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CROWNS AREN'T ALWAYS FOREVER:

When teeth are broken down or when they have cracks that are symptomatic or after root canal treatment they often need to be covered with full or partial crowns to prevent further damage. If these teeth go untreated, it is possible that the opposing dentition will fracture that tooth and it will be lost. After all, the biting forces of teeth on teeth can be up to 800 psi with the heaviest load on the back molars (the "nutcracker" effect).

Crowns aren't usually "budget breakers" for patients, especially if they have a dental reimbursement plan, but most people would rather spend their money on something more fun. But, a crown is certainly more economical than having to replace a tooth that is lost due to trauma or for any other reason. However, crowns may (and probably won't) last forever. As a rule, metal crowns last longer that tooth -colored crowns because metal won't break under those heavy forces and the tooth-colored ones (which are various forms of porcelain glass) certainly can. (I always recommend all-metal crowns on back teeth for that reason). However, the tooth-colored option has to be used on teeth that will be in the "esthetic zone" because most patients do not want metal showing when they smile and neither do I. The good news is that as we come forward into that "esthetic zone", the biting forces diminish making the porcelain less likely to fracture over time.

Unfortunately, crowns can fail and that failure is usually due to decay germs which work their way under the edges of the crowns usually below the gum line (this is called "secondary decay"). This happens most often in areas that are difficult to clean with regular home care devices and, of course, in mouths of those who don't bother to do anything but brush their teeth. The place that I see decay under crowns most often is under the gum on the back of a crowned first molar which is usually the next to the last tooth in patients who have no wisdom teeth. The reason for this is that the contact between those two teeth is almost three forths of an inch wide. When we clean teeth in the dental office, this area invariably bleeds profusely when instrumented due to the chronic inflammation created by the bacteria in and around the gum pocket between these teeth, bacteria that is never effectively removed at home. Patients with otherwise good periodontal health, often have deeper gum pockets in this area which creates an environment for further bacterial accumulation and decay under the margin of the crown. Even in the mouths of the best of flossers, this area is very difficult to clean because of the width of the contact and the physicall limitations of being able to get the hands back back that far in the mouth! Prevention requires thorough irrigation with a Water Pik-type device or the use of interproximal brushes on a daily basis; special floss such as "Superfloss" can also be helpful, again, if used daily.

The big problem with decay under a crown is that, although it first shows up on an X-ray, we do not know how far the decay has "eaten" into the substance of the tooth because the X-ray can't penetrate the material the crown is made of. And, even if the dentist could get to the decayed area he or she can not be sure if they get all of the decay out from under that crown. Also, it is virtually impossible to place a filling that will last any length of time because of limited access and the bleeding that occurs any time gum tissue is minipulated. The only sure way to treat that tooth is to cut off the old crown, remove all of the decay and make a new one. (Trying to salvage an old crown with decay under the margin by "knocking it off" invites breaking the whole tooth off at the gumline rendering it useless. And, even if we could get it off unscathed, there is going to be a really bad void under the edge of the crown where the decay was removed and, although this void can be filled with dental cement, it will eventually be dissolved by the oral fluids leaving another cavity for decay germs to accumulate; i.e. it is a temporary fix at best.)

I've tried to "patch" these up from time to time only to find out on recall X-rays that there was more decay or the filling was washing out or some other problem. Again, a new crown is the only long-term treatment that is dependable.
posted by Steve at 9:06 AM