Lower Extremity Injury Increases Risk of First-time Low Back Pain in the U.S. Army.

Low back pain (LBP) and lower-extremity injuries (LEI) are primary reasons for lost duty days and disability among military populations. Purpose This study examined acute LEI as a risk factor for developing LBP and examined the time to incident LBP between individuals with and without a history of LEI. Methods This retrospective cohort study examined U.S. Army medical and personnel data from the Total Army Injury and Health Outcomes Database for the years 2007 to 2011. Andersen-Gill Cox regression methods were used to examine the change in LEI status over time and changes in demographic covariates. Adjusted hazard ratios (HR) for LBP after LEI were calculated from the Cox regression model for each calendar year. An accelerated failure time (AFT) model was used to describe time to LBP, and mean time to event and adjusted time ratios (TR) after LEI were calculated from the AFT model for each year. Overall HR and TR for LBP after LEI were calculated over the five calendar years using variance-based weighted averages. Results Each yearly analysis included an average of 213,307 soldiers; on average for each year 8.44% of soldiers developed LBP and 11.54% had previous LEI. The pooled TR showed soldiers with a LEI had a 10% decrease in mean survival times to LBP compared to those without a LEI (TR, 0.901; 95% confidence interval, 0.897-0.905). The weighted average HR showed that soldiers with a LEI had 1.7 times the hazard of LBP compared with those without LEI (HR, 1.70; 95% confidence interval, 1.66-1.74). Conclusions These findings suggest that a potential second-order effect of LEI is an increased short-term risk for developing LBP, which should be considered during rehabilitation planning.