Tendon transfer is used to replace the deficient motor unit and to restore function. The functional loss may occur secondary to tendon rupture, peripheral nerve injury, plexus injury or other neuropathies.
In tendon transfer, a healthy functional tendon to be transferred is transected then transferred to another tendon or bone to restore function. The neurovascular supply of the donor unit is preserved, in contrast to a free tendon graft which functions primarily as a spacer or connection.
The transferred motor unit must have sufficient strength and excursion to restore function, and it must be expendable.
It must be kept in the mind that only the insertion of a tendon is moved, but the origin remains in the same location.
A tendon transfer should not significantly decrease the remaining function or create a deformity. Therefore tendon transfer is governed by few principles.
Tendon transfer aims to recover lost function and is specific to the injury and joint involved for each individual patient.
In upper extremity, the aim is to allow motion of the hand and fingers and allow patients better strength and dexterity.
However, for lower extremity aim of tendon transfers is to brace ankle and/or foot for walking and standing.
Principles of Tendon Transfer
Fundamental principles of muscle-tendon units include the following.
Optimal Tissue Bed
In order for a tendon transfer to function optimally, the soft tissue bed must be sufficient, the skeletal support must be stable, and all the joints must be mobile.
In a patient with peripheral nerve palsy, all efforts must be made to kept the joints supple because it is easier to prevent a contracture than to correct.
All joints should be moved passively throughout the palsy because stiff joint would not be corrected by surgery.
A muscle will lose one grade of strength following the transfer. The tendon chosen as a donor for transfer must be strong enough to perform its new function in its altered position.
(Power grading – grade 5 is normal, grade 4 is motion against resistance, grade 3 is motion against gravity, grade 2 is motion with gravity eliminated, grade 1 is trace motion or fasciculations, grade 0 is flaccid).
Therefore, if a muscle was grade 5 (normal) it will be grade 4 (motion against resistance yet deficient strength) after transfer.
Ideally, transferred muscles should be of normal strength.
Excursion must also be considered when choosing a donor. Wrist flexors and extensors have approximately 3 cm of active excursion, finger extensors have 5 cm and the finger flexors have 7 cm of excursion. As a result, a wrist motor is not likely to function well as a finger flexor unless supported by active wrist extension.
Effective excursion can be augmented by extensively dissecting a muscle from its surrounding fascial attachments.
The straight line of pull
Tendon transfers should pass in a straight line if possible to maximize strength. Any joint that is crossed by the tendon transfer must be stable and have active control to avoid a collapse deformity.
One tendon, one function
A single tendon can have only one function. A muscle cannot be inserted into two tendons with separate functions.
The use of synergistic muscles has been advocated for transfer. Muscle function is easier to retain after synergistic muscle transfer. For example, finger and wrist extensors are synergistic. So are finger extensors and wrist flexors. Therefore, using a wrist extensor for finger flexor makes a sense.
There should be a donor muscle available. A tendon when harvested should not result in in an unacceptable loss of function. There must be sufficient remaining muscles available to substitute for the donor muscle.
Timing of tendon transfer
No transfer should be performed until the local tissues are in optimal condition.
The tendon transfers are performed after the permanent motor deficit becomes clear based on clinical evaluation as well as nerve conduction studies and electromyography. A tendon transfer should not be done when it is not certain whether the nerve function would return or not.
Course of Tendon
Tendon transfers function best when passed between subcutaneous fat and the deep fascial layer; they are not likely to be functional if placed in the pathway of a scar.