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Technical Discussion: Common Cause of Distortions

Technical Discussion: Common Cause of Distortions

If you use a two-stage impression material, make sure one doesn’t set up before you apply the other.

L.T. Armstrong, DMD: Always inject the wash material around the preparation and then immediately seat the putty-filled tray before either can set up. This always allows the materials to set together. If one material (the base) is allowed to set before the other material (injection) is used, you are essentially trying to “reline” your impressions.

The reason this causes distortion is that the putty is firm but not rigid and will compress (distort). When a liquid (wash material) is compressed between the two solid materials (tooth and tray), hydrolic pressure is created, compressing the putty. When the tray is removed, the putty will spring back to its original shape and the impression is distorted.

As a result, the crown fits the model but, in the mouth, it’s high, contacts are overly tight, and you will have to grind the crown internally.

A similar problem exists when you take an impression for a relined denture. If you struggle to remove the impression from the mouth but the finished reline has no retention, click here to find the solution.

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