Abstract
Background
The dimensional accuracy of elastomeric impression materials has been evaluated by different methods but their reliability is still questionable. The aim of this study was to evaluate the dimensional accuracy of elastomeric impression material using 3D laser scanner.
Method
In the present study, a metal die with its custom tray were designed. Using this die and tray, 10 impressions each were made from addition silicone (Aquasil LV; Dentsply), condensation silicone (Speedex coltene, Whaledent) and polyether (3M; ESPE). All the impressions were poured with Type IV die stone and total of 30 die replicas were obtained. These were scanned with a Picza 3D laser scanner (LPX 600, Roland,California) and the difference between the stone model was calculated by determining its volumetric changes using CAD-CAM pero version 2.0. One way analysis of variance (ANOVA) was used to compare the groups, whereas significance of mean difference between the groups was done by Tukey HSD.
Results
All the three groups showed mean decrease indicating a dimensional shrinkage from the master die. Mean percentage dimensional change in condensation silicone was maximum (-319.02 ±15.50) followed by polyether (-122.59 ± 0.64) and least in addition silicone (-23.83 ± 0.43). All the results were statistically significant (p<0.001).
Conclusion
Picza 3D laser scanner can precisely measure the volumetric changes in all the three elastomeric impression materials.
Keywords: Elastomeric, Silicone, Polymerisation, Shrinkage
Introduction
Elastomeric impression materials have been considered as the material of choice because they are dimensional stable and have excellent reproducibility.1,2 The advantages are their accuracy, dimensional stability and multiple dies can be reproduced from a single impression. Earlier studies evaluated the accuracy by measuring the linear dimensions of the elastomeric impression materials, however they neglect to account for the dimensional changes that occurred along a three-dimensional surface. All other manual measuring devices are easy to use and readily available but are more time-consuming and error can be incorporated due to operator fatigue. Pereira et al.10 used individual shells of acrylic resin coping for making impressions with different elastomeric impression materials. They evaluated the linear dimensional alterations in the resultant gypsum dies and concluded that addition silicone provides best stability followed by polyether, polysulfide and condensation silicone. The three-dimensional scanner used in this study not only provides precised measurements but reduces operator fatigue and are less time-consuming. The present study was conducted to assess the three dimensional volumetric changes of various elastomeric impression materials using a 3D laser scanner. The laser scanner is a step ahead in most accurate metric measurement.
Materials and methods
In the dental literature, several experiments were conducted to know the effect of material and impression technique on the cast accuracy. Some researchers concentrated on the impression techniques while others focused more on the materials but their reliability is still questionable, so the aim of this study was to evaluate the dimensional accuracy of elastomeric impression materials using 3D laser scanner. Institutional ethical clearance was obtained for this study. A die of standardised dimensions was prepared in brass metal. Details of dimensions of the brass die were diameter of upper circular area 10 mm, diameter of base circular area 15 mm, height of mold 15 mm for evaluation of an impression material (Fig. 1a). A cylindrical perforated tray was fabricated using copper metal which was used to maintain the same position for each impression on the master model (Fig. 1b). Ten molds were made from each of the three elastomeric impression materials. Before taking impressions the inner surface and the peripheral margins of the custom copper tray were painted with universal tray adhesive which was subsequently dried.
Fig. 1.
a: Metallic die made of brass. b: Custom tray for the die.
The impressions were made at first with addition silicone impression material (Aquasil LV; Dentsply) followed by condensation silicone impression material (Speedex coltene, Whaledent) using the putty wash technique. To compensate for the impression being made at room temperature (25 °C) instead of mouth temperature, the tray was held for 12 min which was double the time recommended by the manufacturer. Subsequently the materials were allowed to set. Furthermore, the polyether elastomeric impression material (elastomer; 3M; ESPE) was loaded in a syringe and injected from the top of the custom tray. As all the elastomeric impression material have the inherent property of rebound, the tray was held down to prevent any kind of distortion from lifting off the model. The impressions were kept for 12 min for complete polymerization.
The impressions obtained were preserved for 24 h at a room temperature of 25°C (Fig. 2a). High strength type IV gypsum (Kalabhai Karson, Mumbai, India) was selected for pouring of the impressions and was mixed in accordance with manufacturer's instructions with a water/powder ratio of 23 ml/100 gm. The process of mixing the die stone with water was first by the hand for 10 s to allow all the particles to be completely dissolved in the water and then mechanically for 20 s under vacuum to minimize any chances of void. Vibrator was used during pouring to get condensed and void-free model. The poured impressions were allowed to set for 1 h before being removed from the impression (Fig. 2b).
Fig. 2.
a: Impressions of addition silicone, condensation silicone and polyether (left to right). b: Die models of addition silicone, condensation silicone, and polyether (left to right).
For the purpose of testing the sample, Picza 3D LASER SCANNER (LPX-600) was used having maximum scanning area width of 254 mm and height of 406.4 mm. Wavelength of laser scanner was 645–660 nm with noncontacting laser sensor having spot beam triangulation in which maximum head rotation speed is 37 mm/s (Fig. 3a). All the molds were scanned with laser scanner. The scanning mode for different objects varies, that is object whose shape is close to sphere or cylinder plane scanning is used and that with little unevenness rotary scanning is used. The accuracy of the scanning is dependent on the parameters such as scanning pitch, scanning area, scanning surfaces and scanning angle. New polygons were created using all the scanning points and are called as ''polygon mesh''. To create a polygon mesh with a high degree of completion, the object was scanned at as fine as possible to increase the number of scanning points. The scanning data, polygon mesh data, line-scan data, and point-scan data were chosen and were export via DXF format.
Fig. 3.
a: Initial scanning of die under laser scanner. b: Initial scanned images of dies made up of brass, addition silicone, condensation silicone, and polyether (left to right).
The Picza 3D Laser Scanner (LPX 600) used in this study, processed the image into a 3D meshwork of the mold. The images of the mold were viewed with power mill viewer software. This was repeated for the master die and for all the model made from addition silicone, condensation silicone and polyether elastomeric impression material (Fig. 3b). The difference between the stone model was calculated by determining its volumetric changes using CAD-CAM pero version 2.0.
CAD-CAM pero version 2.0 is an arbitrary based 3D polymerization. During the sampling of the 3D model many points were generated which were actually representing the preparation line (Fig. 4a). All the points were progressively added to obtain a distinctly defined boundary. This procedure was carried out for each models from which milling path were calculated. The duplicated milled paths were first aligned to the virtual model followed by the polygonized surface they were originating from. The 3D volumetric differences of model were calculated using pero version 2.0 (Fig. 4b).
Fig. 4.
a: Final scanning of die under laser scanner. b: Final scanned images of dies made up of brass, addition silicone, condensation silicone and polyether (left to right).
Results
Table 1 shows the mean dimensional changes (absolute and proportional) in 3 different groups (addition silicone, condensation silicone and polyether). All the three groups showed mean change indicating shrinkage from master die. Mean change in condensation silicone (both absolute as well as proportional) was maximum (−319.62 ± 15.50) followed by polyether (−122.59 ± 0.64) and least in addition silicone (−23.83 ± 0.43).
Table 1.
Comparison of dimensional changes (absolute and proportional) in different groups and between group differences (Tukey HSD).
| SN | Group | Dimensional change (as compared with master die) |
|||||
|---|---|---|---|---|---|---|---|
| Absolute (mean ± SD) cubic units |
Proportional (mean ± SD) % |
||||||
| 1. | Addition silicone (n = 10) | −23.83 ± 0.43 | −0.31 ± 0.01 | ||||
| 2. | Condensation silicone (n = 10) | −319.62 ± 15.50 | −4.15 ± 0.20 | ||||
| 3. |
Polyether (n = 10) |
−122.59 ± 0.64 |
−1.59 ± 0.01 |
||||
|
Mean Diff |
SE |
“P” |
Mean Diff |
SE |
“P” |
||
| 4. | Addition silicone vs condensation silicone | 296 | 4.0 | <0.001 | 3.84 | 0.05 | <0.001 |
| 5. | Addition silicone vs polyether | 98.8 | 4.0 | <0.001 | 1.28 | 0.05 | <0.001 |
| 6. | Condensation silicone vs polyether | 197 | 4.0 | <0.001 | 2.56 | 0.05 | <0.001 |
F = 2824.18; p < 0.001 (ANOVA).
Between the groups, addition and condensation silicone showed a maximum mean difference of 3.84% followed by condensation silicone and polyether (2.56) and addition silicone and polyether (1.28) (Table 1). The results obtained were statistically significant (p < 0.001).
On the basis of above evaluation, the following order of dimensional change was observed in different groups:
| Addition silicone < polyether < condensation silicone. |
Discussion
The continued polymerization of elastomeric impressions could be a reason for distortion leading to shrinkage. A die was made to compare the dimensional accuracy of the elastomeric impression materials. Impression was taken on the die with different elastomeric impression material (addition silicone, condensation silicone, and polyether) and was poured with type IV die stone.
In case of polyvinylsiloxane impression after 24 h, change in dimension of the stone cast could be due to polymerization shrinkage. Studies have shown that there are stresses generated by retrieval of multiple pours from elastomeric impressions. Clancy et al.3 analyzed the dimensional stability of 3 materials (polyvinylsiloxane, polydimethylsiloxane, and a polyether) at 8 time intervals up to 4 weeks after the impression was taken, and concluded that the impressions of all the materials that are immediately emptied have greater dimensional accuracy, and that after 4 weeks, addition silicone maintained the best surface detail and had very small dimensional changes.
Craig4 stated that the selection of technique was the key factor. His study specifically focused on the effect of one step and two step putty/wash impression technique on accuracy differences with President addition silicone material. Hung et al.5 conducted studies by using a variety of addition type silicone impression materials to verify whether the effects of technique or choice of material determines the accuracy and concluded that the selection of the material is important with the addition silicone materials for the accuracy.
Purk et al.6 conducted a study by subjecting the addition silicone and a polyether impression materials to −10 °C, 24 °C and 66 °C temperature and analysed the effect of temperature on dimensional accuracy of the materials. He concluded that after the thermal treatment, the dimensional changes of addition silicone was greater than that of polyether and these dimensional changes could have lot of clinical implications.
Purk et al,6 Marcinak et al.7 Chen et al.8 Lapria et al.9 all evaluated the dimensional accuracy of elastomers with different time intervals not greater than 24 h and found that time limitation may be related to the possible dimensional changes in the viscoelastic materials over a long period.
Pereira et al.10 used the resin coping impression technique to determine the linear dimensional alterations in gypsum dies. In their study, stainless steel master cast with two prepared abutment teeth were fabricated and the impressions were taken with polyether, mercaptan-polysulfide, addition silicone, and condensation silicone impression materials. Two identification points were selected in the gypsum dies and the distances between the points were calibrated using an optical microscope. The results obtained were statistically analyzed by ANOVA (p < 0.05) and Tukey's test. They concluded that the addition silicone gave the best accuracy among all the elastomers followed by polyether, polysulfide, and condensation silicone impression materials.
Kumar et al.11 compared various elastomeric impression materials in terms of accuracy and dimensional stability from a single elastomeric impression at various time of pour. Three elastomeric impression materials were chosen for the study. Each impression was poured at various time periods and the resultant models were evaluated under travelling microscope. They concluded that all the elastomeric impression materials showed a consistent behavior up to the fourth pour but polyether showed lesser ability than both the addition and condensation silicone to recover from induced deformation.
Garrofe et al.12 evaluated the linear dimensional stability of different elastomeric impression materials over time. Three impression were taken with each of the following polyvinylsiloxane: Examix-GC-(AdPa), Aquasil-Dentsply-(AdAq) and Panasil-Kettenbach-(AdPa), and three impressions with each of the polydimethylsiloxane: Densell-Dental Medrano-(CoDe), Sppedex-Coltene-(CoSp) and Lastic-Kettenbach-(CoLa). All impression were made with one step technique using putty and light body materials. Using an “ad-hoc” device standardized digital photographs were taken at different time intervals of 0, 15, 30, 60,120 min; 24 h; 7 and 14 days. The photographs were then analyzed using the software. The study concluded that the lineal dimensional stability of elastomeric impression materials would be significantly affected by time.
Markovic et al.13 conducted a study on two elastomers, the addition and condensation—cured silicones, for their dimensional stability over a period. Two cylinders with the spherical top which made of stainless steel were fabricated. Individual custom tray was constructed with the acrylic resin. By using Carl Zeiss Coordinate measuring machine (Contura G2) the master model and dental stone replica models were scanned having a volumetric probing tolerance of 1 μm. Calypso software was used for processing the data. They concluded that the dimensional variations were significantly greater in case of condensation cured silicone in comparison with the addition cured silicone impression material.
Much of the existing literature studying impression materials focuses on evaluating the influence of the following variables: effect of time, relationship between percentage of filling-dimensional stability, use of some mechanism of adhesion of the material to the trays, different techniques, different materials, and type of tray used.
Several studies have been conducted till date to compare the dimensional stability and accuracy of different elastomeric impression materials. Most of them used devices measuring along two-dimensional surfaces, which do not take into account for the dimensional variations that exist along a three-dimensional surface. All other manual measuring devices are easy to operate and readily available but errors can occur due to operator fatigue and the operational use of these devices are more time consuming. In addition to that they predict only the linear dimensional changes of elastomeric impression materials. For a successful fixed partial denture, an ideal mutual orientation of the maxillary arch in relation to the mandibular cast is mandatory which requires impression materials that reproduce the prepared tooth surfaces with accuracy. Elastomeric impression materials polymerized from monomers to macromolecules resulted with a substantial loss of spatial volume and should be described in a 3-D manner. As the 3D laser scanner evaluate an object three-dimensionally, impression materials with less volumetric shrinkage might result in more accurate simulations of mounted casts. Because the 3-D laser scanner take the complex anatomy into consideration, it becomes more clinically relevant than the previously used one- or two-dimensional approaches.
The authors focused on the dimensional accuracy of elastomeric impression material which was recorded at same interval of time using 3D laser scanner and only attempted to analyze the effect regarding the impression. The LPX-600 3D laser scanner can digitize objects with a dimension of 25 cm × 40 cm. The data generated by the LPX-600 are highly precised and can be utilized in multiple CAD/CAM applications. With the help of EZ Studio–software the LPX-600-scanner has the ability to convert the 3D models into a pixel cloud, which in turn can be used in various CAD applications. PICZA 3D laser scanner has Rotary and Plane Scan which can scan almost any surface and shape. In “Rotary”-scan mode, the LPX-600 scans the objects in one shot whereas the object is scanned from six different angles in “Plane Scan”-mode. Therefore, it is advisable to use it for scanning more complex shapes, straight flanks and notches on an object. Usage of the LPX-600 scanner avoids scratches and other types of damage as it uses a laser for scanning. The properties of elastomeric impression material in the clinical situation still differ from the laboratory testing conditions. Further investigations should incorporate more closely simulated clinical conditions.
Conclusion
ADA specification no. 19 recommends that, a maximum negative alteration in dimension after a minimum of 24 h should be 0.50%.14 The mean three-dimensional volumetric shrinkage of condensation silicone was found to be maximum (−319.62 ± 15.50) and least for addition silicone (-23.83 ± 0.43). Comparison between the groups showed that addition and condensation silicone showed a maximum mean difference of 3.84% and minimum for addition silicone and polyether (1.28%) and were found to be statistically significant. The present study evaluated the dimensional accuracy of elastomeric impression material in three dimension giving volumetric changes of all the three elastomeric impression material as compared with previous studies in which only linear dimensional accuracy were analyzed.
Disclosure of competing interest
The authors have none to declare.
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