Comparative Study of Microradiography and Nano-hardness Profiles for Remineralized Enamel

Objectives: An equation has been reported between √KHN and mineral content for demineralized enamel. However, it is unknown whether mineral content profiles of remineralized enamel are equivalent to hardness profiles. Our objective was to compare profiles of transversal microradiography and hardness for remineralized enamel cross sections by using a nano-indentation technique. Materials & Methods: Bovine enamel subsurface lesions were created in lactic acid gel system at pH4.6 for 10 days. After the period, lesions were remineralized for 4 weeks at 37°C(1.5mM Ca, 0.9mM PO₄, 130mM KCl, 20mM Hepes, pH 7.0). Before and after the remineralization, 150mm specimens were cut from the enamel blocks and transversal microradiographs were taken. Simultaneously, nano-indentation testing at a load of 200mgf was performed on the surface, and across the lesion into the underlying sound enamel, starting from 10mm in from the anatomical surface at 10mm intervals. The obtained data were analyzed statistically using one-way ANOVA and Games-Howell′s multiple comparison test. Results: Nano-hardness of the demineralized surface decreased to 10% of sound surface, and remineralization recovered the value to 50% of sound (p<0.05). Microradiography of the thin specimens and nano-hardness values at the cross section resulted in similar subsurface lesion profiles after demineralization. Mineral volume of the remineralized surface and lesion body gave recoveries up to 55-65%, however recovery rates of nano-hardness value were limited to 15-35%. Conclusion: In this study, an extremely low load was used and precise profiles of nano-hardness value across the section were obtained. Remineralization recovered the mineral volume percent remarkably, however the nano-hardness values did not recover to the same degree as the volume percentage mineral. From an ultrastructural point, the remineralization process seems to be more complex than the demineralization process. Therefore, remineralization strategies to recover the mineral should be developed with concomitant recovery of lesion consolidation.