Nano-indentation characterisation of natural carious white spot lesions

The aim of this paper was to characterise the mechanical properties (MPs) and microstructural features of natural enamel white spot lesions (WSLs) using nano-indentation. Five natural WSLs from 4 extracted sound premolar teeth were cross-sectioned. Elastic modulus and hardness were measured systematically across the WSLs at intervals of 25 and 50 microm. The WSLs showed a large reduction in MPs compared to sound counterpart: elastic modulus was up to 83% lower (77.0 +/- 2.2 to 13.4 +/- 2.1 GPa) and hardness up to 91% lower (3.39 +/- 0.41 to 0.29 +/- 0.07 GPa). The intact surface layer above WSLs had less than 34% reduction in MPs. Detailed MP measurements were mapped as two-dimensional contour plots. Typically, WSLs displayed steep MP gradients at the lesion boundary and more uniform MPs with isolated variations in the lesion body. Despite the large reduction in MPs, the microstructure of WSLs remained intact. We postulate that loss of mineral and change of the hierarchical microstructure at the crystallite level contributed to the overall MPs of WSLs.