| Abstract|| |
Background and Objectives: A limitation of vinyl polysiloxane (VPS) impression materials is hydrophobicity, and manufacturers have added surfactants and labeled these new products as "hydrophilic." The purpose of this investigation was to evaluate and compare the dimensional accuracy and surface detail reproduction of two hydrophilic VPS impression materials under dry, moist, and wet conditions.
Materials and Methods: Ten impressions were made under dry, moist, and wet conditions respectively, with monophase, and regular body VPS impression material using a stainless steel metal die similar to that described in American Dental Association (ADA) specification 19, with lines scribed on it. Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression to the same line on the metal die, by using a measuring microscope. The surface detail was evaluated. A one-way analysis of variance and Student t-test were used to compare mean dimensional changes (α = 0.05).
Results: Conditions (dry, moist, and wet) did not cause significant adverse effects on the dimensional accuracy of either material. The mean dimensional changes were 0.00084% (+0.00041%) for monophase and 0.00119% (+0.00033%) for regular body. Monophase material was satisfactory in detail reproduction 100% of the time in dry conditions, 90% in moist, and only 20% in wet conditions. The regular body showed 100% satisfactory impressions in dry, 80% in moist, and 10% in wet conditions. With the additional smooth surface evaluation, only under dry conditions impressions with clinically acceptable surface quality were produced.
Conclusions: Dimensional changes for both materials were well within ADA standards of minimal shrinkage value of 0.5%.
Keywords: Dimensional accuracy, detail reproduction, moisture control, polyvinyl siloxane
|How to cite this article:|
Katyayan PA, Kalavathy N, Katyayan M. Dimensional accuracy and detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under different conditions. Indian J Dent Res 2011;22:881-2
There are two aspects of hydrophobic nature of vinyl polysiloxane (VPS) impression materials. The first aspect relates to the surface energy of the unpolymerized liquid phase of the impression material, and the lack of its ability to wet oral tissues during impression making.  The second aspect relates to the surface energy of the solid, polymerized VPS, and the high contact angle that typically forms when the VPS impressions are wetted with dental gypsum materials. ,
|How to cite this URL:|
Katyayan PA, Kalavathy N, Katyayan M. Dimensional accuracy and detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under different conditions. Indian J Dent Res [serial online] 2011 [cited 2013 May 22];22:881-2. Available from: http://www.ijdr.in/text.asp?2011/22/6/881/94697
To overcome the limitation of hydrophobicity, manufacturers have added surfactants (nonylphenoxy polyethanol homologues) to reduce the contact angle and improve the wettability.  These hydrophilic VPS materials have exhibited increased wettability of the polymerized impressions with gypsum slurries. However, when hydrophilic vinyl polysiloxane impression material was used clinically in the presence of saliva, blood, water, or crevicular fluid, decreased accuracy of the produced impression was reported. This suggests that the hydrophilic additives may not enhance the ability of unpolymerized VPS to wet the oral tissues under partial or complete moisture conditions, which affects the accuracy and detail reproduction of the impression.  This necessitates the evaluation of these two parameters under dry, moist, and wet conditions.
| Materials and Methods|| |
A standardized stainless steel die (similar to that described in ADA specification 19), scored with three horizontal lines [Figure 1], was used for impression making.  The horizontal lines were named A, B, and C. The width of each horizontal line was 0.016 mm. Two cross points at the intersection of the vertical lines with the horizontal line B were labeled as X and Y and served as the beginning and end points of measurements for dimensional accuracy. The die consisted of three components: the test block, the mold and the metal riser [Figure 2].
Before impression making, the die was ultrasonically cleaned to remove any residue and allowed to air dry. Care was taken to avoid contamination of the surface of the die before making impressions. Impressions were made using an automixing impression gun (GC America Inc., Alsip, Illinois) and prepacked cartridges of the impression material. Latex gloves were not worn during material application because of their potential inhibitory effect on polymerization of VPS materials. The cartridge was used in compliance with the manufacturer's recommendations to ensure proper dispensing ratios.
For impressions made under dry conditions, the material was loaded into a fine tipped impression syringe (GC America Inc., Alsip, Illinois) and applied to the lined areas of the die in a zigzag pattern. The material was pushed ahead of the syringe tip.
The mold was then placed onto the test block to contain the material and to ensure a uniform thickness of 3 mm of impression. A cellophane sheet was placed on the impression and a metal plate was pressed over this impression so that the excess material got extruded out. The cellophane acted as a separator for easy removal from the metal plate. The plate and die were held together using a clamp [Figure 3].
This assembly was placed in a water bath in a glass beaker filled with distilled water. The bath was maintained at 32 + 2°C in order to simulate oral conditions (open mouth temperature) in which the impression material would polymerize in an aqueous environment. The mean room temperature was maintained at 22°C. The above procedure was carried out for 10 samples each, of both the monophase and the regular body consistency materials, respectively.
For the impressions made under moist conditions, a fine mist of water (32 + 2°C) from a spray bottle was applied to the surface of the die before the impression material was syringed onto the die surface [Figure 4]. Care was taken to ensure that the entire die surface was covered with a uniform mist of water, avoiding any excess or beading. The same procedure as described above was followed to make the impressions and was done for 10 samples each, for both the monophase and the regular body consistency materials, respectively.
|Figure 4: Mist of water on the die surface before impression making under moist conditions|
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For making the impressions under wet conditions, the metal die was immersed in a water bath before the application of the impression material. With the tip of the syringe immersed under water, the material was injected onto the surface of the die, following the procedure described previously, and was done for 10 samples each, for both the monophase and the regular body consistency materials, respectively [Figure 5].
|Figure 5: Application of impression material onto lined areas of die immersed in water under wet conditions|
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The impressions were allowed to set 3 minutes longer than the manufacturer's recommended minimal removal time, as indicated in ADA specification 19 for laboratory testing. The mold and test block were then separated. The impression was pressed out of the mold using the riser. The side opposite to the reference marks was dusted with talcum powder and transferred impression side up onto a flat plate also dusted with talcum powder.
Ten impressions of each material were made under each of the three conditions: dry, moist, and wet. Dimensional accuracy was evaluated 24 hours after making each impression. A single investigator measures the length of line B between cross points X and Y for each impression. This measurement was made three times to the nearest 0.001 mm at original magnification ×10 using a measuring microscope [Figure 6]. The three measured lengths were averaged and compared with the instrument of line B on the metal die used to make the impression. The percent change from the metal die was computed.
|Figure 6: Evaluation of dimensional accuracy under a measuring microscope|
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Two independent examiners also evaluated the surface detail reproduction. Surface detail reproduction was evaluated immediately after the impressions were recovered from the die. Evaluation was achieved using two methods. The first evaluation was an assessment of the continuity of line replication according to the ADA specification 19 with a slight modification. Rather than only evaluating the continuity of one of the three horizontal lines in two out of three specimens, all three lines were assessed for each specimen. If at least two of the three horizontal lines were reproduced continuously between the cross points, the impression was considered satisfactory. All others were rated unsatisfactory. This modification was made to attain the power analysis parameters and maintain a manageable sample size. ,
An additional evaluation of the impression was necessary to evaluate the surface characteristics such as roughness, pits, or voids on other areas of the impression. These imperfections, if located in critical areas like finish lines in a clinical impression, would render the impression unacceptable. For this macroscopic evaluation, impressions were rated satisfactory if the entire impression surface was smooth, shiny, and free of voids or pits; and impressions were rated unsatisfactory if the impression surface was rough or contained any voids or pits.
Method of statistical analysis
A one-way analysis of variance and Student t-test were used to compare mean dimensional changes (α = 0.05)
| Results|| |
Statistical analyses revealed [Table 1] that there was no statistically significant difference between materials for the moist and wet conditions (P>0.05), but there was a statistically significant difference between materials under the dry condition, indicating that only in dry atmosphere, the monophase consistency shows lesser dimensional change than the regular body consistency [Table 2].
|Table 1: Comparison between the two materials, of the mean dimensional change under similar conditions|
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When the dimensional changes in the three conditions within a given material were compared, though dry conditions gave a lesser dimensional change value followed by moist and then wet conditions, this difference was not statistically significant (P>0.005) [Table 3]. Also, the mean values showed that both the materials in all conditions performed well within the standards of ADA specification 19, which states that acceptable dimensional change should be well below 0.5% [Table 4].
|Table 3: Comparison between the three conditions, of the mean dimensional change for each of the materials tested|
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The three conditions (dry, moist, and wet) have a significant effects on surface detail reproduction evaluated on the basis of line continuity [Table 5]. For the regular body material, under dry conditions, 100% of the impressions were satisfactory, while 80% were satisfactory under moist conditions and only 10% were satisfactory under wet conditions. For the monophase material, under dry conditions, 100% of the impressions were satisfactory, while 90% were satisfactory under moist conditions and only 20% under wet conditions.
|Table 5: Percentage of satisfactory and unsatisfactory impressions for acceptable surface detail reproduction|
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For additional smooth surface evaluation (based on the presence of voids or pits on the impression surface) the results indicated that for the monophase material, under dry conditions, 100% of the impressions were satisfactory, while 20% were satisfactory under moist conditions and none were satisfactory under wet conditions. For the regular bodied material, under dry conditions, 100% of the impressions were satisfactory, while 10% were satisfactory under moist conditions and none were satisfactory under wet conditions [Table 6].
|Table 6: Percentage of satisfactory and unsatisfactory impressions for additional smooth surface evaluation|
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| Discussion|| |
Earlier studies have indicated that VPS materials have demonstrated very good dimensional accuracy, ranking next to only polyether materials. ,,,,,,,,,, It has also been shown that under dry conditions, hydrophilic VPS, and conventional VPS have comparable dimensional accuracies.  This study evaluated the dimensional accuracy of the hydrophilic VPS not only in dry, but also in moist and wet condition. The results of this study were in agreement with similar investigations done earlier, to prove that the dimensional accuracies of the hydrophilic VPS materials were not adversely affected by the presence of moisture. ,, ADA specification 19 criteria states that elastomeric impression materials should not display a more than 0.5% dimensional change after 24 hours of polymerization of the material.  Both materials used in this study were well within these standards, displaying mean dimensional changes of 0.00084% (+0.00041%) for monophase and 0.00119% (+0.00033%) for regular body material. On comparing materials, only under dry conditions, monophase material performed significantly better than regular body material. There was no significant difference between conditions for each of the materials tested, implying that moisture does not affect its accuracy.
In addition to the measurement of dimensional accuracy, this study also examined the detail reproduction of the hydrophilic VPS impression materials. Clinically, several impression material investigations have concentrated on the replication of the finish line of a wet tooth preparation or gingival sulcus reproduction in the presence of crevicular moisture. , These studies have reported conflicting results regarding the ability of VPS impression materials to obtain complete impressions in the presence of moisture. Some investigations reported that hydrophilic VPS impression materials when used on moist or wet dentin surfaces did not always produce acceptable impressions. ,,, Others have found that even though there appeared to be differences in the contact angle formed between different VPS impression materials and moist tooth surfaces, the hydrophilic VPS always obtained complete impressions.  The results of this investigation disagree with the latter finding. The two impression materials used in this study did not always yield satisfactory impressions under moist and wet conditions.
In some impressions, there were areas of pits, voids, and roughness associated with the three horizontal lines used for the ADA detail reproduction evaluation. If such pits or voids were located in the preparation margin, the impression would be unacceptable. Therefore, an additional macroscopic evaluation of the detail reproduction of the smooth surface of the impression was also included in the study.  The results of this additional evaluation were not consistent with the results of the detail reproduction based on the continuous replication of lines. This suggests that an additional evaluation of the smoothness of the impression may be beneficial. The results obtained from this additional evaluation suggested that a dry field is necessary to produce a clinically acceptable impression. Both materials produced the greatest number of smooth and shiny impressions under the dry condition, and both failed to produce a smooth and shiny surface under moist and wet conditions.
The results of this investigation appear to reinforce the idea suggested by Chee et al.,  that the so-called hydrophilic impression materials remain hydrophobic in the unpolymerized state and will not adequately wet surfaces with moisture. Although additive surfactants may have improved the polymerized VPS material's wettability with gypsum materials, , it appears that the impression material still cannot accurately produce the surface detail in the presence of moisture. These results indicate that during use of these materials, one should maintain strict moisture control during impression making.
The results of this in vitro investigation should be viewed cautiously because laboratory testing cannot actually model clinical situations. In this investigation, impressions were made of standardized stainless steel dies. Although the metal dies are calibrated surfaces for the comparisons, they do not resemble the behavior of oral tissues. For example, metal dies do not absorb fluids. In addition, the intrinsic surface-free energy of a metal die will be much higher than the surface-free energy of the proteinaceous surfaces of the prepared teeth and soft tissues. This surface energy of the impressed surface will also affect how well the impression material will wet that surface. Another limitation of this in vitro study is that water instead of saliva was used as the source of moisture. It is well known that the properties of saliva are quite different from those of water, and these differences could potentially have affected the behavior of impression materials.
The main focus of this study was to evaluate the ability of the hydrophilic VPS impression material to perform against wet surfaces. This study helps us to understand the limitation of hydrophilic VPS impression material when used to record the surface of wet oral substrates. Although the moist surface method used in this study may appear more clinically relevant, the wet surface method, in which the die was placed in water and then impression was made, was included to account for a very wet substrate. For wet conditions, it was intended to produce a surface that was completely coated with water. This is in contrast to the oral tissues where there is water at the surface, as well as water within the bulk of the tissue. Water within the bulk can diffuse to the surface during the recording of the impression. It would be very difficult to duplicate this type of moisture contamination in the laboratory, but it does indicate that there are other sources of water present in the mouth that could interfere with the recording of the impressions. ,
The experimental method used in this study should be considered as a preliminary testing of the accuracy and behavior of the hydrophilic impression materials. Other investigations are necessary to assess how the properties of the material are affected by the presence of saliva or moisture in the oral cavity.
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Preeti Agarwal Katyayan
Department of Prosthodontics, Government Dental College & Hospital, Ahmedabad, Gujarat
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]