Four adults who had symptomatic instability of the ankle had an
associated os subfibulare. Operative exploration revealed the ossicle to
represent a non-union of an avulsion fracture of the anterior talofibular
ligament. These findings suggest that an os subfibulare represents an
avulsion fracture that may or may not be associated with laxity of the
anterior talofibular ligament, rather than being a normal variant.