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The Value of a Restoration
Refraction is one of the keys to replicating the look of natural dentition
Adental restorative combines art and science to replace the natural function and appearance of a human tooth. The success of this procedure is based on how well it replicates the natural dentition in terms of appearance and longevity. Manufacturers of composite systems strive to duplicate the manner in which light interacts with enamel and dentin. Some systems consist of one resin to replace both the dentin and enamel layers, whereas other systems have separate resins for each layer. The color of a tooth is multifaceted because of the complex interaction between light waves that reflect, scatter, and refract differently in each of the enamel and dentin layers.1
The human eye can detect subtle changes in value more readily than any other color dimension. Visible light falls between 380 and 770 nm on the electromagnetic spectrum. Light is refracted by the lens of the eye and projected on the retina or the inner surface of the eyeball. There, the light is absorbed by pigments in photoreceptive cells called rods and cones and converted into electrochemical signals, which are then processed by neural circuits in the retina and transmitted to the brain. There are approximately 6 million light-sensitive cone cells (which perceive color) and 125 million rod cells (which perceive black and white).2 Rod cells outnumber cone cells 20:1, which explains why the human eye can detect changes in lightness and darkness (ie, black and white) more readily than variances in hue or chroma. This also explains how visual tension can easily be sensed when the value of a restoration and the adjacent tooth structure are inconsistent.
The refractive index is determined by how much a wavelength bends when passing through a material.3 It has an impact on the appearance of restorative materials by affecting their light-scattering characteristics and translucency.4 For example, water has a refractive index of 1.33, cubic zirconia 2.15, glass 1.52, and diamond 3.00. The refractive index of human enamel is 1.62.4-6 Neuber and colleagues7 used spectroscopic ellipsometry to demonstrate that direct resin composites have a different index of refraction than enamel. The average refractive index ranges between 1.49 and 1.541 for composite resins.
When a composite resin restoration is placed next to human enamel, creating a butt joint, the interface or margin is visible due to the difference in their refractive indexes. To offset this effect, there have been numerous techniques described in which the butt joint margin is replaced with a beveled edge, which creates a gradual transition from the composite to the enamel.8-10 Use of this technique allows the margins to blend in with the surrounding tooth structure.11
From an esthetic viewpoint, however, this technique carries the potential of creating a “glass effect,” which is described as a graying at the margin between the natural tooth material and the restorative composite.1 Because composite resins have an index of refraction that is closer to that of glass than natural enamel, they appear to act more like glass than enamel. As the thickness of the composite resin increases, the greater the glass effect, or lowering of the value, that occurs. In nature, the opposite takes place; the value of a young tooth is much brighter, or higher in value, where the enamel is at its thickest and tends to darken, or lower in value, as it gets older and the enamel becomes thinner.1
Enamel Plus HRi
The Italian manufacturer Micerium SpA and Dr. Lorenzo Vanini collaborated on the development of a composite resin that strove to combat the glass effect. Together, they modified the formulation of their previously introduced composite, Vit-l-escence® (Ultradent Products, Inc., www.ultradent.com), by changing the refraction index to match that of human enamel. Inspired by the high refractive index, they called the new composite Enamel Plus HRi and introduced it in 2008. It is marketed in North America under the name ENA HRi (Synca, www.micerium.synca.com) and has both enamel and dentin components. Because the enamel resin shade of this product matches the refractive index of human enamel, use of a bevel is not necessary during preparation.
Using the ENA HRi system, selecting the correct enamel shade can be done using several techniques. There are 3 achromatic shades that are not included in the VITA Classical Shade Guide (Vident, www.vident.com) that range in value from low (UE1) to medium (UE2) to high (UE3). One method of shade selection includes placing a small amount of the HRi material adjacent to the area of the tooth to be restored and verifying the match after light polymerization (Figure 1 through Figure 4). The VITA Classical Shade Guide can be used as a starting point when selecting the initial shade. The lower-value UE1 shade is in the VITA C1 shade range, the UE2 shade is in the VITA A2 shade range, and the UE3 shade encompasses the VITA B1 shades (Figure 5).
If the VITA 3D-Master® Shade Guide (Vident) is used for identifying the correct enamel shade, selection will be based on value, not hue or chroma. The UE3 enamel shade is in the 0M to 1M range (Figure 6), the UE2 shade is in the 2M to 3M range, and the UE1 shade is in the 4M to 5M range. Because value is a measurement from 0 (black) to 10 (white), the UE1 shade has the lowest value, while the UE3 shade has the highest, or brightest, value. It is also helpful to keep in mind during shade selection that, generally, the younger the tooth, the higher the value. According to the manufacturer, to maintain control of the value, the thickness of the enamel layer should not exceed 0.6 mm, even when the actual enamel may be of greater thickness, especially for anterior restorations (Figure 7). For posterior teeth, a thicker enamel layer may be desirable when the restoration includes the tip of a cusp restoration, where the value is naturally higher. When used for ceramic repair, a very thin layer of enamel can be applied, as most of the restoration’s thickness will be a body or dentin ceramic (Figure 8 and Figure 9).
1. Vanini L. Conservative composite restorations that mimic nature: a step-by-step anatomical stratification technique. J Cosmetic Dent. 2010;26(3):80-98.
2. Kandel ER, Schwartz JH, Jessell TM. Visual Processing by the Retina. Principles of Neural Science. 4th ed. New York, NY: McGraw-Hill; 2000:507-513.
3. Bekefi G, Barrett AH. Waves in dielectrics. Electromagnetic Vibrations, Waves, and Radiation. Cambridge, MA: MIT Press; 1987:426-440.
4. Arikawa H, Shinohara N, Takahashi H, et al. Light transmittance characteristics and refractive indices of light-activated pit and fissure sealants. Dent Mater J. 2010;29(1):89-96.
5. Meng Z, Tao XS, Yao H, et al. Measurement of the refractive index of human teeth by optical coherence tomography. J Biomed Opt. 2009;14(3):034010.
6. Wang XJ, Milner TE, de Boer JF, et al. Characterization of dentin and enamel by use of optical coherence tomography. Appl Opt. 1999;38(10):
7. Neuber M, Sarembe S, Schädel M, Kiesow A. Optical properties of enamel and composites determined by spectroscopic ellipsometry. Poster presented at: IADR General Session; June 20-23, 2012; Iguaçu Falls, Brazil.
8. Vargas M. Conservative aesthetic enhancement of the anterior dentition using predictable using direct resin protocol. Pract Proced Aesthet Dent. 2006;18(8):501-507.
9. LeSage BP. Aesthetic anterior composite restorations: a guide to direct placement. Dent Clin North Am. 2007;51(2):359-378, vii.
10. Milnar FJ. Achieving natural aesthetics in Class IV restorations. Dent Today. 2012;31(1):144, 146-147.
11. de Araujo EM Jr, Baratieri LN, Monteiro S Jr, et al. Direct adhesive restoration of anterior teeth: Part 2. Clinical protocol. Pract Proced Aesthet Dent. 2003;15(5):351-357; quiz 359.
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About the Author
Gregg A. Helvey, DDS, MAGD