Risk for lower extremity injury after concussion: A matched cohort study in soldiers

BACKGROUND: Rates of lower extremity musculoskeletal injury are reportedly higher in professional and collegiate athletes following concussions. However, there is a paucity of evidence on this relationship in individuals who are not high-level athletes. OBJECTIVES: To examine the risk of acute lower extremity musculoskeletal injury in soldiers within 2 years of an incident concussion, compared to matched nonconcussed soldiers. METHODS: This was a matched-cohort study that used the medical encounter and personnel data of active-duty US Army soldiers from 2005 to 2011. Incident concussions were identified using International Classification of Diseases-Ninth Revision codes in medical encounter data of all soldiers from 2005 to 2009. One nonconcussed soldier in the US Army during the same month was matched by age, sex, rank, length of service, deployment status, and military career field to each concussed soldier. Hazard ratio (HR) and 95% confidence interval (CI) were calculated for the risk of lower extremity injury within 2 years of the incident concussion. Monthly HRs were compared to identify differences in injury rates between the groups, and an HR for the period of greatest difference was also calculated. RESULTS: A total of 23 044 individuals (11 522 concussed and 11 522 nonconcussed) were included in the study. Within 2 years of concussion, the hazard of lower extremity injury was 38% greater in concussed compared to nonconcussed soldiers (HR, 1.38; 95% CI: 1.30, 1.46), while the 15-month hazard was 45% greater (HR, 1.45; 95% CI: 1.36, 1.56). CONCLUSION: The rate of lower extremity musculoskeletal injury among this population of physically active adults is higher following concussion, and the risk remains elevated for more than a year following injury. LEVEL OF EVIDENCE: Prognosis, level 2b.