The goal of a tendon transfer is to restore to the extremity a needed motor function that has been lost secondary to paralysis from nerve injury or disease or by muscle or tendon loss. To be considered is the effect that the transfer will have on the extremity and whether the function of the transferred motor can be spared. This technique in which the motor power of one muscle is transferred to a deficient muscle is now well established, with most of the procedures standardized. Until recently, only the tendinous portion of the musculotendinous unit was transferred, but today there is also a capability to transplant a complete muscle with its origin and insertion to a new location. This latter technique, free muscle transfer, is now a potentially useful tool when simpler techniques are not available. The gracilis, latissimus dorsi, and pectoralis major have been used as free muscle transfers on neurovascular pedicles into the upper extremity. Manktelow et al. have presented a series of patients with severe problems in whom these techniques were used to recover finger and thumb flexion.
Study of the evolution of these tendon transfer techniques takes one through an exciting period in the history of reconstructive extremity surgery. Excellent reviews have summarized the early efforts of workers in this field. Based on the work of these early surgeons, the modern principles of tendon transfer were developed. Although many of the original procedures were developed for use in patients having deficits caused by poliomyelitis, today most transfers are applied in patients with peripheral nerve injury or traumatic muscle loss. Patients with central nervous system disorders are probably a third group in whom tendon transfers are done today. The more recent refiners of the work of the masters include Brand, Brand et al, Riordan, Curtis, Littler, Omer, White, and Zancolli. Students who plan to work in this field should familiarize themselves with their work.
Indications for Tendon Transfer
The absence of a particular needed function after irreparable nerve injury, nerve disease, or muscle loss should bring up a consideration for tendon transfer. Time is allowed for wound healing to occur and for recovery of function, either spontaneously in the case of neuropraxia or axonotmesis or for reinnervation after a nerve repair. This usually requires a delay of 4 to 6 months. Although there are some who have advocated tendon transfer while awaiting recovery, I do not believe that there are many indications for early transfers if nerve regeneration is expected. During this waiting period, it is important that the physician, therapist, and patient work to maintain passive mobility in the involved joints. This would be achieved through passive motion exercises and by the use of splints when indicated. Nerve damage in itself is not always an indication for tendon transfer surgery because patients can sometimes perform unexpectedly through substitutive or adaptive methods or can function through dual-innervated musculature and/or variations in innervation. For example, solitary median nerve injury at the wrist may not result in loss of true thumb opposition in many patients because this action may be performed through the use of thenar eminence muscles partially or completely ulnar nerve innervated. The point here is that each problem must be individualized in terms of the loss of function, its severity, the reconstructive possibilities, and the patient’s needs and desires. Also to be stressed is the fact that tendon transfer is a palliative procedure that does not restore full and normal function. It is a redistribution of available power in an attempt to reduce a functional impairment.
When tendon transfer is being considered for the treatment of a particular patient’s problem, certain basic conditions should be satisfied, including joint mobility and adequate soft tissue coverage. There must also be available motor tendons for the transfer.
Basic Conditions for Tendon Transfer
The participating joints in the motor function in question should be freely capable of passive motion if the transfer is to succeed. Hand therapy using physical modalities, including dynamic splinting, may be needed to keep the joints mobile. This therapy can be taught to the patient and carried out by himself or herself, but the supervision and guidance of trained therapists are often necessary and are the ideal. At times, if joints have contracted and do not respond to therapy, the surgeon will do preliminary surgical releases to restore passive mobility.
Adequate Soft Tissue Coverage
Well-healed and pliable soft tissues must be present to provide gliding planes through which the transfer can function. This may require the shifting of skin and the use of local, distant, or free vascularized skin flaps before the actual tendon transfer.
Available Motor Tendons
A donor muscle must be available in the extremity. In single-nerve injury in the upper extremity, there may be a choice of motor tendons available for transfer and suitable for the function needed to be restored. If two of the major nerves in the extremity are involved and irreparably damaged, choices become more limited and the prognosis in terms of function will be greatly reduced. Tendons available in the upper extremity vary with the injury or disease and may include wrist flexors, wrist extensors, flexor digitorum superficialis (FDS), proprius extensors (extensor indicis proprius and extensor digiti minimi), and brachioradialis.
Selection of a Motor Tendon For Transfer
Although most of the transfers performed today have been well described for particular needs, it is useful to understand the decision-making processes that led to the development of specific tendon transfers that are most commonly applied. Boyes, in 1962, discussed those factors involved in the selection of a motor tendon. Amplitude of the donor tendon and power are the prime factors in tendon selection.
The distance that a muscle can shorten from its maximum length is the excursion or amplitude. A muscle contracts to approximately 40% of its resting length. Average amplitude of the wrist flexors and extensors is about 3.5 cm. Full finger extension at the metacarpophalangeal (MP) joints requires 5 cm and the flexor digitorum profundus (FDP) group needs an amplitude of 7 cm for full flexor function. As motor tendons, the FDS group, which provide an amplitude of about 6 cm, is the only group of available tendons that can approach the necessary amplitude to replace the profundus. Therefore, when the extensor carpi radialis longus is used to power the FDP, some sacrifice in range of motion (ROM) must be accepted ( Fig. 47-1 ). This deficit is partially overcome by the additional amplitude provided to the system by the tenodesis effect of the mobile wrist joint. Restoration of the long extensor system by transfer of a wrist flexor is a better matched situation as far as their respective amplitudes of motion are concerned.
Power ability of a muscle to perform work is directly proportional to its cross-sectional diameter. Examples of the power of muscles relative to each other in the forearm are as follows :
Pronator teres, 1.2 m/kg
Brachioradialis, 1.9 m/kg
Flexor carpi ulnaris, 2.0 m/kg
Flexor carpi radialis, 0.8 m/kg
Extensor carpi radialis longus, 1.1 m/kg
Extensor carpi ulnaris, 1.1 m/kg
In the preoperative period, accurate muscle testing is essential in the evaluation of a potential motor tendon. Each available muscle must be graded to ensure adequate strength for the transfer. The rating system 0 to 5 is used:
Flicker of muscle action
Muscle contracts and moves joint with gravity eliminated
Muscle moves joint through full ROM against gravity
Muscle moves joint through full ROM against resistance
It is widely believed that a transferred muscle loses one grade in strength, but this is questioned by Brand, who believes that the loss of effectiveness experienced after transfer is probably brought about by adhesions and drag rather than by actual loss of muscle strength. Whatever the reason, a four-power muscle may be inadequate for transfer and should be used with caution.