Spectral Reflectance and Colour of Dentine Porcelains for All-Ceramic Restorations

Objectives: The present research was aimed to observe spectral reflectance curves in the visible spectrum and evaluate colour coordinates in the three-dimensional CIELAB colour space of the fired VM7® (VITA, Germany) dentine porcelains for all-ceramic restorations in the whole range 3D-master shade system. Methods: Three disk samples with the size of 13 mm in diameter and 1.4 mm in thickness from each shade were fired following the manufacturer's instructions. Both faces of the fired disks were ground and polished down to the thickness of 1.0 mm. Spectral reflectance data in the visible spectrum were collected at 1 nm intervals by using a computer-controlled spectrophotometer. Colour coordinates, L*, a*, b*, C*, h, in the three-dimensional CIELAB colour space were also obtained under the standard illuminant D65. Results: Principal results are summarized as follows: (1) A negative relationship was recognized between lightness (L*) and chromaticity index a* (red-green direction). That is, with increasing a* coordinate from approximately 1 for the 1M1 sample to 13 for the 5M3 sample, the corresponding lightness systematically decreased from approximately 87 to 69. (2) With increasing a* coordinate, hue angle (h) significantly decreased from about 85-degrees for the 1M1 sample to about 69-degrees for the 5M3 sample. (3) Lightness apparently increased with increasing hue angle (h) throughout the whole range of the 3D-master shade system. (4) A strong positive relationship between chroma (C*) and chromaticity index b* (yellow-blue direction) was found, demonstrating that the chroma of the present porcelain is mainly controlled by the b* coordinate. Conclusion: Systematic variations of colour coordinates of the present dentine porcelains throughout the 3D-master shade system was suggested to be caused by the fine structure of the sample which can interfere with shorter wavelengths in the visible spectrum.