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Truth in Leading
 

Dr. Robert Kelly speaks out on the ADA standard for dental ceramics

 

Dr. Robert Kelly DDS, MS, D.M. Sc. is professor of Biomaterials at the University of Connecticut. He also chairs the working group that writes the international standards on dental ceramics and ceramic systems and has been involved in the ADA ceramic standards subcommittee since 1991.

DLP: How did the ADA establish the 69-1991 standard of 300 ppm lead allowance in ceramics?

Kelly: Standards are what are called consensus documents. That means the established standard is fairly generally agreed upon by the standards committee. The standards are written by groups of people representing the manufacturing community, users and interested parties such as academics. At the point in time that the 300 ppm was established, it was widely recognized that pretty much any mined mineral such as feldspar would contain trace contaminants of a lot of things, lead being only one of those contaminants. Contaminants in non-syntehtic materials is simply a fact of life. Even the diet we eat provides somewhere around 14-18 micrograms a day of lead. You can’t avoid it, but it is not harmful in any way at those levels.

DLP: Why was the ADA standard changed to a lead solubility standard in 1999?

Dr. Kelly: The reason the 300 ppm lead content was not retained in the 1999 revision was because it was such a non-issue. The only way contaminants in ceramics pose any sort of risk to humans would be if the ceramic dissolved in the mouth and the contaminants leached out. Therefore, the true risk assessment would be a solubility standard that conforms to existing ISO solubility limits. The solubility standard test for dental ceramics exposes a ceramic restoration to a 4% acetic acid solution, heated to 80˚C for 16 hours. Even if you assume that this extreme test is going to somehow equate to a day of exposure, which it does not, what we end up with are lead levels that are less than those found in drinking water, which is 15 parts per billion (ppb). The 8 to 10 ppb of lead that does leach does out of the porcelain under this severe test equates in reality to decades and decades of exposure and is thousands of times below what you get daily in your diet.

The reason this test was chosen was that the 4% acetic solution was already cited in ISO standards to examine leaded glassware and dinnerware. It was chosen specifically because it was known that the solution could cause leaching, which would allow the ADA to make concentration decisions. However, keep in mind that the test for glassware and dinnerware are carried out at room temperature which is much less harsh than the testing criteria applied to dental ceramics.

That’s why the recent media attention given to the Ohio story about an overseas crown reportedly made in China containing lead is so disturbing. I am very confident that this story is not a health issue and is certainly not a China issue. It has been completely misrepresented by the press even though they have been told by any number of people why it is not as they are representing it. If you had taken those crowns and soaked them in a saliva-like solution for 10 years, the amount of lead leaching out would have been undetectable.

The international standards committee has a meeting in Gothanberg, Sweden in October and I’m sure this issue will come up. I think what we will do is go back to a concentration limit but to analyze the solution and set limits that would screen against a product that somehow had been really adulterated.

DLP: Is it possible to mishandle ceramic in the laboratory in such a way that the final restoration would be porous enough to leach?

Kelly: If you were to mishandle a porcelain product to that extent, its quality and appearance would be such that you couldn’t sell it to your client base. For example, if you underfire a porcelain crown, it will be porous but porous to the extent that the particle size of the porcelain remains at the same size as the original particle size of the powder. The outcome would not be a highly esthetic, quality restoration.

One of the concerns the committee had to address 10 to 12 years ago was that of low-fusing porcelains. In order to produce a low-fusing porcelain manufacturers had to bugger up the glass matrix of the powder and one of the prices you pay is increasing the solubility. We wanted to set a limit on that solubility and so introduced the solubity standard to screen products that might have a problem clinically.

DLP: Are there inferior porcelains in the marketplace that would be porous enough to cause problems clinically?

Kelly: Dental porcelains are a specialty product. To manufacture a highly esthetic set of dental ceramics in a useful color range that fire at the right temperature and have the correct coefficient of thermal expansion so they don’t crack and break when they cool out of the oven, you really have to know what you are doing. And so you don’t find a lot of secondary manufacturers. The porcelains that are being used in China are very likely to be European, U.S. or Japan manufactured.