Local Refractive Index of Human Enamel and Dentin by OCT

Objectives: The aim of this study was to evaluate Swept Source Optical Coherence Tomography (SS-OCT) for measuring Refractive Index of sound Enamel and Dentin at different locations. Materials & Method: Enamel and Dentin slabs (300µm-thick) were sectioned to obtain enamel prisms cross-cut or long-cut and dentin with tubule orientations parallel, oblique or perpendicular to the cutting surface. Each slice was fine polished under running water, and then each placed on a metal plate in order to take B-scan images by SS-OCT (Proto type, Panasonic Shikoku Electronics Co., Ltd). The refractive index (n) at each location on a slab was calculated via dividing the optical path length (OPL) to the real thickness (d). Results: We investigated 16 human teeth slices separately, and the resulting average local refractive indices were 1.61±0.03, 1.62±0.02 for cross-cut and long-cut prism enamel, and 1.68 ± 0.1, 1.49±0.07, 1.51±0.05 for parallel, oblique and perpendicular dentin tubule orientation respectively. T-test showed no significant difference between cross-cut and long-cut enamel groups, while one-way ANOVA (Tukey HSD) revealed that for dentin, there was a significant difference between parallel tubule orientation and the other two groups (p<0.05). Overall average refractive indices of Enamel & Dentin were 1.62±0.02, 1.56 ± 0.11 respectively at the wavelength of 1330 nm. Conclusion: Unlike enamel, the refractive index is not constant throughout human dentin. This finding may provide useful information for depth measurements on the tooth structure by optical coherence tomography. Acknowledgments: This work was supported by the grant from Japanese Ministry of Education, Global Center of Excellence (GCOE) Program, “International Research Center for Molecular Science in Tooth and Bone Diseases”,Tokyo Medical and Dental university and by the research Grant for Longevity Sciences (21A-8)from the ministry of Health, Labour and Welfare.