Laboratory Considerations for Anterior Full-Contour Zirconia Restorations
The effects of translucency and shade on restoration esthetics
Gregg A. Helvey, DDS, MAGD, CDT
There is a constant change in technique, materials, and instrumentation in restorative dentistry. The recent introduction of zirconia materials with higher levels of translucency has brought full-contour zirconia restorations into the anterior regions. In the past, high-strength zirconia could only be used as a framework for single- and multiple-unit anterior restorations. Because of the inherent level of opacity, translucent porcelain was veneered over the framework to develop the anatomical form and esthetics in the restoration. This process duplicated the porcelain-to-metal technique while increasing the level of esthetics. Problems have been constant with the porcelain-to-zirconia restoration, mainly with the veneered porcelain chipping and debonding as well as fractures occurring in the frameworks. There have been a number of framework design changes and reformulations of the veneering porcelains but problems still exist.
After short-term success with full-contour zirconia posterior crowns, manufacturers have now focused on the use of the full-contour concept in the anterior region by increasing the translucency of the material. The reformulation of zirconia has brought about a more translucent material while sacrificing one of its promotional assets, flexural strength. The flexural strength of 3mol% yttria-stabilized zirconia (full-contour posterior) ranges from 900 MPa to 1,200 MPa.1 But using that formula decreases the translucency level, making it too low to provide acceptable esthetics in a full-contour version in the anterior region.
The translucency of a restorative material relies on its light-scattering properties.2 For instance, when incident light is exposed to a non-metallic anterior crown, part of the light is directly reflected, part is absorbed, and another part is transmitted. The transmitted portion will experience internal reflection and refraction, which is referred to as scattering. An increase in light scattering, which is a decrease in translucency, is attributed to the size of internal pores between the material grains, the amount and type of impurities and defects in the material.3 A material that has different internal reflective angles due to the nature of its composition will produce numerous indices of refraction (anisotropic); this is known as birefringent.4 This is what creates the opacity with tetragonal-phase zirconia, which is the strongest of the three possible phases—monoclinic and cubic being the other two phases. When adjacent zirconia grains can be aligned to have the same crystallographic orientation, there is a continuance of the refraction index at their borders that will then increase the translucency.5
Manufacturers are now producing zirconia milling materials that have lower porosity with changes to the size of the particles. Smaller particles can be compacted closer together, thereby reducing the number of reflections and refractions, creating a more isotropic material. Other factors that contribute to increasing the level of translucency are the sintering temperature,6 heating method,5 and atmospheric conditions during the sintering procedure.7 Lastly, the type and amount of stabilizer (yttria) can affect the translucency of zirconia.8 Increasing the amount of yttria and adding cubic-phase zirconia to the milling material can improve the level of translucency. However, this formulation decreases the amount of the tetragonal phase of zirconia, which decreases the flexural strength and fracture toughness. This is why the anterior full-contour zirconia restoratives have lower strength characteristics.9 Strength is sacrificed for esthetics. For example, the Tosoh Corporation (www.tosoh.com), the largest zirconia powder supplier in the world, increased the yttria content in Zpex® zirconia from 5.2 wt% to 9.32 wt%, creating ZpexSmile®. In doing so, the bending strength (flexural strength) dropped from 1,100 MPa to 600 MPa and the fracture toughness decreased from 5 MPam0.5 to 2.4 MPam0.5.
Clinicians planning to use the higher translucent full-contour zirconia restoratives in the anterior region must consider the importance of the shade of the prepared tooth (“stump shade”). This facet of the overall shade selection process is often disregarded and can have an enormous effect on the esthetic success of a restoration. In the past, the bulk of restorations were porcelain-fused-to-metal crowns-and-bridges that were able to mask 100% of the prepared teeth. Depending on the level of translucency, the final shade of all-ceramic restorations will be affected by a certain amount of “bleed-through” from the underlying shade of the prepared tooth.
Of the three elements of color—hue (the color family), chroma (the saturation level of that color), and value (the brightness to darkness of that color)—value is the most visually demonstrable. The reason for this is that the retina in the human eye has two types of photoreceptor—rods and cones. The rods are responsible for dark-adapted or scotopic vision10 and are more numerous (approximately 120 million) than cones. The cones (6 to 7 million) provide the sensitivity to color (photopic). That is why the eye can perceive the level of brightness to darkness (value) in an object much more readily than the color (hue) or the saturation (chroma) because there are 20 times more rods than cones.