To investigate human enamel and dental composites concerning the optical properties: complex refractive index (n), coefficient of extinction (k) in the visible and near infrared region by spectroscopic ellipsometry.
Method:
Extracted human teeth (molars and incisors) were prepared by cutting, grinding and polishing (surface roughness Ra < 5 nm) to enable optical measurements on different depths of the tooth. The measurements were performed at a wavelength range from 380 to 1000 nm on an imaging ellipsometer (nanofilm-ep3se from nanofilm) with a measure spot size less than 100 µm and on a spectroscopic ellipsometer (M2000 from J.A. Woollam, Inc.). A physical model was used to fit the experimental data and to determine n and k. In order to compare the optical properties of enamel with restorative materials, 10 commercial dental composites were examined. Composite specimens (disks) were fabricated according the manufacture instructions and measured under same conditions.
Result:
The refractive index of human enamel decreases with increasing wavelength in the range from 1.67 ± 0.03 (at 380 nm) to 1.62 ± 0.02 (at 1000 nm). There was no significant difference in the values for n measured on different depths or positions of the tooth. Also, no significant difference was observed for the enamel of molars and incisors. The appertaining extinction coefficient k exhibits very small values with k < 0.03. With exception of one composite having a refractive index close to human enamel, the dental composites have shown a significant lower refractive index of about 0.1 to 0.2.
Conclusion:
Both the findings and the new experimental approach to determine the refractive index by the spectroscopic ellipsometry may be useful for the development of dental composites with optimized optical properties.
The study was supported by GDF GmbH, Rosbach, Germany, and Micerium S.p.A., Avegno, Italy.