March 2011, Volume 7, Issue 3
Published by AEGIS Communications
Coming Closer to Nature
Opalescent lithium disilicate matches the optical properties of natural teeth.
By Jim Fondriest, DDS | Matt Roberts, CDT
Restorative dentists continue to search for the optimal porcelain with which to restore teeth in the esthetic zone. The ideal restorative porcelain would be easy to work with, strong, and able to faithfully duplicate the optical characteristics of each layer of a natural tooth. If the surface of teeth were opaque, it would be far easier to replicate a natural appearance.1 Enamel, however, has varying amounts of anisotrophic translucency, which introduces many optical effects that are difficult to replicate with a ceramic material. The property of opalescence is one optical characteristic of natural enamel that can create a highly complex visual display.
This case report shows how the new opalescent lithium-disilicate ingots with creative coping design and layering can help in the effort to create a natural restorative result. The patient agenda was to achieve a natural-looking smile that would be more symmetrical and brighter than her existing teeth (Figure 1, Figure 2, Figure 3 and Figure 4). Bleaching had been only slightly effective. She desired a “clean translucence” to her new smile, but wanted to cover the chromatic “blotchiness” of her teeth. This patient had a very good grasp of the characteristics that commonly exist in natural teeth, and was motivated to duplicate these normal characteristics. She understood the natural phenomenon of chroma gradients that occur from gingival crest to incisal edge, and from the cuspids to the midline. She sought an exaggerated chroma gradient going from the darker upper left posterior teeth that got much brighter going to the midline.
One primary goal of performing restorative dentistry is to maintain as much natural tooth structure as possible. While less tooth reduction is desirable, there are times when more reduction better serves the overall restorative agenda. Before the tooth is prepared, decisions are made as to the desired level of brightness and translucency. Strength requirements are assessed, as are the color and condition of the existing dentition. The amount of color change required of a bonded restoration will determine the amount of tooth reduction. The more change in color desired, the thicker the ceramic layer must be to provide adequate filtering of underlying color. The dentist and technician team makes depth of preparation choices based on the masking ability of the preferred material selection.
To begin the restorative phase, a connective tissue graft was done to fix the gingival clefts and lower the scallops above teeth Nos. 4 through 6 (Figure 5). Mixed-coverage porcelain restorations were planned for teeth Nos. 6 through 11.
As preparation began, it was presumed that the chromatic blemishes within the teeth might actually increase in magnitude. For the chosen translucent porcelain, the depth of preparation necessary to completely mask the dark spots depended on the severity of the discolorations within her teeth, as well as the amount of color change being attempted in the treatment (Figure 6). Less preparation was done on the gingival portions of teeth Nos. 6, 7, 10, and 11, so that a chroma gradient could filter through the translucent porcelain going from gingival to incisal.
The provisionals were too long, and the patient agreed to allow some shortening from her original desired length. Shortening the length of the cuspids and premolars greatly diminished the reverse smile line (Figure 7, Figure 8 and Figure 9).
Addressing Patient Agendas
Cosmetic dentist/technician teams can thrive by making client patients happy and fulfilling patients’ requested cosmetic agendas. This particular patient came into the authors’ office using a magnifying mirror. She used the mirror to describe things she saw in her mouth, and to describe her agenda. It is difficult to cover over blotchy tooth stumps using translucent porcelain, to provide both bright and translucent porcelain, and to create and blend a believable chroma gradient from A-3 to OM3 (Figure 10 and Figure 11). Thoughtful tooth preparation and the use of a pressed opalescent lithium-disilicate coping allowed the dentist and technician to achieve most of this patient’s goals (Figure 12 and Figure 13).
Opalescence can be described as a phenomenon whereby a material appears to be one color when light is observed reflected from it, but looks to be another color when light is seen being transmitted through it.2 A natural opal is an aqueous disilicate that breaks transilluminated light down into its component light spectrum by refraction. Opals act like prisms and refract or bend different wavelengths to varying degrees (Figure 14 and Figure 15).
Translucent enamel displays the characteristic of opalescence. Opalescence causes tooth enamel to reflect blue light back to the observer (Figure 16). Enamel has a primary mineral makeup of hydroxyapatite, which is crystalline calcium phosphate. Hydroxyapatite crystals align in organized, tightly packed masses to form prismatic enamel rods. Refraction will occur as light passes through each enamel rod and at the surface of a tooth. Shorter wavelengths bend more and longer wavelengths bend less.1
Blue light tends to bounce around or scatter within the enamel body (Figure 17). The red-yellows do not bend as much at prismatic interfaces. As such, a higher percentage will escape the enamel. The blue accumulates within the enamel, causing it to appear bluish even though it is intrinsically colorless.1,3,4 This also explains why the lingual view will be red-yellow, as all the blue is filtered out and only the red-yellows transilluminate out the lingual side.
Opalescence and the Fiber Optic Effect of Enamel
When light travels from one translucent material to another, the light will be either reflected at the surface or it will bend as it passes through the surface. The more optically dense a translucent material, the more light will enter and stay within its body.
Diamonds, for example, are optically dense, and their tendency to collect light makes them valuable as jewels. The “critical angle” of incidence must be close to perpendicular for light to escape from one translucent material of high optical density going to another material of low optical density (air). Therefore, any light rays closer to parallel to that boundary will not cross the boundary, and will stay within and be totally reflected back internally. This effect is used in optical fibers to confine light within a cable. Light travels along the fiber, bouncing off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle to escape, the light will travel down the fiber without leaking out once entering the fiber at a low angle virtually parallel to its walls.
The opalescent effects of enamel brighten the tooth, and give it optical depth and vitality.5 When natural teeth are illuminated, all of the visible wavelengths scatter to a certain degree and remain within the body of enamel. Incident light will enter one side of a tooth and scatter to such a degree that it reaches the opposite external surface of enamel at an acute angle. Significant amounts of all wavelengths will fail to escape, thus continuing to scatter within the body of enamel.6 This porcelain has a higher optical density than other dental porcelains and, similar to enamel, will act like a fiber-optic cable holding the light within its body. This provides this porcelain an improved ability to be both translucent and bright (Figure 18).
The Halo Effect
The halo effect is another natural visual effect commonly mimicked in porcelain restorations. In natural teeth, the halo effect is caused by the reflection of red-yellow wavelengths off of the internal lingual–incisal surface of enamel (Figure 19). The actual incisal enamel has no difference in colorants/stains than the rest of the body of enamel. The red-yellow light hits the surface at too low of an incidence angle to transilluminate, and instead reflects or scatters off the buccal–lingual–incisal surface. The scattering halo effect will occur in natural teeth when there is buccal-facing lingual–incisal surface of enamel at the right angle. Most unworn incisors will exhibit a halo, but not all teeth with incisal wear facets will yield the visual halo effect. The wear facet must have a buccal-facing angulation (Figure 20).
Technicians have traditionally created an artificial halo in porcelain restorations by either employing more opaque porcelain or layering stains into the incisal edge. These halo effects can be easily lost, however, through wear of the restoration or by occlusal adjustments in crossover movements made by the restorative dentist. Ideally, all visual effects of natural teeth should be recreated in restorations by using multi-layer ceramics with the same optical properties as the teeth being duplicated, and not by employing stains.7
The refractive characteristics of this porcelain are high, given its opalescent effects. A natural halo will form without stain if light can reflect off a scattering surface. As seen in Figure 21 and Figure 22, the technician has created an internal angled surface at the incisal edge and interproximally with the pressed coping to scatter the red-yellows.
The light in which a porcelain restoration is viewed can affect its appearance. Metamerism is the phenomenon by which an artificial restoration can match in one lighting condition—in the laboratory or at the office—but appear different with different lighting.8 Natural teeth have the ability to opalesce more than most dental porcelains. The visual display seen depends on the nature and vector of the light source illuminating the object. A porcelain restoration might reflect light off its surface exactly as enamel does at one angle, but the two objects may look quite different under different illumination and at many different angulations.9 A good match is created not only by surface reflectivity, but by how the light is scattered within the porcelain body. Mimicking the layers of the tooth by employing materials with the same optical properties will minimize metamerism when reconstructing a tooth with dental porcelain. The closer the optical properties of the two materials to be matched, the more successful the color match will be.10 The use of opaque surface stains to correct mismatches will increase metamerism, because it limits the light entering the deeper layers of porcelain.11
Matching Optical Characteristics of Natural Teeth
Artificial restorations are often most vulnerable to detection in natural sunlight. This opalescent porcelain has positive characteristics in bright full-spectrum light. Closely assessing this patient’s anterior restorations with illumination from an opposing angulation, and at different vectors, reveals the optical effects that make this porcelain so interesting (Figure 23). As the incident light enters these restorations, the blue opalescent qualities appear superficially, without placing blue stains within the ceramic layers. A halo is evident along the incisal perimeter. The blue opalescent properties of this porcelain accentuate the halo effect created by the technician’s placement of internal angles on the copings as well as the angled incisal edge facets created by the dentist, both of which scatter the red-yellow wavelengths.
The higher the optical density of porcelain, the greater the optical depth and overall brightness. Significant amounts of incident light are kept within the body of porcelain that does not have the critical angle to escape.6 Red-yellow light that had transilluminated through the buccal layers of porcelain is being bent around the lingual surface of the restoration, and shows as a brighter signal on the distal incisal of tooth No. 8. This is a manifestation of the fiber optic effect of this porcelain.
The final result fulfilled the patient-driven agendas of morphological symmetry, chromatic uniformity with chroma gradients, translucency, and a natural incisal display with opalescent blue and an incisal halo.
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About the Authors
Jim Fondriest, DDS
Matt Roberts, CDT
CMR Dental Lab
Idaho Falls, Idaho