A sprain is an injury to ankle ligaments - the structures responsible for holding the ankle in the appropriate alignment. Depending on the position of the foot and the motion of the body, different ligaments may be injured. The most commonly affected ligaments are those on the outside or lateral aspect of the ankle. The typical mechanism by which these occur is rolling the ankle with the foot pointed down and inward. The severity of the ankle sprain is dictated by multiple factors, the most important of which is the energy imparted by the injury to the ankle.