Presenting a Very Accurate Impression Procedure and Testing the Accuracy of the Impression Obtained

Accurate impressions are an essential part of many restorative procedures. Many new materials require an intimate fit with the tooth or the restoration will be subject to stress concentrations, leading to fracture.1 Current impression materials are very accurate under standard testing conditions, but combinations of tray, materials, and clinical technique may result in models that are not true representations of the patient’s teeth. Even with details sharply recorded, the impression could still be distorted. Only when the crown or bridge is “tried-in” and it does not seat properly does the distortion become evident.


Because of movement in the periodontal ligament, teeth are in a different position during maximum intercuspation than when the mouth is open. Opposing models poured from impressions taken with an open mouth will not fit together precisely, and bite registration records will not fit the models precisely. Therefore, a closed-mouth impression theoretically registers a more precise occlusal relationship. Recording this “functional” occlusion can be helpful when fabricating implant-supported crowns so that the implant is placed in function after the natural teeth are “loaded.”

A closed-mouth impression is accomplished with a “dual-arch” tray. There are many designs of dual-arch trays, but they are inherently prone to distortion due to flexibility. When placed in the mouth, there is frequently some bending of the tray during maximum intercuspation. The impression material then sets while the tray is in the flexed condition. When the tray is removed from the mouth, it will tend to return to its original position, thus distorting the impression. Only a fraction of a millimeter of distortion will result in a significant misfit at the tolerances that crowns and bridges require. Therefore, whenever such a technique is presented, passive placement of the tray is emphasized. Yet, in actual clinical situations passive placement can be difficult to achieve.2-4 Furthermore, dual-arch trays do not work well when multiple teeth are prepared.

This article presents a technique for achieving highly accurate impressions using a dual-arch tray, and describes the results of a testing protocol for determining the accuracy of this impression technique.

MATERIALS AND METHODS
The impression procedure presented ensures accuracy even if the tray is not placed passively. It is capable of providing a successful impression for “difficult situations” such as a gagger, heavy salivator, large lingual glands, and large tongue. It is an update of a technique first described by Cannistraci5 in 1965 and Hoffman6 in 1992. They lined a methyl methacrylate tooth form with impression material and placed a tray with more impression material over it to “pick it up.”


The proposed impression procedure incorporates a bis-acryl tooth form and polyether impression material. A dual-arch tray (for example, Premier Triple Tray or Sultan Dental Three Way Tray) can obtain an impression of the teeth in a functionally loaded maximum intercuspation. Bis-acryl (for example, A.P.M. Sterngold Insta-Temp or Bosworth Ultra Trim) captures details, sets rapidly, has barely any polymerization shrinkage, has minimal heat released during polymerization, and flows easily. Therefore, there is minimal adjustment needed to achieve proper occlusion.

Freshly mixed bis-acryl polymerizes well and separates easily from a polysiloxane mold (for example, DENTSPLY Reprosil Quixx or Heraeus-Kulzer Flexitime). These have a rapid setting time and a good balance between stiffness and flexibility for the indicated use. Polyether (for example, 3M ESPE Impregum or DENTSPLY Polyjel) is hydrophilic and tear resistant. It also polymerizes well adjacent to bis-acryl material (polysiloxanes leave an unpolymerized zone adjacent to a bis-acryl mold). 

THE PROCEDURE

Figure 1. Patient biting on dual-arch tray to form a preoperative impression with polysiloxane impression material. Figure 2. Dual-arch tray with polysiloxane and bis-acryl bridge.
Figure 3. Bis-acryl form and Iwanson millimeter caliper. Figure 4. Dual-arch tray with bis-acryl form surrounded by polyether impression material that had set and was removed from the mouth.

Step 1
The preoperative tooth shape is corrected directly with composite or cement. The shape is recorded with a heavy-body polysiloxane in a dual-arch tray (Figure 1). Or, full models are mounted on an articulator for a diagnostic wax-up, the model is duplicated, and a clear shell that includes the palate is vacuum-formed to transfer the design to the mouth.

Step 2
The tooth is prepared and bis-acryl material is placed in the polysiloxane impression or vacuum-formed shell. The patient bites down and forms a reproduction of the originally recorded shape (Figure 2). Note: Unpolymerized bis-acryl material placed into a polyether mold results in a polymerized material that adheres too well to the mold.

Step 3
The occlusion of the bis-acryl tooth form is adjusted. Additions can be accomplished with composite. The thickness of the trimmed provisional restoration is measured with a millimeter caliper (Figure 3). The dentist can thereby ascertain whether there is adequate clearance for the desired crown material. This is much more accurate than using direct observation.

Step 4
A second provisional restoration is fabricated in the same manner as the first. One is for the patient to wear, and the other is for incorporation into the impression. This allows the patient to evaluate the shape and function of the proposed tooth or teeth if a significant change was provided. If the patient needs more time, the final impression may be taken at a subsequent visit.

Step 5
The final impression is taken with one of these provisionals and a polyether material. The interior of the bis-acryl tooth form is thinned by about 0.5 mm to make room for adhesive and polyether material. The margin edge is not reduced. It is checked on the tooth to have a nonbinding fit. Adhesive is applied inside the bis-acryl crown and air dried. Polyether is then placed inside the bis-acryl crown and also into a dual-arch tray. The bis-acryl crown with impression material inside is placed on the prepared tooth, the dual-arch tray with impression material inside is placed over it, and the patient bites down until the polyether sets (Figure 4). This impression can be taken 15 minutes after the bis-acryl form was fabricated. (Polysiloxane is not used for this impression. A chemical interaction between the unpolymerized polysiloxane and the hard bis-acryl material prevents polymerization of the polysiloxane).

Step 6
The impression is poured. After separating the impression from the model, the bis-acryl material usually remains in the impression. It can be pulled out of the impression and placed on the model before the die is trimmed. (Impression material inside the bis-acryl crown needs to be removed, but there still remains good contact between the bis-acryl margin and the stone model to allow for an accurate placement on the model.) The models are mounted on an articulator with the bis-acryl crown on the model as a bite registration. A putty material is placed on the buccal and lingual surfaces of the bis-acryl and adjacent teeth to use as a guide for the final shape.


There is no need for a separate bite registration, an index of midline orientation, a lip position record, or a horizontal orientation record. All of the subtle details regarding the patient-approved outer shape have been captured and accurately fitted to the model because the model was, in effect, poured-up into the record.

TROUBLESHOOTING
No technique is perfect, and here are problems that need to be avoided and/or overcome.

Problem: Stone model of thin, long preps may easily fracture.

Solution: Glycerin liquid painted on the impression prevents the usual adhesion of the stone to polyether material, and allows for easier separation of the model. Otherwise, carefully remove the fractured die from the impression and a second pour can be made and the bis-acryl form can be burned off (it powders up and can be flaked off). 

Problem: Bis-acryl tends to bond to fresh-placed composite core material.

Solution: Place glycerin gel (KY Gel) over the composite and spray with air/water combination, and then place the unset bis-acryl.

Other Concerns

•Is there polymerization shrinkage of the bis-acryl?


Results of the experiment described below show that, for this impression technique, after 15 minutes at most, all significant shrinkage has been completed.
•Can the impression be left unpoured for a few days?
Yes.
•Can the impression tolerate temperature extremes during transport; eg, freezing or a hot car in the summer?
Freezing: Yes; then warm slowly to room temperature before pouring up a model.
Heat: Yes, for placing in a hot automobile during the summer; then cool slowly to room temperature before pouring up a model.
•Can it handle multiple pours?
Yes.
•How thin can the polyether material be and still have a good impression? Could the bis-acryl material show through without causing distortion?

Extremely thin areas still form accurate impressions. The bis-acryl material could show through without causing distortions. It does not distort because the bis-acryl material does not flex as long as there is no crack in the bis-acryl, and it was relieved to sit on the prepared site without binding before the impression was taken.

EXPERIMENTAL DESIGN
The following experiment was designed to (1) test the fit of a bridge made from a model of an impression taken as proposed, and (2) evaluate whether the impression distorts if subjected to commonly occurring conditions before it is poured up.

METHOD
First, bridge framework is cast from a wax-up directly from a Typodont (Columbia Dentoform) model. Then, impressions are taken of the Typodont “patient” and subjected to different conditions. Models are poured. Theoretically, if the dies on the model are smaller than the original Typodont, a casting could fit the Typodont closely and then still seat all the way on the model. However, a casting made on such a model would not seat completely on the Typodont “patient.” Rather than casting a new bridge for each model and then fitting it on the Typodont, the one casting made for the Typodont will be adapted to fit each model.


To duplicate the usual sequence of fabricating a casting on a model and then placing it in a patient, the procedure is as follows: (a) each model is coated with Derma-Scrub (Jones Medical Industries), which acts as a separator for temporary cement; (b) temporary cement (Nogenol) is placed into the casting that is placed on a model. Composite is now placed between the pontic bar and the model; and (c) the casting, with composite attached, is removed from the model with the temporary cement still inside, and tried-in the Typodont “patient.”

MODEL = TYPODONT OR MODEL > TYPODONT
The fit on the Typodont is the final criterion regarding whether the impression procedure was accurate. Therefore, the casting needs to be fitted with temporary cement, and composite under the pontic bar, on each model individually, and then placed onto the Typodont (thereby duplicating the sequence of fabricating a casting on a model and then placing it in a patient). If the casting binds prematurely, it is an indication that there would be very significant open margins, and of course a prosthesis that would require a “major grind” to seat. If the frame fits the Typodont completely, it indicates that the model is the same or larger than the Typodont.

MODEL = TYPODONT OR MODEL < TYPODONT
If the model’s abutment teeth are simply larger than the Typodont, the frame would seat completely on the Typodont, but could have open margins. To determine if this is the case, the frame is lined with temporary cement to fit the Typodont and then seated on the model to see whether it is able to seat completely. If the frame does not seat completely this way, the model is larger than the Typodont, and there is potential for open margins. If the frame fits the model completely, it indicates that the model is the same or smaller than the Typodont.

MODEL = TYPODONT
If the frame fits both ways as discussed, the model must be the same as the Typodont because it cannot be larger and smaller at the same time.

CONDUCTING THE EXPERIMENT

Figures 5 and 6. Master framework with a pontic bar and space between the bar and the Typodont.

The Typodont had teeth Nos. 13 and 15 prepared for crowns. Tooth No. 14 was removed, and Sil-Tek (Ivoclar Vivadent) was placed into the socket (Figure 5). The Typodont has a hard acrylic “gingival” base. In addition to being secured with screws, the teeth were further secured to the base material with flowable composite to prevent any movement. A cast frame was fabricated to fit slightly loose on the Typodont-prepared teeth Nos. 13 to 15 (Figure 6). Impressions were taken of the Typodont (as if it were a patient) 15 minutes after the bis-acryl was mixed, using this form and a Premier Triple Tray. A model was poured from the impression.

RESULTS

Figure 7. The master framework is fitted to the stone model with temporary cement and composite was placed between the model and the pontic bar. Figure 8. The master framework removed from the stone model; temporary cement adheres to the interior of the crowns and composite adheres to the bar.
Figure 9. The master framework fits on the Typodont without space between the composite pontic and the ridge. Figure 10. Composite pontic custom-fitted to the ridge of the stone model.
Figure 11. The bridge in Fig 10, with Nogenol, after it was pulled off of the stone model. Figure 12. The bridge with Nogenol in Fig 11, was placed on the Typodont. Note the space between the pontic and the Typodont ridge.
Figure 13. An impression using polysiloxane heavy body and wash resulted in a casting that did not seat completely. Figure 14. Composite pontic was custom-fitted to the stone model.
Figure 15. Note the gap between the pontic and Typodont ridge, with the pontic fitted to the stone model in Fig 14.

The impression using the technique described was poured 15 to 20 minutes after it was taken, and the casting seated completely on the Typodont after it was fitted to the model (Figures 7 through 9). The casting also seated completely on the model after it was fitted to the Typodont.


When the same impression was poured a second time, the same results were obtained.

When the same impression was placed in the freezer for 6 hours and allowed to warm up for 12 hours, the same results were obtained.

The same impression was placed for 3 minutes into an autoclave chamber with an open door, and boiling water was steaming the chamber. The impression cooled down for an hour and was then poured up. The same results were obtained.

This experiment was repeated with new impressions, with the same results. 

For comparison, a dual-arch tray with heavy-body polysiloxane alone was used to take an impression. A model was poured and the casting was fitted to the model. The casting did not seat completely on the Typodont (Figures 10 through 12). 

Another impression was taken with polysiloxane heavy-body and wash at the same time. Again, the casting that was fitted to the model did not seat completely on the Typodont (Figures 13 through 15).

CONCLUSION
When an impression is taken with a polyether in a framework of the bridge fabricated in bis-acryl, and picked-up with a dual-arch tray, the model is virtually an exact replica of the prepared teeth area.


The technique described in this article has been used clinically for years and is practical and effective. The results of this investigation verify what has been apparent clinically. In fact, the investigator always had the impressions poured up between 15 and 20 minutes and was surprised that the impression was able to tolerate the environmental stress to which it was subjected.


References
  1. Esquival-Upshaw JE, Anusavice KJ. Ceramic design concepts based on distribution analysis. Compendium. 2000;8:649-654.
  2. Koplowitz GJ. Trouble-shooting dual-arch impression. J Am Dent Assoc. 1996;127: 234-240.
  3. Lee EA. Predictable elastomeric impression in advanced fixed prosthodontics; a comprehensive review. Pract Periodont Aesthet Dent. 1999;11(4):497-504.
  4. Breeding LC, Dixon DL. Accuracy of casts generated from dual arch impressions. J Prosth Dent. 2000;84(4):403-407.
  5. Cannistraci AJ. A new approach to impression taking for crown and bridge. Dent Clin North Am. 1965;3:73-99.
  6. Hoffman JM. Nontraumatic final impressions for fixed partial dentures. J Prosthodont. 1992;1:61-64.

Dr. Stern earned his dental degree from New York University and has had a private practice in general dentistry in Southfield, Mich, since 1983. He can be contacted at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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