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Table of Contents   
ORIGINAL ARTICLE  
Year : 2011  |  Volume : 14  |  Issue : 2  |  Page : 151-155
Effect of frequency and amplitude of vibration and role of a surfactant on void formation in models poured from polyvinyl siloxane impressions


Department of Prosthodontics, Mcods, Manipal, India

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Date of Submission09-Sep-2009
Date of Decision17-Nov-2010
Date of Acceptance25-Dec-2010
Date of Web Publication7-Jul-2011
 

   Abstract 

Aims : To determine whether the frequency and amplitude of vibration and the use of surfactant has any effect on the formation of voids on the cast surface, poured from a polyvinyl siloxane impression material, using a mechanical model vibrator.
Materials and Methods : A total of 100 impressions of a master die were made using Reprosil, Type 1, Medium Viscosity, Regular body, and Dentsply Caulk. The test group was subdivided into a surfactant and non-surfactant group, 50 impressions each. The impressions were poured in a dental stone with a mechanical model vibrator that was set at a vibration frequency of 3000 cycles / minute (low) and 6000 cycles / minute (high) with the help of a knob. The resultant casts were examined with a Stereomicroscope (LABOMED CZM4) under x10 magnifications.
Results : For the low and high frequencies, when surfactant and non-surfactant groups were compared, for all the amplitudes and for both the material groups, the surfactant groups resulted in fewer void formations and showed very high statistical significance (P-value = 0.001).
Conclusions : The lowest mean voids were obtained for high frequency surfactant groups. Reprosil: High frequency amplitude 2 being the most prominent, produced the least mean voids (mean = 23.2000, P = 0.001, very highly significant). In comparison, the surfactant groups produced the least voids for both frequencies.

Keywords: Void-free casts; polyvinyl siloxane impressions; surfactant; mechanical model vibrator

How to cite this article:
Reddy N K, Aparna I N. Effect of frequency and amplitude of vibration and role of a surfactant on void formation in models poured from polyvinyl siloxane impressions. J Conserv Dent 2011;14:151-5

How to cite this URL:
Reddy N K, Aparna I N. Effect of frequency and amplitude of vibration and role of a surfactant on void formation in models poured from polyvinyl siloxane impressions. J Conserv Dent [serial online] 2011 [cited 2019 Aug 19];14:151-5. Available from: http://www.jcd.org.in/text.asp?2011/14/2/151/82620

   Introduction Top


Introduction of elastomeric impression materials into restorative dentistry has brought about a revelation in the field of restorative dentistry. The field of fixed prosthodontics has benefited the most. The reproduction of detail and dimensional accuracy obtained by these materials has made their use a prerequisite for most of the fixed and removable dental prosthesis.

Polyvinyl siloxane impression materials are widely used in restorative dentistry. High surface tension gives rise to poor recording of detail in moist conditions, as well as an increased risk of voids in dies poured from the impression. To overcome the problems associated with the hydrophobic nature of these materials, manufacturers have made hydrophilic variants. However, the hydrophilic materials are not as wettable as hydrocolloid­­­­­­­­­.

Although hydrophilic polyvinyl siloxanes containing a surfactant agent wet a surface adequately, Panichuttara et al. [1] have shown them to be slightly less dimensionally accurate than hydrophobic polyvinyl siloxanes. These investigators have reported that topical surfactants are more effective than intrinsic surfactants. Therefore, to maximize accuracy and reduce the risk of air voids, topical surface agents are indicated.

Numerous factors are involved in making a void-free dental stone cast from a polyvinyl siloxane impression. Voids on the surface of a cast or die affect the accuracy of a cast restoration. Some of the causes of void formation on the surface of a cast poured from silicone impression materials include: poor wetting characteristics, [2] air bubbles in the mixed dental stone, and improper technique of pouring the impression.

There are many techniques in pouring the impression. Irrespective of the techniques used to carry and place the dental stone, a mechanical vibrator is used to cause the gypsum slurry to flow and spread into the impression. The rate of flow of gypsum slurry in a vertical direction and its spread in a horizontal direction, in an impression held on a mechanical vibrator, depends on the frequency and amplitude of vibration set on the instrument. The formation of voids on the surface of the cast poured from elastomeric impression materials has been studied by several investigators. [1],[2] However, the frequency and amplitude of vibration used has not been mentioned. The role of frequency and amplitude of vibration in the formation of voids on the cast or die has seldom been reported in literature. The purpose of the study was to determine the effect of frequency, the amplitude of vibration, and the role of surfactant in the formation of voids on a cast surface made from polyvinyl siloxane impression material.


   Materials and Methods Top


A total of 100 casts was prepared from polyvinyl siloxane impressions in dental stone, under different vibrating conditions. A mechanical model vibrator (Vibromaster No. 24122, Bego Bremer Goldschagerel will GmBA and Co., Bremen, Germany) was used to pour the impressions. The instrument could be set at a vibration frequency of 3000 cycles / minute and 6000 cycles / minute with the help of a control knob. [3] The amplitude of vibration was provided in steps 1 through 5 and remained constant for the step set on the instrument. The values for the amplitude provided by the manufacturer [3] are presented in [Table 1]. The impression material used was; Reprosil, Type 1, Medium Viscosity, Regular body, Dentsply Caulk.
Table 1: Empirical values for amplitude


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Sample preparation

An aluminium master die consisting of 48 point angles was constructed as described in a previous study. [4] A total of 100 custom trays were fabricated in autopolymerizing methylmethacrylate resin (DPI-RR, Cold cure) with a 2-mm thick wax spacer and stops to ensure correct tray centering and uniform material thickness. The inner surface and periphery of the custom tray was coated with tray adhesive (Caulk® tray adhesive, DENTSPLY Caulk, Milford, Delaware, Dentsply Int. Inc) as recommended by the manufacturer. The adhesive was allowed to dry for 15 minutes according to manufacturer's instruction. One hundred impressions of the master model were made in individual custom trays, using the impression material. The impression material chosen was supplied in tube form, base and catalyst. Equal proportions of the base and catalyst were dispensed onto a clean ceramic tile and hand mixed with a stiff bladed spatula according to the manufacturer's instructions. The material was loaded into the custom tray and the impression of the master die was made. The impressions were examined and the defective impressions were discarded. The impressions were aged by a 20-minute bench set before making a cast.

The impressions obtained were divided into a surfactant group (50 samples) and a non-surfactant group (50 samples). Each subgroup was further subdivided into a low frequency (3000 Hz) group (25 samples) and a high frequency (6000 Hz) group (25 samples). The samples included in the surfactant group were sprayed with a surfactant (Debubblizer, Prime dental products Pvt. Ltd., India), dried, and set aside.

One hundred grams of Type IV die stone (Kalrock, Kalabhai Karson Pvt. Ltd., India) was weighed to the nearest ± 0.1 gm using a physical balance, and 24 ml of water was measured to the nearest ± 0.1 ml. The Die stone was added to distilled water in a clean, rubber mixing bowl. The mix was allowed to soak, and then hand spatulated for 10 seconds with a round-ended steel blade spatula, and then spatulated in a mechanical vacuum spatulator (Bego vacuum mixer) for 25 seconds to obtain a creamy, bubble-free mix.

A paint brush was used to place the mixed dental stone on the periphery of the impression. The vibration frequency and amplitude of the mechanical vibrator was set according to the manufacturer's instruction before pouring the impression. [3] With the impression held on the model vibrator in a tilted position, the mixed dental stone was carefully expressed over the periphery of the impression to flow into its deepest portion. Additional increments of the mixed dental stone were added over the periphery of the impression until the impression was covered, for over a period of 30 seconds.

With the vibration frequency set at 3000 Hz (low frequency), five impressions were poured for each of the five amplitude steps. The instrument was then set at a vibration frequency of 6000 Hz (high frequency); five impressions were poured for each of the five amplitude steps.

Thus, a total of 100 impressions were poured that is, five from each step of amplitude, low and high frequency, surfactant and non surfactant groups. After pouring the impressions, the casts were allowed to set for 24 hours and then removed from the impressions and allowed to dry on a table top. The casts were then numbered for identification and distribution.

Die evaluation

Each cast was examined with a Stereomicroscope (LABOMED CZM4) under ×10 magnification. [5] The casts were examined for the presence of voids by one investigator, and the total number of voids on the surface of the casts were counted and recorded.


   Results Top


The data obtained from the study was subjected to both descriptive and analytical statistics. The mean for the voids present on the specimens was calculated. One-factor and two-factors of the analyses of variance (ANOVA) were performed to compare the means, followed by the Students't-test, to determine the significant differences between the groups of dies. All computations were conducted with the SPSS software (version 11.5).

Means for the voids in all the cast groups calculated for low and high frequencies indicated that an increase in frequency caused a decrease in the number of voids. A univariate analysis of variance was performed to establish how frequency and amplitude affected void formation.

An increase in frequency had a significant- to highly-significant effect on the decrease in void formation (alpha < 0.05). The most prominent effect was seen in the surfactant groups (alpha value = 0.021, significant), where the increasing frequency resulted in a statistically significant decrease in void formation. The increase in amplitude alone had no statistically significant effect on the increase of voids. When both frequency and amplitude were considered, the statistical significance increased (alpha value = 0.004, highly significant).

For the used material, Reprosil, the lowest mean voids were obtained for the high frequency surfactant groups. High frequency amplitude 2 being the most prominent, produced the least mean voids (mean = 23.2000, P = 0.001, highly significant). In comparison the surfactant groups produced the least voids for both frequencies.


   Discussion Top


Polyvinyl siloxanes are inherently hydrophobic­­­­­. [6],[7],[8] However, in recent times, new 'hydrophilic' polyvinyl siloxanes have been introduced, with claims by the manufacturer that they better wet the moist dental surfaces. These new formulations have intrinsic surfactants added to them. Typically these are non-ionic surfactants of nonylphenoxypolyethanol homologs. [8],[9],[10] Millar and co-workers [11] reported a significant reduction in the number of voids and an overall increased quality of polyvinyl siloxane impression, when a modified polydimethyl siloxane wetting agent was applied to the prepared tooth surfaces, before the impressions were made.

The present study can be broadly divided into two sections. The effect of frequency and amplitude of vibration set on a mechanical model vibrator on void formation in dies poured from two types of polyvinyl impression materials was tested. The role of the surfactant in the reduction of void formation was additionally tested.

A total of 200 casts was prepared from polyvinyl siloxane impressions in die stone under different vibrating conditions. A mechanical model vibrator was used to pour the impressions. The instrument could be set at a vibration frequency of 3000 cycles/minute and 6000 cycles/minute with the help of a control knob. The amplitude of vibration was provided in steps 1 through 5 and remained constant for the step set on the instrument.

Two impression materials were used and 100 impressions were made for each material. The surfactant was sprayed on 50 samples of each material. The dies obtained were examined under 10x magnification and the surface voids were counted. The data obtained from the study was subjected to both descriptive and analytical statistics. The mean for the voids present on the specimens were calculated. One-factor and two-factor of the analyses of variance (ANOVA) were performed to compare the means, followed by a Students't-test, to determine the significant differences between the groups of dies. All computations were conducted with the SPSS software (version 11.5).

In this study, the formation of voids on the cast surface poured from polyvinyl siloxane impression material with a mechanical vibrator indicated a relation between the number of voids, vibration frequency, and magnitude of amplitude. When the vibration was induced in the metal plate of a mechanical vibrator, the gypsum slurry placed at the top of an impression moved in a repetitive cycle that caused the gypsum slurry to flow along the walls of the impression. The rate of flow of the gypsum slurry bore a strong resemblance to the periodic motion (vibration) of the metal plate.

The means for the voids in all the cast groups calculated for low and high frequencies indicated that an increase in frequency caused a decrease in the number of voids for the material under consideration.

Void formation may be explained on the basis of the rate of pouring, that is, the rate of flow of the gypsum slurry in a vertical direction along the walls of the impression and the rate of spreading in the mold in a horizontal direction. If the rate of evening out or spreading is equal or greater than the rate of vertical flow, there is less turbulence in the gypsum slurry, as a result of which there is less chance for void formation. However, if the rate of vertical flow is greater than the rate of spreading, then there is a possibility of an increased turbulence in the material, which will result in more void formation.

At a low frequency of vibration, the metal plate of the mechanical vibrator vibrates more violently. However, if the frequency of vibration is increased from zero to infinity, the metal plate may almost appear stationary and the gypsum slurry will not respond to the exciting vibration. Consequently, the rate of flow of the gypsum slurry is greater at a low frequency of vibration than at a high frequency of vibration. The results of this study indicate the disadvantage of pouring an impression at low frequency (3000 cycles/minute) as a method of reducing the voids in a cast. This is in accordance with a study done by Abdullah MA in 1998. [13]

Increase in frequency had a significant- to highly-significant effect on the decrease in void formation in both the test materials (alpha < 0.05). The most prominent effect was seen in the surfactant groups of material one (Reprosil, alpha value = 0.021, significant) and material two (Aquasil, alpha value = 0.009, highly significant), where the increasing frequency resulted in a statistically significant decrease in void formation. For both the material groups, the increase in amplitude alone had no statistically significant effect on the increase of voids. When both frequency and amplitude were considered, the statistical significance increased (material one: alpha value = 0.004, material two: alpha value = 0.006, highly significant).

This finding may be due to the fact that energy is proportional to the square of the amplitude; therefore, more energy is transmitted to the gypsum slurry, causing it to flow at a faster rate. When the rate of flow of the gypsum slurry is greater than the rate of spreading, there is a greater chance of void formation.

The results also indicated that the means of all the casts prepared with low and high frequency groups were not the same for both the materials. To determine whether the groups were different from each other, a Students't test was used.

For the casts prepared in low frequency, both the materials showed no significant statistical difference for both surfactant and non-surfactant groups under all amplitudes. The surfactant group for amplitude 2 showed Reprosil to be statistically significant with fewer voids than Aquasil.

For the casts prepared in high frequency both the materials showed no significant statistical difference for both surfactant and non-surfactant groups under all amplitudes, barring non- surfactant groups amplitude 2 and 3, where Reprosil produced statistically significant fewer voids than Aquasil. For the surfactant groups, although not statistically significant, Reprosil produced comparatively fewer voids than Aquasil.

The role played by the surfactant used, in the formation of voids, was analyzed using the Student's t test by comparing the surfactant and non-surfactant groups.

At a low frequency, when surfactant and non-surfactant groups were compared, for all the amplitudes and for both the material groups, the surfactant groups resulted in fewer void formations, which showed high- to very high statistical significance, barring material one: Reprosil, for amplitudes 4 and 5, material two: Aquasil for amplitude 3, where although the number of voids formed in the surfactant group were less than in the non-surfactant group, they were not statistically significant.

At a high frequency, when the surfactant and non-surfactant groups were compared, for all the amplitudes and for both the material groups, the surfactant groups resulted in fewer void formations and showed a very high statistical significance (P value = 0.001). These findings were in accordance with those in the previous published studies. [2],[4],[11],[13],[14],[15]

The performance of the material was then considered individually. The Students't test was applied to gain further information. For material one: Reprosil, the lowest mean voids were obtained for the high frequency surfactant groups. High frequency amplitude 2 being the most prominent, produced the least mean voids (mean = 23.2000, P = 0.001, very highly significant). In comparison, the surfactant groups produced the least voids for both frequencies.

For material two: Aquasil, the lowest mean voids were obtained for the high frequency surfactant groups. High frequency amplitudes 2 and 4 being the most prominent, produced the least mean voids (means = 25.0000 and 22.6000, P = 0.001 and 0.004, very highly significant and highly significant). In comparison, the surfactant groups produced the least voids for both frequencies.


   Conclusions Top


The lowest mean voids were obtained for the high frequency surfactant groups. Reprosil: High frequency amplitude 2 being the most prominent, produced the least mean voids (mean = 23.2000, P = 0.001, very highly significant). In comparison, the surfactant groups produced the least voids for both frequencies.

From the results obtained, the following clinical implications can be derived:

  • Mechanical model vibrators with adjustable frequency and amplitude settings should be selected for regular laboratory use. High frequency settings result in reduced void formation in poured casts.
  • Surfactants can be sprayed on the disinfected addition silicone impressions before pouring them in die stone, to reduce air entrapment.


Limitations of the study include

  • Operator variability may result in increased or decreased void formation
  • The results were obtained by using one die stone and one impression material and might or might not be extrapolated to other die stones and impression materials.


 
   References Top

1.Panichuttara R, Jones RM, Goodacre C, Munoz CA, Moore BK. Hydrophilic poly (vinyl siloxane) impression materials: dimensional accuracy, wettability, and effect on gypsum hardness. Int J Prosthodont 1991;4:240-8.   Back to cited text no. 1
    
2.Robinson PB, Dunne SM, Millar BJ. An in vitro study of a surface wetting agent for addition reaction silicone impressions. J Prosthet Dent 1994;71:390-3.  Back to cited text no. 2
[PUBMED]    
3.Vibromaster No. 24122. Operating and maintenance manual. Bremen, Germany: Bego Bremer Goldschagerel will GmBH and Co.  Back to cited text no. 3
    
4.Cullen DR, Mikesell JW, Sandrik JL. Wettability of elastomeric impression materials and voids in gypsum casts. J Prosthet Dent 1991;66:261-5.   Back to cited text no. 4
[PUBMED]  [FULLTEXT]  
5.Soh G, Chong YH. Determination of an optimal magnification for examining voids in elastomeric impressions. Int J Prosthodont 1990;3:573-6.  Back to cited text no. 5
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6.Michael N, Mandikos MN. Polyvinyl siloxane impression materials: An update on clinical use. Aust Dent J 1998;43:428-34.  Back to cited text no. 6
    
7.O'Brien WJ. Dental materials. Properties and selection. Chicago: Quintessence Books; 1989.  Back to cited text no. 7
    
8.Craig RG, O'Brien WJ, Powers JM. Dental materials. Impression Materials. 11th ed. St Louis: Mosby;2002;329.  Back to cited text no. 8
    
9.Chai JY, Yeung T. Wettability of nonaqueous elastomeric impression materials. Int J Prosthodont 1991;4:555-60.  Back to cited text no. 9
    
10.Takahashi H, Finger WJ. Dentin surface reproduction with hydrophilic and hydrophobic impression materials. Dent Mater 1991;7:197-201.  Back to cited text no. 10
[PUBMED]  [FULLTEXT]  
11.Millar BJ, Dunne SM, Robinson PB. The effect of a surface wetting agent on void formation in impressions. J Prosthet Dent 1997;77:54-6.   Back to cited text no. 11
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12.Serway RA. Physics for scientist and engineers. 3rd ed. Philadelphia: Saunders College Publishing; 1992. p. 325-460.  Back to cited text no. 12
    
13.Abdullah MA. Effect of frequency and amplitude of vibration on void formation in dies poured from polyvinyl siloxane impressions. J Prosthet Dent 1998;80:490-4.  Back to cited text no. 13
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14.Norling BK, Reisbick MH. The effect of non-ionic surfactants on bubble entrapment in elastomeric impression materials. J Prosthet Dent 1979;42:342-7.  Back to cited text no. 14
[PUBMED]    
15.Millar BJ, Dunne SM, Nesbit M. A comparison of three wetting agents used to facilitate the pouring of dies. J Prosthet Dent 1995;74:341-4.  Back to cited text no. 15
[PUBMED]    

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Correspondence Address:
N Kulashekar Reddy
Flat no 302, Sandhyagiri Appartments, Kalan Nagar, Gaddianaram, Hyderabad - 60
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.82620

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