BAZOS/MAGNE
Dentin tubule direction at the DEC
parallel
0°
oblique
45°
POV
perpendicular
90°
Fig 7 Dentin subadjacent to the DEC exhibits a transitional orthogonal rotational orientation from cervical to incisal with regards to the dentin tubules. Hence, despite the fact that cervical dentin is thicker, due to the parallel orientation of of the tubules, it is rendered more translucent. The opposite happens with respect to incisal dentin, despite the fact that it is thinner, due to the perpendicular orientation of the tubules, it is rendered more opacious.
In the case of enamel, major random scattering occurs on the ultrastructur- al level from the hydroxyapatite (HAp) crystal subunits, whereas minor random scattering occurs on the microstructural level due to the prism sheaths/interpris- matic material orientation in conjunc- tion with the sinuous paths of Hunter- Schreger bands. 14,15 The scattering coefficient appears to increase with shorter wavelengths, 16 while thin enam- el sections also exhibit a Fraunhofer dif- fraction pattern in the plane perpendicu- lar to the enamel prisms, 17 acting as a diffraction grating which in turn may be responsible for generating iridescent re- flections (Fig 6). In the case of dentin, multiple direc- tional scattering occurs on the micro- structural level due to the presence and spatial arrangement of the dentin tubules and the collagen fiber mesh. 14,18,19 In contrast with enamel, the scattering co- efficient does not change significantly with wavelength. 20 Directly below the
DEC, scattering is decreased due to low tubule density with small tubule size, compared to the dentin directly adja- cent to or above the pulp due to high tubule density with large tubule size. 21 Thus there exists a significant translu- cency gradient, that of superficial den- tin which is more translucent and that of deep dentin, which is three times more opacious. 22 Additionally, the regional variation of dentin tubule orientation is of particular relevance with regards to light transmission, rendering cervical dentin as highly transmissive, middle dentin as moderate, and incisal dentin as low (Fig 7). Conversely, the DEC lacks significant scatterers. The elevated lateral light dif- fusion that occurs at the DEC has been described as the “glass layer” or “high diffusion layer” or “brilliance zone.” 23,24 Factors that may be considered for this enhanced light diffusion is that the in- ner aprismatic enamel presents a more uniform HAp crystal orientation, con-
337 THE INTERNATIONAL JOURNAL OF ESTHETIC DENTISTRY VOLUME 9 • NUMBER 3 • AUTUMN 2014
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