Questions & Answers on Zirconia

(Provided by THE DENTAL ADVISOR)

Q: Does zirconia ceramic degrade in strength intraorally in 5-10 years?

A: Yes, all-ceramic materials degrade in strength over time - some more than others. Zirconia ceramic  has an initial flexural strength of over 1200 MPa, so even if it degrades 5-10% or more, it is still significantly stronger than other ceramics.


Q: Does it takes longer to remove a zirconia crown with a bur compared to a PFM crown?

A: Fine diamond burs can cut thru a zirconia crown just as fast as a PFM crown.


Q: Does shading or coloring zirconia ceramic reduce the strength as compared to the uncolored zircona?

A: Weakening of colored zirconia ceramic copings has not been observed.


Q: Is the fit of crowns and bridges based on zirconia copings as good as that of PFM crowns and bridges?

A: Yes, the marginal fit of crowns and bridges fabricated from zirconia copings can be as good or better than PFM crowns and bridges.


Q: Is the fit of a zirconia coping important since the zirconia ceramic is so strong?

A: The thicker the cement the weaker the all-ceramic restoration. Cements are not as rigid as zirconia ceramic, so a thicker cement layer will cause more flexural deformation of a ceramic core, increasing the potential for fracture.


Q: Does silane treatment improve the bond strength of a resin cement to a zirconia coping?

A: No, but the use of Rocatec/Cojet (3M ESPE) does. Silica particles are embedded into the surface of zirconia and then silanated. This treatment can increase the bond strength of a resin cement to a zirconia coping.


Q: Is it ok to grind on a zirconia coping without water spray?

A: Water spray is necessary when grinding on a zirconia ceramic coping. The white light or hot spot you see between a diamond and a zirconia ceramic has been measured at 1500 ºC. If the temperature of zirconia ceramic rises above 1000 ºC, a phase change can occur that will induce cracking and surface defects, lowering the strength of the coping.


Q: Is preparation design of a zirconia coping important since the zirconia ceramic is so strong?

A: Preparation design of the zirconia coping is very important. The thickness of unsupported porcelain should not exceed 2 mm. The zirconia coping should be as thick as needed to avoid unsupported porcelain.


Q: What is zirconia?

A: Zirconia is another name for zirconium dioxide (ZrO2), an oxide of the metal zirconium.


Q: Are there differences among zirconia ceramics?

A: Differences among zirconia ceramics can occur with the level of purity of zirconia; grain size, trace elements and stabilizing compounds; and consistency of formulation. Differences can exist in shrinkage from batch to batch because of differences in the coefficient of thermal expansion (CTE).


Q: Zirconia substructures have high flexural strength, 900-1200 MPa, but what is the flexural strength of the zirconia/porcelain system?

A: Flexural strength is typically performed on a three-point bend specimen and is a function of relative thickness of the zirconia and overlaid ceramic, but it would not exceed the flexural strength of the zirconia.


Q: Some clinical studies have shown that there is a weakness in the overlay materials that causes fractures at the marginal ridge areas. How much strength do you really need with zirconia if the overlay materials have less strength that can result in fractures?

A: The fractures are most likely caused by insufficient support of the overlay porcelain in the marginal ridge area. Porcelain is much stronger in compression that in is in tension and shear. If the marginal ridge area isn’t built out with sufficient zirconia, it will leave the porcelain in that area unsupported and vulnerable to tensile and shear forces. If the framework is designed in such a way as to provide support for porcelain at the marginal ridge, you won’t  see as many fractures. Another cause of fracture is internal stress caused by the heating and cooling of the zirconia and porcelain during firing.


Q: What is the best substructure design to provide sufficient support for the proximal overlay ceramics?

A: The overlay porcelain should not be thicker than 1.5 to 2 mm depending on the location and should be adequately supported by the zirconia coping.


Q: What type of porcelain is used for the overlay materials for zirconia copings?

A: There are a number of zirconia-optimized porcelain systems available. Most zirconia ceramics have a coefficient of thermal expansion (CTE) of about 10. Make sure the overlay porcelain has a CTE close to 10. Do not use porcelain intended for PFM.


Q: Do the new feldspathic veneering porcelains for zirconia offer a different (better/worse) bond strength to zirconia substructures?

A: New veneering porcelains for zirconia are formulated to match the coefficient of thermal expansion of the zirconia and to bond to the zirconia coping. Some systems require an initial application of a wash layer.


Q: Are there any testing protocols that a dental laboratory can use to determine that his substructure has been processed to provide maximum strength and durability? If so, what are those protocols?

A: Flexural properties of sintered zirconia specimens can be measured but this test is destructive in nature. It is crucial that the dental laboratory work with a reputable milling center.


Q: What is the difference in strength between pressed-porcelain-to-zirconia versus traditional stacked or layered porcelain-to-zirconia?

A: Both techniques should produce adequate strength if the heating and cooling temperatures encountered during processing do not create adverse thermal gradients that cause residual stresses.


Q: Does the wiggle room of a milled restoration compromise the retention of a crown?

A: The better the fit, the better the retention. Highly retentive zirconia-based crowns can be cemented, whereas poorly fitting crowns should be bonded to tooth structure. Use a self-adhesive resin cement or bonded resin cement for cases with poor retention.


Q: Why use zirconia – is this clinician driven or manufacturer driven?

A: The use of zirconia copings is driven by improved esthetics, biocompatibility, and durability as compared to PFM.


Q: How strong is strong enough when it comes to zirconia?

A: It is important that zirconia-based restorations be stronger initially than PFM due to a small amount of potential deterioration of properties intraorally over time. A zirconia with a flexural strength of 800 MPa should be adequate.


Q: What are the long-term clinical results of using zirconia in the anterior or posterior?

A: THE DENTAL ADVISOR has been monitoring approximately 500 Lava (3M ESPE) restorations over a five-year period with no failures of the zirconia in copings or three- or four-unit bridges.


Q: Where do the fractures of the bonded zirconia restorations compared to the PFM restoration occur?

A: Failures of zirconia-based restorations typically involve chipping or delaminating of the overlaid porcelain. These failures can be minimized by proper design of the zirconia coping to properly support the overlaid ceramic.


Q: What is the connector size for multiple-unit zirconia fixed restorations?

A: A cross section of 9 mm2 is recommended.


Q: How does bonding affect a zirconia restoration?

A: Bonding of zirconia-based restorations can improve retention when additional strength is needed. Use self-adhesive resin cements or bonded resin cements.


Q: What type of bonding material should be used with zirconia-based restorations?

A: Self-adhesive resin cements and bonded resin cements bond well to both tooth structure and zirconia when increased retention is desired. Zirconia can be treated with special primers (Clearfil Ceramic Primer, Kuraray America; Metal-Zirconia Primer, Ivoclar Vivadent) to improve bonding of resin cements to the intaglio surface of the zirconia coping.


Q: How does blasting or grinding affect the zirconia?

A: Sand blasting improves the bond strength of resin cements to the intaglio surface of zirconia-based restorations as does grinding. However, excessive grinding can cause a crystalline transformation of zirconia that can compromise its strength. Water spray during grinding is recommended.


Q: What blasting or grinding procedures are recommended for zirconia?

A: Sandblast the intaglio surfaces with 50-micron aluminum oxide at no more than 30 psi. Grinding should be done with fine or extra-fine diamonds with only light pressure on the area being adjusted and only for a short period of time. Water spray is recommended.


Q: What is a typical fracture rate in zirconia restorations?

A: An typical rate of fracture of zirconia-based restorations is 1 to 2% per year of service.


Q: How many unsupported elements (pontics) can you put side by side in a bridge using zirconia?

A: Maximum span length is typically 88 mm.


Q: Do different zirconia products have different CTEs?

A: Yes, there are variations in coefficients of thermal expansion (CTE) among zirconia products as well as among overlay porcelains. Compatible systems must be used.


Q: How strong is a zirconia-based restoration when the substructure has a flexural strength of 1,200 MPa and the surface ceramic has a flexural strength of 110 MPa?

A: Zirconia is stiff. Because it does not bend easily, it helps the weaker overlaid porcelain resist fracture.


Q: What's a closed zirconia milling system and what is the benefit of an open system?

A: Open and closed architectures refer to the way scanners/design stations communicate with the mills and printers. An ‘open’ system provides a standardized file type and structure that can be understood by other open systems and equipment. A 'closed'-architecture (proprietary) system will only work with equipment that uses the same proprietary file types and structure. Open- architecture systems may provide the ability to use a number of different scanners, mills and printers, perhaps the best one for the desired task. However, if there is a problem with a final output, it may be difficult to track down the source of the problem and obtain assistance in resolving it. With a closed-architecture system, you are relying on one manufacturer for each piece of hardware. They shouldn’t have any issues helping the user identify problems. A closed-architecture manufacturer is probably more likely to have brand recognition in their favor and the abilityto provide marketing support. Recently, at least one closed-architecture manufacturer has opened up their system to work with selective other open-architecture products.


Q: With porcelain-fused-to-metal systems, ceramic bonds to the oxide layer on the metal. What does the zirconia veneering ceramic bond to?

A: Usually the overlaid porcelain bonds to a compatible porcelain wash material that is applied to the zirconia first.