Distal anterior interosseous nerve transfer to motor branch of ulnar nerve

An anterior interosseous nerve (AIN) transfer to the motor branch of the ulnar nerve is used to restore ulnar intrinsic hand function.

It is performed for both chronic ulnar nerve compression and acute high ulnar nerve injury.

For chronic nerve compression, AIN transfer is performed when patients have evidence of acute muscle denervation on electrodiagnostic studies, which indicates that muscle can be reinnervated. If a patient has no evidence of acute denervation, the transferred median axons will not have distal muscle targets to reinnervate.

For high acute ulnar nerve trauma, the AIN to deep motor transfer is indicated if normal reinnervation will not take place within 12 to 18 months of the injury. For instance, if the injury causes a significant nerve gap and requires grafting, this will delay normal reinnervation and the AIN to deep motor transfer is indicated. The coaptation is performed 10 cm proximal to the wrist crease. Therefore lacerations distal to this level cannot be treated effectively with this procedure.

AIN transfer to the motor branch of the ulnar nerve may be performed in an end-to-end or end-to-side manner. End-to-end coaptation is indicated for injuries proximal to the elbow, in which reinnervation will not reach the intrinsic musculature of the hand before the loss of motor end plates. End-to-side coaptation may also be considered for patients with more proximal injuries, when the goal is to preserve motor endplates until normal reinnervation can occur. This method is also useful when compression causes intrinsic muscle denervation, as indicated by electrodiagnostic testing and weakness on physical examination, although the ulnar nerve remains in continuity.

Nerve transfer procedures are ideally performed in a timely manner so that reinnervation of the target muscles can occur within 12 to 18 months after the time of injury. After 18 months, loss of motor end plates at the distal targets occurs, and tendon transfer procedures provide more predictable outcomes.

The function of the AIN is confirmed by testing the strength of the flexor pollicis longus (FPL; flexion of the thumb at the interphalangeal joint) and the independent strength of flexor digitorum profundus (FDP) to the index and middle finger (flexion of the distal interphalangeal joint). Strength of the pronator quadratus is tested by examining the patient’s ability to pronate the forearm with the elbow in flexion.

Passive range of motion (ROM) of the hand is examined to ensure that all joints are supple and without contracture.

Electromyography (EMG) is often obtained to determine whether the intrinsic muscles are actively denervating and can potentially be reinnervated or to determine whether the ulnar nerve can regenerate after an acute closed injury.

Active denervation is required for this procedure to be effective because empty motor-endplates are necessary for reinnervation. This is indicated by increased insertional activity, positive sharp waves, and fibrillations.

The EMG can be helpful for identifying chronic reinnervation, which is indicated by polyphasic motor units of longer duration and/or increased amplitude.

For closed ulnar nerve injuries, an EMG is obtained at 3 months, at which point the presence of motor units indicates that the ulnar nerve will regenerate and observation can be continued. If there are no motor units at 3 months, then an end-to-end AIN to deep motor transfer can be performed to reinnervate the ulnar intrinsic muscles. The exception is gunshot injuries, for which we recommend waiting 6 months for electrodiagnostic results to predict recovery.

Within the forearm, the ulnar nerve lies between the two heads of the flexor carpi ulnaris (FCU) and innervates the FCU and FDP to the ring and small fingers. The ulnar nerve and artery run together along the volar ulnar aspect of the forearm. The ulnar artery lies radial to the ulnar nerve, and the neurovascular bundle is located deep to the tendon of the FCU.

The dorsal sensory branch of the ulnar nerve arises 5 cm proximal to the pisiform to supply cutaneous sensation to the dorsoulnar aspect of the hand.

At the wrist, the ulnar nerve and artery lie superficial to the flexor retinaculum (transverse carpal ligament) within Guyon canal. The ulnar nerve divides at the hook of the hamate into the superficial branch, which contributes sensation to the ring and small fingers, and the deep motor branch, which courses under the hypothenar muscles to innervate the intrinsic muscles of the hand (interosseous muscles, third and fourth lumbricals), adductor pollicis, and the deep head of the flexor pollicis brevis.

The ulnar nerve at the forearm has predictable topography with respect to the orientation of the fascicles. At the mid and proximal forearm, the motor fascicle group is flanked by two sensory fascicular groups (sensory-motor-sensory). The sensory fascicles that innervate the small finger and ulnar aspect of the ring finger lie along the radial aspect of the motor fascicular group. The sensory fascicles that contribute to the dorsal sensory branch of the ulnar nerve lie ulnar to the motor fascicular group.

The anterior interosseous nerve arises from the median nerve approximately 4 cm distal to the elbow. It lies along the volar aspect of FDP and the interosseous membrane after passing through the two heads of the pronator teres. AIN function is primarily motor, and it innervates the FPL and the FDP to the index and long finger, Its terminal motor branch supplies the pronator quadratus.