[Aplasia of the posterior arch of the atlas--report on two cases].

Two cases of aplasia of the posterior arch of the atlas incidentally found are reported. Case 1: A 37-year-old man hit the forehead and the right mandible against the front glass at a head-on collision of his car and was brought to the emergency room. He was alert and neurologically normal except for multiple incision wounds with glass fragments in the face. CT, and cervical X-rays revealed total absence of the posterior arch of the atlas. Case 2: A 73-year-old woman, who sustained whip lash injury at an automobile accident, was admitted several hours after injury, complaining of neck and occipital pain. No neurological deficit was seen. CT demonstrated hemi-aplasia of the posterior arch of the atlas and isolated rudimentary posterior tubercle. Review of the literature and discussions were made on the incidence and genesis of this anomaly.