Why put a filling in the tooth to build it up if you are going to crown it?
First, we need to accept that the reason you need a crown is most often because there is not enough tooth structure left to do a filling or your tooth is cracked.
Do you think a crown would stay for long on this tooth?
After the patient is numbed, the first step in preparing a tooth for a crown in our office is to remove all the old filling and decay. Some dentists feel it all right to leave the old filling if it “looks OK” and just work around it. We do not. We know that underneath fillings we sometimes find surprises, decay or fractures. Radiographs (x-rays) can not reveal this decay because the filling materials block out the x-rays. Other dentists take out all the old fillings as in the photo and simply make the crown to ‘fill in” the voids created by removing the old filling/decay. We choose to build-up the remaining tooth for several reasons.
Bonding vs Mechanical Retention or Both?
Some claim bonding the crown to the tooth makes building up unnecessary. Often times a tooth will need a crown because a portion of the tooth has broken off leaving very little tooth. Then what do you bond to? The longevity of the bonding process is debated, but some studies show a significant breakdown over time.Do you think a crown could stay supported on this?
If the bond does breakdown what is going to hold the crown on? I like to follow established engineering principles that have been used for ages to design and prepare crowns. I just do not think you can hold onto a hole no matter how good your “glue/bond” is. I know that crowns have been held on for years prior to the bonding process by retention created and based on these engineering principles that involve a precise slip fit between the walls of the tooth and the crown. Often without building the tooth up we have no walls to hold the crown on.
We want the foundation on which the crown rests to be clean, solid and provide retention based on these tried and true engineering principles. After removing all the old filling/decay we place a specialized filling building up the tooth to provide a foundation that we can shape ideally for the best mechanical retention in addition to bonding the crown to the tooth.
Other Reasons to Build Up a Tooth
I do not know if simply bonding would be adequate to hold a crown on but I figure why take a chance when placing a build up has so many advantages: 1. It seals and protects the nerve insulating it from the outside world 2. It provides a uniform thickness of porcelain reducing the stress produced during the production of the crown and thus reducing the chance of fractures created by different thicknesses of porcelain. 3. If the prepared tooth has irregularities the impression can get torn or distorted. 4. Perhaps most importantly, before I entered dental school I worked in a dental lab as a model maker, and I saw first hand how much easier it was to work on a tooth that had been built up and shaped properly. This enables the laboratory to get a more consistent and better fit for the crown. The resulting crown will fit better and last longer and ultimately that is our goal.