Carpal Tunnel Biomechanics – Displacement and Associated Intra-Carpal Tunnel Pressure Studies during Finger Flexion

Abstract

Idiopathic carpal tunnel syndrome (CTS) affects a significant portion of the population, although its exact etiology remains unknown. Based on dynamic ultrasonographic examination of the carpal tunnel inlet, in thumb and index finger (IF) flexion, we observed that the tendon(s) moves in the volar-ulnar direction, displacing the median nerve in the same direction. In middle finger (MF) flexion, however, the tendon moves dorsa-radially, leading to the volar-ulnar movement of the nerve. Median nerve and tendon displacements increased from thumb, IF and MF flexion. These general motion patterns were applied to a patient-specific computer-generated model of the carpal tunnel to determine the associated pressures. Motions in which the tendons move volarly, as in IF and thumb flexion, elicit greater nerve contact and von Mises stresses than those in which the tendons move in a dorsal-ulnar direction, as in MF flexion. Nerve displacement was also not directly correlated with nerve stress. The utilization of finite element modeling allows direct quantification of stresses within the tunnel without any invasive procedures, and may provide insight into the changing intra-carpal mechanics as CTS develops, which may lead to more effective CTS treatment.