Displacement and strain of the median nerve at the wrist

Median nerve displacement and strain in the carpal tunnel region were measured as functions of wrist position and carpal tunnel pressure in 5 cadaver forearms during simulated active finger flexion. The positions of spherical stainless-steel markers embedded within the median nerve and flexor digitorum superficialis of the long finger were measured in 3 dimensions by a radiographic direct linear transformation technique. Each limb was tested in 3 wrist positions (60 degrees extension, neutral, and 60 degrees flexion) and 4 carpal tunnel pressures (0, 30, 60, and 90 mmHg). Carpal tunnel pressure was controlled with a balloon angiocatheter inserted deep to the flexor digitorum profundus. The ratio of median nerve to flexor tendon excursion was linear and was affected by wrist position but not carpal tunnel pressure. Patterns of strain in the median nerve proximal to the flexor retinaculum were different from those of strain within the carpal tunnel. Nerve strains were affected by wrist position, but carpal tunnel pressure had no effect. The hydrostatic pressure effect associated with carpal tunnel syndrome does not appear to influence median nerve kinetics or kinematics for the wrist positions studied.