Subdental synchondrosis and anatomy of the axis in aging: a histomorphometric study on 30 autopsy cases

Abstract

IntroductionFractures of the odontoid process of the second cervical vertebra comprise 7-13 % of all cervical spine fractures [22]. Moreover, fractures of the axis' odontoid basis-classified as Anderson and D'Alonzo type II fractures-represent the most frequent type of dens fractures with about 66% [3,9,18]. Odontoid fractures occur at all ages with a bimodal distribution. In the younger group they are often secondary to high-energy trauma as motor vehicle accidents. The second peak in the incidence of odontoid fractures is in the elderly, where they appear as a result from low-energy injuries, such as falls from standing height [4,12,13,19]. Abstract During skeletal development the two ossification centers of the odontoid process are separated from the corpus of the axis by a subdental synchondrosis. This synchondrosis is thought to close and disappear spontaneously in adolescence although this has never been studied in detail. The basis of the dens is of clinical relevance as type II dens fractures are located here. To characterize the morphological architecture of the axis with particular attention to the subdental synchondrosis, the complete axis was harvested from thirty age-matched and gender-matched patients of the three different age groups at autopsy. The subdental synchondrosis and the bone structure of the dens, the basis of the dens and the body of C2 were analyzed by radiography, histology and quantitative histomorphometry. At the macroscopic level the persistency of the subdental synchondrosis in the adult cervical spine was detected in 87% (26 of 30) of the specimens. Histomorphometry revealed a residual disc blastema with an average size of 25.8% of the sagittal depth of the basis of the dens at this level. Bony integration of the synchondrosis was poor throughout all ages. Histologically a cartilaginous matrix composition of the subdental synchondrosis persisted throughout all groups. The trabecular microarchitecture demonstrated a significant reduction of bone volume and trabecular number as well as an increased trabecular separation within the basis of the dens as compared to the corpus or the dens of C2. This histomorphometric data regarding a poor integration of the synchondrosis into the trabecular network and the reduced bone mass within the basis of the dens might offer a previously underestimated explanation for the occurrence of type II dens fractures and their association with pseudoarthrosis, respectively. Matthias Gebauer

Letter to the editor concerning: “Subdental synchondrosis and anatomy of the axis in aging: a histomorphometric study on 30 autopsy cases by M. Gebauer et al. (2006) Eur Spine J 15(3):292–298”

“…We have identified several items of contention in regard to this project and would like to put the results in proper context. First, we argue that Gebauer et al inaccurately characterized the regions on the samples of the axis in the sagittal plane [8]. In the sagittal plane they have distinguished three regions in the axis sample: the odontoid process, the base of the odontoid process and the body of the axis.…”

“…Gebauer et al [8] are to be commended for their initial study focusing on the morphology of the subdental synchondrosis in human cadaveric cervical spines. As they have pointed out [8], the remnants of the subdental synchondrosis could contribute to the high number of Type II fractures (according to the classification of Anderson and D'Alonzo [4]) and to the frequency of nonunion of these fractures. After carefully reading the authors' paper, we questioned their methods and results.…”

“…Since different classification schemes have been used in the region of the base of the dens, Heggeness and Doherty [12] obtained the contradictory data to the results from Gebauer et al [8] and Amling et al [1][2][3]. Gebauer et al [8] stated that the base of the dens was the region of low bone mass within the axis at all ages.…”

“…Gebauer et al [8] stated that the base of the dens was the region of low bone mass within the axis at all ages. The descriptions of the low bone density region in the axis were almost the same in the [12] and Gebauer et al [8], but were classified as different regions.…”

“…Gebauer et al [8] stated that the base of the dens was the region of low bone mass within the axis at all ages. The descriptions of the low bone density region in the axis were almost the same in the [12] and Gebauer et al [8], but were classified as different regions. Gebauer et al [8] argued that this markedly weaker trabecular region within the base of the dens is a region of least biomechanical resistance that might explain the development of Type II fractures.…”