Dental updates from Dr. Mckeever.

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CROWN (CAP) SELECTION

If I asked fifty patients needing crowns on a tooth if they want me to make them a tooth-colored crown (which is metal covered with a white porcelain which is glass) or a metal crown (gold or silver metal), forty nine of them would say "tooth colored, of course" with most of them, but not all, leaving out the "you idiot" on the end. However, there are some places in the mouth where using metal to fabricate a crown is the smart thing to do because it gives us a restoration that could be servicable a lot longer.
Now nobody wants a whole lot of metal, if any, showing on their teeth. We try to place tooth-colored crowns on all of the upper teeth from first molar to first molar and, in the lower arch at least from second premolar to second premolar and the first molars if the patient so desires. When a patient must decide on what to place over a second molar, however, I have to give my speech to try to talk him or her into metal.
The chewing mechanism is somewhat like a nutcracker with the fulcrum in front of the jaw joint but in back of the last back tooth (which is usually the second molar). These back teeth take a lot of the biting force when the jaws (nutcracker) closes (up to one thousand p.s.i. or more). Around 70% of all of the broken teeth that I see are second molars (which, unfortunately, are also the hardest teeth to work on). There are two potential problems associated with placing a tooth-colored crown on the back tooth in a dental arch. First of all, the porcelain (glass) part of the crown will eventually get tiny cracks in it due to the crushing forces from its position in the "nut zone". These small cracks can turn into big cracks over time resulting in a piece of the porcelain breaking off rendering that crown non-functional. The other consideration in the preparation of a crown is how much the dentist has to shorten the tooth to have enough room for the materials that crown is to be made from. Teeth get shorter as they get further back in the arch; a second molar can be so short that the gum tissue behind it is even with the top of the back edge of the tooth. When preparing a tooth for a metal crown, it is only necessary to remove about one millimeter of tooth from the biting surface which is the width of a dime. When grinding the tooth for a porcelain/metal crown (or an all-porcelain crown which is going to be weaker yet), the dentist has to remove almost two millimeters from the biting surface. The height and the taper of the remaining tooth after it has been shortened determines how well the crown will stay cemented over the years considering that the more surface are the cement can utilize the better it adheres. If a tooth that has to be reduced to six millimeters in preparing for a metal crown is cut down another millimeter for a tooth colored crown, there is a loss of ssurface area of over 20% (assuming the taper is the same) resulting in a crown that just isn't as stable over time. The shorter the tooth, the higher that percentage goes.
I warranty my crowns for a certain period of time but I feel so strongly about placing metal only on these second molars that, if a patient is really insistant about having glass placed in that area of the occlusion, I will certainly do so but it will not be on my warranty list.
posted by Steve at 7:53 PM