, within the main trunk of the median nerve, for 2.5 to 10 cm.
The nerve and artery pass through the antecubital fossa underneath the lacertus fibrosus and give off branches to the palmaris longus (PL), FCR, FDS, and rarely the flexor digitorum profundus (FDP). The nerve then dives between the deep and superficial heads of the PT, to which it supplies one to four branches.
The fibrous arch of the PT lies 3 to 7.5 cm below the humeral epicondylar line. The fibrous arch of the superficialis arch lies 6.5 cm below the humeral epicondylar line. The median nerve enters the forearm deep to the fibrous arch of the FDS and emerges beneath the radial side of the muscle belly of the middle finger superficialis, where it is quite superficial and near the PL tendon.
In a dissection of 31 cadaver arms, Tung and Mackinnon noted that double innervation of the FDS was found in 94% of the specimens.2 The most common branching pattern was a proximal branch that also carried the branch to the PL and a distal branch that arose from the median nerve distal to the origin of the AIN branch. The proximal branch was noted to arise 3.1 cm ± 1.3 cm distal to the medial epicondyle and was 2.1 ± 0.7 cm long. The distal branch arose 7.4 cm ± 2.7 cm distal to the medial epicondyle and was 2.3 ± 0.8 cm long.
In the study by Ukrit et al,1 the proximal FDS branch arose 3.5 ± 0.8 cm below the interepicondylar line. The distal FDS branch arose 6.9 ±2.2 cm below the interepicondylar line. The proximal FDS/PL branch arose 3.6 ± 1.1 cm below the interepicondylar line.
The PL muscle was present in nine specimens. The PL branch was present in five specimens and was combined with the proximal FDS branch in four specimens. It arose at 3.4 ± 1.6 cm below the interepicondylar line.
The FCR branch was present in all specimens; it arose 4.6 ± 0.5 cm below the interepicondylar line. It was 4.2 ± 1.7 cm long and contained 2,971 ± 345 myelinated nerve fibers, which represents ~ 39% of the axons of the PIN branch.
The average number of myelinated nerve fibers of the proximal FDS branch was 983 ± 224, which represents ~ 35% of the axons of the ECRB branch.1
In the arm region, the radial nerve is often injured in association with some form of unconsciousness. In a Saturday night palsy, an obtunded patient sits with an arm over a chair back or rests the head on the lateral surface of the arm. Alternatively, the radial nerve can be compressed in the groove between the brachialis and forearm muscles when one person rests his or her head on the middle third of the arm of another (honeymooner′s palsy). The patient will typically present with a wrist drop and an inability to extend the fingers, thumb, or wrist. In addition, the brachioradialis will be affected along with variable involvement of the triceps. The patient will also have diminished sensation over the dorsum of the first web space.
The nerve conduction study (NCS) typically demonstrates the absence of the superficial radial sensory nerve action potential (SNAP). Motor recordings are more difficult, since no muscle is sufficiently isolated from other radially innervated muscles. A surface electrode over the extensor indicis proprius (EIP) results in a volume-conducted response from the adjacent radial innervated muscles, which makes side-to-side amplitude comparisons difficult. Radial nerve recordings using needle electrodes in the EIP are more common as a result, which makes it difficult to approximate the degree of axonal loss by assessing the amplitudes. The electromyogram (EMG), however, is quite useful and permits a relatively accurate localization of the lesion. In a spiral groove lesion, for example, all three heads of the triceps should be normal, with denervation of the brachioradialis and all muscles distal to it.
In PIN syndrome, the presenting symptoms are weakness and/or paralysis of the extensor muscles, which results in a wrist or finger drop. There may be a history of a fall onto an extended and pronated arm, although many cases are spontaneous, especially if due to an underlying lipoma, ganglion, or rheumatoid nodule arising from the radiocapitellar joint. The patient will present with variable weakness or paralysis of the EPL, EIP, EDC, and ECU. Motor function of the ECRB and ECRL should be preserved, since they are innervated before the PIN dives between the two heads of the supinator muscle. The patient will therefore extend the wrist in radial deviation. As it travels distally through the radial tunnel, the PIN may potentially be entrapped by fibrous bands anterior to the radiocapitellar joint, the radial recurrent leash of vessels, the fibrous edge of the ECRB, the proximal border of the supinator (i.e., the arcade of Frohse), or the distal edge of the supinator muscle.
PIN lesions do not affect the superficial radial SNAP, which should be normal. The compound motor action potential of PIN-innervated muscles may show a drop of conduction velocity or amplitude, but this is difficult to assess with surface electrodes. Needle EMG is the best technique for localization, especially with partial lesions. In acute denervation, decreased recruitment, increased insertional activity, and fibrillation potentials with or without positive sharp waves are present. In chronic lesions seen after 3–6 months, decreased recruitment may still be seen, along with giant motor unit potentials and polyphasia due to peripheral axonal ingrowth.