Biomechanics of the thumb flexor pulley system

The purpose of this study was to quantify the contributions of the thumb flexor pulleys on the mechanical efficiency and angular joint displacement of the human thumb. Fourteen human (below elbow) cadaveric specimens were divided into two study groups. Four experimental trials were performed with each thumb. Group 1 was tested with an intact pulley system, as well as after sequential sectioning of the oblique, A1, and A2 pulleys. Group 2 was tested with all pulleys intact and following the sequential sectioning of the (A1), oblique, and (A2) pulleys. Each forearm was mounted to a dynamic testing apparatus, and an 815 g load was applied at the thumb tip. The flexor pollicus longus tendon was attached to a force transducer and computer-driven stepper motor, and tension was recorded through full range of motion of the interphalangeal and metacarpophalangeal joints. The initial sectioning of either the A1 or oblique pulley did not significantly alter the thumb efficiency or its interphalangeal or metacarpophalangeal joint angular displacement. When both the A1 and oblique pulleys were cut, significant bowstringing was observed, as well as decreased interphalangeal flexion and increased metacarpophalangeal flexion; total angular motion remained unchanged. The tension required to achieve a given displacement also decreased following the sectioning of both the A1 and oblique pulleys. There was no statistical difference between an isolated, intact A2 pulley and no intact pulleys. These findings indicate that repair or reconstruction of either the A1 or oblique pulley after injury will restore normal thumb kinematics.