Editor's note: This is the second in a two-part series drilling into the controversy around amalgam restorations. Part I traced the debate back to its origins in the 19th century, explaining how amalgam's low cost has kept it in dental offices despite concerns about toxicity.
Like the monster in some Grade B horror flick, the argument over amalgam fillings just won't die. Every year, a scientist steps forward brandishing the results of a new study designed to drive a stake through its heart. But before the journal leaves the printing press, opposing scientists are picking it apart. What makes this question so hard to answer?
No one disputes that chewing on amalgam fillings releases mercury vapor, or that some of the vapor finds its way into the lungs. And no one argues that mercury can cause damage. From the lungs, mercury vapor passes into red blood cells where it oxidizes, becoming mercuric ion. Mercuric ion tends to accumulate in the brain and kidneys, where it binds to sulfur in essential proteins, denying them to cells. Deprived of these proteins, cells die. And while some of the mercury is eventually eliminated, a portion remains sequestered in these organs.
But is the mercury released from amalgam enough to cause significant damage? That's where experts disagree. Rodway Mackert, D.D.S., Ph.D., a professor of oral rehabilitation at the Medical College of Georgia’s School of Dentistry and a spokesman for the American Dental Association (ADA), argues that the mercury in amalgam is so tightly bound to other materials that only safe levels find their way into human tissues.
Other researchers suggest no level is safe, especially since continuous exposure to mercury could, in theory, cause mercury to steadily accumulate in internal organs. They also theorize mercury released into saliva can be converted by microorganisms in the human body to methylmercury which can enter the bloodstream through the intestinal wall. “Amalgam puts out much more mercury than what the ADA tells the American people,” says Boyd Haley, Ph.D., a professor of chemistry at the University of Kentucky.
Research hasn't resolved the dispute. A 1989 study in the Journal of Applied Electrochemistry found that a single filling with an average surface area of 0.4 square centimeters puts out 15 micrograms of mercury a day through a combination of vapor and dissolution in saliva. But it's much harder to measure mercury in the human body. Amalgam wearers probably exhale much of the vapor emitted in their mouths, lessening their exposure. And elemental mercury that's swallowed in saliva is likely to be eliminated. So estimates of the amount people absorb from an average number of amalgam fillings ranges from 1.2 micrograms to 27 micrograms a day, according to review published in the April 1995 Journal of the Federation of American Societies for Experimental Biology.
Even if we knew exactly how much mercury people absorb from their fillings, it wouldn't tell us whether it's causing significant damage. Some research suggests it does: A study coauthored by Dr. Haley and published in the September 2006 issue of the journal Neuroendocrinology Letters, examined 465 patients, each of whom had an average of 10 amalgam fillings. Suffering from a combination of symptoms including severe fatigue, memory loss, and depression, the patients were divided into five groups. One group was left untreated. Patients in the four other groups either had all their amalgam fillings replaced, underwent one or two types of mercury "detoxification" (including homeopathic treatment) regimens, or received both amalgam replacement and detoxification.
The symptoms of untreated patients remained the same. Those who had their amalgam replaced showed significant improvement. Combined with detoxification, patients fared even better.
But the study accounted poorly for the placebo effect, which might be particularly strong in a group of patients undergoing multiple surgeries to resolve a mostly subjective set of symptoms. And other studies have looked in vain for effects of amalgam. For example, researchers described in the February 1999 Journal of the American Dental Association how they autopsied 68 people with Alzheimer's Disease and 34 people without the disease. They found no correlation between Alzheimer's Disease and the number of amalgam fillings or the amount of mercury in the subjects' bodies.
Gold standard
Among all the studies conducted so far, two studies published in the April, 2006 Journal of the American Medical Association come closest to the gold standard of medical research: both were long-term, prospective, randomized and placebo-controlled with a combined total of 1,041 patients. In both, researchers followed two groups of children for at least five years as they received either composite or amalgam fillings. Children who received amalgam fillings did have more mercury in their urine. But that didn’t make a difference in their performance on neurobehavioral and psychological tests or IQ scores in either study.
But even these studies can't quite lay the controversy to rest. They don't account for the possibility that mercury from fillings takes decades to accumulate to a toxic level, or that some fraction of the population -- too tiny to register in studies this size -- is particularly susceptible to mercury poisoning. But they do provide powerful evidence that amalgam fillings are not causing any immediate or powerful damage to patients. "Can we say unequivocally that there is nothing there?" asks Timothy A. DeRouen, Ph.D., professor of biostatistics at the University of Washington, who led one of the JAMA studies. "No. But we can say we have eliminated [the possibility] of a clinical effect of the size that people would be concerned about."
Though it is unlikely amalgam opponents will ever surrender, the debate may gradually fade for other reasons. Many governmental agencies around the globe are starting to restrict amalgam fillings because of concerns that the mercury (from amalgam waste) ends up polluting the environment.
And even without such restrictions, the use of amalgam is declining, largely because the newer white resin composites look so much more natural than a mouth full of metal. In 2005, silver amalgam made up nearly 32 percent of all fillings while resin composites made up nearly 47 percent, according to a paper published in the September-October 2007 issue of Public Health Reports. Just six years earlier, amalgam was used for almost 60 percent of fillings, with resin composites making up nearly 40 percent, according to the July 2004 issue of the Journal of the California Dental Association.
Other materials are catching on as well. In 1999, 1 percent of cavity restorations were completed with substances such as gold, glass ionomer, and ceramics, according to the 2004 paper. By 2005, 22 percent of restorations were done with these materials.
All the materials -- new and old -- have different strengths and weaknesses. For example, in September 1999, Clinical Oral Investigations published a laboratory study showing that composite resins killed cells on contact, even after the materials were aged in artificial saliva for two weeks.
"The takeaway message: everything in this world has some level of risk associated with it," says Dr. DeRouen. In the case of mercury exposure from amalgam, he argues, "the benefit still outweighs the risk."
Laura Lane has bared her fangs for the nearly 10 years spent in journalism. She first sank her incisors into newspapers and then moved on to the molar-grinding dot-com world. She has written for The Dallas Morning News, Harvard Women's Health Letter, Shape magazine, WebMD, as well as a variety of trade publications. She earned a B.S. in biology from the University of California, Los Angeles and a M.S. in biological sciences from Stanford University.
