1. The frequency of os naviculare stress fractures of the high jumper, who jumps at extreme position of plantarflexion and pronation, is therefore not astonishing.

2. We report on two cases in which the differential diagnosis between an os tibiale externum (os naviculare accessorius) and a post-traumatic pseudarthrosis of the os naviculare was subject to discussion.Both patients had suffered an acute trauma in the region of the middle foot and showed identical clinical symptoms.Conventional X-ray did not allow a definite classification of the diagnosis either.However, this was possible in both cases by virtue of typical signs evidenced by computed tomography and magnetic resonance imaging.