An important as well as critical challenge in the clinical setting is restoring the natural shape of a tooth. This involves rendering proper anatomy, depth, and translucency in the typical adverse conditions encountered in dental restorations. Clinicians must achieve perfect rendering and contouring in an area that presents a lack of space, little material thickness, and the need to employ opaque materials under thin outer layers. If done incorrectly, the result may be inexpressive, lifeless restorations because translucency and depth are important characteristics when attempting to replicate what was created naturally.
When dealing with depth, or “depth effect,” a challenge often faced is the distance between the extreme positions of a screen, over which an optical system can chart images. This is the result of the different levels of translucency that a structure may present. It is necessary to note that translucency and depth are intertwined and are responsible for the simultaneous perception of various special situations, such as “close and far” and “front or back.”
In applying this concept to natural dentition, depth is said to be all structures that are part of the tooth. This is attributed to the different levels of translucency and depth that the enamel and dentin present, which also contribute to the natural appearance of the dentition.
Translucency and Opacity of Dental Structures
The analysis of the interaction of light and dental structures is immensely important, as it is necessary to understand the optical properties of teeth. Regarding this interaction, it can be said that dentin is the color and enamel is the color modifier. Although there are variations of composition and mineralization, it is known that enamel allows a 70.1% average light passage, whereas 52.6% of light can be transmitted through the dentin structure. There are few reports in the literature that study teeth as a whole, yet for correct understanding of the phenomena that occur in enamel and dentin, they should be studied separately.
In dealing with enamel, the rods that comprise the basic structure of the enamel generally rise at a right angle from the dentin surface. In cervical areas, the rods divert from horizontal orientation and lean apically. Near the incisal or cusp tip, the rods change direction gradually, becoming oblique and nearly vertical over the edges. Rod groups can present wrinkling, referred to as Hunter–Schreger bands, throughout their course of movement. Because of this orientation change, less light is transmitted, which decreases the translucency of the enamel.
The translucency of the enamel is often expressed by the transmission coefficient, or the relative amount of light that passes through a certain thickness. The transmission coefficient of enamel is dependent upon the wavelength of incident light, as the total transmission of light through human enamel.
Enamel also modifies the chromatic aspects of the teeth because of phenomena such as reflection, transmission, refraction, thickness, and surface texture. It is also important to note that enamel has the ability to attenuate underlying colors, which can affect the chromatic aspects of the teeth. Therefore, properties of light reflection, or transmission of enamel, are dependent on its texture, orientation of enamel rods, and its ability to refract light, in addition to histological characteristics.
Dentin, however, can be considered the dental tissue of higher relevance when concerned with color. From an optical point of view, dentin is a low-translucency structure with various chroma and saturation variations. Dentin tubules are cylindrical structures that are spread throughout the entire depth of the dentin. Their course on the coronal portion commonly assumes the smooth curved shape of an italic “S,” becoming even smoother near the root. The first convex curvature of this double-bent course begins at a right angle with the pulp surface and is oriented toward the tooth apex.
These tubules achieve an ending point at the dento-enamel junction. Near the incisal and cusp edges, the tubules are nearly straight. Throughout their length, they present relatively regular small secondary bending, with sinusoidal shaping. This dentin tubule arrangement enables the dentin to demonstrate selective light diffraction, as certain rays are reflected whereas others are absorbed. This phenomenon produces relative opacity, which is a special property of the dentin.