Epidemiology of ankle sprains and the risk of separation from service in US Army soldiers

STUDY DESIGN: Retrospective cohort study. OBJECTIVES: To report the incidence rate of ankle sprains in active-duty soldiers and to examine if soldiers who sustain ankle sprain injuries are more likely to leave the Army than those who do not sustain an ankle sprain. BACKGROUND: Ankle sprains are one of the most common musculoskeletal injuries in physically active people and have been identified as the most common foot or ankle injury in active-duty Army personnel, with a rate of 103 sprains per 1000 soldiers per year. METHODS: Data were analyzed on the entire active-duty US Army population from 2000 to 2006 (n = 1 014 042). A semi-parametric Cox proportional hazard model was built. RESULTS: The overall incidence rate for ankle sprains was 45.14 per 1000 person-years. After controlling for length of service prior to the study period, soldiers who sustained a single ankle sprain were 27% less likely (relative risk ratio = 0.73; 95% confidence interval: 0.73, 0.75) to leave the service than soldiers who had no documented history of an ankle sprain. However, this trend toward increased service time no longer held true for those who sustained a recurrent sprain (risk ratio = 1.07; 95% confidence interval: 0.99, 1.15). CONCLUSION: It appears that individuals who sustain an incident ankle sprain have longer time in service in the Army than those who do not sustain this injury. However, this trend toward longer service time no longer held true for soldiers who sustained a recurrent sprain. This could be an indication that preventing recurrent injury could factor into longer periods of military service. LEVEL OF EVIDENCE: Prognosis, level 2b.