Structural heterogeneity within the axis: the main cause in the etiology of dens fractures. A histomorphometric analysis of 37 normal and osteoporotic autopsy cases.

Fractures of the odontoid process are potentially serious injuries; Type II and III fractures, as described by Anderson and D'Alonzo, are seen in the emergency room especially in young adolescents and individuals over 60 years of age. The etiology of these fractures is still controversial. Malunion and nonunion in both types of fractures are presumed to be due to insufficient external or internal fixation, but this theory has not been fully explained. To examine these issues, the authors expanded their prior studies of the anatomy of the axis. For histomorphometric analysis of cancellous and cortical bone, the axis was removed in 37 autopsies (26 normal and 11 osteoporotic cases) and sectioned in the sagittal plane to a thickness of 1 mm using a surface-stained block-grinding technique. The base of the dens is the region of least resistance for fractures because of its reduced trabecular bone volume, a poorer trabecular interconnection, and a cortical thickness one-third that of the axis. In all cases, trabeculae were disconnected from the trabecular lattice, and in 30%, microcallus formations were demonstrated in the base of the dens. A special filigree type of trabeculae 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 increased. The authors infer from the data that the bone structure of the axis is responsible for the location, distribution, and frequency of fractures 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.

The microarchitecture of the axis as the predisposing factor for fracture of the base of the odontoid process. A histomorphometric analysis of twenty-two autopsy specimens.

The axis from twenty-two cadavera was removed at autopsy and was sectioned in the sagittal plane to a thickness of one millimeter with use of a surface-stained block-grinding technique. Combined two and three-dimensional analysis included an evaluation of the volume of the trabecular bone, the trabecular interconnection, and the cortical thickness as well as qualitative investigation of the structure of the cancellous bone. The body of the axis, the base of the odontoid process, and the odontoid process were analyzed separately. The base of the odontoid process is a region of least resistance for fractures because of its unique microarchitecture. The mean volume of trabecular bone of the base of the odontoid process is 55 per cent less than that of the axis and the odontoid process. The base also has a markedly poorer trabecular interconnection and a cortical thickness that is one-third that of the odontoid process. In all of the specimens, trabeculae that were disconnected from the trabecular lattice (trabeculae with free ends) were demonstrated in the base of the odontoid process. The formation of microcallus in six (27 per cent) of the specimens supports the hypothesis that microfractures occur as a result of stress peaks, mechanical fatigue, and the relative insufficiency of bone in the static condition. Therefore, the base of the odontoid process can be considered as a site of predilection for fractures.

Tissue interactions during axial structure pattern formation in amphibia.

Tissue interactions have traditionally been assigned important roles in establishing the pattern of amphibian axial structure morphogenesis. Those interactions have been postulated to generate the patterns of neural fold morphogenesis, neural tube formation, and somite development. A review of axial structure development together with a brief discussion of the classical viewpoint, is presented. A re-examination of axis formation has recently been carried out with the SEM. Embryos which displayed major defects in notochord development, ranging from diminished length to complete obliteration, were produced by irradiating fertile eggs prior to first cleavage. A comparative SEM analysis of normal and "notochord defective" embryos revealed that, contrary to previous reports, the notochord is apparently a dispensable component of the developing axial structure system. Lastly, TEM examination of the notochord defective embryos allowed some insight into the ultrastructural alterations which occur in the notochord and neural tube cells of irradiated embryos. Additional information about the structure of the notochord, and the cellular mechanics of somitogenesis emerged from those studies.