What is a “buildup (or, why do crowns and bridges fall off)?”

Let’s flash back in time. Bonding is not yet in widespread use. Teeth are routinely filled with non-bonded silver mercury amalgam fillings. When a corner of the remaining tooth structure fractures, the dentist reshapes the silver that remains in the tooth and places a crown to cover the tooth.

Several years later the patient returns to the dentist with the crown in his or her hand. The silver is inside the crown, since it came out of the tooth when the crown loosened. There’s almost nothing left of the tooth. How do I know this story? Been there, done that; won’t do it that way any more. For years, my practice has been to remove all the restorative material in a tooth before putting a crown on it. Why? Because you don’t know what’s under the material (we didn’t have decay-detecting dyes until relatively recently—about the last 12-15 years); decay may have been left in under the silver, or the silver wasn’t bonded to the tooth and can easily pull out once some of its retention is drilled away.

Once I’ve determined that a tooth needs a crown or bridge, I’ll remove all the old restorative material, use decay detecting dye (and remove any decay I may find), and bond an epoxy-like material to reinforce the tooth. Then I’ll reshape that material and place the crown over it, secure in the knowledge that that material is unlikely to dislodge and the crown is unlikely to ever fall off.

Why else might cause a crown or bridge to fall off?

• Insufficient mechanical retention. The remaining tooth structure is too short to hold a crown.
• The cement (glue) that held the crown in has dissolved. That’s right; until about 12 years ago, all crown cements on the market could slowly dissolve when exposed to saliva. That’s not a very good property for a crown cement to possess. Today’s cements actually bond the crown in place and are far superior. Some of the cements even time-release fluoride; another good property to have.
• Bite problems. If you bite on the crown in such a way as to torque the tooth back in forth, something has to give. Either the tooth will become loose, the jaw muscles will start to hurt, or the crown will be forced to come off the tooth because the cement or bonding agent gradually fatigues from excessive pressure.