The Dental Advisor Answers Your Zirconia Questions

1. What is zirconia?   
Zirconia is another name for zirconium dioxide (ZrO2), an oxide of the metal zirconium. 

2. Are there differences among zirconias?
Differences among zirconia ceramics can occur with the level of purity of zirconia; grain size, trace elements and stabilizing compounds; and consistency of formulation. Batch to batch variances can affect shrinkage during sintering, and potentially, the coefficient of thermal expansion (CTE). Which means these factors can ultimately influence the strength and fit of the substructure and its thermal stability with the overlaying porcelain.

3. Is there a difference between zirconia and zirconium? 
(See #1)

4. Does zirconia degrade when exposed in the oral fluids?
Degradation of zirconia have been reported when zirconia is exposed to oral fluids.  However, the initial strength of zirconia is high enough that the loss in strength due to oral fluids does not jeopardize the structural stability of the framework.

5. Does the bond strength of veneering porcealain to a zirconia substrate degrade with exposure to oral fluids?
The interface between the zirconia and overlay porcelain is fused so degradation due to exposure to oral fluids would not be expected.

6. At what temperature during firing does zirconia begin degrading? 
Zirconia does not degrade during firing as long as manufacturer’s instructions are followed. Green state zirconia fuses together and densifies during sintering.

7. Is there any degradation of the zirconia substructure after repeated firing cycles?
Yes, repeated firing could weaken zirconia through phase transformation.

8. Zirconia substructures have high flexural strength, 900-1200 MPa, but what is the flexural strength of the zirconia/porcelain system? 
Flexural strength is typically performed on a three-point bend specimen and would be a function of relative thickness of the two materials but it would not exceed that of zirconia.

9.What is the shear bond strength of the overlay ceramic to the zirconia substructure?
It could be determined by a bending test but it would be dependent on the thickness and firing temperature.

10. Are there any standardized testing protocols for testing zirconia and its systems?
If so, what are those protocols?  Protocols include three-point or four-point bending and ring-on-ring biaxial tension tests.

11. 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? 
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 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 be seeing as many fractures. Another cause of fracture is internal stress caused by the heating and cooling of the zirconia and porcelain.

12. What is the best substructure design to provide sufficient support for the proximal overlay ceramics?
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.

13. Have there been any studies completed regarding substructure designs? If so, what are the findings?
The coping should be designed to minimize the thickness of the overlaid porcelain and keep it uniform.

14. What is the chemistry of the current YTZP Zirconia used in dentistry?
YTZP refers to yttria-stabilized tetragonal zirconia polycrystals plus proprietary trace elements.

15. Are all zirconia blocks the same? If not, how can I differentiate a high-quality block from a low-quality block?
Composition and density are all critical to quality outcomes.  It is not easy for the lab technician and dentist to measure these qualities.  The end user must rely on the company’s reputation.

16. What type of porcelain or glass is used for the overlay materials? 
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.

17. Do the new feldspathic veneering porcelains for zirconia offer a different (better/worse) bond strength to zirconia substructures?
New veneering porcelains for zirconia are formulated to match the coefficient of thermal expansion of the zirconia and to bond to the coping.

18. Is there a standard material specification for YTZP used in dentistry? 
No.

19. Are there any testing protocols that a laboratory can use to determine that his substructure has been processed to provide maximum strength and durability? If so, what are those protocols? 
Flexural properties can be measured but this test is destructive in nature. It is crucial that the laboratory work with a reputable milling center.

20. What is the strength difference between pressed-to-zirconia versus traditional stacked or layered porcelain-to-zirconia?
Both techniques should produce adequate strength if the heating and cooling temperatures do not create adverse thermal gradients.

21. Please provide any empirical and non-biased bond strength data of buildable and pressable ceramics to zirconia.

Please provide any non-biased data on the strength of zirconia copings/crowns cemented on NBS dies.

23. Does the wiggle room of a milled restoration compromise the retention factor of a crown?
The better the fit, the better the retention. Highly retentive crowns can be cemented, whereas poorly fitting crowns should be bonded to tooth structure.
 
24. There are rumblings from Europe that some are experiencing failures of zirconia implant abutments at the interface with the fixture. What could account for this and have there been any studies?
This only pertains to connections that are zirconia and not the titanium basket design.

25. Why use zirconia – is this clinician driven or manufacturer driven? 
The use of zirconia copings is driven by improved esthetics, biocompatibility, and durability.

26. How strong is strong enough when it comes to zirconia?
It is important that all-ceramic restorations be stronger initially than PFM due to a small amount of potential deterioration of properties intraorally over time. A flexural strength of 800 MPa should be adequate.

27. What are the long-term, five-year clinical results of using zirconia in the anterior or posterior?
THE DENTAL ADVISOR has been monitoring approximately 500 restorations over a five-year period with no failures of the zirconia copings or three- or four-unit bridges.

28. Where do the fractures occur of the bonded zirconia restorations compared to the PFM restoration?
Failures of zirconia restorations include chipping or delamination of the overlaid porcelain.

29. What is the research on connector size/strength for multiple unit zirconia fixed restorations?
A cross section of 9 mm2 is recommended.

30. Do the milled or sintered zirconia abutments fit the implant the best? 

31. How dimensionally accurate is zirconia when used as an abutment for an implant?
Zirconia abutments are very accurate.

32. What is the research demonstrating that zirconium can transmit light?
Companies are now developing translucent zirconia ceramics.
 
33. How does bonding affect a zirconia restoration?
Bonding of zirconia can improve retention when additional strength is needed.
What type of bonding material should be used? Self-adhesive resin cements bond well to both tooth structure and zirconia.

34. How does blasting or grinding affect the zirconia?
Sand blasting improves the bond strength as does grinding. However, excessive grinding can cause a crystalline transformation of zirconia that can compromise the strength. Water spray is recommended.

35. Some research has shown that the cements being used for zirconia are not doing the job well. Can you elaborate?
Self-adhesive resin cements are working well.

36. What blasting or grinding procedures are recommended? 
Sandblast the outside and inside 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.

37. Because of the bonding problems with cement, can composite be used?
Bad question.

38. What makes one brand of zirconia superior to another?
A superior product has consistent, reliable results.  Consistency of composition and quality of components is critical. Zirconia should be strong; uniform in density, color and esthetics; and provide excellent fit. Impurities and variations in material consistency can lead to deficiencies in the final product.

39. What is an acceptable fracture rate in zirconia restorations?
An acceptable rate is 1 to 2% per year of service.

40. What is the average life of a zirconia restoration?
No one knows but it is not unreasonable to expect an average life similar to PFM.

41. What is the cheapest block of zirconia you’ve heard of and would you recommend its use?

42. What is the science behind zirconia?

43. Is anyone tracking the data about zirconia remakes?
44. What is the strength difference between more translucent and less translucent zirconia blocks?

45. How many unsupported elements (pontics) can you put side by side in a bridge using zirconia?
Maximum span length is 88 mm.

46. What are the limits of radioactivity in zirconium oxide and how can I measure that?

47. Are there required strength limits to different indications?

48. Do different ZrO2 have different CTE's?
Yes, there are variations in CTE among zirconia products as well as among overlay porcelain. Compatible systems must be used.

49. How strong is my restoration when my substructure has 1,200 MPa and the surface ceramic has 110 MPa? 
The advantage of zirconia is that it is stiff. It does not bend easily and therefore helps the weaker overlaid porcelain resist fracturing.

50. Does infiltration of color, prior to the sintering process, influence the strength of the zirconia? If it does not why isn’t everybody doing it?
Published studies indicate no significant difference.

51. How can I increase the bond strength between ZrO2 and the cover ceramic?

52. What's a closed zirconia milling system and what is the benefit of an open system? 
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 ability/willingness to provide marketing support. Recently, at least one closed architecture manufacturer has opened up their system to work with selective other open architecture products. 

53. Does a cheaper zirconia block necessarily mean that it is not the same quality as a higher priced block? 
No, but one would be wise to determine why it can be made and sold more cheaply. Does the manufacturer guarantee the product?

54. With porcelain-to-metal systems, ceramic bonds to the oxide layer on the metal. What does the zirconia veneering ceramic bond to?
Usually the porcelain bonds to a compatible porcelain wash material that is applied to the zirconia first.