When It's Time to Fill Your Cavities, All Options Are Toxic

When your dentist finds a cavity, besides the immediate fear of the drill or the painful pinch of the novocaine injection, the choice of filling material can also raise anxieties. And it's not easy—you either opt for plastic composite resins, which use dangerous plasticizers like bisphenol A, or metal amalgam fillings containing mercury, which needs little introduction as a toxic heavy metal. Both types of fillings bring with them concerns about long-term health effects and both show risks and benefits in terms of their durability and other factors affecting long-term oral health.


Most people know dental amalgams as “silver” fillings; they're made up of a mix of mercury, silver, tin and copper. Mercury, which is about half of the amalgam compound, binds the other metals together in such a way that they can be easily manipulated. Traditionally, dentists have preferred them over the plastic composite resins as they're the more durable of the two and cost effective. They're still the preferred choice for back teeth fillings.

The downside of the long-standard silver amalgam fillings is the elemental mercury they contain and the long-running scientific debate over the health significance of trace amounts that are released in the dentist’s office, and over the long chewing life of a given filling.

Mercury is among the most toxic nonradioactive elements and can be harmful even at low doses. According to the federal Agency for Toxic Substances and Disease Registry, long-term exposure to mercury in either organic or inorganic form "can permanently damage the brain, kidneys, and developing fetuses." For those reasons, mercury is slowly being phased out of many commercial and consumer uses, including thermometers and home thermostats, but it is still used in many industrial processes and is in such products as fluorescent lights.

In the solid, elemental form, ingested mercury generally moves quickly through the gut with little adverse effects. However, in the body and after it’s excreted into the biome, a portion gets converted by bacteria to a much more toxic form called methyl mercury. Small quantities may also be inhaled as elemental mercury which is more toxic when it bypasses the gut and makes a beeline to the nervous system where, like methyl mercury it can cause subtle but measurable effects on brain development in fetuses and young children. It may also sequester in adult brains, possibly adding to risks for mental illness and neurodegenerative disease like Alzheimer’s. Best known as a bioaccumulative contaminant in tuna, shark, swordfish and other fish species high on the food chain, methyl mercury has also been linked to heart disease and neurotoxicity in adults who eat a lot of these fish.

Concerns like these have sent some patients running to the dentist to replace their silvers with composite fillings, which are completely mercury-free. But removing amalgams increases the oral exposure to mercury vapor, and alternative resin-based fillings have their own uncertain risks and benefits that patients and dentists will have to weigh against the profile for silver amalgams until science can deliver more definitive conclusions.

Alternative filling materials are usually made with one of two kinds of complex polymers – light- and chemical-cured composite resins that combine plastic and ceramic components and glass ionomer cements that have a polyacrylate component. However, the variety of available chemical combinations are complex and in many cases the full list of components is considered proprietary and not fully disclosed, even to government regulators. That said, a common monomer that’s used is bisphenol A—the same BPA that’s made polycarbonate baby bottles and canned foods public enemy number one for many parents due to concerns about its ability to mimic the hormonal activity of estrogen, an effect that is not associated with mercury.

Researchers have also found that bisphenol A, the chemical building block of polycarbonate plastics, may be linked to other health problems including cancer, reproductive dysfunction and heart disease. 

But on the upside, bisphenol A does not accumulate in the body or mobilize to brain tissue like mercury does. In any case, research on both chemicals indicates that the amount released over the life of either mercury or resin fillings is small relative to higher exposures that occur when the filling is first applied. Government regulators in the US and the EU have deemed both to be of insignificant risk, while acknowledging that further research is needed.

While the controversy over relative risks continues, a parallel debate concerns durability and the long-term risk of complications and more serious, expensive and painful dental work that may result when dental restorations leak or deteriorate prematurely. Many dentists report, and some research suggests that resin composites may not be as durable for certain types of restorations, especially larger fillings in posterior chewing surfaces, compared to less impacted front teeth, and smaller-sized fillings.

Some practitioners argue they put some patients at risk for more invasive root canal procedures if filling materials degrade faster and in ways that make it harder to detect developing decay in adjacent tissues. However, the situation may be improving as dentists become more familiar with alternative materials and better trained. Likewise, material chemistries have evolved over the 30 years they’ve been in use and may lead to better outcomes.

In any case, it pays for dentists and the cavity-prone consumer to stay tuned. The European Union is revisiting the issue in November when it has scheduled a public consultation to review its 2008 position which should promise to bring out more debate and hopefully tangible research with more conclusive findings. 

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