What's in Your Teeth Doesn't Affect Your Brain

During the past decade or so, increasingly sensitive technology has shown that some of the mercury in silver dental fillings is vaporized with chewing. And links have been found between urine, blood, and tissue levels of mercury and the number of silver fillings people have.

Nonetheless, studies that have attempted to answer the question of whether mercury in silver fillings is harming people's health have come up with contradictory results. None of the studies, however, was a randomized clinical trial.

Now the results of two such trials have been published in the April 19 JAMA. Neither found neurocognitive damage from silver fillings containing mercury.

Sonja McKinlay, Ph.D., president of the New England Research Institutes, and her colleagues recruited approximately 500 children for their study. The children were aged 6 to 10, had never had fillings before, and had decay in at least two posterior teeth that needed to be removal and filled.

The children were randomized to receive, as replacement material, either silver fillings or white fillings (a resin composite material that was developed as an alternative to silver fillings and does not contain mercury). Children in both groups were matched for age, gender, race, I.Q., and other possibly confounding variables. If they needed fillings during the next five years, they received the same kind they had initially received.

Both at the start of the study and at the five-year follow-up, subjects were measured for urine levels of mercury and tested for I.Q., learning, memory, and visual-motor performance.

After five years, the silver-filling group had significantly higher levels of mercury in their urine than did the white-filling group, the investigators found—that is, 0.9 ug/g versus 0.6 ug/g creatinine-adjusted levels. Nonetheless, the 0.9 ug/g was comparable to average levels reported for American adults and within the range generally considered safe. Furthermore, the silver-filling group, like the white-filling one, experienced increases in I.Q., learning, memory, and visual-motor scores from the start of the study. Moreover, there were no statistically significant differences in five-year scores between the two groups. In fact, children in the silver-filling group who had received the most silver fillings scored slightly higher on I.Q. at five years than did children who had received fewer silver fillings.

Finding Surprised Researchers

This surprising finding, McKinlay told Psychiatric News, means either that silver fillings have “very small” positive health effects or that “there could be adverse effects of resin composites that we could not measure.”

Indeed, while resin composite material does not contain mercury, it does contain chemicals that might be released either during the placement process or subsequently and adversely affect health, Timothy DeRouen, Ph.D., a professor of biostatistics and dental public health sciences at the University of Washington, told Psychiatric News.

DeRouen headed the second randomized clinical trial exploring the impact of silver fillings on neurobehavioral health. The trial was conducted on more than 500 children aged 8 to 10 who had no previous exposure to silver fillings and were in need of replacement for a carie in at least one permanent tooth. They were assigned to receive either silver or white fillings.

But before the children received fillings, the levels of mercury in their urine were measured, and they were tested for memory, attention/concentration, and motor/visual-motor skills, as well as nerve-conduction speed. Through the following seven years, the measurements and tests were repeated. If any youngsters in the silver-filling group needed more fillings during that time, they received silver ones, and if any youngsters in the white-filling group needed more, they got white ones.

The researchers found that mercury levels were similar for both treatment groups at the start of the study—that is, 1.8 μg/g creatinine-adjusted for the silver-filling group and 1.9 ug/g creatinine-adjusted for the white-filling group—but during followup were 1.0 to 1.5 μg/g higher in the silver-filling group—a statistically significant difference. Nonetheless, the higher levels in the silver-filling group were within the range generally considered to be safe. Moreover, there were no significant differences between the two groups, at any time point, on neurobehavioral-test or nerve-conduction-speed scores.

Questions Remain

As reassuring as the results from both studies are, they do not prove that silver fillings are totally innocuous, Herbert Needleman, M.D., cautioned in an accompanying editorial. In addition to being a professor of psychiatry and pediatrics at the University of Pittsburgh, Needleman has made noteworthy contributions to the understanding and prevention of lead poisoning in children over the years.

For example, he pointed out, “Although both studies were sufficiently powered to rule out clinically important neurocognitive effects, the statistical power may be insufficient for detecting smaller effects.” And while the two studies followed children for five to seven years, exposure to mercury-containing fillings early in life might still inflict neurotoxic damage later. Indeed, mercury in silver fillings might play a role in Alzheimer's disease, other studies have suggested.

The DeRouen trial was funded by the National Institute of Dental and Craniofacial Research. The McKinlay trial was funded by the same, plus the New England Research Institutes.

An abstract of McKinlay's article, “Neuropsychological and Renal Effects of Dental Amalgam in Children,” is posted at<http://jama.ama-assn.org/cgi/content/abstract/295/15/1775>. An abstract of the DeRouen trial, “Neurobehavioral Effects of Dental Amalgam in Children,” is posted at<http://jama-ama-assn.org/cgi/content/abstract/295/15/1784>.▪