Comparative Evaluation of Surface Detail Reproduction and Dimensional Stability of Poly Ether, Vinyl Siloxane, and Vinyl Siloxane Ether Impression Materials: An In vitro Study.

INTRODUCTION: Impression making is an integral part of prosthetic dentistry. Development of material science has allowed integrating qualities of hydrophilic polyether and hydrophobic polyvinyl siloxane into a newer hybrid material, vinyl polysiloxane (VPS) ether. This study was aimed to compare the VPS ether with the polyether and polyvinyl siloxane in terms of accuracy and dimensional stability.
MATERIALS AND METHODS: Stainless steel dies with the American Dental Association specification 19 were made. Die has three horizontal and two vertical lines which are used for taking the impression. Two cross-points at the junction of the vertical lines with line 2 were marked as x and x' and served as beginning and end points of measurements for dimensional accuracy. Accuracy was evaluated 30 min after making each impression. If at least two of the three horizontal lines were reproduced continuously between cross-points, this impression was considered satisfactory. The specimens are poured with Type IV gypsum product and allowed to set completely for 24 h. Then, dimensional stability was measured in the model by measuring the distance between the two lines and comparing the distance with the measurement of line on metal die, which was used to make the impression.
RESULTS: The mean value obtained for light- and medium-bodied VPS ether was 0.05370 and 0.05330 and for light and medium-bodied polyvinyl siloxane was 0.06370 and 0.07150, respectively. The mean value for polyether monophase was 0.06430. Two-way ANOVA and post hoc test showed statistical significance.
CONCLUSION: The newer VPS ether material showed good surface detail reproduction and dimensional stability when compared with polyvinyl siloxane and polyether. Copyright:

INTRODUCTION

Making an impression to duplicate oral condition and tooth morphology is an integral part of prosthetic dentistry. For better accuracy of an impression, the impression materials, as well as the techniques, are equally important. Failure to create an ideal impression can effect in a poorly fitting prosthesis, increased chair time, and expensive remakes. It is up to a dentist to select the best impression material to produce the desired result, while taking into deliberation the clinical objectives of the case. There is an advancement in the impression materials from the reversible hydrocolloid near the beginning of the nineteenth century to the polyether materials launched in the 1960s and most newly the polyvinyl siloxane in the 1970s.

Polyether is a hydrophilic and rigid material with high modulus of elasticity, but because of its high stiffness after setting, short working and setting time and high cost limit their use.[1] Polyvinyl siloxane is used widely because of their excellent elastic recovery, good surface detail reproducibility, case of handling, dimensional accuracy, and moderately short working and setting time, and it can produce multiple casts from the single impression.[23]

Advancement in material science has given origin to an invention of new impression material, integrating the hydrophilic qualities of polyether and polyvinyl siloxane into a newer material, vinyl polysiloxane (VPS) ether.[2] It merges some of the most desired properties into solitary material. Studies on the new vinyl siloxane polyether material are few as they are new to the market. Hence, this original study was aimed to evaluate and compare the surface detail reproduction and dimensional accuracy of three elastomeric impression materials, namely polyether, polyvinyl siloxane and Vinyl siloxanether

MATERIALS AND METHODS

A total of five different viscosities of elastomeric impression materials were used which include polyether (Monophase) 3M ESPE, Germany; polyvinyl siloxane (medium and light body) 3M ESPE, Germany; and Vinyl siloxanether (medium body and light body) Identium, Germany. The impressions were stored under the manufacturer's recommended conditions. The elastomeric impression materials were divided into five groups

Group 1: Vinyl siloxanether – Light

Group 2: Vinyl siloxanether Medium

Group 3: polyvinyl siloxane Light

Group 4: polyvinyl siloxane – Medium

Group 5: Polyether – Monophase

The specimens were made using a stainless steel die having three parts: a block part, mold, and riser. Each specimen was made using a stainless steel mold. The die had three parallel lines set 2.5mm apart and two perpendicular lines set 20mm apart which were used for evaluating the test parameters. The parallel lines were labeled as 1, 2, and 3 and the width of each parallel line was 0.60 mm. Two cross points at the intersection of the perpendicular lines with line 2 were marked x and x' and served as the beginning and end points of measurements for dimensional accuracy [Figure 1].

Figure 1

(a) Stainless steel die. (b) Stainless steel die with three horizontal lines

A total of fifty impressions were prepared with 10 impressions in all the five groups. The tray adhesive supplied by the manufacturer was evenly applied over the inner surface of the tray allowed to dry for 5 min before loading. The one step single mix technique was performed by mixing impression material using rubber base mixing gun (GC, India) and loaded into the block. All impression materials are mixed according to the manufacturer's directions by using clean dispensing auto mix gun. Standardized impression making technique was performed. To simulate oral condition, the dies with the applied impression material were transferred into a water bath maintained at 32°C ± 2°C. The impression material was then allowed to set twice the manufacturer's recommended setting time as indicated in the American Dental Association (ADA) specification number 19 for laboratory testing to compensate for the difference in a room (21°C ± 2°C) and mouth (37°C) conditions.

After the impression material has set, the impression was gently removed. Surface detail reproduction was evaluated 30 min after making each impression stored at room temperature. Continuity of line of duplication was calculated according to the ADA requirement 19. If at least two of the three parallel lines were reproduced continuously between cross points, this impression was measured acceptable, and others were rated unacceptable [Figure 2]. Macroscopic assessment of the impression's smooth surface was also integrated in this study. For this added macroscopic assessment, impressions were rated satisfactory if the entire impression surface was even, glossy, and free of voids or pits; and impressions were rated as unsatisfactory if the impression surface was uneven or contained any pits or voids.

Figure 2

(a) Vinyl siloxanether specimen. (b) Polyvinyl siloxane specimen. (c) Poly ether specimen

All impressions were stored at room temperature for 30 min before pouring with Type IV gypsum product. To homogenize the setting expansion of the improved stone, the powder was precisely weighed, and the water was dispensed using a measuring jar at a ratio of 100 g/20 ml in a mixing bowl. The mix was poured into the impression and was allowed to set absolutely for a day before being removed from the impression. Then, dimensional stability was calculated in the model by measuring the distance between the two lines, and comparing the distance with the line on metal die used to make the impression. The measurements were done using the traveling microscope to a precision level of 0.01mm. [Figure 3]. This study used a 2-factor completely randomized design. A 2-way analysis of variance (ANOVA) was used to compare the mean dimensional changes of the 5 materials at p < .05 level. The least significant difference (LSD) test was used as a post hoc test for pairwise comparisons. Pearson X2 (< .05) was used to evaluate surface detail reproduction of the 5 materials as determined by the American Dental Association requirement 19 and the added smooth surface characteristic assessments.

Figure 3

Dimensional stability evaluation through travelling microscope

RESULTS

Mean value of the light bodied Vinyl siloxanether (Identium) was 0.05370, whereas the mean value of the medium bodied Vinyl siloxanether (Identium) was 0.05330. The medium bodied Vinyl siloxanether (Identium) exhibited less change in dimensional accuracy compared to the light bodied Vinyl siloxanether (Identium). Mean value light bodied polyvinyl siloxane (Aquasil) was 0.06370, whereas the mean value of the medium bodied polyvinyl siloxane (Aquasil) was 0.07150. The light bodied polyvinyl siloxane (Aquasil) exhibited less the change in dimensional accuracy compared to the medium bodied polyvinyl siloxane (Aquasil). Mean change of polyether was 0.06430. The monophase polyether (Impregum) exhibited less the dimensional accuracy compared to the medium bodied polyvinyl siloxane. All the materials had a significant effect on detail reproduction (Pearson Chi square, P < 0.05). 90% of the light and medium bodied vinyl siloxanether (Identium), and monophase poly ether (Impregum) impressions were acceptable whereas 80% of the light bodied, polyvinyl siloxane (Aquasil) and 70% of the medium bodied, VPS (Aquasil) were found acceptable.

The mean standard deviation and standard error of all the five groups are specified in the table. A 2-way ANOVA was performed to test the significance of the obtained data [Table 1]. This result indicated that the dimensional accuracy, as given by the American Dental Association specification 19, was not affected. However, statistically significant differences (p<.05) were found between the 5 materials. The medium-bodied, Vinyl siloxanether (Identium) exhibited less change in dimensional accuracy compared to the light body, Vinyl siloxanether (Identium). Data for the 5 materials are shown in Table 2.

Table 1

Percentage of satisfactory and unsatisfactory impressions assessed with additional smooth surface evaluation

Percentage of satisfactory and unsatisfactory impressions assessed with additional smooth surface evaluation	Group					Total
	Group I	Group II	Group III	Group IV	Group V
Satisfactory
Count	9	9	8	7	9	42
Percentage within group	90.0	90.0	80.0	70.0	90.0	84.0
Unsatisfactory
Count	1	1	2	3	1	8
Percentage within group	10.0	10.0	20.0	30.0	10.0	16.0
Total
Count	10	10	10	10	10	50
Percentage within group	100.0	100.0	100.0	100.0	100.0	100.0

Table 2

Statistical evaluation for dimensional stability

Groups	n	Mean	SD	SE	Significance
Group I	10	0.05370	0.011691	0.003697	0.005
Group II	10	0.05330	0.003917	0.001239
Group III	10	0.06370	0.016573	0.005241
Group IV	10	0.07150	0.008734	0.002762
Group V	10	0.06430	0.014469	0.004575
Total	50	0.06130	0.013414	0.001897

SD: Standard deviation, SE: Standard error

Post Hoc test between each group showed a significance only between group 1 and IV (P =0.014) and group II and IV (P=0.011). Data for the 5 materials are shown in Table 3.

Table 3

Post HOC test for dimensional stability

Materials	Material	Mean differance	Sth . error	Sig .
Group I	GROUP II	0.000400	0.005338	1.000
	GROUP III	- 0.010000	0.005338	0.346
	GROUP IV	- 0.17800	0.005338	0.014
	GROUP V	- 0.010600	0.005338	0.289
Group II	GROUP I	-0.000400	0.005338	1.000
	GROUP III	- 0.010400	0.005338	0.308
	GROUP IV	- 0.018200	0.005338	0.011
	GROUP V	- 0.011000	0.005338	0.255
Group III	GROUP I	0.010000	0.005338	0.346
	GROUP II	0.010400	0.005338	0.308
	GROUP IV	- 0.007800	0.005338	0.592
	GROUP V	- 0.000600	0.005338	0.663
Group IV	GROUP I	0.017800	0.005338	0.014
	GROUP II	0.018200	0.005338	0.011
	GROUP III	0.007800	0.005338	0.592
	GROUP V	0.007200	0.005338	0.663
Group V	GROUP I	0.010600	0.005338	0.289
	GROUP II	0.011000	0.005338	0.255
	GROUP III	0.000600	0.005338	1.000
	GROUP IV	- 0.007200	0.005338	0.663

DISCUSSION

A discussion on the basic requirements of impression materials can be approached from four main angles. On the one hand, it deals with factors that influence the dimensional accuracy of the impression; on the other hand, it deals with parameters that impact the dimensional stability. Furthermore, the handling and setting characteristics of the impression material, as well as other variables, such as cost, taste, and color, play a role. Recent development and advances have led to the introduction of newer impression materials, which claim to have better and with more clinical applications than the conventional elastomeric impression material. Most commonly used impression material, polyvinyl siloxane has proven to have good dimensional accuracy, but it lacks in tear strength, while polyether has proven to have better tear strength of all the elastomeric impression materials. A new impression material which is a blend of both the positive properties of addition polyvinyl siloxane and polyether has been developed which has better tear strength than additional polysiloxane. There is no available literature that claims the dimensional stability and accuracy of the newly formulated VPS ether; hence, the purpose of the present study was aimed to evaluate and compare the dimensional stability and accuracy of three elastomeric impression materials, namely polyether, polyvinyl siloxane, and VPS ether.

Balkenhol et al. stated that the use of metal tray is one of the best in regarding the dimensional accuracy and reliability of impression making.[4] Impression taking with disposable plastic stock trays is becoming increasingly popular for daily impression procedure. Carrotte et al. stated that the impressions made with flexible plastic trays produced considerable discrepancies due to flexibility of the tray under heavy impressions. Custom-made trays provide uniformity of materials which minimize the dimensional changes which might distort an impression. Dimensional accuracy of the impressions in the custom tray was found to be more accurate when compared to stock trays.[567] When addition silicone impressions were used the dimensional accuracy was same with putty/wash, single-mix, and double-mix techniques when they used custom-made perforated trays, the most replicative impression and resultant die were found with the full adhesive application. In this study, we used a standardized model with a stainless steel base is fabricated, and each specimen was made using a stainless steel mold.

Bomberg et al. stated that to minimize marginal opening, the use of full application of adhesive and the use of perforated trays were one of the essential factors. The use of stock or custom-made trays and the use of the putty/wash or single-mix technique had no significant effects on the observed marginal opening.[8] Hence, in this study, tray adhesive sponsored by the company was applied to the surface of the stainless steel custom-made block and allowed to dry for 5 min before loading the impression in the block.

Craig et al. suggested that auto mixing tips are important because of its simplicity, convenient to use, cost-effectiveness, no spatulation, and consistent mix.[9] Chee et al. stated that the use of automixing system reduced the number of bubbles incorporated in the mix for all the impression materials.[10] Hence, in this study, we used automixing system to reduce the number of bubbles in the impression material.

In most of the studies reported in the literature so far, a precision measurement was done using instruments such as traveling microscope, micrometer, vernier caliper, and laser probes. In the present study, a traveling microscope (ELFO, India) was used. It had the least count of 0.01 mm, fitted with × 10 magnification.

Chandur et al. stated that the working casts and working dies from regular and fast-setting poly ether demonstrated an increase in all dimensions when compared to the master model and stainless steel complete crown preparation. The working casts from the fast-set polyvinyl siloxane were larger than the master model, whereas working dies showed a reduction in mesiodistal dimension and height compared to the stainless steel complete crown preparation. The new fast-setting polyether and polyvinyl siloxane materials demonstrated dimensional accuracy equivalent to a traditional polyether.[11] In our study, the addition of polyether improved the dimensional stability of the newer material.

Shillingburg et al. stated that polyvinyl siloxane impression materials are extremely precise when used in clinical dental practice. The dimensional stability of the material was typically time reliant. Dentists have been reported to delay pouring of impressions up to 72 h; therefore, it is important that an impression material should remain dimensionally accurate for this stage of time. Polyvinyl siloxane impression materials have demonstrated finer dimensional stability when evaluated with other elastomeric materials, principally because they do not discharge any by-products. Both the polyvinyl siloxane materials showed good dimensional stability over the time period of the study.[12] In our study, when compared with polyvinyl siloxane and polyether, newer hybrid material was dimensionally more stable.

Gonçalves et al. and Hamalian et al. suggested that polyvinyl siloxane was more stable even after 4 weeks when compared to polyether, which had dimensional stability only within 24 h.[1314] In our study, VPS ether showed good dimensional stability when compared with polyvinyl siloxane and polyether, so further studies can be done regarding the dimensional stability of the material once a long period of time following mixing.

Petrie et al. stated that the dimensional stability for both hydrophilic polyvinyl siloxane impression materials was not considerably affected by the dry, moist, or wet atmospheres. There was a statistically considerable difference in the dimensional stability between the two polyvinyl siloxane materials. However, dimensional changes for both materials were well below ADA values of maximal shrinkage value of 0.5%. Both materials tested acceptably with respect to detail reproduction under dry and moist conditions but not in wet conditions.[15] In this study, dimensional stability and accuracy for VPS ether were superior to polyether and polyvinyl siloxane. Measurements of casts obtained from all five groups showed a slight increase in dimensions. However, when these changes in dimensional stability were compared with ADA requirement 19, all the materials revealed satisfactory dimensional stability, well below 0.5% dimensional change. Surface detail reproduction was first assessed based on the criteria similar to the ADA requirement 19 (2 of the 3 parallel lines were reproduced continuously between cross-points).

CONCLUSION

In this in vitro study, when comparing the surface detail reproduction and dimensional stability of polyether and polyvinyl siloxane with new VPS ether impression material, the VPS ether impression material showed good surface detail reproduction and dimensional stability.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.