Is a filling the only way to protect the margin of a crowned-bridge-anchoring tooth from cavities?

It is not uncommon to have gums shrink and expose roots of teeth beyond the margin of crowns and bridges, but often the situation is cleansable and won't decay.  If the gums cover the area currently, then the risk is very low (strep mutans, the bacteria involved in decay, is aerobic.  This means margins below the gums rarely decay).I would simply etch and bond the surface, but not drill/remove tooth structure, if I was trying to protect/prevent decay.  If decay is already present, I would remake the entire bridge and make the margin lower on the tooth.  The filling will eventually fail.

Hi:Without X-ray and photo it is difficult to give you a very accurate advice.  However, filling can be an inexpensive way of fixing a margin of a crown that is either slightly open or has very small cavity.  Best,Dr. Maddahi

As a preventative measure placing a filling around a crown is at best "iffy". Esthetic fillings around crowns are  a compromise aesthetically. I would only place a filling around a crown if there was decay. If your dentist is finding erosion around your crown then he may feel that a filling to prevent food trapping and causing future decay is warranted. You could use a prescription strength fluoride gel around the root surface of your teeth to prevent decay instead of the filling. Discuss your concerns with your dentist and I am sure he will help you.