Tendon transfers for high median nerve injury

Indications

• In contrast to low median nerve injuries, the anterior interosseous nerve (AIN) is harmed in high median nerve injuries. This causes a loss of thumb interphalangeal (IP) joint flexion and index finger IP joint flexion. Both high and low median nerve injuries also cause sensory deficits, but these are treated with nerve transfers.

• In general, tendon transfer procedures are indicated for patients whose deficiencies are not able to recover with reinnervation. Patients without joint stiffness, who have minimal scarring along the path of the tendon transfer, who experience sufficient sensation of the hand, and who are able to comply with rehabilitation protocols for motor reeducation are ideal candidates.

• Many options exist for reconstruction of thumb opposition (see Chapter 63 ), but loss of thumb IP flexion and index finger flexion is specific to high median nerve injuries.

• Unlike patients with low median nerve injuries, suitable donors are sparse and mainly derived from the radial nerve innervated muscles, which include the brachioradialis (BR), extensor indicis proprius (EIP), extensor carpi radialis longus (ECRL), and extensor carpi ulnaris (ECU). Common transfers include:

  • BR to flexor pollicis longus (FPL)

  • Flexor digitorum profundus (FDP) tenodesis: side-to-side transfer of ulnar-innervated FDP to median-innervated FDP

  • ECRL to median FDP transfer (typically only the index finger)

• The ECRL to profundus transfer is an elegant procedure that gives patients independent pincer function. The radial nerve innervated muscles are over the dorsum of the arm and are often spared from injury, whereas the ulnar nerve is close to the median nerve in the forearm, which makes the combined median and ulnar nerve injury a common occurrence.

• Forearm pronation is also a median nerve function, powered by the distal pronator quadratus (anterior interosseous nerve, AIN) and proximal pronator teres (median nerve proper). This pronation function may be spared if the injury is distal to the elbow because the pronator teres nerve branches come off close to the elbow.

Clinical examination

• Soft tissue equilibrium: The transferred tendons will only glide if they are covered by supple, soft tissues. Transfers will not function as well if covered by skin graft or scarred tissue.

• Joints : All joints across which the tendon transfer acts must have full passive range of motion (ROM) before the transfer is performed.

• Strength: Although standard strength comparisons exist ( Table 64.1 ), the actual strength of potential donor tendons should be examined thoroughly before selection. Donors should be at least as strong as the recipient because a muscle-tendon unit loses strength after transfer.

  TABLE 64.1

  Relative Strength of Tendons

  Muscles	Relative Strength
  Brachioradialis	2
  Flexor carpi ulnaris
  Wrist extensors Extensor carpi radialis longus/brevis Extensor carpi ulnaris	1
  Digital flexors Flexor pollicis longus Flexor digitorum superficialis Flexor digitorum profundus	1
  Digital extensors Extensor digitorum communis Extensor indicis proprius Extensor digiti minimi	0.5

• Excursion: Wrist flexors and extensors have 3 cm of excursion, digital extensors have 5 cm of excursion, and digital flexors have 7 cm of excursion. Although donor and recipient excursion should match whenever possible, excursion can also be augmented with other motion (e.g., finger flexion excursion can improve with wrist extension/tenodesis effect).

Imaging

Radiographs are useful to identify the presence of arthritic joint changes or other bony abnormalities that may preclude full passive ROM.

Surgical anatomy

Patients with high median nerve injuries suffer denervation of the abductor pollicis brevis (APB), the opponens pollicis, the superficial (radial) head of the flexor pollicis brevis (FPB), the radial two lumbricals, the FPL, the index and long finger FDP, the flexor digitorum superficialis (FDS) to all fingers, the flexor-pronator group (pronator teres, flexor carpi radialis [FCR]), the palmaris longus (PL), and the pronator quadratus (PQ; Fig. 64.1 ).

Positioning

• The patient is placed in a supine position with the upper extremity on a hand table. A nonsterile tourniquet is used.

• Procedures are performed under either general or regional anesthesia.