Effect of wrist activity on median nerve function

Background - Hand intense occupational activities have been associated with an increase in the incidence of carpal tunnel syndrome (CTS). CTS is characterized by an impairment of median nerve function. To date, a dose-response relationship between wrist activity and median nerve performance has not been documented. Since repetitive hand/wrist activity in the workplace has significant implications, it is important to establish a scientific basis for the aetiology of work-related carpal tunnel syndrome. Methods -- In a laboratory environment, twenty-seven clinically confirmed asymptomatic female subjects performed continuous repetitive wrist motion in the flexion-extension plane during which an angle of 120 degrees was subtended about the neutral wrist position. Four levels of wrist activity, corresponding with 0 (static), 22 (Iow), 38 (medium) and 49 (high) repetitions per minute, were prescribed. Wrist motion was recorded using a state-of-the-art 3D electromagnetic tracking system (HumanTRAC). Mathematical descriptors of wrist kinematics, including cycle time, amplitude, angular velocity and angular acceleration, were calculated. Sensory median nerve response to imposed physical stressors was monitored antidromically and recorded using a clinical electroneurometer every ten minutes throughout the simulated work activities. Near-nerve skin temperature was recorded at three sites along the distal sensory branch of the median nerve every twenty minutes. Results - After adjusting for changes in near-nerve skin temperature, a significant within-subject effect of duration of exposure (time) was detected. Sensory median nerve conduction velocity differed statistically by 2.1 ms-1 between the static and high wrist activity conditions after 120 minutes of exposure, signifying adverse effects on nerve conduction that are uniquely attributable to repetitive hand motion. Wrist activity measures of mean angular acceleration presented a highly significant association with nerve performance, where nerve conduction decreased as wrist activity increased. Using regression analysis, a maximum safe wrist-workload exposure limit of 0.91 repetitions per minute is proposed. Limitations of this result are discussed. A biomechanical model is presented to calculate the effect of physical risk factors on tendon forces at the wrist. This model offers a method by which findings of the study can be employed for workplace exposure surveillance and development of ergonomic workstation design recommendations. Conclusions -- Across the study population of clinically asymptomatic female participants, a change in median nerve performance was observed. This significant effect was evoked due to imposed physical stressors. A dose response relationship between work intensity, exposure time and median nerve conduction velocity was demonstrated. The research explored in this thesis presents a foundation for the future development of a "Dynamic Median Nerve Stress Test". This test would involve the performance of a repetitive motion activity of the wrist during which changes in the function of the median nerve are closely monitored. The Dynamic Median Nerve Stress Test might prove to be valuable both as a provocative clinical test as well as an important research tool.