Hydraulic and Hydrophobic Impressions
(Continued)Multiple Single Units
Previously, we introduced an impression technique that relied on hydraulic pressure to produce highly accurate impressions of subgingival finish lines.With this technique, you do not have to pack retraction cord and wait for the gingival tissue to be stretched out. You do not have to apply caustic, foul tasting hemostatic agents repeatedly to achieve hemostasis. You do not even need to completely isolate the prepared teeth from saliva.
The hydraulic and hydrophobic impression technique (a/k/a H&H) will consistently reproduce subgingival finish lines with unparalleled accuracy and definition, even in a hemorrhagic and wet field. And you will need only a fraction of the time and materials that are part and parcel of conventional crown and bridge impression techniques. And best of all, your frustration factor with capturing subgingival finish lines in the impression will drop to zero.
You should also consider just how much more comfortable the H&H impression procedure is compared with conventional impression techniques. Have you ever sat in the chair and had somebody shove retraction cord down into your sulcus? Hurts quite a bit during the procedure (and for several days after). And, have you ever tasted some of these hemostatic agents?
And then what happens when you do not get the impression the first time? You have to pack cord again and stretch out the gingiva some more, maybe tear some more of the periodontal attachment. Apply more of the foul tasting hemostatics. Waste more chair time. Is there anything you can say to the patient that will really make them understand why they have to go through all of that again?
Today, time is very important for most patients. With H&H you will have your patients out of the chair in record time. You can use impression materials that set rapidly so the entire impression procedure will require under four minutes! You will make a perfect impression the first time around and will not need to repeat the impression sequence a second or third time. Never a need for cord or hemostatics. No other impression technique can compete with the H&H impression technique for maximizing patient comfort and reducing chair time as well as reducing your frustration.
In this article we describe how you can use the H&H for multiple single units. To demonstrate this technique, we have selected a case with four adjacent teeth to be prepared for metal ceramic crowns, all with finish lines located subgingivally and all with gingival hemorrhaging immediately after the teeth have been prepared.
Clinical Case
The preoperative view shows the teeth in the maxillary left quadrant with failing amalgam restorations (Figure 22). Figure 22 |
The teeth were prepared for single crowns, the finish lines were placed subgingivally as dictated by the preoperative condition of the teeth (Figure 23).
 Figure 23 |
A dual arch impression tray was selected that passively fit the quadrant containing the prepared teeth. A correctly sized tray will fit loosely and you should be able to wiggle the tray around without it binding against the teeth or arch. Have the patient practice biting all the way down into MI with the tray in place.
Both sides of the tray were loaded with 30-Second Blue Velvet (J. Morita, 1-800-752-9729), a vinyl polysiloxane bite registration material and inserted to the proper position. The patient was coached into biting all the way down into MI (Maximum Intercuspation).
When the patient bites all the way down into MI during the impression procedure, observe how the teeth interdigitate on the contralateral side. The pattern of cusp-fossa interdigitation should indicate that the patient has closed all the way down into MI.
After the bite registration material had set, the tray and bite registration impression complex was held on the arch opposing the prepared teeth and the patient opened wide. The tray and impression is stabilized on the opposing arch and is not removed. It is held in position on the opposing arch.
The bite registration impressions of the prepared teeth were washed and dried. These bite registration impressions of the prepared teeth act as a highly customized tray. The prepared teeth were washed and dried to remove excess blood and saliva. A small amount of 90-Second Flexi-Velvet, a hydrophobic, low viscosity, vinyl polysiloxane material was injected into each of the bite registration impressions of the prepared teeth.
Do not inject low viscosity material into the bite registration impressions of the unprepared teeth or around the prepared teeth. Only a small amount of low viscosity material is required. The patient then closed again all the way down into the bite registration impression.
In this second step of the hydraulic and hydrophobic impression procedure, when the patient closes into MI, the prepared teeth are thrust into the low viscosity impression material like pistons and generate a hydraulic force. This force propels the low viscosity material into the sulcus under sufficient pressure to displace blood and saliva. The low viscosity material is propelled with such a strong hydraulic force that it fills the entire sulcus and reproduces the entire subgingival finish line as well as the submarginal anatomy of the root.
It is critical during this second step in the impression procedure that the patient closes all the way down into MI. You should again coach the patient to close all the way down and tell the patient to "bite down hard" until the low viscosity material has completely set. We have found it helpful during this phase for the dentist to cup his hand under the chin of the patient to help the patient to remember to keep closed all the way down.
In the final impression, the subgingival finish lines were reproduced with great detail and were sharply defined (Figure 24). Since the low viscosity material is hydrophobic, it will not incorporate blood or saliva but rather will bodily displace them when propelled into the sulcus under hydraulic force.
 Figure 24 |
With this technique you can proceed directly from preparing the teeth to making the final impression. Do not stop to pack cord, apply hemostatic agents, etc. Just prep and impress. Insert your temps and the patient is out the door. The final restorations will require minimal adjustments because of the high degree of accuracy with which the dual arch impression procedure records the occlusal relationships of the opposing and prepared teeth.
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