Identification and Characterization of Surface Zones in Rodent Caries

Objectives: We aim to establish that intact surface zones (SZs), a well-known and important feature of human enamel caries, are also common to rodent caries. Furthermore, we seek to characterize the compositional nanostructure of SZ enamel as compared to sound surface enamel (SSE).
Methods: Carious lesions were generated in rats through a sugar-rich diet and cariogenic Streptococcus mutans inoculations. Synchrotron x-ray computed microtomography (µ-CT) was performed to non-destructively identify lesions and characterize intact surface zones. Informed by µ-CT, mechanical sectioning and standard focused ion beam lift-out techniques were applied to generate tips suitable for characterization via atom probe tomography (APT). APT then provided three-dimensional reconstructions of SZ and SSE volumes, resolving the relative location of chemical species with high spatial resolution.
Results: Analysis of µ-CT identified 130 lesions across 30 scanned samples, and 95 (73%) displayed intact surface zones. SZs were observed in 12 biological replicates, in multiple teeth from the same animal, in multiple lesions within the same tooth, and across lesions of varying severity taken from 2 subsequent time points. 11 SZ and 9 SSE specimens have been characterized via APT. These preliminary results show some co-localization of minority species like Mg and F, but neither SZ nor SSE specimens have yet displayed the nanowire structure expected from previous APT of sound enamel in rodents and humans.
Conclusions: We present definitive evidence that surface zones exist during rodent enamel caries, and their prevalence across the dataset suggests they remain intact for a significant portion of lesion development. This confirmation supports the use of such rodent models as a human analog to explore disease pathology and evaluate potential treatments. Preliminary APT results suggest that SSE and SZ enamel may be too drastically altered from bulk sound enamel to be easily compared at such high resolution, but a larger dataset is necessary to confirm these conclusions.