Characterisation of Carious White Spot Lesions by XRMT and Nano-indentation

Objectives: To characterise the mineral density (MD), mechanical properties (MP) and micro-structural features of natural enamel white spot lesions (WSLs) using X-ray micro-tomography (XRMT), nano-indentation and scanning electron microscopy (SEM). Methods: Five natural WSLs from four extracted non-carious premolar teeth were scanned at a voxel size of 7.6 µm using a desktop XRMT system. Five hydroxyapatite (HA) phantoms with densities ranging from 1.52 to 3.14 g/cm³ were used as calibration standards for each scan. MD gradients throughout the WSLs were quantified using a calibration equation derived from HA phantoms. Subsequently, teeth were cross-sectioned and elastic modulus (E) and hardness (H) were measured systematically across the WSLs at intervals of 25 and 50 µm using nano-indentation and followed by SEM investigation. Results: The MD of sound enamel, apparent intact surface layer of WSL, and lowest level of WSL was found to be 2.65 - 2.89, 2.23 - 2.58 and 1.48 - 2.03 g/cm³ respectively. Similarly, the WSLs showed a large reduction in MP: E was 83% lower (77.0 ± 2.2 to 13.4 ± 2.1 GPa) and H 91% lower (3.39 ± 0.41 to 0.29 ± 0.07 GPa). Typically, WSLs displayed steep MP gradients at the lesion boundary and more uniform MP with isolated variations in the lesion body. SEM images showed loss of mineral in different locations of enamel micro-structure as the lesion progressed into more severely demineralised regions. Conclusion: XRMT is a sensitive in-vitro technique capable of characterising and quantifying MD of small non-cavitated WSLs. Detailed analysis of WSLs demonstrated intact micro-structure despite the large MP reduction. We postulate that loss of mineral and change of the hierarchical micro-structure at the crystallite level contributed to the overall MP of WSLs. Acknowledgement to Biodental Solutions Pty Ltd, ARC (LP0561184) & Colgate-Palmolive for their financial support.