Functional implications of the musculoskeletal anatomy and passive tissue properties of the forearm

This thesis explored anatomical factors thought to contribute to hand dysfunction in three major studies which examined: (1) the effects of wrist posture and tendon loading on carpal tunnel pressure; (2) analysis of the carpal tunnel contents using magnetic resonance imaging (MRI); and (3) the passive properties of the forearm musculature.

Hydrostatic pressures and localized loading in the carpal tunnel were measured in cadaver arms. Hydrostatic pressure increased with flexion, extension and radio-ulnar deviation. Hydrostatic pressures were further increased by loading the finger flexor and palmaris longus muscles. Localized loading also increased with tendon loading, specifically, with tension on the finger flexors with the wrist flexed and tension on palmaris longus with the wrist in extension. Increases in pressure and point loading by tension in palmaris longus were attributed to its tendon altering the shape of the carpal tunnel, while the increases from the flexor tendon loading were attributed to direct transmission from the tendons to the supporting surfaces in the wrist. Both hydrostatic pressure and point loading measures are required to fully describe possible damage to the median nerve.

Loading of wrist structures was also determined using MRI data of three healthy men. Digitizing the flexor tendons as they passed through the carpal tunnel allowed digital recreation of the tendon paths and calculation of the radius of curvature along the tendon paths. The radii of curvature of the flexor tendon paths was found to be reduced with wrist flexion thereby increasing the force per unit length imparted normal to the tendon path. A low level pinch grip, used to load the flexor tendons, was found to further decrease the radius of curvature. This increased the transmitted force per unit length by two modes: increased tendon tension and decreased radius of curvature.

The MRI study prompted a re-evaluation of tendon-joint models and determined that the values for radius of tendon curvature in the literature have been underestimated, thus the force per unit length has been overestimated. A model that allows the tendon to "bowstring" at the joint is necessary for a better estimate of the radius of curvature.

A comprehensive examination of the passive properties of the forearm muscles indicated that measured passive force curves were well predicted by exponential regression equations. Forces were calculated for functional hand posture by incorporating the equations into an excursion model where the finger extensors were found to have sizable passive forces (up to 10 N) in postures requiring flexion of multiple finger joints, such as tip pinch, key pinch and briefcase grip. Application of these data, via a computer model of the finger, to typing postures and keyboard data entry postures determined the wrist extensor musculature to support static loads up to 30% of maximum capacity. (Abstract shortened by UMI.)