Most of us learned in school that things expand and contract when exposed to changes in temperature. Bridge builders knew long ago that if they didn’t allow for it, a bridge would fall into the chasm when temperatures plummet in winter! Conversely, the bridge would buckle when exposed to the heat of summer. That’s why they have expansion joints–the familiar “cha-chunk, cha-chunk” when you drive your car on and off of a bridge.
Dentists have had to deal with this problem for years. Unfortunately, our patients don’t abide by the seasons! They expose their teeth to extremes of heat (coffee, soup, tea) and cold (ice cream, ice water, high-speed snow skiing!!) all the time. Have you ever been to a nice restaurant where the doting waiter offers you ice cream AND coffee for dessert?!? (Pity your poor teeth with those old silver fillings!) In fact, after ice and popcorn kernels, the most common cause of tooth fracture in my office over the years comes after Thanksgiving–“But, Doc, I was only eating mashed potatoes when it broke!” Of course, they were homemade–full of chunky, “hot-spots” that caused excessive expansion that broke off an already weakened piece of tooth.
Even more insidious is the contraction to cold. When the metal filling contracts, a microscopic gap opens to allow for fluids, bacteria and sugars to enter. The scary part is that this “leakage” is the reason that silver fillings work in the first place! The leakage of fluids permits oxidation (“rust”) that “seals” the natural space between the tooth and the silver filling. The end result, however, is recurrent tooth decay that weakens the tooth and/or leads to infection of the root canal–which is why even small fillings don’t last forever.
I advise patients that my “cut-off” for determination whether a tooth should be crowned or “just have a filling” is my 50% Rule:
When the volume of the filling exceeds 50% of the available, remaining healthy tooth structure, the tooth should be restored with a crown to minimize the risk of cracking the tooth when heat, chewing or clenching forces are applied.
Of course, it is only a guideline. And, it doesn’t mean that treatment can’t be staged over time–it just increases the risk of fracture “while you wait.” I will usually offer my best “estimate of longevity” when a patient asks me, “How long can I wait?” But in my heart, I know my response should call to mind an age-old problem:
My crystal ball is broken, let’s use yours! 😉
If only life were that easy…
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Next time: Commandment #4–Do it once; do it the right way.