Neck injuries: I. Occipitoatlantal dislocation--a pathologic study of twelve traffic fatalities.

Twelve of 155 persons killed in traffic crashes had occipitoatlantal dislocations. Nine were vehicular occupants, 2 were cyclists, and one was a pedestrian. The dislocations involved various combinations of lacerations of the alar ligaments, the occipitoatlantal joint capsules, the dura mater, the tectorial membrane, the rectus capitis muscles, and the suboccipital muscles. In 2 instances, an occipital condyle failed instead of the corresponding alar ligament, producing condyle fractures. Atlas ring fractures occurred in 3 instances. Axial and subaxial cervical trauma were uncommon. Facial or mandibular fractures occurred in a majority of cases, vault skull fractures were uncommon, and basilar fractures were absent. Pontomedullary brainstem lacerations occurred in 9 of the 12, and 4 had midbrain lacerations. The majority of the victims succumbed to acute neurogenic shock as the sole or the major mechanism of death. The biomechanical basis for occipitoatlantal dislocation is discussed, and the author suggests that distraction, in concert with variable combinations of extension, rotation, and posterior translation is responsible for occipitoatlantal dislocations.