The association between a lifetime history of work-related low back injury and future low back pain: a population-based cohort study

This population-based cohort study investigated the association between a lifetime history of a work-related low back injury, in those who had recovered to have no or mild low back pain, and the development of troublesome low back pain (LBP). A secondary analysis explored the possible effects of misclassification of the exposure by examining the association between a lifetime history of having taken time off work or performed light duties at work because of a work-related low back injury. Current evidence from cross-sectional studies suggests that individuals with a history of a work-related low back injury are more likely to experience future LBP. However, there is a need to examine this association prospectively in a large population-based cohort with adequate control of known confounders.

We formed a cohort of 810 randomly sampled Saskatchewan adults with no or mild LBP in September 1995. At baseline, participants were asked if they had ever injured their low back at work. The secondary analysis asked if they had ever had to take time off work or perform light duties at work because of a work-related low back injury. Prospective follow-up 6 and 12 months later, asked about the presence of troublesome LBP (grade II–IV) on the Chronic Pain Grade Questionnaire. Multivariable Cox proportional hazards regression analysis was used to estimate these associations while controlling for known confounders.

The proportion followed up at 6 and 12 months was 76 and 65 %, respectively. We found an association between a history of work-related low back injury and the onset of troublesome LBP after controlling for gender (adjusted HRR = 2.24; 95 % CI 1.41–3.56). When covariates that may also be mediators of the association were added to the model, the effect estimate was attenuated (adjusted HRR = 1.37; 95 % CI 1.41–3.56). We found a similar association between a lifetime history of having taken time off work or had to work light duties at work because of a work-related low back injury, adjusted for gender (adjusted HRR = 2.31; 95 % CI 1.39–3.85) which was also diluted by the further adjustment for covariates that may also be mediators of the association (adjusted HRR = 1.80; 95 % CI 1.08–3.01).

Our study suggests that a history of work-related low back injury or taking time off work or having to perform light duties at work due to a work-related low back injury may be a risk factor for the development of troublesome LBP. Residual confounding may account for some of the observed associations, but this was less in the group who took time off work or had to work light duties due to a work-related low back injury.