[Atlas arch aplasia].

A bilateral aplasia of the dorsal arches of the atlas with persisting tuberculum posterius has been observed by the authors. Since the patient had suffered a severe cranio-cerebral trauma, it became necessary to exclude a possible fracture by differential diagnosis. This was possible by means of CT and conventional x-ray diagnosis. According to the classification of dorsal atlas arch aplasia by von Torklus and Gehle (1975) into 6 types, the diagnosis was, in the case under report, Keller's type of aplasia of the arch of the atlas.