Anterior Interosseous Nerve Transfer to Thenar/Hypothenar Motor Branch
Nerve transfer procedures have gained increasing acceptance as efficacious treatment options for the reconstruction of certain peripheral nerve injuries. These procedures are of particular value in patients where orthotopic nerve repair or reconstruction either is not possible or is likely to result in a poor functional outcome. In general terms, motor nerve transfers share three advantageous features as compared with graft reconstruction of a mixed motor-sensory nerve with a proximal level of injury:
A source of functionally appropriate axons (for example, a predominantly motor donor nerve to a predominantly motor recipient nerve without the random likelihood of functionally useless sensory axons occupying distal motor axon tubules)
A nerve coaptation site that may be significantly more distal to the orthotopic nerve injury (thus in much closer proximity to the neuromuscular end plates), resulting in an abbreviated period of denervation atrophy of the target muscle
A single neurorrhaphy site as opposed to the two neurorrhaphy sites required with a graft, and thus half the potential for extraneural axon wandering at the coaptation site
The anterior interosseous nerve (AIN) in the distal forearm is a functionally expendable, nearly pure motor nerve that is of established value for reanimation of the intrinsic muscles of the hand. To the author′s knowledge, the first clinical application of distal AIN transfer was reported by Wang and colleagues as occurring in 1979.1 The first published literature reference to the utility of this transfer is that of Wang and Zhu in 1997.1
Indications
The primary indication for distal AIN transfer is reanimation of the intrinsic muscles of the hand. The most common indication is for reconstruction of the deep motor branch (DMB) of the ulnar nerve, which most commonly provides innervation for the palmar and dorsal interosseous muscles, the adductor pollicis muscle, and a portion of the flexor pollicis brevis muscle. As a nerve transfer to the DMB of the ulnar nerve, it should be chiefly utilized in patients with irreparable ulnar nerve injuries proximal to the elbow or as an adjunctive procedure with ulnar nerve repair or reconstruction at a proximal level. Clinical experience has demonstrated that recovery of useful intrinsic motor function in adult patients undergoing ulnar nerve repair or reconstruction proximal to the elbow, and particularly in those with nerve defects, is achieved in a small percentage of patients. A poor outcome in such a situation is readily explained by the facts that one is dealing with a mixed motor-sensory nerve trunk, with a transection level that is very far distant from the neuromuscular end plate and that, for successful reinnervation, requires axons to cross two nerve coaptation sites. The previously mentioned advantageous features of a distal nerve transfer directly address these challenges.
An additional, but much less common, clinical application is distal AIN transfer for reanimation of the thenar branch (TB) of the median nerve in patients with irreparable injury to the median nerve distal to the origin of the AIN near the elbow. Most patients with median nerve injuries in the forearm either have concomitant injury to the more deeply situated AIN or have injury at a level that is sufficiently distal that a transfer of the AIN does not really place the neurorrhaphy site appreciably closer to the thenar muscle motor end plates than would be the case with orthotopic nerve reconstruction. Moreover, the functional goal of reanimation of the TB of the median nerve is simple thumb opposition, and this function can in many cases be reliably achieved by any of several well-known tendon transfers.
As is a general rule, with all complete peripheral nerve injuries, nerve-related surgical intervention, whether nerve repair, reconstruction, or nerve transfer, should be performed as soon as possible after it is established that an irreparable nerve injury exists. With few exceptions, this should be within 6 months of the initial injury. However, because nerve transfers allow placement of the neurorrhaphy site closer to the neuromuscular end plates, there is greater latitude for the acceptable time delay. Successful results with nerve transfer have been reported as late as 1 year.
Contraindications
There are few absolute contraindications to AIN transfer.
One is an extreme delay from the time of injury to surgery. Most surgeons agree that 1 year is the upper limit of delay, but some may stretch this period to 18 months. I believe most surgeons would regard nerve transfer beyond 2 years as futile.
Another absolute contraindication is a nonfunctional AIN. This is the case in a patient with a direct injury to the AIN in the forearm, a patient with a complete median nerve injury proximal to the elbow, or a patient with a brachial plexus palsy involving the C8 and T1 roots, lower trunk or medial cord.
A third absolute contraindication is that of a patient with extensive direct injury to either the DMB of the ulnar nerve or the TB of the median nerve in the proximal palm of the hand.
In addition, there is a category of relative contraindications, which would include injuries to the ulnar or median nerve, the nature and level of which could be addressed by an orthotopic nerve repair or reconstruction that has the likelihood of an equal or better result than that expected by AIN transfer. Examples of this category include:
An ulnar nerve laceration, seen within a few weeks of injury and amenable to primary repair, within 6 cm of the wrist. In such a case, there is the opportunity for precise topographical orientation of the transected nerve ends, with a single neurorrhaphy site and possibly at a level even closer to the neuromuscular end plates than could be achieved by AIN transfer.
A young child with a tidy ulnar nerve laceration in the mid-forearm. The enhanced potential for recovery of peripheral nerve function in this age group would likely trump the advantages of a more distal neurorrhaphy site with a pure motor nerve by AIN transfer.
Examination/Diagnostic Aids
Confirmation of unimpaired function of the distal AIN is most commonly accomplished by assessing the strength of the pronator quadratus (PQ) muscle. Usually this is done indirectly by clinical examination. The strength of forearm pronation is compared with the contralateral limb with the elbow fully flexed to minimize the contribution of the pronator teres muscle. If there is significant concern that the distal AIN may not be functional, cross-sectional magnetic resonance (MR) imaging, comparing the mass of the PQ muscle in both limbs, may be helpful. Rarely, evaluation of the distal AIN may require electromyographic evaluation of the PQ using an ultrasound-guided recording electrode in the muscle.
Anatomy
The motor axons supplying the distal AIN originate from the anterior horn cells that supply the C8 and T1 nerve roots. These axons course distally within the lower trunk of the brachial plexus, the medial cord of the brachial plexus, the median nerve proper, and ultimately the anterior interosseous branch of the median nerve arising just proximal to the elbow. The AIN typically innervates the flexor pollicis longus muscle, the radial portion of the flexor digitorum profundus muscle, and the pronator quadratus muscle, and it carries some scant nonmotor fibers that terminate at the wrist joint. In the distal forearm the AIN lies on the interosseous membrane adjacent to the anterior interosseous vessels and passes deep to the pronator quadratus muscle, sending multiple branches to the muscle within its substance. At the level of the proximal border of the PQ muscle, the nerve has a caliber of 1.3 ± 0.1 mm and is composed of 912 ± 88 axon fibers. By comparison, the diameter and axon fiber count of the DMB of the ulnar nerve are 1.9 ± 0.17 and 1,216 ± 108, respectively, and those of the TB of the median nerve are 1.4 ± 0.12 and 1,020 ± 93.2
Pearls
This is a technically simple surgical procedure with very little in the way of downside risk.
The alternatives to AIN transfer should be judiciously considered. AIN transfer should be selected as an alternative to a well-executed ulnar nerve repair or graft only in clinical situations that are likely to result in a poor outcome—above-elbow ulnar nerve injuries associated with a significant gap in an adult seen several months after injury. It should be considered for median TB reanimation when reasonable traditional opponens plasty tendon transfer options are lacking.
The AIN identification and isolation should be done at the initial stage of the procedure so that confirmation of a PQ muscle contraction upon AIN stimulation can be accomplished prior to the onset of nerve dysfunction related to tourniquet-induced ischemia.
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