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Scientific Principles of Optical Esthetics
Restorative dentistry involves repairing and replacing natural
tooth structure, dentin and enamel with crowns & bridges
fabricated from man-made materials with machine-generated
anatomy, while minimizing perturbation or alteration of the
mechanical function, reliability and esthetics of natural tooth
structures. A primary objective of engineering and manufacturing
dental restorative ceramic materials, then, is to mimic
the optical properties of natural dentition. For this purpose, an
understanding of the fundamental structure and optical properties
of tooth enamel and dentin is paramount, along with a
thorough knowledge of scientific principles pertaining to the
study of light (electromagnetic radiation) and its interaction
with biomimetic dental materials.
Organic Structure
The intrinsic optical properties and characterization of natural
dentition enamel and dentin is limited to date. Synthetic crown
& bridge restorations generally replace the entire enamel layer
(which is approximately 1.5 mm thick, depending on factors
such as tooth location) and part of the dentin. 1 Natural tooth
enamel is considered optically transparent, in that it transmits
approximately 50 percent of the light it encounters (Fig. 2).
Enamel is comprised of inorganic calcium phosphate (96
weight percent) and organic molecules and water (4 weight
percent). 2–7 The calcium phosphate crystals function as optical
nanoprisms that are approximately 26 nm in diameter and
about 100–1000 nm long, formed into columnar structures
called nanorods. 2–4 These structures are aligned and parallel
to one another, oriented with the nanorod’s long axis perpendicular
to the outer surface of the tooth. Due to its structure
and composition, enamel calcium phosphate nanorod material
exhibits a unique, optical light-guiding effect. Figure 3 shows
a cross section of a natural molar. When compared to enamel,
the underlying dentin structure consists of a decreased
ratio of inorganic calcium phosphate (70 weight percent)
to organic molecules and water (30 weight percent).
Optical Terminology
Standardized language used by dentists and dental technicians
to describe the esthetics of dental material include the optical
properties of hue, value, chroma, translucency and opacity.
Hue (see Glossary, page 53) is the color or dominant wavelength
in the visible electromagnetic spectrum (400–700 nm)
of a material (Fig. 4). Value is the brightness level, where
white is assigned a higher value and black is assigned a lower
value. 8,9 Chroma is the intensity of hue or color. In 1931, the
Commission Internationale de l’Eclairage (International Commission
on Illumination, or CIE) formulated a standardized
colorimetry system by which to better quantify and physically
describe the human color perception. 10 Hue, value and chroma
terms are used by the CIE 1976 (L*, a*, b*) color space system
(more commonly known as CIELAB), based on measurements
made on a spectrophotometer. Translucency is the amount
of light that transmits through a material, and opacity is the
lack of light transmitting through a material.
Figure 2: Optical transmission of tooth enamel, 1 mm thick
Nanorod structures
Figure 3: Optical image of a human molar
Figure 4: Electromagnetic spectrum
Enamel
Dentin
– Understanding Zirconia Crown Esthetics and Optical Properties – 51