Finishing Zirconia Chairside

No material has had as dramatic a rise in popularity as zirconia. In the last 10 years, there has been a rapid development in the use of zirconia in medical and dental technology. Because of its improved strength, biocompatibility, and enhanced esthetics, zirconia is included in the term bioceramics.¹ Zirconia’s primary use in dentistry was and still is as a substructure for single crowns and frameworks for fixed partial dentures. Material science and CAD/CAM technology have now propelled the use of zirconia in monolithic single-unit crowns and multiple-unit fixed partial dentures. Even though zirconia is considered a ceramic, technically, it is a metal oxide and has different properties than the other true ceramic materials used today.

Depending on temperature and pressure formation, zirconia can exist in three forms—cubic, tetragonal, and monoclinic.² Cubic, which has a straight prism and straight-sided form, is stable at temperatures above 2370°C and exhibits moderate mechanical properties. Tetragonal, which has a straight prism and rectangular-sided form, is stable between 1170°C and 2370°C and possesses improved mechanical properties. Monoclinic, which has a deformed prism with a parallelepiped form (prism with six faces), is stable at room temperatures up to 1170°C and has reduced mechanical properties.^1-2 The tetragonal form is used in dentistry and medicine.

Zirconia possesses a unique characteristic called transformation toughening, which is the material’s unique ability to stop the growth of cracks. Tensile stresses generated by an imminent crack induce a change from the tetragonal configuration to a monoclinic configuration and a localized volume increase of 3% to 5% (Figure 1 and Figure 2). This volume increase results in a change of tensile stresses to compressive stresses generated around the tip of the crack (Figure 3 ). The compressive forces counter the external tensile forces and stop the crack from advancing.^3-4 This characteristic accounts for the material’s low susceptibility to stress fatigue and high flexural strength of 900 MPa to 1200 MPa.^5-6

To control phase transformations and stabilize zirconia at room temperature, different metal oxides such as yittria (Y2 O3) or ceria (CeO2), are added to the crystal structure.⁷ The addition of stabilizing oxides yields multiphase materials, called partially stabilized zirconia.⁸ Technically, if yittria is added for stabilization, then it is referred to as Yittria-stabilized Tetragonal Polycrystals (Y-TZP).²

Grinding can increase the strength of Y-TZP zirconia.⁹ Micro cracks are produced by grinding, which results in the development of compressive strains from the transformation-related volume increase several microns below the surface.¹⁰ Micro-crack propagation is prevented by the surface compressive strains, resulting in an increase of the zirconia’s flexural strength.² But before technicians begin to advise their doctors to have their way with grinding zirconia, certain criteria are essential. The severity (depth) of the grinding and the rise in the locally developed temperature will have an effect on the volume percentage of transformed zirconia (tetragonal phase to monoclinic phase).^11-15 Using a 25-µ diamond bur increases the zirconia’s strength, whereas coarse grinding results in decreased strength. Grinding with a water coolant can promote the tetragonal-monoclinic transformation, which increases the surface compressive strength.^11-12,16

The aging of Y-TZP zirconia is a low-temperature degradation phenomenon. It is characterized by a progressive, spontaneous transformation of the tetragonal phase into the monoclinic phase (T-M), which results in diminished mechanical properties.¹⁷ Sato et al¹⁸ proposed that a slow T-M transformation occurs when Y-TZP is in contact with water, vapor, body fluid, or steam sterilization, which leads to surface damage. The reaction of water with zirconia at the tip causes a formation of zirconia hydroxide, which accelerates crack growth of pre-existing flaws and promotes the T-M phase transition.¹ The most critical temperature range for the development of aging is 200°C to 300°C.¹⁹ Grinding zirconia without a water coolant can produce a white light or hot spot measuring up to 1500°C, which can induce a T-M phase change.

The literature shows conflicting studies concerning surface treatment of zirconia ceramics. For example, Guazzato et al²⁰ suggest that the flexural strength of Y-TZP ceramics is increased by sandblasting and wet grinding, provided that the material is not followed with heat treatment. The heat relieves the compressive forces by reversing the monoclinic phase, which improves the flexural strength. On the other hand, Wang et al²¹ state that heat has no influence on the flexural strength whether it is performed before or after particle abrasion. This procedure is in need of further study. After evaluating the influence of surface grinding and sandblasting, Karakoca and Yilmaz²² found that the biaxial flexural strength of materials decreased after grinding and increased after sandblasting.

Guazzato et al²⁰ also state that fine polishing may remove the layer of compressive stresses and therefore lower the mean flexural strength. Does one give up strength for a smoother surface to the opposing dentition?

Giordano²³ suggests that any procedure that affects the surface of zirconia can cause a varying degree of phase change. He notes that there is an increase in the compressive stress, which increases the strength when zirconia surface is ground with a 125-µ grit diamond wheel. The strength is a result of the phase transformation. Subsequent heating will relieve the compressive stress, causing a reversal of the transformation; in fact, the strength will be reduced 30% from the original surface. This is because the damaged surface is still present. If a 50-µ and a 5-µ wheel are used to polish, then the damaged surface is removed and the original strength is returned. Giordano’s group also found that grinding, without polishing or heat-treating before porcelain application, increases the chances of the veneering porcelain cracking. Sandblasting also affects the surface, but does not create the same level of damage that grinding does.

Finishing with diamond burs involves the indenting and scratching of the ceramic surface with sharp diamond particles, which can lead to subsurface damage.²⁴ The diamond’s particle size influences the induced damage.²⁵ Severe edge chipping or flaking occurs on feldspathic porcelain with both coarse and fine diamonds, but is absent with ultrafine diamond burs. Y-TZP zirconia is free of edge chipping for all grit sizes of diamond burs.²⁵ A microscopic examination of the surface after finishing by a dental handpiece and diamond bur reveals a combination of ductile and brittle-type chip formation.^25-27 The removal mechanism in the feldspathic porcelain is dominated by brittle fracture, while the removal process in Y-TZP zirconia is primarily achieved by a ductile mode.²⁸

There are a number of finishing and polishing systems commercially available that are specific for zirconia-based crowns. These systems contain a series of diamond burs of various shapes and diamond-impregnated silicone cups and points. There are two classes of diamond burs—the common, less-expensive bonded diamond bur and the sintered diamond bur. They come in fine, medium, and coarse grit. The sintered diamond bur is made from a slurry of diamond particles and a binding material that is hot-pressed onto a bur shank. As a sintered diamond wears, the diamond chips are worn or abraded from the matrix, and new subsurface diamond particles are exposed to continue the cutting process.²⁹

Not all sintered diamond burs are the same. Depending on the manufacturer, the diamond particles can be natural or synthetic and can vary in chip size and shape as well as individual particle faceting.²³ For example, the Diablo (Bredent, www.bredent.com) is a friction-grip sintered diamond bur that has an auto-sharpening property, according to the manufacturer. This means that as the diamond grit wears, it is automatically removed and replaced with a new diamond.

When comparing numerous diamond burs indicated for use on zirconia, Christensen³⁰ notes the larger grit size instruments demonstrated more chatter and less control than medium and fine diamonds. According to information the author obtained from various manufacturers, their particular diamond fabrication was superior to their competition, the difference being in the quality of the diamond particle, the hardness of the binding material, and the precision and concentricity of the shaft. The longer the particle is embedded in the binding material, the more efficient the instrument’s cutting ability is. That could explain why some finer grit diamonds actually cut more efficiently than larger grit diamonds that have softer binding materials.

A literature search did not produce any scientific data that showed any one particular diamond bur, made specifically for zirconia, to be superior over another. The one point that all manufacturers share—no matter what diamond instrument is used—is that copious amounts of water, with very light or no pressure, should be used on zirconia. In other words, let the diamond bur do the work.

It is also recommended that the undersurface of the zirconia crown never be touched. This is because, as previously mentioned, all ground surfaces should be polished. It would be difficult to use polishing wheels inside the crown. Instead, the prepared tooth should be altered to completely seat the crown. More studies are needed to follow the handling of zirconia and the effects it can have on the long-term use of this material.

Gregg A. Helvey, DDS, is an adjunct associate professor at the Virginia Commonwealth University School of Dentistry in Richmond, Virginia and maintains a private practice in Middleburg, Virginia.

Gregg A. Helvey, DDS
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry
Richmond, Virginia

Private Practice
Middleburg, Virginia