Understanding Zirconia Crown Esthetics and Optical Properties
INTRODUCTION
Today's state-of-the-art esthetic restorative crown & bridge materials consist of monolithic ceramics such as zirconia (ceramic) and lithium disilicate (glass-ceramic). Monolithic ceramic restorations made of partially stabilized zirconia (typically 3 percent yttria, 97 percent zirconia, by weight) are increasingly being used as an alternative to traditional PFM restorations and other porcelain-veneered ceramic substructures, such as porcelain-veneered zirconia frameworks. When compared to other all-ceramic crown & bridge materials, monolithic zirconia exhibits a unique combination of high flexural strength, fracture toughness, and toothlike esthetics that is redefining what constitutes a reliable and esthetic ceramic crown & bridge material (Fig. 1). Moreover, advanced CAD/CAM manufacturing technology provides for a cost-effective alternative to conventional PFM and full-cast restorations.
BruxZir® Solid Zirconia, the monolithic nanocrystalline zirconia material developed by Glidewell Laboratories, has helped to revolutionize crown & bridge manufacturing technology and set a new benchmark for esthetic monolithic ceramics. According to Robin Carden, senior director of research and development at the lab, "BruxZir Solid Zirconia … represents the state-of-the-art all-ceramic crown & bridge material, with its unique combination of toothlike esthetics and superior mechanical reliability." BruxZir Solid Zirconia is helping to usher in a new age of biomimetic dental materials that are being engineered to emulate the optical, mechanical and biological characteristics and integrity of natural dentition. The future of dental restorative materials will be newly engineered nanostructures and nanocrystalline materials that replicate the esthetics, reliability and biological function of naturally occurring tooth structure.
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 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.
Spectrophotometry
A spectrophotometer (Fig. 5) is a sophisticated analytical optical instrument consisting of a stable and precise light source and light detector (photo detector) capable of measuring discrete colors or wavelengths with a 1 nm resolution measured between the ultraviolet and infrared wavelengths within the visible light spectrum. True colors, shades and optical properties can be reproducibly quantified by a spectrophotometer, which engineers and scientists use to characterize optical ceramic and dental ceramic materials by measuring the transmission, reflection and absorption of light as a function of wavelength. The latest digital dental shade-taking systems use a simplified spectrophotometer to quantify tooth shades.
Determining Shade
Ceramic crown & bridge restorations are fabricated to the desired shade and esthetic detail based typically on a standardized shade guide system, such as the VITA Classical Shade Guide or VITA 3D-Master Shade Guide. The VITA Classical Shade Guide was introduced in 1956 and is considered the gold standard for dental tooth shade quantification. This guide classifies dental shades into 16 different shade guide tabs. The VITA 3D-Master Shade Guide introduced in the 1990s is based on the earlier Munsell color space and the CIELAB colorimetry system. The VITA 3D-Master Shade Guide tabs are categorized by value, hue and chroma colorimetry parameters. The dentist and dental laboratory technician use the shade guide tabs to determine the appropriate tooth shade by visually comparing the shade guide tabs against the patient's tooth and against the ceramic dental restoration – in ambient background lighting. This method, although common, is inherently subjective, dependent on myriad variations arising from ambient light interactions and color sensitivity of the human eye.
Color and the Human Eye
The eye detects three primary colors: red, green and blue. The optical focal plane is the retina, which contains two kinds of light detection sites: rods and cones. Rods make up the majority of light sensors in the retina. They are extremely sensitive to light intensity variation, but are not color-sensitive. Cones are sensitive to specific colors – or rather the electromagnetic wavelengths that are responsible for color vision. Cones consist of three primary lengths attuned to specific colors or wavelengths: long (L-cones), medium (M-cones) and short (S-cones). L-cones detect red colors at a peak wavelength of 565 nm, while M-cones detect green colors at a peak of 540 nm and S-cones detect blue colors at a peak of 440 nm. The majority of these retinal cones are of the long and medium variety. The superpositioning of the three different color signals results in a peak sensitivity to light wavelength at 550 nm. The human brain can perceive the continuum of visible colors between blue (the shortest wavelength the retina is sensitive to at 400 nm) and red (the longest light wavelength the retina is sensitive to at 700 nm). Except for red, green and blue, color is a perception, a result of the brain processing the additive intensities of the three primary colors detected by the cones.
Visible Light and the Electromagnetic Spectrum
The color-related esthetics of natural dentition and dental ceramic materials result from the interaction of light with the material in the visible part of the electromagnetic spectrum (400-700 nm). This visible range, of course, represents only a small portion of the larger electromagnetic spectrum, which includes ultraviolet rays, X-rays, and gamma rays smaller than 400 nm in wavelength; and near-infrared, infrared, microwaves, radio waves, and longwaves larger than 700 nm in wavelength (Fig. 4). Wavelengths that fall within the visible electromagnetic spectrum are expressed in nanometer units, where 1 nm equals one billionth of a meter (1 nm=1x10-9 m). Expressed in colors, the visible electromagnetic spectrum ranges from violet (400 nm) to red (700 nm). As described earlier, the human eye is most sensitive to the yellow-orange color at a wavelength of 550 nm.
Dielectrics
Dental ceramic materials such as veneering porcelains and alumina/zirconia crown & bridge frameworks are electrically insulating materials commonly known as dielectrics. Dielectric materials are generally comprised of inorganic oxides, such as silicon dioxide (SiO2), zirconium dioxide (ZrO2), titanium dioxide (TiO2) and many others. Additionally, the calcium phosphate of natural dentition and polymer materials such as composites are typically dielectrics consisting also of inorganic nitrides and
some carbides.
Refraction and Permittivity
The refractive index (n) of a material can be defined as the ratio of the speed of light (c) propagating in a vacuum to the velocity of light (v) propagating in that material, expressed as n=c/v.11 The refractive index is a function of permittivity, which is the measure of the resistance that is encountered when forming an electric field in a medium. More specifically, permittivity is determined by a material's ability to polarize in an applied alternating electric field. Dielectric materials are susceptible to this polarization from the electric field component of the propagating electromagnetic waves. The relative permittivity (εr) of a dielectric material is determined by its composition, crystal structure and the applied electromagnetic field wavelength or frequency. In nonmagnetic dielectric materials, the refractive index is equal to the square root of its relative permittivity (expressed as n=√εr).12 The index of refraction for some common dental materials can be compared to that of natural dentition in Figure 1.
Transmission vs. Reflection
When visible light interacts with dielectric dentition and dental ceramic materials, wavelengths are either transmitted, reflected or absorbed, the sum of these values equaling the incident light source energy. This can be expressed as 1=T+R+A, where T is the value of transmission, R is the value of reflection and A is the value of absorption. A schematic of optical material-light interaction is shown in Figure 6. The optical esthetics of dental ceramics are based on the wavelength dependence of light reflected from the ceramic restoration and the depth perception of transmitted light. An observer's eye will see light reflected from the ceramic. This reflected light is comprised of first (initial) surface reflection, and also light partially transmitting from the dental ceramic and reflecting from a second interface or lightscattering surface. Additionally, backlighting of the ceramic is transmitted through the material. The observer's visual perception is based on the summation of these optical reflections and transmissions that result from interaction with the restorative ceramic material.
Light Interaction in Dielectric Materials
The wavelength dependence of light interaction with dielectric materials is a complex phenomenon. Light-dielectric material interaction ranges from light scattering from material porosity on the order of the effective wavelength (Mie theory) to quantum mechanical interaction with light and the crystalline order and atomic structure of the ceramic.12-18 Additionally, light scattering caused by birefringence (double refraction) is the driving force behind recent research into optically transparent nanocrystalline ceramic materials. Birefringence is caused by anisotropic crystalline index of refraction, as found in non-symmetric crystal structures – typically noncubic or strained. This results in the refractive index being different for various crystallographic plane orientations with respect to the direction of light propagation (Fig. 7).
RESTORATIVE CERAMIC MATERIALS OF TODAY
There are primarily three different types of esthetic dental ceramic materials used today for crown & bridge restorations: glassy phase porcelains used in veneering, glass ceramics such as lithium disilicate, and ceramics such as zirconia and alumina.
Glassy Phase Porcelains
Materials comprised of mainly glassy silica in the amorphous solid state (glassy matrix) make up the majority of the dental ceramics used today. Glassy phase veneering porcelains and pressable ceramics over substructures typically consist of glassy matrix feldspathic material with a leucite crystal phase. Typically, feldspathic leucite-reinforced glassy materials consist mainly of an amorphous silica phase (approximately 70 weight percent amorphous silica). The glassy phase veneering porcelains have been used for more than 30 years, and have become the benchmark for esthetics in dental restorations. These are typically used as the esthetic layer over substructures. Their low flexural strength and fracture toughness limit their indication to veneering layers, inlays and onlays (Fig. 1). However, glassy phase porcelains have the most esthetic optical properties in terms of simulating the optical transmission and color of natural dentition, due largely to the fact that these materials have the highest optical transmission when compared to other restorative ceramic materials (Fig. 8).
Glass Ceramics
Lithium disilicate glass-ceramics are approximately 30 weight percent amorphous silica and 70 weight percent crystalline lithium disilicate crystals, as shown via scanning electron microscopy (Fig. 9). These glass-ceramics are typically used for pressable all-ceramic crowns and monolithic ceramic CAD/CAM crown restorations. The optical transmission and resulting esthetics of shaded lithium disilicate are currently the benchmark for esthetic monolithic CAD/CAM ceramic crowns (Fig. 8). However, the flexural strength and fracture toughness limit their indication to single crown restorations (Fig. 1).
Ceramics
Zirconia and alumina dental ceramic materials are typically comprised of a nearly 100 weight percent crystalline phase, as shown via scanning electron microscopy (Fig. 10). These ceramics have been used in dentistry for the last 15 years due to their high flexural strength and fracture toughness (Fig. 1). The indications for partially stabilized tetragonal zirconia range from single crown restorations to full-arch frameworks, due to the material's unique blend of strength and color when compared to other crystalline dental ceramics. The limitation to date has been that zirconia tends to have a higher opacity, which would seem to contraindicate use in the esthetic zone. Nevertheless, recent advances in zirconia ceramic processing technology have resulted in highly popular monolithic crown & bridge products, prescribed for use primarily in the posterior tooth region, and strides have been made to improve the material's esthetic qualities. BruxZir Solid Zirconia, for example, which undergoes a unique, colloidal processing technique (patent pending), has been shown to exhibit improved optical transmission compared to zirconia that has undergone conventional processing such as cold isostatic pressing (CIP) (Fig. 11, Fig. 12).
THE FUTURE OF ZIRCONIA RESTORATIONS
Ongoing advances in zirconia processing technology are resulting in dramatically improved zirconia esthetics, which in turn has caused a paradigm shift in CAD/CAM all-ceramic crown & bridge restoration materials and technology. The intrinsic physical limit for toothlike optical transmission of tetragonal zirconia is currently within reach. The theoretical optical transmission limit of tetragonal zirconia is governed by birefringence as a result of grain or crystal size. The optical transmission of tetragonal zirconia is increased exponentially as the sintered grain size is reduced (Fig. 13).13,14 Toward this end, the research and development team at Glidewell Laboratories is among those known to be leading the effort to develop sub-50 nm crystalline sintered zirconia by developing sub-4 nm crystal zirconia starting powder.
As it appears today, crown & bridge restorations with tooth-like esthetics created from monolithic tetragonal zirconia are the future of dental restorations. The inherent mechanical strength and reliability of tetragonal zirconia, along with the continued development of more natural, toothlike esthetics, will make monolithic zirconia a dominant choice for the majority of crown & bridge restorations, expanding from the posterior tooth region to crowns, bridges and veneers placed in the anterior esthetic zone.
References
Ken Knapp joined Glidewell Laboratories in March 2008 as a program manager in the Materials Research and Development group. He has 30 years' experience in the synthesis of magnetic and dental nanomaterials and microfabricated magnetic recording head devices. The last 10 years of his career have been focused on researching and developing nanostructured titanium dental devices and nanozirconia materials. Ken is an inventor on 23 U.S. patents in the areas of magnetic and dental nanostructured materials and devices. Contact him at 
