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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 6  |  Issue : 2  |  Page : 78-84

Reliability of multiple pour between two classes of polyvinyl siloxane impression materials: A preliminary stereomicroscopic study


Department of Prosthetic Dental Sciences, College of Dentistry, Jazan University, Jazan, Saudi Arabia

Date of Submission07-Apr-2021
Date of Acceptance27-Oct-2021
Date of Web Publication12-Jan-2022

Correspondence Address:
Mohammed E Sayed
Department of Prosthetic Dental Sciences, College of Dentistry, Jazan University, Jazan
Saudi Arabia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/KKUJHS.KKUJHS_16_21

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  Abstract 


Objective: To evaluate the accuracy and dimensional stability of casts made from regular set alginate alternative (AA) and monophase additional silicone (MAS) impression materials following multiple pours. Materials and Methods: A hard typodont master cast with index holes was used. A total of 20 impressions were made using a modified perforated metal tray with 10 impressions of each material. Each impression was poured four times with type IV die stone with a total of 80 stone casts (40 for each material). The horizontal anterior–posterior (HAP), horizontal cross-arch (HCA), and vertical distances were measured in the recovered stone casts using a stereomicroscope with an accuracy of 0.001 mm. The obtained data were statistically analyzed using two-way analysis of variance and Tukey's HSD post hoc test (P < 0.05). Results: The dimensions on the typodont master cast and stone casts were significantly different (P < 0.001), except for the first pour from MAS (P > 0.05). There was a statistically significant difference in the dimensions of the stone casts made from the AA and MAS impression materials among the four pour groups (P < 0.001), with exception of MAS-based casts between first pour and second pour groups at HAP and HCA dimensions. Pairwise comparisons showed statistically significant difference between the two impression materials at all measurement locations, with only one exception of HAP measurements for second-poured casts. Conclusion: A clinically acceptable casts can be made by pouring AA and MAS impressions twice. In addition, the third and fourth pours were accurate in the AA group when compared to MAS group. Clinical Significance: According the knowledge of the authors, this study is the first to evaluate the predictability of re-pouring alginate alternative impression material and compare to a standard polyvinyl siloxane impression material such as monophase. Therefore, the statement of clinical significance in the abstract will help the dentists to realize the potential uses for alginate alternative impression material and recommendations of this study.

Keywords: Alginate alternative, dimensional stability, monophase additional silicone, multiple pour


How to cite this article:
Sayed ME, Abdullah AL-Makramani BM, Al-Sanabani FA, Shaabi FI. Reliability of multiple pour between two classes of polyvinyl siloxane impression materials: A preliminary stereomicroscopic study. King Khalid Univ J Health Sci 2021;6:78-84

How to cite this URL:
Sayed ME, Abdullah AL-Makramani BM, Al-Sanabani FA, Shaabi FI. Reliability of multiple pour between two classes of polyvinyl siloxane impression materials: A preliminary stereomicroscopic study. King Khalid Univ J Health Sci [serial online] 2021 [cited 2022 Dec 8];6:78-84. Available from: https://www.kkujhs.org/text.asp?2021/6/2/78/335628




  Introduction Top


Making an accurate impression is an important step in the fabrication and fitting of dental prosthesis. The accuracy and passive fit of dental prostheses is essential for the long-term performance of the restoration and health of oral soft tissues. The final impression should be accurate and dimensionally stable to fabricate restorations with perfect marginal integrity, internal fit, and precise occlusal and interproximal contacts.[1],[2],[3]

Dental impression is a record or a replica of the mouth tissues taken at an unstrained rest position or in several positions of displacement.[4] Excellent teeth preparation is worthless unless its details are accurately captured and transferred to the dental laboratory. An accurate working cast enables a dental technician to construct perfect restorations. It is well known that an inaccurate and poorly fitted dental prosthesis leads to compromised esthetics and eventually to biological failure of the dental prosthesis.[5],[6]

Due to high development in dental materials, the dentists have a range of impression materials to choose from to fit the intended clinical applications. The impression materials are entirely selected at the choice of the dentist based on skill and personal preference.[7],[8] Several types of impression materials are available for crowns and fixed dental prosthesis impressions including silicones, polyether, polysulfide, and reversible and irreversible hydrocolloid impression materials. The polyvinyl siloxane (PVS, addition-type) silicone impression material is the most common material in prosthodontics field, owing to its ease of handling, high level of dentist and patient acceptance, and reliability. PVS impression materials have precise surface reproduction, superior accuracy, dimensional stability, a relatively short setting time, low creep, elastic recovery from undercuts, and moderate-to-high tear resistance.[3],[9],[10],[11],[12],[13]

Alginate, irreversible hydrocolloid, is an elastic impression material which has many advantages including hydrophilicity, pleasing taste and odor, nonstaining, low-cost, ease of handling, and less affected by the presence of saliva. The generated casts from alginate impression can be used to for several dental procedures such as preparing diagnostic casts, interim crowns and bridges, orthodontic study models, night guards, bleaching trays, and fabrication of removable prosthesis. However, the alginate main limitations are its low dimensional stability and low tear resistance.[14],[15],[16] Recently, a new brand of modified silicone impression material known as alginate alternative (AA) has been developed with claims of manufacturers that they have outstanding dimensional stability, exceptional elastic recovery, and remarkable tear resistance and can be re-poured.

The accuracy of stone casts generated from repeated pour of a single impression is of great importance for both dentists and dental technicians. It helps reduce the clinical time, patient inconvenience, and extra cost. The duplicate casts can be used for diagnostic purposes or as working or refractory casts so that the master cast remains intact. During the process of working casts and dies preparation, the sectioning of the cast and defining the margin by ditching may cause changes in the inter-abutments distance and loss of gingival reference. Therefore, another intact duplicate cast made from the same impression will be helpful to verify the fit and contour of the finished restoration.[1],[5],[17],[18] The dimensions of a cast from the second pour can be affected by continuing polymerization of the impression material and/or by distortion of the impression during removal of the first cast.[19]

The maintenance of dimensional accuracy of the impression material that undergoes multiple pouring is of clinical importance because the dentist often wants to have two or more casts of the same impression and to avoid problems associated with repeating the impression. To the best of our knowledge, this study is the first to evaluate the predictability of re-pouring AA impression material and compare to a standard PVS impression material such as monophase. The purpose of this study is to compare between AA and monophase additional silicone (MAS) impression materials in terms of accuracy, dimensional stability, and ability of each of them to produce clinically acceptable multiple casts from single impression. This study examined the following null hypotheses:

  1. There will be no statistically significant differences in the accuracy between casts made by the two impression materials and the typodont master model at each of the three locations (horizontal anteroposterior [HAP], horizontal cross-arch [HCA], and vertical [V]) for all four pours
  2. There will be no statistically significant differences between AA-generated casts at each of the three locations for all four pours; neither will there be any statistically significant differences between MAS-generated casts at each of the three locations for all four pours
  3. There will be no statistically significant differences in dimensions measured at each of the three locations between the casts generated from the AA and MAS impression materials.



  Materials and Methods Top


A mandibular typodont hard model (Model A-3, Frasaco GmbH, Tettnang, Germany) was used in this study and modified by acrylic resin material around buccal and lingual vestibules to create 2 mm deep grooves for inserting the metal tray to be used as a reference and vertical stop during impressions taking.

Five index holes were made on the typodont model, i.e., 3 for the horizontal measurements (HAP, HCA) and 2 for the vertical (V) measurement [Figure 1]. Two types of impression materials were used in this study; AA (Maxill, Cortland, OH, USA) and MAS (Maxill, Cortland, OH, USA) impression materials. A marking on the outer side of the impression cartridge was used to make sure that an equal amount of impression material was used for all impressions. A manual impression gun was used for dispensing the material, and compatible mixing tips were used following manufacturer's instruction. Tray adhesive (3M ESPE, Seefeld, Germany) was applied over the tray and allowed to dry for 10 min before the impression procedure. Ten impressions were taken for the typodont master model by each material, and then, each impression was poured four times. The impressions were poured in type IV die stone (Elite Rock, Badia Polesine, Zhermack Rovigo, Italy), which was mixed under vacuum mixture (Mix-R, Dentalfarm, Torino, Italy) and poured on vibrator (Plaster Vibrator A0120 VIT, Dentalfarm, Torino, Italy) following the manufacturer's recommendations. A total of 80 stone casts were produced (40 from each impression material).
Figure 1: Measurements on a re-poured cast image using stereomicroscopic and its accompanied special software

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Measurements

A small ruler was used for software calibration at each measurement session. Putty indexes were used to maintain the same angulation and distance from the camera. A computer system consisting of a stereomicroscope, a connected USB CCD camera (Amscope, Irvine, California, USA), a personal computer, and special software was used for measurements of the HAP, HCA, and V distances.

The HAP, HCA, and V dimensions on the typodont master models were measured 10 times for each dimension. The mean for each measurement of the typodont master model was calculated and used as the control to compare among the eight corresponding groups of poured stone casts. To ensure reproducibility, each stone cast measurement was repeated three times, and the corresponding mean values were considered as the statistical units. The accuracy of casts fabricated was expressed as the percentage of deviation from the corresponding typodont master model's values.

For each dimension, the difference between the mean value of the cast model (MCM) and the mean value of the typodontmaster model (MTMM) divided by the MTMM and multiplied by 100, expressed as the percentage of deviation from the typodont master model for each test group of each dimension.

Percentage of deviation = [(MCM − MTMM)/MTMM)] × 100

Statistics

This study has two independent variables (impression material type and number of pours). A t-test was used to test the first hypothesis. Simple main effects (individual error terms) for the specific comparisons relative to our second and third hypotheses were attained from a two-factor analysis of variance (ANOVA). Tukey's HSD post hoc test was used. The data were processed using a statistical software package SPSS for Windows, Version 21 (SPSS Inc., Chicago, Illinois, USA). An α level of 0.05 was maintained for all analyses.


  Results Top


[Table 1] provides descriptive statistics reported on the stone casts and typodont master cast at each measurement location (HAP, HCA, and V) under each combination of material and pouring times.
Table 1: Measurements of stone casts and typodont master cast (mm)

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Testing of the first null hypothesis required eight comparisons (impressions subjected to four times pouring vs. typodont master model for each material) for each measurement location. Outcomes of the t-tests revealed that the HCA, HAP, and V dimensions on the typodont master cast and stone casts were significantly different (P < 0.001), except for the first pour from MAS (P > 0.05). For both impression materials, all dimensions of the stone casts in the second, third, and fourth pour groups were significantly higher than those for the master cast (P < 0.001). However, the dimensions in the first pour group were smaller than those for the master cast, with AA material being significantly lower than those for the master cast (P < 0.001) and MAS not significantly different from the master cast (P > 0.05).

Regarding the second hypothesis, a two-factor ANOVA was employed to determine the effects of impression material and number of pours on the accuracy of stone casts. The main effects of the independent variables (impressions and number of pours) were statistically significant, as was their interaction [Table 2]. The main effect suggests that the four pours and impression materials did not all perform similarly. However, to know which pairs of means that differ significantly, a Tukey's HSD post hoc test was used for this comparison. There was a statistically significant difference in each of the three dimensions (HAP, HCA, and V) of the stone casts made from the AA impression material among the four pours groups (P < 0.001). Similarly, a statistically significant difference in each of the three dimensions (HAP, HCA, and V) of the stone casts made from the MAS impression material was detected among the four pour groups (P < 0.001), with exception between first pour and second pour group at HAP and HCA dimensions. In general, both impression materials experienced contraction in the first pour group but undergone expansion in the second, third, and fourth pour groups, at all three locations (HAP, HCA, and V).
Table 2: Two-way analysis of variance for the interaction effect of impression materials and number of pours

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The third hypothesis involved mean comparisons of the three dimensions (HAP, HCA, and V) of the stone casts made from the two impression materials (AA and MAS) at each of the four pours. Pairwise comparisons showed that these comparisons were statistically significant at all measurement locations with only one exception at the HAP for second-poured casts. In general, the MAS impression material was more accurate than AA impression in the first and second pour casts, with the exception in the second pour at HAP dimension; however, AA was more accurate in the third and fourth pour groups. Therefore, the results of the present study negate all three null hypotheses because statistically significant differences were observed.

[Table 3] shows the percentage of deviation (%) and absolute change (in μm) from typodont master cast of both AA and MAS impression materials at each pour. American Dental Association (ADA) Specification No. 19 for elastomeric type I impressions allows a maximum of 0.5% contraction after 24 h.[20] Thus, a clinically significant distortion could be considered any dimensional change (contraction or expansion) when it was greater than 0.5%. Based on this criterion, [Figure 2] and [Figure 3] summarize the percentage of deviations of the tested groups. The first pour group was the only group that did not produce a distortion above the 0.5% level for stone casts made by AA impression material [Figure 2]. However, the first and second pour groups were the only two groups that did not produce a distortion above the 0.5% level for stone casts made by MAS impression material [Figure 3].
Table 3: The percentage of deviation and absolute change (μm) from typodont master cast

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Figure 2: Percentage of deviation from the typodont master cast for alginate alternative impression material at three different locations in the four different poured groups

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Figure 3: Percentage of deviation from the typodont master cast for monophase additional silicone impression material at three different locations in the four different poured groups

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  Discussion Top


The capability of impression materials to produce accurate and dimensionally stable casts generated from repeated pour of a single impression is of great importance for both dentists and dental technicians.[1],[5] New types of impression materials have been introduced to the market such as AA with claims of manufacturers that they have outstanding dimensional stability, exceptional elastic recovery, and remarkable tear resistance and can be re-poured. To select the appropriate impression material, different studies have been conducted including the current study.

The AA impression material has been marketed as substitutes for traditional alginate impression materials and has provided an additional choice of materials for multiple clinical uses. The AA impressions are PVS-based materials with medium-body consistency. Their manufacturers claim that they can be poured multiple times with consistent accuracy, are stable over time, and can be re-poured to produce multiple casts.[21]

The present study evaluated the accuracy and dimensional stability of AA and MAS impression materials and ability of each of them to reproduce clinically acceptable multiple casts from single impression. All impressions were taken from a mandibular typodont hard model to simulate an impression procedure for clinical scenario of mandibular arch. Tjan and Whang have reported that the use of tray adhesive is crucial to maintain accuracy upon multiple impression pours.[22] This study has maintained this protocol and used a perforated tray to allow multiple pours with accuracy for all study groups and samples.

In the current study, the horizontal (HAP and HCA) and vertical (V) dimensions of the stone casts made from AA were significantly different from those for the master cast (P < 0.001). However, the horizontal and vertical dimensions of the stone casts made from MAS impression material were significantly different from those for the master cast (P < 0.001), except in the first pour group, in which there were no significant differences (P < 0.05). In general, the third pour and fourth pour groups yielded the highest percent of deviations, while the first pour and second pour groups yielded the lowest percent of deviations for each of the horizontal and vertical dimensions. The first and second pour groups were more accurate than the third and fourth pour groups.

In the present study, the differences in the horizontal and vertical measurements on the stone casts generated by MAS impression material were significantly lower than those generated by first and second pours of AA impression material, except at HAP dimension of casts made from second pour in which no statistically significant difference exists. Interestingly, the differences in the horizontal and vertical measurements on the stone casts generated by MAS impression materials were significantly higher than those generated by third and fourth pours of AA impression material. The MAS impression material was more accurate than AA impression in the first and second pour, with the exception of the second pour at HAP dimension, but AA was more accurate in the third and fourth pour groups.

This can be explained by the higher elastic recovery of AA material than MAS, which was subjected to high deformation during retrieval of the casts from the impression. When comparing the results of the horizontal and vertical dimensions of the first and second pours, it can be seen that the distortion started in the MAS impression after retrieval of the first cast from the impression and become more progressive in the third and fourth pours. The AA also has been subjected to distortion, but it was less than that for MAS, which makes that AA was more accurate than MAS after the second pour.

Studies evaluated the accuracy of different AA impression materials reported that these materials showed excellent dimensional stability with immediate or delayed pouring.[21],[23] In terms of accuracy, several studies reported that addition reaction of silicone had greater accuracy in comparison to alginate which concurs with the findings of the current study.[24],[25],[26]

The results of the present study supported the findings of other studies. Tjan et al. evaluated the accuracy of MAS and reported that repeated pour did not affect the dimensional accuracy and stability of the impression made from these materials.[27] Haralur et al. assessed the accuracy of multiple pour cast from putty reline technique (two steps): heavy–light body (1 step) and MAS impression material.[5] The putty reline technique and heavy–light body techniques showed that the lowest percentage of deviation from a master model and monophase impression technique had the least dimensional accuracy. They found that all impression techniques including monophase showed a statistically significant difference in the dimension over the repeated four pours.[5]

However, the result of the present study was not in agreement with the result of another study. Mehta et al. evaluated the dimensional accuracy of generated casts from additional silicone impressions after repeat pours.[1] It was concluded that repeated pouring of the impressions at the five different times showed statistically insignificant effect on the accuracy of the generated casts.[1]

Although statistical analyses were used to test significant differences between the study groups, an evidence-based cut-off limit should be considered to obtain clinically significant outcomes. Therefore, in a clinical situation, the ADA Specification No. 19 recommends a maximum allowable dimensional change for elastomeric impression materials to be 0.5% after 24 h and this was used as the standard in this study.[20],[28]

The deviations in horizontal and vertical dimensions of the AA with first pour group were less than 0.5% and were clinically acceptable, but with second, third, and fourth pours groups, the deviations in the vertical dimension were not clinically acceptable [Table 2], [Table 3] and [Figure 2]. The deviations in horizontal and vertical dimensions of MAS with first and second pour groups were clinically acceptable (>0.5%), but with third pour group, the deviations in the vertical dimension were not clinically acceptable, and in the fourth pour group, the deviation in all dimensions were not clinically acceptable [Table 2], [Table 3] and [Figure 3]. However, according to Tjan et al., a dimensional deviation from the master model of approximately 50.0 μm was clinically acceptable and did not prevent the complete seating of the master casting.[27]

The present study evaluated only one brand of AA and MAS impression materials. Further studies comprising more brands of both AA and MAS impression materials are recommended.


  Conclusion Top


The result of this in vitro study showed that AA impression material is dimensionally accurate and showed comparable accuracy to that of the MAS material. This dimensional stability of AA impression material may be beneficial for use in several aspects of prosthetic dentistry that go beyond the traditional use of primary casts to reduce the cost factor. However, further studies are required to correlate the finding of this study in clinical setting.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Mehta R, Wadhwa S, Duggal N, Kumar A, Goel M, Pande S. Influence of repeat pours of addition silicone impressions on the dimensional accuracy of casts. J Interdiscip Med Dent Sci 2014;2:108.  Back to cited text no. 1
    
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Haralur SB, Saad Toman M, Ali Al-Shahrani A, Ali Al-Qarni A. Accuracy of multiple pour cast from various elastomer impression methods. Int J Dent 2016;2016:7414737.  Back to cited text no. 5
    
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[PUBMED]  [Full text]  
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Choudhary S, Sivakumar I, Buzayan MM, Choudhary P. Dimensional accuracy of double poured casts obtained from extended pour alginate impressions: An in vitro study. J Int Oral Health 2018;10:272-7.  Back to cited text no. 16
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  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

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