Implantology is now an established discipline in dentistry. It is beyond the experimental stage so that risks can now be calculated and a prognosis made.
Standardised impression techniques are now also used in implantology. There are other important aspects to be considered in comparison with taking impressions of prepared teeth.
It is particularly important to fix the three-dimensional implant position as accurately as possible and transfer the position precisely to the working model, as implants are osseointegrated and, unlike natural teeth, they do not move at all. On the other hand, the implant surface and margin do not have to be reproduced exactly as required with the preparation margin of a natural tooth, as most modern implant systems use prefabricated precision components(12).
The type of implant system selected should have undercut transfer abutments for the impression that are secured exactly in position on the implant with a retention screw. This retention screw is taken out before removal of the impression from the oral cavity so that the transfer abutment can remain in the impression. This pick-up technique therefore requires a custom tray11 that is perforated at the transfer abutment areas.
The screws project from the abutments and the perforated custom resin tray. This allows the transfer abutments to be secured stably in position before taking the impression and easily removed after the impression is taken. Clinical studies also confirm the high accuracy of the pick-up technique for transferring the threedimensional implant position to the situation on the model.(1, 8, 15)
The following one-step impression procedures can always be used for this technique:
• Single phase or monophase technique
• Heavy-body wash technique.
The two impression techniques only differ in the use of the impression material (single and dual phase). The heavy-body wash technique can be regarded as a modified monophase technique.
With the monophase technique the impression is taken in one working stage (one-step monophase technique) using one material (same viscosity of tray and application syringe material). The disadvantage of this technique, however, is that it produces ‘flow folds’ in the impression(7).
With the heavy-body wash technique the impression is also taken in one step (one-step dual phase technique) using two materials with different viscosities (high viscosity tray material and low viscosity application syringe material). The higher viscosity of the tray material counteracts the formation of flow folds and contributes greatly to successful implant impressions(13).
In implant prosthetics, the heavy-body wash technique does not require any more time than the monophase impression technique, so it is also an alternative in this respect. This will be clearly demonstrated using the following clinical example.
Clinical case
In collaboration with the Department of Oral and Maxillofacial Surgery, an implant-borne prosthetic restoration was fabricated on three Xive implants (Dentsply Friadent) in the 45, 46 and 47 region for a 65-year-old female patient with a unilaterally reduced dentition in quadrant IV (Figure 1). Figures 2 and 3 show the situation after a healing period of approximately six months and exposure of the implants at the beginning of prosthetic treatment. First of all, study model impressions were taken of the upper and lower jaw with alginate (Palgat Plus, 3M Espe). The lower model was used for fabricating a custom impression tray in a resin material (Palatray, Heraeus Kulzer) in the dental laboratory and the upper model was used as the antagonist.
At the second appointment the gingiva formers were first replaced with transfer abutments (Figures 4 and 5). The heavy-body wash technique was used to take an impression of the implants and other anatomical structures relevant for the restoration using the custom impression tray and an A-silicone (Aquasil Ultra Heavy in combination with Aquasil Ultra XLV, Dentsply).
The custom impression tray was loaded first with the high viscosity impression material (Aquasil Ultra Heavy) (Figure 6). The low viscosity material (Aquasil Ultra XLV) was then added to the tray material as the second step (Figure 7) and at the same time this material was syringed intra-orally round the transfer abutments (Figure 8). The loaded impression tray was then inserted into the patient’s mouth (Figure 9).
After the impression material had cured, the retention screws were loosened and the impression removed carefully from the mouth. Finally, the impression was checked visually to ensure all the relevant structures had been perfectly reproduced (Figure 10).
At the crown framework try-in stage (Figures 11 and 12), a laboratory fabricated resin index was used to check that the model situation and oral situation corresponded. At the final appointment the finished crowns were tried in and then fitted using a torque wrench (Figures 13 and 14).
Discussion
Nowadays polyethers, which have high adhesive and hydrophilic properties, are often used when taking impressions of implants. The adhesive properties contribute to the secure fixation of the impression abutments in the impression; the hydrophilic properties increase the flow of the material onto the structures to be reproduced.
With large implant restorations (e.g. splinted crowns, bridges) there are advantages in using A-silicone as the impression material, as this class of material tends to produce more accurate impression results in vitro than a polyether15 and is more manipulative in the production process due to its high shore hardness. A-silicones do, however, have a greater tendency to form flow folds when taking monophase impressions.
This risk is reduced if a one-step dual phase impression (heavy-body tray material in conjunction with a light-body syringe material) is used instead of a monophase impression. (6, 13) Tests in our department have demonstrated that the heavy-body wash technique is ideal as a practical impression taking technique in implant prosthetics.
The consistently excellent impression results produced with the heavy-body wash technique or one-step putty wash technique when taking impressions in crown and bridge prosthetics (2, 3, 4, 5, 7, 9, 10, 14) can therefore also be achieved in implant prosthetics.
In summary, the one-step heavy-body wash technique, in conjunction with a pick-up technique, was found to be a proven impression technique in implantology; the illustrated case history based on the use of Aquasil Ultra Heavy combined with Aquasil Ultra XLV provides an excellent example of the technique.
References
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11. Wirz, J.: Die Bedeutung des individuellen Abformlöffels. Quintessenz 1998; 24: 907-915
12. Wöstmann, B: Die Abformung in der Implantologie. ZM Fachinfos, Chirurgische ZHK, Implantologie, 2000
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14. Wöstmann, B., Höing, M., Ferger, P.: Vergleich von hand- und maschinengemischten Abformmassen (Pentamix-System). Dtsch Zahnärztl Z 1998; 53: 753-756
15. Wöstmann, B., Hassfurth, U., Balkenhol, M., Ferger, P.: Influence of Impression Technique and Material on the Transfer Accuracy of the Implant Position onto the Working Cast. J Dent Res 2003; 82: B-392.
