Do you always need a crown after a root canal?

I was taught, and many of you have heard, that any tooth that’s had a root canal needs a crown. Recent research has disproved this, at least in some cases.

If a front tooth is basically structurally sound (i.e., not a mass of old fillings), a crown gives no benefit except when it’s needed for aesthetics. I bond a filling into the access hole I drilled in the back of the tooth to do the root canal, and the tooth becomes structurally sound again.

When a root canal is done on a back tooth, the access hole is made in the chewing surface of the tooth. Back teeth are subject to much greater chewing pressures than front teeth, so it’s not enough to just fill up the access hole in a back tooth that has had a root canal. The chewing surface of the tooth has to be covered completely. This can be done by an onlay, which covers the chewing surface of the tooth and part of the sides of the tooth, or by a crown, which covers the entire tooth, including all the way down the sides.

Onlays are more conservative because they preserve more of the original tooth structure, and they protect the tooth just as well as a crowns do. For this reason, I try to do onlays on back teeth wherever possible, and only resort to crowns where there is no other choice.

By the way, the above column reminds me of one of my favorite dental course questions. “What’s the biggest block to new knowledge?” Answer: “Old knowledge.” There is always new research and new information; that’s why I take so many courses.
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.

Recent courses

In December, I took a course on “Ceramic Science 2010.” As dentistry moves away from porcelain in many cases, it’s especially important to keep up on the developments in high-strength dental ceramics. These materials are used more and more as a replacement for porcelain-fused-to-metal in crowns. Porcelain and ceramics are both used for veneers; each material has advantages and disadvantages.

The lecturer, from NYU dental school, had a tremendous amount of new information on choosing the best dental ceramic for specific needs, and how to make ceramics perform to their optimum potential. A good course, and information every cosmetic dentist can use every day.