Nerve Transfers for Median Nerve Injury

Due to its role in hand function, median nerve injuries can have a significant impact on patients’ lives. Nerve transfers can be performed alone or in conjunction with tendon transfers to restore critical functions supplied by the median nerve, with a particular emphasis on restoring grasp, pinch, and sensation. Continued study into possible nerve transfers for median nerve injury has led to multiple options that can be tailored to the patient’s demands, deficits, and injury pattern.

Introduction and Anatomy

The median nerve originates from the lateral and medial cords of the brachial plexus and plays a crucial role in hand and forearm function. The median nerve proper travels down the upper arm between the biceps and brachialis muscles, running alongside the brachial artery; after crossing the elbow, it passes between the 2 heads of the pronator teres (PT) and then between the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). The median nerve proper provides innervation to the palmaris longus (PL), pronator teres (PT), flexor carpi radialis (FCR) and flexor digitorum superficialis (FDS). Approximately 6-8 cm distal to the medial epicondyle of the elbow, the anterior interosseous nerve (AIN) branches from the median nerve and provides motor innervation to the flexor digitorum profundus (FDP) to the index and middle fingers, flexor pollicis longus (FPL), and pronator quadratus (PQ) muscles. Further distally, the median nerve divides into the palmar cutaneous branch (PCB) a few centimeters proximal to the wrist and the recurrent motor branch at the level of the carpal tunnel. ^, The recurrent motor branch provides innervation to the intrinsic muscles of the hand, including the abductor pollicis brevis (APB), the superficial head of the flexor pollicis brevis (FPB), opponens pollicis (OP), and the first and second lumbricals. The distal branches of the median nerve provide sensory innervation to the volar aspect of the thumb, index, middle, and radial half of the ring finger and the palmar cutaneous branch (PCB) provides sensory innervation to the radial palm and thenar eminence.

Median nerve injuries are broadly classified as “high” or “low,” and refer to whether the injury is proximal or distal to the anterior interosseous nerve (AIN). An assessment of muscle involvement, pattern of sensory loss, presence of pronation, wrist and finger flexion, and anatomic location of injury categorize the injury ( Table 1 ).

Table 1

Comparison of low Versus High Median Nerve Injuries

	Low-Median Nerve Injuries	High-Median Nerve Injuries
Causes	• Carpal tunnel syndrome • Wrist fractures • Penetrating injuries at the wrist	• Fracture-dislocations of the humerus or elbow • Deep penetrating injuries of the upper arm • Brachial plexus injuries
Anatomical location of injury	Distal to the origin of the anterior interosseous nerve in the wrist region	Proximal to the origin of the anterior interosseous nerve in the upper arm or elbow region
Affected muscles and nerves	• Intrinsic thenar muscles (APB, FPB, OP) • Two radial lumbricals	• Extrinsic flexors of forearm (FDS, radial half FDP, FPL) • Pronator quadratus and teres • Intrinsic thenar muscles (APB, FPB, OP) • Two radial lumbricals • Palmar cutaneous branch
Spared muscles and nerves	• Extrinsic flexors of forearm (FDS, radial half FDP, FPL) • Pronator quadratus and teres • Palmar cutaneous branch	• None
Clinical exam	• Weakness in thumb abduction and opposition (“ape hand deformity”) • Sensory loss on the volar surface of the thumb, index, middle, and radial half of the ring finger	• Loss of pronation • Weak wrist flexion • Absent flexion of the thumb, index, and middle fingers • Weak thumb opposition • Sensory loss over the radial hand • Sensory loss on the volar surface of the thumb, index, middle and radial half of the ring finger
Restoration goals	• Thumb opposition • Protective sensation of digital nerves involved in pinch	• Thumb opposition • Flexion of the index and middle fingers • Protective sensation of digital nerves involved in pinch

APB, abductor pollicis brevis; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; FPB, flexor pollicis brevis; FPL, flexor pollicis longus; OP, opponens pollicis.

Low median nerve injuries affect the intrinsic muscles of the thumb and the 2 radial lumbricals, while the extrinsic flexors of the forearm, pronator teres and the palmar cutaneous branch are spared. On clinical examination, patient will demonstrate weakness in thumb abduction and opposition (sometimes referred to as “ape hand deformity”) and sensory loss in the thumb, index, middle, and radial half of the ring finger. Common causes of low median nerve injuries include carpal tunnel syndrome, wrist fractures, or penetrating injuries at the wrist.

High median nerve injuries typically occur in the upper arm or elbow region, proximal to the origin of the anterior interosseous nerve. ^, This is most commonly due to fractures or dislocations of the humerus or elbow, deep penetrating injuries to the upper arm, or brachial plexus injuries. ^, Both the extrinsic and intrinsic median-nerve innervated muscles and the palmar cutaneous branch are affected in this injury pattern.

Historically, Tinel proposed that a high median nerve injury would result in: (1) loss of pronation; (2) weak wrist flexion; (3) absent flexion of thumb, index, and middle fingers; (4) compromised thumb opposition; and (5) soss of sensation over the radial hand and fingers. However, clinical observations have shown discrepancies between these classical descriptions and actual patient presentations. ^, Variations in presentation can be due to incomplete injury, differences in preinjury function and anatomic variations. Several studies have demonstrated that isolated high median nerve injuries reliably cause deficits in thumb interphalangeal joint and index finger distal interphalangeal joint flexion, grasp, pinch, and opposition, while pronation more than 50 degrees, wrist flexion, and middle finger flexion are largely preserved.

Recent clinical observations have also challenged traditional views on the expected sensory loss after median nerve injury. Some prior literature suggests anesthesia occurs over the radial aspect of the palm, while others have found sensory preservation at this anatomic location. ^, ^, ^, The most clinically relevant sensory deficits have been observed over the volar aspect of the distal phalanx of the thumb, and over the volar aspect of the middle and distal phalanges of the index and middle fingers. ^, ^, ^, This pattern of sensory loss, particularly the loss of protective sensation in these areas, has important functional implications and directs attempts at restoring sensation in these areas. Interestingly, sensory function in the ring finger is often preserved in high median nerve injuries due to the ulnar nerve’s contribution to the ulnar neurovascular bundle, which can maintain sensation across the entire pulp of the finger.

Indications for Nerve Transfer

Indications for nerve transfer after median nerve injury are determined by 2 crucial factors: the time elapsed since the initial injury and the distance to the target site. Nerve transfers are particularly effective for proximal nerve injuries in the upper arm with distal targets when nerve repair alone is unlikely to restore innervation prior to irreversible motor end-plate degeneration. In some cases, a combination of nerve transfers and median nerve repair may be necessary, especially if pain from a symptomatic neuroma is present. Timing is critical in these procedures with the optimal window for nerve transfers being 6-9 months postinjury but may be considered for up to 12 months. Sensory nerve transfers, although controversial, may be considered even up to 3 years after the injury due to their low morbidity and preserved distal sensory organs. For motor deficits beyond the 12-month mark, tendon transfers remain the preferred treatment option.

As there are multiple options for donor nerves for the following nerve transfers, the donor should be chosen based on a case-by-case basis, depending on residual function following the injury ( Table 2 ). When selecting a donor nerve, the clinician must ensure that the donor nerve is healthy and has a redundant function. Other important considerations are anatomic feasibility and preserving possible donors for tendon transfer in the setting of nerve transfer failure.

Table 2

Overview of Potential Nerve Transfers for Specific Median Nerve Deficits

Deficit	Affected Median Nerve Innervated Muscles	Potential Nerve Transfers
Pronation	PT PQ	• Brachialis to PT • ECRB to PT • FDS to PT • FCR to PT • FCU to PT
Wrist flexion	FCR PL	n/a – motion is preserved via FCU
Finger flexion Grip strength Pinch strength	FDS FDP FPL	• Brachialis to AIN • Brachioradialis to AIN • Supinator to AIN • ECRB to AIN • FDS to AIN • Supinator to FDS • FDS to FPL
Thumb opposition	Thenar musculature	• Branch to third lumbrical to thenar branch of median nerve (TBMN) • Hypothenar (ADM) branch of ulnar nerve to TBMN
Sensation	Radial palm, thumb, index, middle and radial surface of ring finger	• Dorsal digital nerve branches to palmar digital nerve branches • Dorsal radial sensory nerve to palmar digital nerve branches • Ulnar palmar digital nerve branches to affected radial palmar digital nerve branches

ADM, abductor digiti quinti; ECRB, extensor carpi radialis brevis; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; FDS, flexor digitorum superficialis; FPL, flexor pollicis longus; PL, palmaris longus; PQ, pronator quadratus; PT, pronator teres.

Goals of Median Nerve Transfers

The restoration of pinch and grasp strength and protective sensation are the major goals of reconstruction after median nerve injury. Other functions, such as pronation or wrist flexion, are typically preserved due to redundant muscle function and thus are not routinely performed. Our preferred transfers are described below.

Restoration of index finger and thumb flexion

Restoring index finger and thumb flexion is crucial for grasp and pinch strength. Our preferred transfers are the ECRB to AIN and supinator to FDS. Successful transfer to AIN will restore function of the FPL and radial FDP muscle. As the FDS branches from the median nerve proximal to the takeoff of the AIN, transfer of the supinator branch to FDS aims to improve grip strength. Depending on the patient’s deficits and available donors, these transfers can be “mixed and matched” as necessary, with the AIN taking priority as a recipient nerve. If neither of these donors are available, the brachioradialis branch is located proximally and can also serve as a possible donor.

Extensor carpi radialis brevis (ECRB) branch to anterior interosseous nerve (AIN) with supinator branch to flexor digitorum superficialis (FDS) branch nerve transfer

ECRB is our preferred donor for AIN nerve transfer. Our technique for ECRB to AIN with supinator to FDS transfer begins with the patient positioned in a supine position while a tourniquet is applied to the upper arm. Anesthesia is then induced without any muscle relaxation to allow for the use of a nerve stimulator. A curvilinear incision is designed crossing the cubital fossa running from radial to ulnar and skin flaps are raised in the subcutaneous plane. The lacertus fibrosis is then divided. The superficial branch of the radial nerve (SBRN) is identified deep to the brachioradialis muscle origin and is protected. Deep to the SBRN, the ECRB branch and the supinator branch are identified and confirmed using a nerve stimulator. Our recipient branch, the AIN arises from the median nerve at the cubital fossa and branches from the median nerve at a posterior, radial, and dorsal location ( Fig. 1 ). The pronator teres tendon is then step-lengthened to allow for adequate retraction and visualization. The FDS nerve branch is visualized branching from the ulnar aspect of the median nerve. Once the desired donor and recipient nerves are identified and confirmed using a nerve stimulator, the donor branch is divided sharply as distally as possible proximally to any branching and the recipient branch is divided proximally. Nerve coaptations are then performed under loupe magnification without tension utilizing 9-0 nylon suture.