BAZOS/MAGNE
Light guiding by scattering in enamel and dentin
minescence, photoelectric effect etc) – re-radiated with a lower energy state (eg, fluorescence)
The heterogenous composition and asymmetric directional distribution of the hard dental tissue structural compo- nents add to the level of complexity with regards to microscopic light interactivity. Natural waveguides, 8 such as enamel and dentin, are differentiated from con- ventional optical fibers by being non- uniform and containing scattering parti- cles. Nonetheless, they have the ability to collect light and transport it purposely towards the pulp chamber (Fig 5). 9-12 Scattering generally implies a forced deviation of light from a straight trajec- tory by localized non-uniformities (scat- terers), found upon or within the me- dium through which it interacts, without the loss of energy. Reflection, refraction and diffraction represent various forms of scattering. With regards to enamel and dentin, multiple scattering path- ways are prevalent. In the quantum pic- ture, when the wavelength (frequency) of the scattered light is the same as the incident light, elastic scattering occurs. Conversely, when the emitted radiation has a wavelength different from that of the incident radiation, inelastic scatter- ing occurs. The inorganic component of the den- tal hard tissues is responsible for elastic scattering; via Rayleigh scattering (rath- er isotropic, only depending on the po- larization of the wavelength) in the case of enamel and via Mie scattering (rather anisotropic, forward scattering is pre- dominant) in the case of dentin, 13 while the organic component of the dental hard tissues is responsible for inelastic scattering; via fluorescence.
Relative refractive index
Due to the fact that enamel and dentin are heterogeneous hydrated substrates of variable inorganic and organic com- position, one must consider collectively the volume fraction of their individual elemental component’s refractive index (RI), resulting in their respective relative refractive index (RRI). Moreover, de- pending on the localized mineral content of these substrates, minor fluctuations in the RI may ensue, with the highest values always occurring within the more mineralized locations. 1,2 The structural orientation and the ar- rangement of the enamel prisms do not seem to have a significant effect on light attenuation, resulting in an RRI value of 1.63. 3,4 Unlike enamel, the structural ori- entation and arrangement of the dentin tubules seem to play a significant role with regards to the RRI of dentin. Tra- ditionally, dentin has been cited with a generalized RRI value of 1.54. 3 Contem- porary localized RRI values for dentin subadjacent to the dentinoenamel com- plex (DEC) include 1.60 (cervical), 1.56 (middle), and 1.49 (incisal). 4 The DEC, being an organic proteina- ceous continuum 5 that is dominated pri- marily by Type I collagen, has an RRI value of 1.43. 6,7
335 THE INTERNATIONAL JOURNAL OF ESTHETIC DENTISTRY VOLUME 9 • NUMBER 3 • AUTUMN 2014
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