Opposition of the thumb: an anatomic and biomechanical study of tendon transfers.

Tendon transfers for opposition of the thumb were anatomically and biomechanically studied to help determine the optimal criteria for selecting the best motor unit for a transfer. Forearm and hand muscle volume, mean fiber length, and cross-sectional area were measured in eight fresh specimens of the upper extremity to determine which muscles best replace lost thenar muscle strength. In a separate group of 18 specimens, the effective moment arms for abduction and flexion of the first metacarpal were calculated in vitro and from biplanar radiographic techniques to determine the effect of eight different opposition transfers on thumb abduction, rotation, and strength. Results of these studies demonstrate that the transfers of flexor digitorum superficialis (FDS) of the long finger and extensor carpi ulnaris best replaced lost thenar muscle strength and provided maximal abduction and near full thumb rotation. The transfers of the extensor carpi radialis longus and the FDS of the ring finger replaced 60% and 40% of required thenar muscle strength, respectively. The palmaris longus was the least effective transfer, having good abduction but weak flexion and opposition. Motion, balance, and strength of tendon transfers must be considered for effective thumb opposition.