Tendon transfer surgeries aimed at restoring key pinch in persons with tetraplegia are frequently successful. Nevertheless, surgical outcomes would be improved with knowledge of the forces produced at the thumb-tip by muscles because the ability to pinch an object depends on the direction of the thumb-tip force produced against the object. The aims of this dissertation were:​ (1) to measure thumb-tip forces produced by thumb muscles;​ (2) to use these forces to evaluate surgical approaches to restoring key pinch in persons with tetraplegia;​ and (3) to estimate how anatomic and postural factors contribute to inter-specimen variability in thumb-tip force.

With the thumb in a flexed key pinch posture, 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 plane of flexion-extension of the thumb. We also measured how two surgeries—stabilization of the interphalangeal (IP) joint, and release of the A1 and the oblique pulleys of the flexor pollicis longus (FPL)—affected thumb-tip force produced by FPL. In addition, based on the thumb-tip forces measured in the nonsurgically altered thumb, we developed a two-dimensional, mathematical model to theoretically evaluate the variation in thumb-tip force caused by inter-specimen differences in thumb anatomy and posture.

The magnitudes of the thumb-tip forces produced ranged from 9% (extensor pollicis longus) to 34% (adductor pollicis) of the applied force across the nine muscles studied. Muscles produced forces in all directions at the thumb-tip (i.e., the palmar, distal, dorsal and proximal force directions). The FPL and the ulnar head of the flexor pollicis brevis (FPBu) produced forces with the two largest palmar force components. Most intrinsic muscles primarily produced forces in the distal direction;​ the extensor pollicis longus and brevis, in the dorsal direction;​ and the abductor pollicis longus (APL), in the proximal direction. Stabilization of the IP joint and release of the pulleys of FPL significantly increased (p ≤ 0.05) the magnitude of the thumb-tip force produced by FPL. Stabilization of the IP joint also produced a significant shift (p ≤ 0.05) in FPL’s thumb-tip force orientation toward the palmar direction. Inter-specimen variations in the force components produced by APL and FPL represented the two extremes in measured variability. The interquartile ranges for the proximal and palmar force components of APL’s thumb-tip force were 0.5 N and 0.4 N;​ and of FPL’s, 2.7 N and 0.8 N. The mathematical model indicated that inter-specimen differences in the angles of the MP and IP joints could largely account for the variations in thumb-tip force magnitude and direction measured in situ.

Tendon transfer surgeries aimed at restoring key pinch in persons with tetraplegia are typically designed with one paralyzed muscle in mind because, often, only one donor muscle is available for transfer. This dissertation indicates that FPL is a natural choice because it produces one of the largest palmar force components. The palmar force component makes key pinch possible, whereas, the other components tend to cause the thumb to slip unless friction at the contact prevents it. This dissertation suggests that stabilization of the IP joint facilitates key pinch by rotating FPL’s thumb-tip force toward the palmar direction. This dissertation also suggests that if FPBu were involved in a tendon transfer surgery with FPL, then the pinch would be more desirable than with FPL alone because the resultant thumb-tip force would have a larger palmar force component.