Measurement of thumb -tip forces produced by individual muscles: Application to restoring key pinch in persons with tetraplegia

Tendon transfer surgeries to restore key pinch in persons with tetraplegia are frequently successful. Nevertheless, surgical outcomes can be improved with knowledge of the forces produced at the thumb-tip by muscles. The aims of this dissertation were to measure such forces and to evaluate surgical approaches aimed at restoring key pinch in persons with tetraplegia. The thumb was placed in a flexed posture that simulated key pinch. We applied 10 N of force to the insertion tendons of the nine thumb muscles in cadaveric specimens and measured the resulting thumb-tip force in the flexion-extension plane of the thumb. The magnitudes of the thumb-tip forces produced ranged from 9% (extensor pollicis brevis) to 34% (adductor pollicis) of the applied tendon force. Muscles produced force in all directions at the thumb-tip (i.e., the proximal, distal, palmar and dorsal force directions). Most intrinsic muscles produced force in the distal direction; the extensor pollicis longus and brevis in the dorsal direction; and the abductor pollicis longus in the proximal direction. The flexor pollicis longus (FPL) and the ulnar head of the flexor pollicis brevis (FPB) produced the largest palmar force components. In tendon transfers aimed at restoring pinch in persons with tetraplegia, the donor muscle is most commonly attached to the insertion tendon of the paralyzed FPL. Despite the large desirable palmar force component produced by FPL, its proximal force component may be too large to facilitate a stable pinch (i.e., the thumb may slip). Thus, the palmar force component is most important to pinch execution and the proximal-distal component tends to cause the thumb to slip unless contact friction prevents it. We found that the undesirable non-palmar force component would indeed be small if FPL and both heads of FPB produced force simultaneously. Customarily, the muscle functions of both heads of FPB are not considered for restoration of pinch in tetraplegic individuals. Therefore, this dissertation suggests a new approach to restoring pinch function that has the potential to improve surgical outcome.