Odontoid base hypodensity and its role in type II fracture risk and nonunion: a CT study

Abstract Background Type II odontoid fractures show high incidence and a notable risk of nonunion. Whether regional variation in trabecular density at the odontoid base contributes to this vulnerability remains unclear. Methods We retrospectively analyzed cervical CT scans from 136 adults. Standardized oval regions of interest (ROIs) were placed within cancellous bone while avoiding cortex and sclerosis. Mean Hounsfield unit (HU) values were compared across three predefined regions—the odontoid tip, odontoid base, and C2 vertebral body—with age- and sex-stratified analyses using analysis of variance and multiple-comparison adjustment. Two blinded observers performed the measurements, and reproducibility was quantified with intraclass correlation coefficients (ICCs). Results HU values were highest at the odontoid tip, lowest at the base, and intermediate at the C2 body. On average, HU declined by ~ 51% from tip to base and then partially increased at the body. This regional pattern persisted across age and sex strata, and older individuals showed lower base HU. All primary regional comparisons remained statistically significant after adjustment. Conclusions The odontoid base appears to be a structurally vulnerable zone with reduced trabecular density. This hypodensity may help explain the high incidence and may be associated with nonunion of type II odontoid fractures, particularly in elderly patients; however, biomechanical and clinical studies are required before informing definitive treatment recommendations.