Department of Prosthodontics, University of Hamburg, Hamburg, Germany
Braz Dent J (2000) 11(1): 19-27 ISSN 0103-6440
Introduction | Materials and Methods | Results | Discussion | Conclusions | Resumo | References
A high-grade steel model with different preparation angles of 3°, 7° and 11° in one- and two-step impression techniques were made for the determination of the influence of the preparation angle on the dimension loyalty of the resulting master casts. Using a 3-D-measuring instrument, analysis of the data showed that with the correction technique the model abutments led to a reduction in dependency on the preparation angle. The general reduction in stump height of the abutments was independent of the preparation angle. The double-mix technique showed decent increased abutments and is thus recommended for impressions of supragingival preparations.
Key Words: impression technique, one-step impression technique, two-step impression technique, polyvinylsiloxane.
Since the introduction of the impression technique in dentistry, exact working models for the fabrication of accurate restorations are of great importance because even minimal dimensional changes of the master models in relation to the preparation angle may lead to significant vertical discrepancies (Marxkors, 1985, Figure 1). Errors in the impressions may be caused by the dentist himself (Lim et al., 1992), by the impression material (Schwickerath, 1976), or by the impression technique (Lee, 1995), due to shrinkage during the polymerization reaction (Wichmann and Borchers, 1992), the building up of internal stress (Meiners, 1977; Petersen and Asmussen, 1991) as well as deformation of the impression tray (Lehmann, 1980; Shigheto, 1989).
In addition to exactness, an impression technique has to be practical to be used clinically. Based on the resulting hydrostatic pressure, the corrective impression technique has a wide application for subgingival preparations especially in Northern Europe (Schwindling, 1971), regardless of cast deformation resulting from the elastic recoil of the first phase (putty) material (Meiners, 1977). Many techniques were designed in the past by different researchers to overcome the problem of this elastic recoil of the first impression material. Schwindling (1971) suggested using linen strips as spacers on the preparation in the so-called supplementary impression technique before the first phase. Stähle (1967), on the other hand, recommended moving the loaded tray during impression taking with the first phase. However, these techniques resulted in unfavorable reduction of the hydrostatic pressure (Meiners, 1977).
Other authors recommend the use of the double-mix and the Sandwich technique for elastomeric impression materials, which were originally developed for hydrocolloid impression materials (Meiners, 1977). By doing so the advantages of the putty phase (high stability, low polymerization shrinkage) and light body (better flow properties especially for subgingival, difficult access areas) were combined. Contrary to the corrective impression technique, these impression techniques showed only minimal elastic recoil caused by elastic effects of the unset material. According to the literature, the one-phase impression technique with polyethers or the polyvinylsiloxanes lead to very accurate impressions (Fenske, 1998; Ishida, 1990).
These investigations concerning the accuracy of different impression techniques were based on a phantom model with almost parallel preparation angles. However, we know from our investigations in regard to the corrective impression technique that the preparation angle has an important influence on the dimension of the resulting casts (Sadat-Khonsari, 1999). On this basis, it was the aim of the present study to investigate the influence of one- and two-step impression techniques on the preparation angle, to check the validity of previous research.
Materials and Methods
The impressions were taken from a phantom model composed of 3 stainless steel cylinders prepared in 3°, 7° and 11° tapering angles (Figure 2). A 3-mm thick brass mold was used to take the impressions in a specially determined impression machine. A-Silicone Dimension H Penta (putty)/Garant L (light) (Fa. ESPE, Seefeld, Germany) with a shore hardness from A=58 for the putty material was used.
Using the corrective impression technique a preliminary impression was made with Dimension H. After setting, the second phase was loaded for 6 s with 100 N and allowed to set without pressure.
For the cutting-out technique, drain grooves were done by the insertion of standard brass profiles on the model during the preliminary impression. Subsequently, the second phase was applied after setting of the putty phase as described above.
For the supplementary technique, the first phase was relieved by a 1-mm thick spacer layer with a plastic sheet, the second phase was loaded with 100 N for 6 s and allowed to set without pressure.
The double-mix and single-step impression techniques were carried out by application of pressure (100 N) and the machine was allowed to reach an end position and then it was supported until the material was fully set. For the monophase impression technique Impregum Penta (Fa. ESPE, Seefeld, Germany) was used.
For each impression technique, 15 impressions were made from the original model using one technique at a time. Because the room temperature was 21°C, the setting time was doubled to insure sufficient polymerization of the materials. The impression trays were then removed and the resulting impression was examined macroscopically for the absence of air bubbles.
After a relaxing time of 30 min, the impressions were poured with super hard gypsum class IV (Fujirock, GC Dental Industries, Tokyo, Japan) which was mixed properly under vacuum and handled according to the manufacturer instructions. After the removal of the working models they were again checked macroscopically for the absence of air bubbles. The diameters of the resulting stumps on the working models were measured using a 3D-coordinate computer measuring machine (Zeiss, Type ZMC 550, Oberkochen, Germany) at 1 mm, 3 mm and 5 mm from the top of each stump. Measurements of the height of the stumps were also made.
The differences between the measurements for each single preparation angle and technique were averaged arithmetically. ANOVA and the Student-Norman-Keuls test for non-normal distributed samples were used for statistical evaluation.
Dimensional deviations of the corrective impression technique
Dimensional changes for the corrective impression technique are shown in Figure 3. The 3° tapering cone was found to have an average height deviation of 53 µm and a reduction of the average diameters ranging from 26 µm to 63 µm, which was the smallest model cast resulting from the corrective impression technique. The 7° tapering cone was found to have an average height reduction of 45 µm and the average diameters ranged from -15 µmto 17 µm. The 11° tapering cone showed 37 µm in height reduction and an enlargement of the diameters from 29 µm to 46 µm. The diameters of the middle range measurement for the 3° and 7° tapering preparations showed lower values than the upper and lower range measurements, which gave the corrective impression technique its characteristic sand glass appearance. Steeper preparation angles showed statistically significant smaller model casts for both the diameters and the heights.
Dimensional deviations of the cutting-out corrective impression technique
Dimensional changes for the cutting-out corrective impression technique are shown in Figure 4. The 3° tapering cone showed an average height deviation of 72 µm and a reduction of the average diameters ranging from 18 µm to 55 µm. The 7° tapering cone was found to have an average height reduction of 38 µm and the average diameters ranged from -14 µm to 10 µm. The 11° tapering cone showed 11 µm in height reduction and an enlargement of the diameters from 4 µm to 32 µm. Steeper preparation angles in relation to the diameter resulted in statistically significant smaller model casts. A difference of the cutting-out technique compared to the corrective technique was not found.
Dimensional deviations in the double-mix impression technique
The results of the double-mix technique are shown in Figure 5. All of the cones were slightly enlarged. For the 7o cone, the heights increased 9 µm, and for the 11o cone 6 µm. The diameters increased from 3 µm to 6 µm (3° cone), 6 µm to 12 µm (7° cone) and 3 µm to 6 µm (11° cone). Statistically significant differences in relation to the preparation angle were not found.
Dimensional deviations in the supplementary impression technique
The values of the supplementary impression technique are displayed in Figure 6. With this impression technique there was less dimensional variation compared to the corrective impression technique; the cast height was only enlarged by 2 µm to 6 µm. The average diameter varied only minimally from the original models: i.e. 3° cone range 4-9 µm, 7° cone range 2-11 µm and 11° cone range -7 µm to 10 µm. In this impression technique, neither the degree of steepness of the preparation nor the measuring distance from the top of the preparation were found to have any statistical significance.
Dimensional deviations of the monophasic impressions
The monophasic impressions (Figure 7) also had a very low enlargement of the diameters (3° cone: 15-21 µm, 7° cone: 15-22 µm, 11° cone: 25-32 µm). The heights showed an enlargement of 17 µm for the 7° cone and 32 µm for the 11° cone. The 3° cone was reduced -9 µm. Statistically significant differences in relation to the preparation angle were also not found.
The results of the corrective impression technique were in agreement with that reported earlier in the literature diminishing the resulting die casts (Mantovani, 1990; Gelbhard et al., 1994; Habib and Shehatz, 1995). However, we found that small tapering degrees resulted in pronounced dimensional deviation. With greater tapering degrees, the impressions with the corrective procedure showed increased dimensional accuracy. Hung et al. (1992), Idris et al. (1995) and Lee (1995) reported no significant differences for the corrective and double-mix techniques: however, the present study indicated that the preparation angle is an important factor for the result of investigations of impression techniques or materials. The dependence of the corrective impression technique on the preparation angle can be explained by the changed pressure conditions at the repositioning of the primary impression. With greater taperings, the repositioning of the primary impression causes increased hydrostatic pressure in the second phase only terminally because of the geometry, which leads to a cast reduction by the elastic recoil as reported by Sadat-Khonsari et al. (1999). A positive influence of the cutting-out technique on the dimensional accuracy of the corrective impression, as reported by Lehmann (1980), was not seen in our experiment. We could not find a superiority of the corrective technique compared to the double-mix, supplementary or monophasic impressions as reported by Penaflor et al. (1998) and Lee (1995). The other techniques showed more dimensional accuracy in the resulting working models with no dependence on the tapering degrees. Therefore, elastic recoil of the supplementary impression or elastic effects of the unset impression material in these techniques are less important. The slight enlargement of the die casts seems to be justified by the shrinkage of the impression material itself as reported in 1977 by Meiners.
The application of results of dental material investigations to clinical practice is questionable, because all parameters can never be simulated in an in vitro model. The almost pressureless impression techniques showed a sufficient exactness of reproduction for clinical application. Because of the possible time-saving factor, the use of monophasic or double-mix impression techniques is advantageous for impressions of at least supragingival preparations. To avoid the occurrence of vertical discrepancies, a thicker layer of die reliefing agent on the working model should be used (Mantovani, 1990), especially in clinically desirable parallel preparations, if the application of the correction technique cannot be avoided.
Um modelo de aço de alta qualidade com diferentes ângulos de preparação (3o, 7o e 11o) foi feito para determinar a influência da preparação do ângulo na estabilidade dimensional nos modelos mestres, durante moldagem de 1 e 2 fases. Usando um instrumento de medida tri-dimensional, a análise dos dados mostrou que com a técnica de correção, os suportes do modelo levaram a uma redução na dependência na preparação do ângulo. A redução geral na altura no coto dos modelos foi independente do ângulo de preparação. A técnica dupla-mista mostrou satisfatório aumento nos suportes e por isso é recomendada para modalgem de preparações supragengivais.
Unitermos: tecnica de moldagem, tecnica de moldagem de uma fase, tecnica de moldagem de duas fases, polivinilsiloxano.
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Correspondence: Dr. Christian Fenske, Department of Prosthodontics, University of Hamburg, Martinistraße 52, D-20246 Hamburg, Germany. Tel: +49-40-42803-2261. Fax: +49-40-42803-4096. E-mail: firstname.lastname@example.org
Accepted March 13, 2000
Eletronic publication July, 2000