Association between wrist angle and carpal tunnel syndrome among workers
Doctor of Philosophy in Environmental Health Science
University of California, Berkeley
Professor David Rempel, Chair
Carpal Tunnel Syndrome (CTS) is the most common work-related peripheral neuropathy among workers who perform hand-intensive tasks. It has a greater impact on time away from work and disability than almost all other workplace injuries. For prevention, the National Institute of Occupational Safety and Health recommends reducing four primary biomechanical risk factors: hand tool vibration, grip and pinch force, repetitive hand exertions, and awkward wrist posture. However, the findings from studies evaluating the relationship between wrist posture and CTS are inconsistent. Few studies have measured wrist postures of workers and very few studies have been prospective in design. This dissertation presents a meta-analysis evaluating the effects of wrist posture on CTS among workers followed by an analysis of wrist posture risk from a large prospective study of blue-collar workers.
A meta-analysis reviewed the findings from prior studies that investigated the association between wrist posture and CTS and calculated a pooled relative risk.
In addition, data was collected from a 28-month prospective study of 447 blue-collar workers at four industrial sites in the U.S. These workers were followed with symptom surveys and nerve conduction studies at the wrist to identify incident cases of CTS. Video recordings of the workers performing their usual tasks were analyzed for grip type and wrist posture. Inter-observer reliability of wrist posture measurement was examined using Interclass Correlation Coefficients (ICC) and Kappa coefficient. Using each worker's task exposure and estimated time on task, a time weighted average exposure metric was calculated for several measures of wrist posture. A survival analysis assessed the relationship between wrist postural risk factors and incident cases of CTS. The final multivariable models were adjusted for confounding.
Nine epidemiologic studies met the inclusion criteria for the meta-analysis. The pooled relative risk of work-related CTS increased with increasing hours of exposure to wrist deviation, or extension/flexion [RR = 2.01; 95% CI: 1.646-2.43; p < 0.01; Shore adjusted 95% CI 1.32 to 2.97]. The inter-rater reliability for wrist posture measures from video found moderate (Kappa = 0.48) to substantial (Kappa = 0.64) agreement between observers across the five different types of grip. The ICC ranged from 0.57 and 0.67 with the highest correlation for high-force power grip.
Job level measures of wrist posture were merged with other data from the prospective study. During the 28 months of follow-up there were 29 incident cases of dominant hand CTS. Multivariable analysis indicated that the hazard ratio for CTS more than doubled with the combined exposures to wrist extension and forceful (> 10 N) pinch. The HR, after adjusting for age, gender, BMI, and medical history, was 2.83 (95% CI 0.91-8.76; p-value <0.2), where the cutoff between low and high exposure was 6.07 degree mean wrist extension.
Discussion and conclusion
We found that exposure to wrist extension, especially during the application of forceful pinch grip increased the risk of CTS. A new method for estimating posture from videos was developed and found to have good inter-rater reliability; levels of reliability that are higher than prior studies. Recommendations are made for video analysis methods for estimating wrist posture. The analysis of the prospective data suggests that reducing mean wrist extension, if above 6 degrees, or reducing the combination of wrist extension and high-force pinching will reduce CTS risk among blue-collar workers in hand intensive jobs. Workplace interventions should incorporate training and engineering interventions to reduce sustained exposure to these factors. These findings may be useful in the design of workplace safety programs to prevent CTS in order to keep our workers healthy and productive.