Fractures of the arches of the atlas: a study of their causation.

The autopsy findings in 32 accidental deaths which showed fracture(s) of the arches of the atlas have been correlated to reconstructions of the course of events in the accidents. Flexion of the head causing fracture(s) of the neural arch or odontoid process of the axis also resulted in fracture(s) of the posterior arch due to downward traction. Extension of the head causing fracture of the odontoid process of the axis gave rise to fractures of the posterior arch due to pressure from below. Tilting of the head caused marginal fractures of the anterior arch due to oblique traction. Tilting of the head also caused fracture(s) of the anterior and/or posterior arch due to transverse extension of the atlas ring and/or superior dislocation of one lateral mass in relation to the other. Oblique flexion or extension of the head resulted in similar fractures. Flexion of the head with some rotation combined with compression of the neck can cause the anterior margin of one upper joint surface of the axis to act as a wedge separating the anterior arch of the atlas from below. Extension of the head with some rotation combined with compression of the neck can lead to a fracture running through one lateral mass due to its tilting-dislocation in posterior direction. It is striking that there was no fracture of the atlas which could be ascribed to a simple and symmetric compression of the neck (classical Jefferson's fracture).