Boston University Professor Answers Your Zirconia Questions
Boston University Professor Answers Your Zirconia Questions
1. What is zirconia?
Zirconia (ZrO2) is the oxidized form of zirconium (Zr) just as alumina (Al2O3) is an oxide of aluminum (Al).Biomedical and structural/functional applications of zirconia typically do not use putre zirconia. We use partially stabilized tetragonal zirconia. Zirconia can exist in several phases depending upon the temperature. Zirconia (ZrO2) may exist in several crystal types (phases) depending upon the addition of minor components such as Calcia (CaO), Magnesia (MgO), Yttria (Y2O3), or Ceria(CeO2). Specific phases are said be stabilized at room temperature by the minor components. If the right amount of component is added, then one can produce a fully stabilized cubic phase – cubic zirconia of mega cart “faux” diamond fame. If smaller amounts are added, 3 – 5 weight%, then a partially stabilized zirconia is produced. The tetragonal zirconia phase is stabilized at room temperature, but under stress the phase my change to monoclinc with a subsequent 3% volumetric size increase. This dimensional change takes energy away from the crack as well and can stop it in its tracks. This is called “transformation toughening”.
2. Is there a difference between zirconias?
Zirconia may differ with respect to the weight% of additives such as yttria. Particle size may be different. This can lead to differences in crystal size, phases and mechanical properties of the sintered zirconia.
With respect to blocks, there is a definite difference in strength, density, and homogeneity. We have tested numerous brand name and no-name blocks and have found some blocks have porosity, are weak and warp after sintering.
3. Is there a difference between zirconia and zirconium?
SEE ANSWER TO QUESTION 1.
Zirconium is a metal.
4. Does zirconia degrade when exposed in the oral fluids?
It is possible that zirconia may degrade over time. Our laboratory performed an accelerated aging test of zirconia and found no degradation of the two materials we tested: LAVA and Vita YZ Invizion. It is possible that other zirconia block materials may show degradation. A batch of hip implants from St. Gobains failed catastrophically after a few years in the body. This may have been due to a problem with the exact composition of the secondary additives like yttria leading to phase instability. It is important that zirconia formulations have a small amount of alumina (0.2% or so) to help prevent instability of the crystal phases.
Our Paper: Papanagiotou HP, Morgano SM, Giordano RA, Pober R. In vitro evaluation of low-temperature aging effects and finishing procedures on the flexural strength and structural stability of Y-TZP dental ceramics. J Prosthet Dent. 2006 Sep;96(3):154-64
A recent study showed fatigue in water but this is true of any material:
Studart AR, Filser F, Kocher P, Gauckler LJ.
In vitro lifetime of dental ceramics under cyclic loading in water.
Biomaterials. 2007 Jun;28(17):2695-705.
Another study estimated a lifetime of 20+ years even under fatigue:
Stuart AR, Filser F, Kocher P, Lüthy H, Gauckler LJ.
Cyclic fatigue in water of veneer-framework composites for all-ceramic dental bridges. Dent Mater. 2007 Feb;23(2):177-85.
5. Does the bond strength of veneering porcealain to a zirconia substrate degrade with exposure to oral fluids?
We are in the process of examining that in our laboratory. Shear bond strenghts that we have measured vary with different manufacturer’s porcelains. We have not seen a degradation in bond strength yet with themal cycling.
R. Giordano, C. Sabrosa , R. Pober and R. Delucas, “Bond Strength Of Porcelain to Zirconia”, J Dent Res 84(Spec Iss A):#544, 2005.
6. At what temperature during firing does zirconia begin degrading?
I am not sure what is intended by this question. The crystal transformation process may be reversed at temperatures above 1000°C.
7. Is there any degradation of the zirconia substructure after repeated firing cycles?
We have not seen any change in the zirconia materials we tested under repeated firing: LAVA and Vita YZ. This may not be true for other blocks.
8. Zirconia substructures have high flexure strength, 900-1200 MPa, what is the flexure strength of the zirconia/porcelain system?
According to one study it is only about 90 MPa but this study also had low values for zirconia. It is important to remember that the veneer porcelain is indeed the week point in ay multi layer system including PFM. Load to failure values are important. Typically the framework is 0.5 – 0.7 mm thick and the connectors and pontic are thicker. These materials have tremendous load bearing capacity. We also must look at other all-ceramic systems as a guide for clinical reliability and survivability. The In-Ceram family has been the subject of many clinical trials which can guide us as to what framework strength we made need for all-ceramics.
90 MPA study:
White SN, Miklus VG, McLaren EA, Lang LA, Caputo AA.
Flexural strength of a layered zirconia and porcelain dental all-ceramic system.
J Prosthet Dent. 2005 Aug;94(2):125-31.
9. What is the shear bond strength of the overlay ceramic to the zirconia substructure?
In our tests shear bond strengths vary depending upon the veneering porcelain. These ranged from about 16 MPa to 40 MPa. Values ranging from about 20 – 40 MPa have been found for porcelain bonding to metal. Numbers vary depending upon the type of test methodology as well as materials.:
R. Giordano, C. Sabrosa , R. Pober and R. Delucas, “Bond Strength Of Porcelain to Zirconia”, J Dent Res 84(Spec Iss A):#544, 2005.
A recent study suggests differences in shear bond between colored and un-colored zirconia. Lowe values for colored versus uncolored:
Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Effect of Zirconia Type on Its Bond Strength with Different Veneer Ceramics. J Prosthodont. 2008 Mar 17.
10. Are there any standardized testing protocols for testing zirconia and its systems? If so, what are those protocols?
There are standardized tests for mechanical properties of dental ceramics but none specifically for dental zirconia, ISO 6872. There are no minimum standards for dental zirconia. I am on the ADA CAD/CAM standards committee and we will be addressing this issue.
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?
Clinical studies of all ceramic materials have demonstrated in general what strength we need for various regions of the mouth. In-Ceram alumina at 450 MPa is successful as an anterior three unit bridge but not as a posterior three unit bridge. In-Ceram zirconia is successful as a posterior three unit bridged, at about 650 MPa. So for a posterior three unit bridge we probably need to be at least 650 MPa. All ceramic bridges typically fail at the connector. We can decrease connector size as we get to stronger materials like zirconia and we can fabricate longer spans.
12. What is the best substructure design to provide sufficient support for the proximal overlay ceramics?
A design which creates an even thickness of porcelain no greater than 2 mm. For example, the new Inlab software allows you to essentially do a cutback of a full thickness wax up to get an even veneer thickness.
13. Have there been any studies completed regarding substructure designs? If so, what are the findings?
We have unpublished study based on a thesis looking at failure load of different designs. In our tests we varied core thickness and connector dimensions. We found that a core thickness of about 0.3 mm and a connector cross section of about 7.2 mm square gave failure loads equivalent to a three unit In-Ceram zirconia framework with recommended Vita cross section. This implies that these thinner values for zirconia may be sufficient for three unit bridges.
14. What is the chemistry of the current YTZP Zirconia used in dentistry?
SEE QUESTION 1 ANSWER.
15. Are all zirconia blocks the same? If not, how can I differentiate a high-quality block from a low-quality block?
A question which is often asked about zirconia is: Is it all the same? Initially, many believed that it probably was very close. Our studies have shown that blocks from name-brand manufacturers, which are approved for use by inLab do not have the large warping problem and low strength we found with no-name blocks. We tested blocks from China, from Canada and from some U.S. manufacturers, which are not approved for use by inLab and found that a number of them had severe warping and much lower strength than one would expect for zirconia. I did not expect to find this result when we first started testing. Almost every month I get an e-mail form a new supplier with "top-quality" zirconia blocks at bargain prices. I would not believe these statements without independent testing to back up these claims - buyer beware. This problem of substandard blocks is not just an issue for inLab, but for other systems as well such as CERCON, LAVA and others.
Unfortunately you cannot tell just by looking at it. Mechanical property tests and microstructural analysis of the sintered material must be performed.
16. What type of porcelain or glass is used for the overlay materials?
The composition varies depending upon the manufacturer but the overlay materails are generally based on a feldpsathic porcelain composition. This can be fine particle a multiphase material with a leucite crystals (VM9) or a glass matrix with some fluorapatite crystals and leucite crystals (?) Ivoclar emax Ceram.
Do the new feldspathic veneering porcelains for zirconia offer a different (better/worse) bond strength to zirconia substructures?
Again bond strength varies with the manufacturer’s porcelain. I believe that firing temperature and the use of a initial bonding alyer can greatly affect the quality of the bond.
Also, surface preparation may affect the bond strength.
17. Is there a standard material specification for YTZP used in dentistry?
NO there is no standard. Most are similar in composition but there is no exact requirement for components.
18. 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?
It is difficult to do this without mechanical testing and an electron microscope but a laboratory might be able to examine the structure under a high powered optical microscope to look for porosity and possibly use a dye to look for cracks.
19. What is the strength difference between pressed-to-zirconia versus traditional stacked or layered porcelain-to-zirconia?
There is no significant difference. There are differences in manufacturers porcelain strength values. For example values range from about 80 MPa for Cerabien ZR to over 100 MPa for a fine particle material such as Vita VM7.
20. Please provide any imperical and non-biased bond strength data of buildable and pressable ceramics to zirconia.
SEE MY PREVIOUS ANSWERS.
Also another study looking at pressed strength was about 30 MPa.:
Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part 3: double veneer technique.
J Prosthodont. 2008 Jan;17(1):9-13. Epub 2007 Oct 11.
21. Please provide any non biased data on the strength of zirconia copings/crowns cemented on NBS dies.
Sorry but what are NBS dies?
22. Does the wiggle room of a milled restoration compromise the retention factor of a crown?
How much wiggle room? Commercial cast restorations often have gaps of 100 – 140 microns. Milled restorations are often better than those values.
30 Labs, Identical Cast, three unit PFM fit 106 – 145 microns:
Northeast SE, Br. Dent. J 172(5):198-204, 1992
SIRONA CAD/CAM 30 – 75 microns
Bindl A, Int J Perio Rest Dent. 2007 Dec;27(6)
Estafan D, Gen Dent. 2003 Sep-Oct;51(5)
Nakamura T, Int J Prosth. 2003 May-Jun;16(3)
Pressed Ceramics: 30 - 67 microns
Mörmann, J Oral Rehab. 2005 Jun;32(6)
Beschnidt SM, J Oral Rehabil. 1999 Jul;26(7)
23. There are rumblings from Europe that some are experiencing failures of zirconia implant abutments at the interface with the fixture. This only pertains to connections that are zirconia and not the titanium basket design. What could account for this and have there been any studies?
Overtorquing can create stress on the brittle zirconia and crack it. Also, many labs and dentists grind the abutments without additional finishing procedures – this weakens the abutment.
24. Why use zirconia – is this clinician driven or manufacturer driven?
Zirconia is biocompatible, esthetic, can stop crack propagation like natural teeth and can potentially be used anywhere in the mouth. These are all aspects which make this material desirable for the PATIENT. I think it is patient driven.
25. How strong is strong enough when it comes to zirconia?
It depends upon the location and the clinical application. Single unit, three unit, six unit? The larger the restoration the more strength is required for an all-ceramic restoration. We know that single unit central and lateral crowns can be made from Empress 1 with a strength of about 120 MPa. High stress areas such as canines and molars require higher strength materials for all ceramics. My opinion, based on clinical trials on In-Ceram, Empress 1 and Empress 2 and Vita MKII, minimum values are:
Single-Unit Central/Lateral: Bonded 120 MPa (glass matrix/glass ceramic type)
Canine, Molars: Bonded conventional glass matrix materials such as high strength pressables, Bonded CAD/CAM such as Vitablocs MKII (130 – 160 MPa) ; Conventional cement: high strength cores 400 MPa
Three-Unit Anterior Bridge: 450 MPa
Three-Unit Posterior Bridge : 600 MPa - 650 MPa
Four-Unit Posterior Bridge: 800 – 850 MPa
Six-Unit Anterior: 800 - 850 MPa
26. What are the long-term, five-year clinical results of using zirconia in the anterior or posterior?
In general, the frameworks do not fail. There are failures, to varying degrees, of the veneering porcelain. Ranging from a few percent to 13% in one study and over 60% in a study being presented by Dr. Rella Christensen at the IADR (Internatonal association of Dental research) in July 2008. In Christensen’s study pressed ceramic chipped at about 20 percent versus about 50 percent in hand-built porcelain.
Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed
The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: a prospective clinical pilot study. J Prosthet Dent. 2006 Oct;96(4):237-44.
Sailer I, Fehér A, Filser F, Gauckler LJ, Lüthy H, Hämmerle CH.
Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont. 2007 Jul-Aug;20(4):383-8.
27. Where do the fractures occur of the bonded zirconia restorations compared to the PFM restoration?
Bonded or un-bonded fractures generally occur in the connector or if the coping thickness is too small, fracture will occur in the coping adjacent to the connector in a bridge.
28. What is the research on connector size/strength for multiple unit zirconia fixed restorations?
SEE QUESTION 13 Answer.
29. Do the milled or sintered zirconia abutments fit the implant the best?
I do not know. I am not aware of any study examining this. Theoretically there should be less error in milling the part out of a dense block of material. However, since zirconia is difficult to mill a rigid milling device is needed.
30. How dimensionally accurate is zirconia when used as an abutment for an implant?
I do not understand the question. You can machine fully dense zirconia to high tolerance, it just takes a while to do this.
31. What is the research demonstrating that zirconium can transmit light?
Zirconium is metal, zirconia is the ceramic. We have a study in progress looking at translucency of various all-ceramic materials. I have attached this slide.
32. How does bonding affect a zirconia restoration?
It is unknown at this time. We know bonding reinforces glass matrix materials like pressables. Bonding may help to reinforce the tooth by creating a strong link with the restoration. This occurs with glass matrix based ceramics.
33. What type of bonding material should be used?
New bonding cements such as Ivoclar Multilink Automix have studies that demonstrate good bonds to zirconia, alumina, metal and glass matrix ceramics. It is a universal cement. Single bottle bonding materials tend to have lower bond values.
34. How does blasting or grinding affect the zirconia?
This is very complicated. Any procedure which affects the surface of the zirconia can cause a phase change from tetragonal to monoclinic to varying degrees. In our work grinding with a 125 micron grit diamond wheel cause an increase in strength. However, if it is then fired, the compressive stress generated by the grinding is relieved and there is reversal of the transformation. Then strength decreases by about 30% relative to the untouched surface. This is because you are now left with a damaged surface. If, however, you polish the surface with a 50 micron and a 5 micron wheel, the damage is removed and you are back at the original strength.
We have also found that grinding cause increased cracking of the veneering porcelain in our thermal shock tests. We have applied porcelain from various manufacturers to zitrconia and performed a thermal shock test to evaluate materials and surface finish with respect to cracking/chipping of veneering porcelain. We have found that there are large differences in cracking resistance between various manufacturer’s porcelains. We have also found that grinding WITHOUT polishing or HEAT TREATING before porcelain application greatly increases the cracking of veneering porcelain. IN MY OPINION: POLISHING and HEAT TREATING a ground surface is recommended before applying porcelain.
Sandblasting also affects the surface, but in our tests the sandblasting does not create damage as is seen with grinding. Again MY OPINION: Try to avoid sandblasting the external surface. If sandblasting is performed on the external surface a HEAT TREATMENT should be performed. We currently use the Vita recommended heat treatment. Vita was the only manufacturer to recommend this procedure. Sandblasting should be performed essentially to clean the surface only and should be done with 50 micron alumina at 30psi.
35. Some research has shown that the cements being used for zirconia are not doing the job well. Can you elaborate?
I am not sure what you are referring to. Glass ionomers do not bond well if at all to zirconia. SEE QUESTION 34.
36. What blasting or grinding procedures are recommended?
SEE QUESTION 35.
37. Because of the bonding problems with cement, can composite be used?
YES, SEE QUESTION 34.
38. What makes one brand of zirconia superior to another?
BLOCKS: Starting powder, block fabrication techniques, quality control. Also, there are differences in minor components but all should have a small amount of alumina to help stabilize the phases.
39. What is an acceptable fracture rate in zirconia restorations?
I do not know. In general the figure of 5% at 5 years is used for any dental restoration to be considered successful – I believe this is based on information from Dr. Gordon Christensen.
40. What is the average life of a zirconia restoration?
I do not know and I do not think we have enough clinical trial information to make a definite conclusion, but based on fatigue studies, the framework could last 20+ years.
41. What is the cheapest block of zirconia you’ve heard of and would you recommend its use?
I do not know the cheapest. YOU GET WHAT YOU PAY FOR! Unless thoroughly tested by an INDEPENDENT source, I would not use the cheapest block.
42. What is the science behind zirconia?
SEE QUESTION 1.
43. Is anyone tracking the data about zirconia remakes?
I am sure Glidewell is and probably Mark Jackson from Precision Ceramics.
44. What is the strength difference between more translucent and less translucent zirconia blocks?
There may or may not be any correlation. Translucency may be affected by pores in the material. More porosity means more interfaces to cross as light passes through the material. This leads to an opaque appearance. However, translucency may also be affected by chemical composition and crystal size. We have not found a definite correlation. I am aware of another study but I will have to track that down for you.
45. How many unsupported elements (pontics) can you put side by side in a bridge using zirconia?
It is unknown at this time. I would not make a “roundhouse”. Zirconia is a stiff material and there might be problems of “stress shielding” the underlying bone causing demineralization. This is speculation on my part but the same can be seen with large stiff orthopedic implants.
Even though it is strong, it is still brittle and I would try to avoid making very long span bridges out of it especially with multiple pontics.
46. What are the limits of radioactivity in zirconium oxide and how can I measure that?
Hafnium is a naturally occurring impurity with minimal radioactivity well below the normal daily exposure we get.
47. Are there required strength limits to different indications?
SEE PREVIOUS ANSWER, QUESTION 26.
48. Do different ZrO2 have different CTE's?
This is possible, manufacturers report slightly different thermal expansion coefficients. There are some differences in percentages of stabilizing materials such as yttria. This may be simply a testing variation or it may be real.
49. How strong is my restoration when my substructure has 1,200 Mpa and the surface ceramic has 110Mpa?
SEE ANSWER TO QUESTION 8.
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?
In our study using 3M and Vita colorants there was no affect.:
S. Farsi, R. Giordano, And R. Pober, Flexure strength of yttria partially stabilized colored/un-colored zirconia J Dent Res 85(Spec Iss A):abstract 1874, 2006.
In another study there was an affect if a very high concentrations was used. Strength decreased significantly by about 20% and there was a change in crystal structure. TConcentartions up to about 5 wt% showed no change, concentrations at 10 wt% showed a significant change. The high concentration is about 10 - 20 of times higher that what is actually used in currently available solutions from 3M/ESPE or Vita.:
Study showing change in strength:
Shah K, Holloway JA, Denry IL.
Effect of coloring with various metal oxides on the microstructure, color, and flexural strength of 3Y-TZP. J Biomed Mater Res B Appl Biomater. 2008 Apr 23.
Everyone is not doing it: 1. Due to potential patent infringement issues. 2. Some concerns about strength.
51. How can I increase the bond strength between ZrO2 and the cover ceramic?
Systems which use a wash fired at a higher temperature, such as the Vita VM9 system, seem to create a higher bond.
52. What's a closed zirconia milling system and what is the benefit of an open system?
CLOSED: You buy everything from a single manufacturer. You are locked into their machine, software, scanner and materials. However, if something goes wrong you make one call. Support may be much better.
OPEN: You mix and match components based on your own need. There is more flexibility, potentially lower cost, and maybe a better product. However, if something goes wrong you have a much more difficult job of trying to get it fixed. Is it the scanner, the software, the milling unit, the furnace, the material or an interaction between the components? This is potentially a much more time consuming process to fix a problem.
53. Does a cheaper zirconia block necessarily mean that it is not the same quality as a higher priced block?
Not necessarily but you need independent testing of the materials to determine if it is of the same quality The block must be homogeneous in order to shrink uniformly so the copings/framework will fit. The block must be of uniform micron/submicron particle size with minimal porosity for strength and finally of the proper chemical composition to maintain long term stability..
54. With porcelain to metal, ceramic bonds to the oxide layer on the metal. What does the zirconia veneering ceramic bond to? Is it as strong as the bond of porcelain to metal?
SEE ANSWER TO QUESTON 9.
55. Since I cannot test the materials in my laboratory, how do I determine if the block has uniform micron and submicron particle size and minimal porosity to ensure strength? What is the standard for the micron and submicron particle size? Also what would the “proper chemical composition” be in order to maintain long-term stability?
This is a problem for all parties involved. Obviously, every lab cannot have the blocks examined by an independent testing organization. Also, FDA approval for “generic” blocks can be obtained by demonstrating that the material is “substantially equivalent” to a pre–existing approved material. There is no quality check of the product other than what the manufacturer supplies with the application.
There also is no standard yet. The ADA formed a committee to develop standards for CAD/CAM systems and materials but the standards are still in development.
Typical grain sizes are around 0.5 - 1 micron. A typical chemical composition is about 3-5 wt% Yttria, a small amount of Hafnium (an impurity), possibly a small amount of alumina (0.25 – 0.5%) to further resist degradation in water, maybe some other impurities <0.1%, and the bulk is zirconia.
If you venture into the world of third party zirconia suppliers then you may or may not be getting a reliable product. “Buyer Beware”. There may or may not be good quality control. Even if you start with a quality powder, the processing of the powder into a block may not be properly performed. Just because a block pops out of the mold does not mean it should be sold.
56. Since the experts had a range of answers regarding the acceptable fracture rate for zirconia restorations, (a) where would a laboratory find this information from a block manufacturer, and (b) what is the median within the range of answers?
Most of the well known manufacturers have data on testing of the lifetime of their materials. Most of the lab data on frameworks of zirconia following recommended wall and connector thickness show that the framework could last well beyond 10 – 15 years, even a lifetime.
It is the veneer porcelain that is a problem.
57. What is the average lifespan of a zirconia restorations? Our experts said that there hasn’t been enough clinical trails to determine – when should we expect to know?
See answer to 56.
Data from Rella Christensen testing about 100 three unit posterior bridges showed about 50% chipping of some zirconia-veneer combinations after 1 year. 1 framework failure after two years.
Clinical performance of PFM, zirconia, and alumina three-unit posterior prostheses. Christensen, K.A. Eriksson, and B.J. Ploeger. J Dent Res Issue#87(Special Issue B):Abstract#1566 , 2008.
I believe these veneer problems are related to firing cycles, the state of the zirconia surface finish, and veneer thickness. Since zirconia is a poor thermal conductor low firing temperatures and short firing cycles may not allow for sufficient firing of the porcelain. There are some porcelains with a peak firing temperature around 800°C. Also, porcelains may vary in their compatibility.
Grinding without heat treating and/or polishing may increase the risk of cracking of the porcelain. Porcelains vary in their resistance to thermal shock and cracking in our tests.
“Thermal Shock of Porcelain Veneered Zirconia with Various Surface Treatments” M. Fahmi, R. Giordano, and R. Pober. J Dent Res Issue #87(Special Issue B),: Abstract# 092, 2008.
58. What would be considered “sufficient support for the overlay porcelain and the marginal ridge area”?
I think you want to try and maintain an even thickness of porcelain of about 1.5 – 2.0 mm.
We all seem to have gotten away from the old rules of support used for metal frameworks. Many copings I see, no matter what the material, are now “thimbles”.
59. How do you avoid the fractures that are caused by “internal stress caused by the heating and cooling of the zirconia and porcelain”?
After sintering, any finished zirconia surface (like grinding) should be heat treated before firing porcelain. Ground surfaces may be polished to minimize/reverse damage caused by grinding.
Also, different veneer porcelains may have different degrees of compatibility with the zirconia related to firing cycle and thermal expansion.
For larger zirconia frameworks it may be advisable to slow the heat rate by 25% – 50% and increase peak temperature by 10°C and/or increase hold time. This will vary depending upon your furnace calibration and porcelain.
I would say that all the manufacturers attempt to do this (match the CTE of the porcelain to that of the underlying zirconia core) but variations in composition lead to large differences in firing temperatures. A low firing temperature may magnify the poor thermal conduction of zirconia leading to under-fired porcelain and more chipping/veneer degradation. Also, many veneer porcelains for zirconia are essentially glasses without a crystal phase. Some do have a crystal phase such as leucite to help prevent crack propagation.
60. Since the experts answered that there is no standard material specification for YTZP is there a minimum or maximum amount for the necessary components (i.e. before the fillers)?
SEE ANSWER about particle size and composition.
61. How/where can a laboratory obtain the manufacturer’s durability information?
The manufacturer. Most have good technical information on their websites concerning composition, properties and handling.
A good site to look for the most current independent tests is the IADR (International Association of Dental Research, www.iadr.org ) website which has a searchable database of abstracts presented at their annual meetings. It often takes 1 year or more for articles to get published in journals. However abstracts contain the most recent information. A note of caution, it is important to reach a conclusion based on the body of evidence not just a single paper or abstract.
62. Where can a laboratory obtain the CTE information on the blocks?
The manufacturer. Most have good technical information on their websites concerning composition, properties and handling.
