Atlanto-occipital disarticulation. Accident characteristics. A retrospective study of cases of atlanto-occipital disarticulation was conducted to describe incident characteristics: 24 cases were identified, including nine motor vehicle drivers, two passengers, seven pedestrians, and five motorcyclists; one other person had fallen four stories. The highest rates were found among motorcyclists and pedestrians. Atlanto-occipital disarticulations occur in high-energy impacts and collisions and are associated with aortic laceration in 25% and basilar skull fracture in 21% of such cases. Current restraint systems and motorcycle helmets do not appear to prevent this generally rapidly fatal injury. 