Impressions

The use of retraction agents before impression procedures is almost always needed. There are many different types of chemical agents available. Discussion of these agents is beyond the scope of this article. However, all agents have the potential to be caustic to tooth structure if left on the tooth preparation for extended periods of time.

It is important that good retraction is made around the tooth preparation with a minimum amount of retraction agent included in the cord or on the gingival tissue. Copious amounts of water are indicated to rinse all remnants of the retraction agent away prior to making an impression. The use of water and/or chlorhexidine gluconate with pumice can be beneficial when cleansing the tooth before and after tooth preparation. Several new products have been introduced as tooth preparation cleaners and surface tension reducers.

If the tooth structure is clean and an impression is properly made, all areas of the preparation will be recorded accurately in the impression. It is important to carefully evaluate the finish line to make sure that there are no areas of debris or tissue tags left on the margin prior to the impression procedure. This can have an adverse effect on the leakage of the crown or retainer after fabrication and cementation. The entire impression should be carefully evaluated after the impression is completed with particular attention given to the finish lines (fig. 2). If there are slight tears, a small bubble, or an incorporation of a foreign particle in the impression, it is very easy to make a second single tooth impression of that particular preparation. This can be done with a copper band or modified stock tray technique (fig. 3). This second impression will then give the laboratory technician a perfect margin for the fabrication of that particular crown or retainer.

Figure 2: Full arch impression showing small voids on facial margin of tooth 25.

Figure 3: Sectional impression of tooth 25.