Standard body-part term; no special register or usage restrictions.
昨日、足首をひねってしまった。
I twisted my ankle yesterday.
彼女は細い足首をしている。
She has slender ankles.
I slipped on the stairs and twisted my ankle.
I sprained my ankle during the basketball game.
My ankle hurts because I walked for a long time.
I rotated my ankles to stretch before exercising.