Outline

Upper extremity extensor tendon injuries are often detrimental to overall hand function and may lead to significant patient disability. Insufficiency of the wrist, thumb, and/or digital extensor tendons may be caused by direct or indirect mechanisms or may be secondarily attributable to systemic disease or radial nerve dysfunction. This chapter will provide an overview of tendon transfer methods to address extensor tendon insufficiency, alternatives to tendon transfers for various clinical scenarios, and guidelines for postoperative rehabilitation after reconstructive techniques.

The extensor muscles of the wrist and hand are contained within the dorsal aspect of the forearm and are all innervated by the radial nerve. At the level of the wrist, the extensor tendons are divided into six compartments by tough fibrous septae of the extensor retinaculum, numbered from radial to ulnar. The first compartment contains the abductor pollicis longus (APL) and extensor pollicis brevis (EPB). Anatomic variation within this compartment includes potential multiple tendon slips of the APL and a distinctly separate compartment for the EPB.¹ The second compartment includes the extensor carpi radialis longus (ECRL) and the extensor carpi radialis brevis (ECRB). The ECRB is oriented ulnar to the ECRL. The third compartment houses the extensor pollicis longus (EPL). This tendon takes a sharp turn radially after it courses distal to the ulnar aspect of Lister’s tubercle. Within the fourth compartment are the extensor digitorum communis (EDC) and the extensor indicis proprius (EIP). The terminal branch of the posterior interosseous nerve (PIN) and accompanying interosseous vessels lie at the floor of this compartment. The EIP is oriented ulnar to the index EDC. The extensor digiti minimi (EDM) is the sole tendon within the fifth compartment at the level of the distal radioulnar joint (DRUJ). The final and most ulnar sixth compartment encloses the extensor carpi ulnaris (ECU). This tendon courses through a fibro-osseous tunnel on the dorsal surface of the distal ulna, creating a groove that is often evident on plain radiographs of the wrist.

Extensor tendons are divided into zones of the hand and forearm, which help to guide treatment. The zones are numbered from 1 to 9 with odd numbered-zones largely corresponding to underlying joints. Zone 1 overlies the distal interphalangeal joint (DIP) of the four digits, and zone 9 extends proximal to the musculotendinous junction. The extensor zones of the thumb are altered because there is one less joint. In the thumb zone T1 is over the interphalangeal joint (IP) and T3 over the metacarpophalangeal (MCP) joint, whereas in the other digits the MCP joint would correspond to zone 5. The carpal level is T5 for the thumb and zone 7 for the other digits.²

Injury or dysfunction of an extensor musculotendinous unit may result from direct laceration, acute closed rupture, chronic attritional wear, vascular insufficiency, systemic inflammatory disease, or radial nerve denervation. There are several clinical scenarios where extensor tendon injuries cannot be adequately repaired in primary fashion and may require tendon transfer reconstruction. A classic example is rupture of the EPL tendon in association with nonoperative treatment of a distal radius fracture. When faced with such a scenario, patient factors such as comorbidities, accompanying injuries, occupation, and compliance must be taken into account. Basic principles of tendon transfer surgery remain paramount for achieving optimal results. The work of Starr among injured military personnel in the early twentieth century established a framework for tendon transfer surgery and is summarized in Table 34-1.

Table 34-1 Principles of Tendon Transfer Surgery Applicable to Extensor Tendon Reconstruction

Expendable donor	The choice of tendon for transfer should not fully compromise existing normal function.
One tendon for one function	Splitting a transferred tendon for separate functions will function only to the shortest excursion of the recipient tendons.
Direction	A straight line of pull maximizes function of the transfer.
Similar excursion	Donor and recipient excursion should be comparable.
Similar strength	Donor and recipient strength should be comparable. One grade of recipient strength will be lost, even for the most successful transfers.
Joint mobility	Tendon transfers will not function in the setting of contracted joints. Near full passive mobility of distal joints must be established preoperatively.
Synergy	Antagonistic muscles must provide a stabilizing effect for the recipient.
Tenodesis	Taking advantage of the tenodesis effect of the hand enhances tendon transfer function; wrist arthrodesis should be avoided if possible.
Tissue equilibrium	Tendon transfers should be delayed until adjacent bone and soft tissue injuries are healed and mature.
Power versus positional motors	Weaker motors are used for position and stronger motors for power.

More recent work has demonstrated that sarcomere length may be measured in vivo and a single sample represents the entire muscle.³ Consequently, transferred musculotendinous units may be set at a sarcomere length specific to the muscle being replaced. Friden⁴ astutely recognized that surgeons typically overtighten transfers, thinking they will relax over time, when in reality the sarcomeres are overstretched and only fire at 28% of the maximum force of the muscle. This may in part account for transfers losing one grade of strength. Overly tightened transfers become somewhat of a passive tenodesis.⁵

Wrist kinematics plays a role in tendon transfer. The “dart thrower’s” arc, described as combined wrist extension and radial deviation through combined wrist flexion and ulnar deviation, is the functional motion for most activity. It is argued that with some tendon transfers, this motion is either not restored or compromised. For example, if the flexor carpi ulnaris (FCU) is sacrificed for transfer in a patient with radial nerve palsy, the dart thrower’s arc may be restricted.

Traumatic extensor tendon injury may be direct or indirect and further classified as acute or chronic. The acute treatment of these injuries is covered earlier in this textbook. Occasionally, the diagnosis is missed or the injury is neglected, especially in the setting of polytrauma, altered mental capacity, or in patients with poor understanding of a condition or limited access to medical resources. The chronicity of the condition often alters the type of treatment that can be rendered. Cases that would ordinarily be treated by direct repair are instead addressed by delayed reconstruction, and tendon transfer surgery is an indispensible option in these clinical scenarios.

The first description of extensor attritional rupture was a case report of two patients by Vaughn-Jackson in 1948. Both patients had rupture of the ring and small EDC in proximity to the distal end of the ulna; the “syndrome” of progressive extensor ruptures from ulnar to radial now bears the author’s name.⁶ Almost a decade later, another case report demonstrated six patients with attritional extensor ruptures associated with rheumatoid arthritis (RA). The authors speculated that rupture resulted from attritional changes caused by localized rheumatoid synovitis.⁷ In patients with RA, the ECU tendon subluxates volarly, the wrist deviates radially, and the digital extensors course directly over a dorsally prominent distal ulna.

Radial nerve dysfunction compromises the extension capabilities of the wrist and hand. The radial nerve essentially innervates the entire dorsal aspect of the forearm musculature. The radial nerve proper innervates the triceps, lateral portion of brachialis, anconeus, brachioradialis, and ECRL. At the level of the elbow, the radial nerve proper divides into its superficial sensory branch and the PIN. The PIN innervates the remaining extensor muscles in the forearm including the ECRB (with some variability), supinator, APL, EPB, EIP, EDC, EDM, and ECU. The radial nerve may be injured directly from a penetrating injury or often indirectly in association with humeral shaft fractures. It may also be injured iatrogenically during surgical approaches to the arm. Depending on the level and extent of the nerve injury, wrist extension, digit extension, and/or thumb extension may be compromised. Lack of active wrist extension will severely affect power grip of the hand. Digit extension is mostly compromised at the level of the MCP joints because IP joint extension receives significant contributions from the median and ulnar-innervated intrinsic musculature (lumbricals and interossei) of the hand. A variety of tendon transfers have been described to improve function without creating imbalance within the hand. In devising an appropriate transfer, available muscles that can substitute for the lost function are considered and the above principles of tendon transfer surgery guide the ultimate choice and technique.