Quantification of fingertip force reduction in the forefinger following simulated paralysis of extensor and intrinsic muscles

Objective estimates of fingertip force reduction following peripheral nerve injuries would assist clinicians in setting realistic expectations for reh abilitating strength of grasp. We quantified the reduction in fingertip for ce that can be biomechanically attributed to paralysis of the groups of mus cles associated with low radial and ulnar palsies. We mounted 11 fresh cada veric hands (5 right, 6 left) on a frame, placed their forefingers in a fun ctional posture (neutral abduction, 45 degrees of flexion at the metacarpop halangeal and proximal interphalangeal joints, and 10 degrees at the distal interphalangeal joint) and pinned the distal phalanx to a six-axis dynamom eter. We pulled on individual tendons with tensions up to 25% of maximal is ometric force of their associated muscle and measured fingertip force and t orque output. Based on these measurements, we predicted the optimal combina tion of tendon tensions that maximized palmar force (analogous to tip pinch force, directed perpendicularly from the midpoint of the distal phalanx, i n the plane of finger flexion-extension) for three cases: non-paretic (all muscles of forefinger available), low radial palsy (extrinsic extensor musc les unavailable) and low ulnar palsy (intrinsic muscles unavailable). We th en applied these combinations of tension to the cadaveric tendons and measu red fingertip output. Measured palmar forces were within 2% and 5 degrees o f the predicted magnitude and direction, respectively, suggesting tendon te nsions superimpose linearly in spite of the complexity of the extensor mech anism. Maximal palmar forces for ulnar and radial palsies were 43 and 85% o f non-paretic magnitude, respectively (p < 0.05). Thus, the reduction in ti p pinch strength seen clinically in low radial palsy may be partly due to l oss of the biomechanical contribution of forefinger extrinsic extensor musc les to palmar force. Fingertip forces in low ulnar palsy were 9<degrees> fu rther from the desired palmar direction than the non-paretic or low radial palsy cases (p < 0.05). (C) 2000 Elsevier Science Ltd. All rights reserved.