[Structure of the axis--key to the etiology of the dens fracture].

Fractures of the dens are seen especially in young adolescents but also in individuals after the sixth decade of life. The etiology of these fractures and the occurrence of non-union after initial treatment is still discussed controversially. To address these issues, the axis was removed from thirty-seven autopsy cases for histomorphometric analysis. The base of the dens is a region of least resistance for fractures due to its reduced trabecular bone volume, a poorer trabecular interconnection and a cortical thickness one third that of the axis. In all of the cases, trabeculae disconnected from the trabecular lattice, and in 30% microcallus formations were demonstrated in the base of the dens. In osteoporotics the microarchitectural differences of cancellous bone between the base of the dens and the other regions of the axis are increased markedly. The obtained data suggest that the bone structure of the axis is responsible for the location, the distribution and the frequency of fractures of the odontoid process. The deficiency of bone mass within the base also offers a new explantation for the occurrence of non-unions even after treatment of fractures of the base of the dens.