Not Filling A Need: The Trouble With Silver Amalgam

(Note to Reader: This essay was in response to a question by a contact on the Comics Curmudgeon, who goes by the nom de guerre of the Reverend Nehemiah Scudder.)

Back when I was a student in dental college, silver amalgam was still the standard dental material for fillings in back teeth – while such things (which we’d never even think of using these days) like silicate cement were used for fillings in front teeth. Today, both have been replaced by light cure composite resin, a material so superior that if I weren’t an atheist I’d ask god to bless whoever first invented it.

Why?

Now, amalgam is basically an alloy of roughly 50% mercury with the remaining 50% comprising various ratios (depending on manufacturer) of silver, tin, copper, and other metals; silver and tin are the major components.  When you mix them together and grind them (either in a mechanical mixer or a mortar and pestle) you get a mix which hardens after a while.

Now, I agree that amalgam fillings appear to last very well. If the conditions are all right, and the filling isn’t too big, the filling can last for many, many years. Or, and more significantly, it can appear to last for many, many years.

These are a few of the problems with amalgam:

1.     The problem of corrosion: As time goes on, the amalgam corrodes. These corrosion products leach into the tooth, which is why teeth with large amalgam fillings go a bluish colour with time. The corrosion also opens up a space between the tooth and the filling margin, allowing liquids and bacteria to leak past into the bottom of the cavity, a process called microleakage. Eventually, though the filling remains in place, decay begins under the filling, without the owner of the tooth being aware of it, until one day the pain starts.

I can see corrosion in this, as well as at least two points of marginal failure from creep. Photo taken from the net.

2.     The problem of creep: As the amalgam corrodes, the percentage of mercury in the corroded portions increases, and this causes increased plasticity of the material; it becomes softer and more likely to deform with chewing pressure. As a result of this deformation, the margins shift (creep) over time, further increasing the gap between tooth and filling and increasing microleakage.

3.     The problem of post-setting expansion/contraction: Now, as I said, silver amalgam is a mixture of the silver/tin alloy and mercury. The proportion of this mix is very important. Use too little mercury, and your mix is a friable, crumbly mess which will not cohere into a hard filling, Use too much, and it’s a soft, plastic lump which will deform with extraordinary rapidity and will corrode with amazing speed. There’s also another problem. Unlike other filling materials, amalgam changes size after emplacement. Usually it has some, and unpredictable, expansion; if there’s water or saliva contamination this expansion is even more. With extremely low mercury content there may even be a contraction. Either way, there will be a change in dimensions from what the dentist wants to place. This change of dimension may be only a fraction of a millimetre, but that’s like a cliff as far as bacteria are concerned. A filling whose final dimensions aren’t under your control isn’t much of a filling.

4.     The problem of conductivity. Amalgam is metallic, and an electric conductor. Now the mouth is obviously not a dry place. Saliva is always present, and it’s a mix of many substances, including substantial numbers of ions. Though gold is rarely used these days, there was a time when it was routinely placed as a filling. If a gold filling was placed near or opposite an amalgam filling, there would be an electrical charge between the two, causing corrosion in the amalgam and (since gold is placed directly on the tooth, without any base material as insulation below the filling) acute sensitivity in the gold filling. Also, since amalgam conducts heat and cold remarkably well, even fairly shallow fillings need a base below them to insulate the tooth,

5.     The problem of retention. Amalgam is only retained in the tooth by the structure of the tooth around it. It has no chemical or close mechanical bond with the tooth tissue. The only way amalgam stays inside the tooth is if the tooth is cut (by a dental “drill”) into a boxlike shape capable of accommodating it and preventing its dislodgement by chewing forces. This means that frequently much more tooth structure has to be removed than would be justified by the decay present. On the other hand, more modern filling materials either have a chemical bond to the tooth or are retained by micro-tags of resin inserted in tiny perforations etched in the tooth itself. Also, resin fillings can be easily attached to fibre-optic posts inset in the root canals for greater retention. If you want to do this with amalgam, it’s much more difficult and nowhere near as successful because unlike resin amalgam has no chemical bond to anything.

6.     The problem of contamination: As I said, amalgam fillings corrode. The older they are the more they corrode. And if they have to be removed, what with the modern high-speed air-driven dental motors, a nice aerosol of water spray and corroded filling powder (including droplets of free mercury) are blown into the room. The clinic staff members are safe, or should be, because of their eye wear and masks. But the patient will certainly breathe in the aerosol, and multiple fillings can cause a health risk.

7.     The problem of aesthetics:  Amalgam is probably the least aesthetic dental material ever invented. I’d name gold, too, but at least gold stays yellow over the years, and doesn’t stain the teeth either. Amalgam steadily darkens over time, and the corrosion products stain the teeth. Resin, on the other hand, can be shade selected to match the tooth colour exactly.

8.     The problem of hidden failure: When a filling made of resin or other material fails, it’s immediately obvious : the filling comes out. When an amalgam filling fails, there’s often no such warning sign. Corrosion or creep can cause failure in a filling which hangs on for years longer, while bacteria under it, sheltered by the filling itself, merrily eat away at the tooth.

9.     The vast superiority of the alternatives: Every single disadvantage of amalgam is missing in its alternatives, especially the resin fillings. Resin is easy to place, and since there’s no mixing involved you can’t ruin it by screwing up the ratios of the mix. Unlike amalgam, you aren’t in a hurry to place it before it hardens. With resin, you place it at your (reasonable) leisure and then harden it with a beam of light. Resin doesn’t change dimensions after placement. You don’t have to overcut a tooth; often you don’t have to cut the tooth at all. It’s tooth-coloured, doesn’t leak, doesn’t conduct heat or electricity, and it can be removed without poisoning anyone. When it fails it’s immediately obvious, and  it can be replaced with no trouble at all.

Sequence of three stages of restoring a badly broken root-canal-treated tooth with a fibre optic post and light-cured resin. Photos by me. I would not attempt this with amalgam with any confidence of success.

I don’t exactly remember when I last placed an amalgam filling. It must be six to eight years or more.

Hope that answers your question, Nehemiah.