STRUCTURAL HETEROGENEITY WITHIN THE AXIS - THE MAIN CAUSE IN THE ETIOLOGY OF DENS FRACTURES - A HISTOMORPHOMETRIC ANALYSIS OF 37 NORMAL ANDOSTEOPOROTIC AUTOPSY CASES

Fractures of the odontoid process are potentially serious injuries; Ty pe II and III fractures, as described by Anderson and D'Alonzo, are se en in the emergency room especially in young adolescents and individua ls over 60 years of age. The etiology of these fractures is still cont roversial. Malunion and nonunion in both types of fractures are presum ed to be due to insufficient external or internal fixation, but this t heory has not been fully explained. To examine these issues, the autho rs expanded their prior studies of the anatomy of the axis. For histom orphometric analysis of cancellous and cortical bone, the axis was rem oved in 37 autopsies (26 normal and 11 osteoporotic cases) and section ed in the sagittal plane to a thickness of 1 mm using a surface-staine d block-grinding technique. The base of the dens is the region of leas t resistance for fractures because of its reduced trabecular bone volu me, a poorer trabecular interconnection, and a cortical thickness one- third that of the axis. in all cases, trabeculae were disconnected fro m the trabecular lattice, and in 30%, microcallus formations were demo nstrated in the base of the dens. A special filigree type of trabecula e in the base of the dens is often seen in patients with osteoporosis; microarchitectural differences of cancellous bone between the base of the dens and the other regions of the axis are also markedly increase d. The authors infer from the data that the bone structure of the axis is responsible for the location, distribution, and frequency of fract ures of the odontoid process in normal healthy bone and this frequency is greatly increased in individuals with osteoporosis. The deficiency of bone mass within the base also suggests a new explanation for the occurrence of nonunions, even after treatment of fractures of the base of the dens.