Micro-CT Mineral Density and Nanoindentation Mechanical Properties in Carious Dentin

Objectives: The objective was to evaluate natural occlusal carious dentin in whole tooth using polychromatic micro-computed tomography (μCT) mineral density and nanoindentation mechanical properties of hardness and elastic modulus.
Methods: Six human molars with non-cavitated enamel-dentin carious lesion were scanned by μCT (SMX-100CT) set to a resolution (12μm/voxel) nondestructively. X-rays were generated at 100kV/100μA with metal filters (0.5-mmAl and 0.3-mmCu) and beam-hardening correction. Gray scale values were converted into mineral density values (g/cm³) using hydroxyapatite phantoms (0.20-3.14g/cm³). The data were reconstructed as 3D images using computer software. The same molars were sectioned and polished were measured for hardness and elastic modulus using the nanoindentation system (ENT-1100a) under 5mN indenter load. On each specimen three centrally located linear arrays of indentations were made from the pulp towards dentin-enamel junction (DEJ) including carious lesion at 50μm depth intervals. The mineral density and the indentation array on each specimen were analyzed at the same spot and compared.
Results: 3D images by μCT reconstructions showed the saucer-shaped occlusal dentin caries lesion adjacent to the DEJ. The mechanical properties of dentin generally appeared to follow the same trend as mineral density on the profiles. The relationship between hardness or elastic modulus and mineral density showed a significant correlation. The hardness and elastic modulus of the carious dentin deteriorated progressively toward the lesion, in the range of 100 to 800MPa and 4.0 to 24.0GPa, respectively. The mineral density of the lesion was determined to be in the range of 0.3 to 1.5g/cm³.
Conclusions: The mineral density of natural carious dentin derived from μCT approach showed similar trend to the mechanical properties derived from nanoindentation. μCT can be used as a reliable method for detecting carious lesions adjacent to the DEJ nondestructively.