a. Synergistic: act together to produce a single composite movement. e.g.
- Finger flexors ↔ Wrist extensors
- Finger extensors ↔ Wrist flexors
b. Expendable: There must be atleast 1 muscle that can continue to perform function of transferred tendon. e.g. 2 out of ECRB, ECRL, ECU (wrist extensors) can be transferred
c. Adequate strength: Transferred muscle should not be stronger (antagonistic function gets restricted) or weaker (not effective) than the host muscle. Muscles usually lose atleast 1 grade MRC muscle power following transfer. The relative strengths (working capacity) of muscle are:
- FDS: 4.8
- FDP: 4.5
- BR, FCU: 2
- FCR, Wrist extensors (ECU, ECRB, ECRL), PT: 1
- Finger extensors (EDC, EI, EDM): 0.5
- APL, EPL, EPB, PL: 0.1
d. Contractures released: Joints should be supple. In hands, MCP joints must have full passive range of motion.
e. One tendon (donor), one function: Transfer of a single donor tendon to 2 recipients that perform opposing functions across a given joint will result in decreased transfer force, amplitude and efficiency.
f. Adequate excursion: Excursion of donor and recipient muscles should be matched. Boyes 3-5-7 rule provides a helpful mnemonic regarding excursion of wrist or finger extensors or flexors.
- Wrist flexors and extensors: 33 mm
- Finger extensors and EPL: 50 mm
- Finger flexors: 70 mm
- Others: Brachioradialis ~60 mm
g. Straight line of pull: Tendon transfer should result in a linear vector of pull to maximize efficiency and minimize potential deforming forces. To achieve this, the muscle might have to be dissected free – till close to its origin (taking care not to injure the main nerves and vessels supplying it). The plane where it lies (above or below surrounding muscles) may also need to be changed.
h. Tissue equilibrium: The transferred tendons should lie in a healthy bed of tissue with no edema, inflammation, or scarring. If the bed is scarred, it should be excised and covered with flap earlier.
i. 1 joint/pulley
We can also use another mnemonic: 13 S
In a Sensible patient, I will transfer a:
- Strong, Sacrificeable, Synergistic tendon with Sufficient excursion
- Straight through a Scarless, Stable bed, Subcutaneously
- Across a Supple, Sensate joint
- To achieve a Single function by Securing distally