Development and Validation of a Cumulative Exposure Shoulder Risk Assessment Tool Based on the Fatigue-Failure Theory

Abstract

Exposure to physical risk factors and the resultant cumulative loading has been identified as a major contributing factor for the development of workplace musculoskeletal disorders (MSDs). Nevertheless, the dose-response relationship of MSDs and their association with workplace risk factors is still not well understood. Previous studies have suggested various methods for addressing cumulative loading where the cumulative damage for the total job is estimated by considering each individual task. However, most of the methods use linear integration methods; which assume that low force, long duration and high force, short duration tasks have the same injury risk. More recent studies support the role of fatigue failure theory in the development of MSDs. Evidence supporting the fatigue failure process in the development of MSDs includes epidemiological studies, tissue loading in animal studies and in vitro testing for MSDs. Those studies explained the fatigue failure behavior of biological tissues including tendons, ligaments, cartilage, and spinal motion segments which also revealed a pattern of force repetition interaction consistent with the fatigue failure process. Thus, cumulative loading effects could be estimated using the fatigue failure process. Upper extremity MSDs, including shoulder injuries, are second highest in terms of injuries causing occupational lost time claims after back injuries. Current observational tools available for upper extremity assessment generally do not consider the impact of cumulative exposure. Furthermore, combined effects of jobs comprising multiple tasks are not available. The aim of this dissertation was to develop and validate a cumulative exposure shoulder risk assessment tool based on the iii fatigue failure theory. Video recordings of jobs from an existing epidemiology study were analyzed to get the required inputs for the tool development. Postural estimates and repetitions per workday were obtained from this video analysis. Moments for both left and right shoulders were then estimated and shoulder strength capability was used to establish the ultimate strength of the shoulders. From this relationship, the number of cycles to failure was calculated for each level of stress so that the damage per cycle (DPC) could be obtained and repetitions used to estimate the cumulative damage (CD) per day. The CD can be easily summed for multiple tasks to get a daily dose of exposure, and the contribution of various tasks can be easily estimated. The tool was validated against the epidemiology database using numerous shoulder outcomes. Both crude and adjusted logistic regression results indicated strong dose-response associations between log CD and all shoulder outcomes (p<0.0001). Odds ratios (ORs) were higher for the self-reported symptoms such as current shoulder pain (1&2 vs 4&5) and current shoulder pain (1 vs 5). When the effect of the personal characteristics was introduced into the model and adjustments were made for the moments and capability strengths, significant results were also obtained. However, greater variability is observed in shoulder CD estimates which can be explained by the sex differences in terms of shoulder capability strength which in turn will affect the CD estimates. Subjective ratings for shoulder strength assessment were used to investigate the impact of subjective ratings on the model adequacy. Significant results were obtained for these subjective measures logistic regressions (P<0.0001). The ORs for the different shoulder outcomes were close to the ones obtained for the original model. All results indicated strong associations between shoulder cumulative damage estimate, and all investigated negative shoulder health outcomes. This provides further support for the impact of the fatigue failure and the estimated cumulative damage in the development of MSDs.