Crown Doesn’t Fit?

A few years back, our lab was located in the lower level of a large dental group. This had some downsides, but for the most part, it was a great experience. As I worked closely with several dentists, I got to see the issues they were up against as well as the results of my own craftsmanship.

In one series of events, a certain doctor began having repeated ill-fitting crowns even though they fit the die perfectly. Since none of the other five doctors were having this issue, I had to rule out the common impression materials that were being used. I didn’t have any special instructions with her cases that would isolate a particular technique, so I had to investigate her clinical procedures more closely.  To re-impression would sometimes remedy the ill-fit but not enough to indicate it was a successful solution.

One thing I did notice was that the compromised crowns mostly seemed to be the first molar and back. The second molar was the most likely candidate to have the problem. When I examined the unwanted crowns, I noticed a pattern in the flaw. They all seemed to be undersized compared to the new impression. This showed me that the impression material was being distorted during the cure.

It turns out that the patients were biting down so hard that the plastic frame of the tray was bending and stretching out while the material was being cured. When the tray was removed, the plastic went back to its original position, taking the vinyl material hostage. This created a smaller record of the preparation.

Our suggestion was to use a more rigid tray on these types of cases. The occasion of this increases as we get closer to the condyle of the mandible. While the local Alligators here in Florida have a bite force of 2125 PSI, we humans still claim an impressive 200 pounds of pressure from the back molars while the force tapers off toward the front of the jaw. That is plenty of pressure that can easily cause some unanticipated flexing if not closely monitored. One Doctor suggests tilting the patient's head back and asking them to “close “rather than “BITE!”

Ned LaMarti, CDT