Tendon transfers are a reconstructive tool to restore function in the setting of impaired intrinsic and/or extrinsic muscles affecting the hand and wrist. In patients with radial nerve dysfunction, the goal of tendon transfer surgery is the restoration of active wrist, thumb, and finger extension. Although the flexor musculature for the fingers is unaffected, these patients lose grip strength, as they are unable to effectively stabilize their wrist in extension during finger flexion. Therefore, they have poor transmission of flexor power to their fingers. Sensory disturbance is not routinely of functional consequence to these patients, unless they develop a symptomatic neuroma. This chapter presents the general principles of tendon transfers and details transfers for radial nerve palsies as a representative example.

The radial nerve originates from the posterior cord of the brachial plexus and constitutes the largest terminal branch of the plexus. It travels along the posterior wall of the axilla and through the triangular space, continuing deep to the long and lateral head of the triceps as it spirals around the posterior-lateral humerus. The radial nerve pierces the lateral intermuscular septum at a relatively constant distance proximal to the lateral epicondyle (known as the lateral nerve height), thereby entering the anterior compartment of the arm between the brachioradialis and brachialis. This height can be determined as a function of the trans-epicondylar distance (TED). The TED is the distance between the lateral and medial epicondyle; the ratio of lateral nerve height to TED averages 1.7. The nerve then crosses the elbow and continues deep to the brachioradialis, splitting into the superficial branch of the radial nerve (SBRN) and the posterior interosseous nerve (PIN). The SBRN provides sensory innervation to the radial aspect of the forearm and hand, while the PIN travels deep to the supinator (the radial tunnel) to innervate the extensors of the wrist and fingers (Table 31.1).

Radial nerve palsies may occur from a variety of causes: a penetrating wound, a closed or open fracture of the humerus, or iatrogenic injuries during surgical procedures around the distal humerus and elbow. Regardless of the cause, it is important to distinguish the level of the radial nerve injury (high vs. low), as this will dictate remaining function and availability of possible donor muscles and tendons. A low injury is typically classified as a lesion distal to the elbow, namely the PIN, whereas a high injury involves the proper radial nerve proximal to its bifurcation. A low injury or isolated PIN lesion will result in lack of finger metacarpophalangeal (MCP) extension and radial deviation with wrist extension, due to loss of extensor carpi ulnaris (ECU) with persistent activity of extensor carpi radialis longus (ECRL) ± extensor carpi radialis brevis (ECRB). Thumb retropulsion and interphalangeal hyperextension are lost secondary to losing the extensor pollicis longus (EPL) function. A high lesion will also lose all wrist extension (ECRB and ECRL) and brachioradialis. Extension at the proximal and distal interphalangeal (PIP and DIP) joints will remain intact following any radial nerve paralysis, as these joints are extended by the lumbrical and interosseous muscles, which are innervated by the median and ulnar nerves.

There are several basic requirements that must be met when planning tendon transfer surgery. These include (1) supple joints; (2) soft-tissue equilibrium; (3) similar excursion
between donor and target muscles; (4) adequate strength; (5) expendable donor muscles and tendons; (6) similar straight line of pull between transferred tendons; (7) synergy; (8) one tendon for one function; and (9) the fact that all fractures must be healed. Each of these will be examined in more detail, as it is imperative that the surgeon address these conditions prior to proceeding in order to maximize postoperative function.