Diagnoses of traumatic brain injury not clearly associated with deployment, active component, U.S. Armed Forces, 2001-2016.

From 2001 through 2016, a total of 276,858 active component service members received first-time diagnoses of traumatic brain injury (TBI). Person-time and incident cases of TBI were assigned to one of three groups. Group 1 included only service members' person-time before their first-ever deployments. Group 2 included service members' person-time during their overseas deployments and the 30 days after their return from deployment. Group 3 included only service members' person-time more than 30 days after return from deployment. The crude overall incidence rate of TBI among deployed service members (1,690.5 cases per 100,000 person-years [p-yrs]) was 1.5 times that of service members in group 1 (1,141.3 cases per 100,000 p-yrs), and 1.2 times that of service members in group 3 (1,451.2 cases per 100,000 p-yrs). The portion of the surveillance period during which the annual incidence rates of TBI in groups 3 and 2 exceeded the rates in group 1 likely represents, at least in part, the increased risk of service in an active combat zone. For group 2, this period extended from 2007 through 2013. For group 3, this period lasted from 2007 through 2016. Examination of the TBI case-defining encounters with recorded injury causes yielded leading causes similar to those of TBIs in same-aged civilians (land transport and slips, trips, and falls). Factors that may explain why the TBI incidence rates among the previously deployed were higher than those of the never-deployed group are discussed.