On to the next sessionOne of the most frustrating problems associated with impression taking is the impression that looks perfect, but is actually distorted. Much time is wasted in making the crown, scheduling the patient, trying to seat the crown and retaking the impression. The question immediately arises, why didn't it fit? Although there are many reasons why a crown doesn't fit, most likely it is not the dental laboratory. Dental labs couldn't stay in business if their crowns didn't fit a vast majority of the time. The same goes for the impression material. Successful crowns and bridges have been made with every major material. This is not to say that dental laboratories and manufacturers don't have a bad day once in a while, just that it is not as common as some dentists would like to believe.
I believe the most likely reason for crowns that do not fit is rebound, or a change in the shape of the preparation in the impression due to the springing back of the rubber material that set under pressure. Shrinkage of an impression material, if the tray is rigid and there is a bond between the tray and material, will most likely lead to a larger die and crowns that go to place more easily on the prepared tooth. The reason for this is that most preparations are extra coronal. If the preparation is intra coronal, shrinkage will lead to a larger unit and a restoration that will be more difficult to seat in the tooth.
How do we get rebound? There are number of ways to create that unwanted rebound. In general terms, pressure was generated in the impression material or placed on the tray during the setting of the material while it was in the mouth. When the tray is removed from the mouth, the pressure is released and the material springs into the space left by both the teeth and preparation, leading to smaller dies and crowns that are tight.
If the pressure to seat the impression material flexed the tray, then the tray could also rebound, leading to a distorted impression. Consequently, when an impression is seated, it should be held with neutral pressure while setting.
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jchamilt@umich.edu © J. Hamilton, University of Michigan, School of Dentistry, 1996