RT Journal Article SR Electronic T1 Unilateral posterior arch fractures of the atlas. JF American Journal of Neuroradiology JO Am. J. Neuroradiol. FD American Society of Neuroradiology SP 783 OP 786 VO 5 IS 6 A1 R A Suss A1 K J Bundy YR 1984 UL http://www.ajnr.org/content/5/6/783.abstract AB Unilateral posterior arch fractures of the atlas are discussed with two clinical examples and an experimental study of their mechanism. Laboratory fracturing of posterior arches of atlas specimens with a specially adapted universal testing machine produced nonsimultaneous fractures of the two sides in four of six specimens. In three of these specimens, a complete fracture on one side was temporarily displaced because the orientation of the leverage acting on the other side changed from sagittal to oblique. The consequent increase in the effective length of the lever arm reduced the angular deformation and strain on the second side. The second fracture occurred only after additional deflection of the posterior tubercle by up to 3 mm reproduced on the second side about the same angle of deformation that had caused the first fracture. A posterior arch fracture occurring by this mechanism will remain unilateral if the deflection is arrested before failure of the second side.