Objective: To determine the hardness (H) and elastic modulus (E) of sound and molar-incisor hypomineralisation (MIH) enamel, and to correlate these mechanical properties with the mineral density (MD) of enamel as measured by x-ray microtomography (XMT).
Methods: The MDs of seven extracted MIH teeth and seven sound teeth were measured using XMT. The teeth were then sectioned sagittally and H and E determined using nanoindentation. H and E were mapped from the cemento-enamel junction to the cusp tip and from the dentine-enamel junction (DEJ) to the outer enamel surface.
Results: H and E in cervical enamel of sound and MIH teeth had comparable values (H = 3.64 ± 0.69 GPa, E = 73.03 ± 16.73 GPa). H and E increased towards the occlusal part (H = 4.1 ± 0.55 GPa, E = 83.98 ± 12.29 GPa) in sound teeth but decreased towards the occlusal part (H = 0.97 ± 0.67 GPa, E = 29.89 ± 15.65 GPa) in MIH enamel. In sound enamel, H and E increased from the DEJ to the outer enamel surface. In contrast, H and E increased from the DEJ to the middle part of enamel and then decreased towards the outer surface. In both sound and MIH enamel, H and E always followed the distribution of the MD.
Conclusions: There is a strong correlation between the MD of enamel and its mechanical properties. The concordance between mechanical properties and MD agrees with Spears' finite element analysis model of enamel. It offers an explanation for the cohesive failure of MIH enamel observed under adhesive restorations.