Butting heads.
D001265:Athletic Injuries / Q000453:epidemiology*; D006259:Craniocerebral Trauma / Q000453:epidemiology*; D003704:Dementia / Q000453:epidemiology*; D005538:Football / Q000293:injuries* / Q000706:statistics & numerical data*; D006801:Humans; D012307:Risk Factors