Hydraulic and Hydrophobic Impressions
(Continued)There is no need to stop and place retraction cord to produce a space between the tooth preparation and the free marginal gingiva. The hydraulic force will drive the low viscosity material even through a narrow gap between the gingiva and the tooth. There is no need to apply hemostatic agents since the low viscosity material is driven with such force that it readily displaces any blood and saliva that may be in the sulcus (Figure 8).
 Figure 8 |
The dentist can proceed directly from preparing the tooth to making the final impression. This reduces the number of steps required to make the final impression, decreases the cost of the procedure, conserves chair time and vastly improves patient comfort. And best of all, this technique yields a final impression with very sharply defined finish lines (Figure 9).
 Figure 9 |
Troubleshooting
A few tips to help things along. Don't overload the tray with bite registration material. You only need enough bite registration material to act as a custom tray and to capture the prepared tooth, adjacent and opposing teeth. In the second step of the impression procedure, the patient will have to bite down into the bite registration impression. If there is too much excess set bite registration material, this may be difficult and the patient may not be able to bite all the way down.Don't inject too much low viscosity impression material into the bite registration impression of the prepared tooth. The bite registration impression will reproduce the axial walls of the preparation but will generally not extend into the sulcus. The objective of using a wash in the second step is to propel the low viscosity material into the sulcus to capture an impression of the subgingival finish lines. You only need a small amount of low viscosity material to accomplish this.
We use 30-Second Blue Velvet (J. Morita), a thixotropic vinylpolysiloxane bite registration material to load the tray. It has the advantage of setting up within one minute after the tray has been inserted. We used 90-Second Flexi Velvet (J. Morita), a thixotropic vinylpolysiloxane low viscosity impression material for the wash in the second step. Because this material is thixotropic, it does not run out of the bite registration impression of the prepared tooth after it has been deposited. It also has the advantage of setting up within two minutes after it has been injected into the bite registration impression of the prepared tooth and the patient closes into it. A typical impression takes us no more than four minutes.
Discussion
This impression technique has enabled us to vastly increase our productivity and decrease our chair time while reducing overhead. We do not stock retraction cord, cord packing instruments, or hemostatic agents. We use only a fraction of the impression material utilized in other conventional impression techniques. Also, we do not need custom acrylic trays. We need only a fraction of the time usually required to make final impressions with conventional impression techniques. We generally get a perfect impression on the first try and rarely have to repeat the impression sequence. Our patients are far more comfortable during our impression procedures and do not have to suffer through cord packing and the application of hemostatic agents. They suffer far less postoperative pain since their gingiva has not been stretched to the tearing point and has not been traumatized with chemicals.Just in the last two years we have successfully inserted over 3,000 units using this impression technique. The cases have sharply defined finished lines that are easy for the technician to read. The occlusion has been very accurate and has required minimal adjustments. And last but not least, our frustration factor with this very challenging aspect of making crowns and bridges has plummeted. In fact, for us, making impressions of subgingival finish lines is no longer a challenge.
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