Comprehending Optimality of Finger Flexor Tendon Pulley System Using Computational Analysis

Abstract

Existing prosthetic/orthotic designs are rarely based on kinetostatics of a biological finger, especially its tendon–pulley system (TPS). Whether a biological TPS is optimal for use as a reference, say for design purposes, and if so, in what sense, is also relatively unknown. We expect an optimal TPS to yield a high range of flexion while operating with lower tendon tension, bowstringing, and pulley stresses. To gain insight into the TPS designs, we present a parametric study which is then used to determine optimal TPS configurations for the flexor mechanism. A compliant, flexure-based computational model is developed and simulated using the pseudo-rigid body method, with various combinations of pulley/tendon attachment point locations, pulley heights, and widths. Results suggest that three distinct types of TPS configurations corresponding to a single stiff pulley, two stiff pulleys, or one stiff and one flexible-inextensible pulley per phalange can be optimal. For a TPS configuration similar to a biological one, the distal pulleys on the proximal and intermediate phalanges need to be like flexible–inextensible string loops that effectively model the behavior of joint and cruciate pulleys. We reckon that a biological flexor TPS may have evolved to maximize flexion range with minimum possible actuation tension, bowstringing, and pulley stress. Our findings may be useful in not only developing efficient hand devices but also in improving TPS reconstruction surgery procedures.