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Table of Contents   
ORIGINAL RESEARCH  
Year : 2012  |  Volume : 23  |  Issue : 3  |  Page : 301-304
Effect of different curing temperatures on the distortion at the posterior peripheral seal: An in vitro study


1 Department of Prosthodontics and Crown and Bridge, Gitam Dental College, Vishakapatnam, India
2 Department of Prosthodontics, Marathamandal Dental College, Belgaum, India
3 Department of Prosthodontics and Crown and Bridge, SGRD Institute of Dental Sciences, Amritsar, India
4 Department of Prosthodontics and Crown and Bridge, Modern Dental College, Indore, India

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Date of Submission11-Jan-2011
Date of Decision28-Apr-2011
Date of Acceptance08-Nov-2011
Date of Web Publication11-Oct-2012
 

   Abstract 

Context: Most of the maxillary complete dentures do not adapt accurately to the cast because of the changes in the resin during polymerization. The amount of heat associated with processing of polymethyl methacrylate has been correlated with the adaptation of the processed denture base to its supporting tissues.
Aims: This study conducted to determine the effect of different curing temperatures on the accuracy of fit of a complete maxillary denture and to compare with that of fiber-reinforced acrylic resins.
Materials and Methods : An ideal maxillary rubber mould used to make an edentulous cast. Forty-eight stone casts made from the ideal rubber mould. Out of 48 identical stone casts, 44 stone casts with wax adapted processed using simple acrylic resin. The remaining four stone casts were processed using fiber-reinforced acrylic resin. However, processing time for 12 hrs was maintained constant for all the samples. The variable to investigate was the temperature. Therefore, four denture bases processed at each temperature from 60°C to 80°C with two-degree increments for a total of 44 samples using simple acrylic resin. Remaining four samples processed at 70°C using fiber-reinforced acrylic resin. A traveling microscope used to measure the discrepancy between each cast and its denture base.
Statistical analysis used: Analyzed with Student's unpaired t test.
Results : Minimum distortion was observed at 70°C. Maximum distortion was observed for the denture bases processed at 80°C. Amount of distortion increases as the processing temperature increases which was highly significant. Distortion was significantly high from 60 to 68°C in the decreasing order.
Conclusion: This study verified the observation that maxillary complete denture base show the greatest discrepancy at the central portion of the posterior palatal seal region as the processing temperature increases.

Keywords: Distortion, posterior palatal seal, processing temperature

How to cite this article:
Pasam N, Hallikerimath R B, Arora A, Gilra S. Effect of different curing temperatures on the distortion at the posterior peripheral seal: An in vitro study. Indian J Dent Res 2012;23:301-4

How to cite this URL:
Pasam N, Hallikerimath R B, Arora A, Gilra S. Effect of different curing temperatures on the distortion at the posterior peripheral seal: An in vitro study. Indian J Dent Res [serial online] 2012 [cited 2019 Dec 10];23:301-4. Available from: http://www.ijdr.in/text.asp?2012/23/3/301/102209
Complete denture fabrication is fraught with intrinsic and extrinsic potential errors. The greater part of this error controlled with careful clinical and laboratory techniques. The key for the success of prosthesis is retention. [1] Dimensional changes during processing are one of the greatest errors that lead to loss of retention of complete denture. Many variables that influence dimensional changes occur in the denture considered during both the processing phase and patient use. Although standards for processing established to minimize dimensional changes during curing of dentures but the effect of curing temperature on the accuracy of fit of a complete denture is still controversial. The accuracy, fit and retention of a denture base fabricated with poly (methyl methacrylate) correlated to the heat involved in processing. The amount of heat associated with processing of acrylic resins correlated with the adaptation of the processed denture base to its supporting tissues. A lower or higher temperature used to adequately process acrylic resin for a denture and still produce an accurate fit.

Therefore, the study conducted to determine the effect of processing temperature, on the accuracy of fit of the complete maxillary denture and to compare it with the accuracy of a high-impact acrylic resins processed by long curing cycle


   Material and Methods Top


The present study was conducted in the Department of Prosthodontics and Crown and Bridge, K.L.E.S's Institute of Dental Sciences, Belgaum, Karnataka.

An ideal rubber mould was used to make edentulous non-undercut maxillary cast to allow easy removal and replacement of processed denture bases [Figure 1].
Figure 1: Ideal maxillary edentulous rubber mould.

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Dental stone measured and mixed with the recommended amount of water. Forty -eight stone casts made from the ideal rubber mould. Each cast was marked for identification and covered with two layers of base plate wax, which were also marked for identification [Figure 2]. Out of 48 identical stone casts, 44 stone casts with wax denture bases processed using simple acrylic resin (Group 1). The remaining four stone cast were processed using fiber-reinforced acrylic resin (Group 2). However, processing time for 12hrs was maintained constant for all the samples.
Figure 2: Ideal maxillary edentulous cast and two layers of base plate wax adapted.

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The variable investigated was the temperature. Therefore, four denture bases processed at each temperature from 60 to 80°C with two-degree increments for a total of 44 samples using simple acrylic resin. Remaining four samples i.e., Group 2 processed at 70°C using fiber-reinforced acrylic resin.

To minimize the possibility of residual monomer, porosity because of incomplete processing at low temperature and shrinkage due to polymerization, each of the 48 samples processed with acrylic resin for 12hrs at specific temperatures. Automatic processing bath was used to maintain a constant processing temperature.

All the maxillary dentures were prepared by standardized technique. Four maxillary dentures were processed in a water bath at 60°C for 12 hours with a two-degree increment in the temperature for each of the four samples were processed until 80°C. Following completion of the polymerization cycle, the denture flasks kept for bench cooling overnight. Subsequently the denture bases were then deflasked.

The maxillary denture base removed from the cast on which it was processed. The cast trimmed using model trimmer approximately until the region of posterior palatal seal. Water used as a coolant. The irregularities at the center of the posterior palatal seal removed with decreasing grades of coarseness of sandpaper. Midline and 2 mm on either side of the midline were marked on the cast and the denture base to measure the discrepancy between the cast and the denture base at the centre of the posterior palatal seal. Traveling microscope used to measure the distance between the cast and its respective denture base [Figure 3].
Figure 3: Travelling microscope.

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The formula used to calculate the distance is Total Reading = Main Scale Reading + (Coinciding Vernier Division × Least Count). The least count of the microscope was 0.001 cm. Five measurements were taken for each sample, to determine the discrepancies between dentures and stone casts. Average of the five values taken for each sample and thus average of twenty values were taken for samples at each temperature.


   Results Top


This study carried out on an ideal edentulous maxillary cast to determine the discrepancy incorporated into the posterior aspect of maxillary denture bases by long curing cycle. Data was statistically analyzed using, Student's unpaired 't' test. [Table 1] shows distortion observed for the denture base cured at different curing temperatures (Group 1).
Table 1: Group 1

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Observations

  • Minimum distortion observed at 70°C and there was no significance difference between the denture bases processed from 68 to 72°C.
  • Maximum distortion observed for the denture bases processed at 80°C.
  • Amount of distortion increases as the processing temperature increases which was highly significant when compared with that at 70°C. The results of this study are enumerated in [Table 1], [Table 2] and depicted in [Graph 1] and [Graph 2].
    Table 2: Group 2

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[Table 2] shows comparison of the distortion for denture bases processed using fiber-reinforced acrylic resin with simple acrylic resin and the observations are:

Observations

Distortion was less with the denture bases processed using fiber-reinforced acrylic resin when compared with simple acrylic resin but the amount of discrepancy was not significant as shown in [Table 2].


   Discussion Top


Retention of a denture is judged by its resistance to removal in a direction opposite to that of its insertion. The amount of denture retention is greatest when the film thickness is as thin as possible, i.e., when the fit of the denture is as close as possible to the oral tissues. [2]

The coefficient of linear expansion of the resin is much greater than that of the gypsum products in which it is formed and thus introduces internal strain. [3] In addition, temperature of the curing cycle affects the polymerization shrinkage. [1] Thickness of the dentures, powder: liquid ratio of the acrylic resin affects the dimensional accuracy. The pressure applied and maintained during flasking tends to improve accuracy of the fit of the denture bases. [4] The anatomic form of the palate also shares the responsibility in maintaining the positive contact with the tissue of the palate. [5] The mandibular denture exhibits no similar phenomenon because of the labial aspect of the alveolar ridge, which is neutralized by the lingual flange. Hence, in this study the maxillary denture base tested for the discrepancies. [6]

It was observed that processing changes that occur in complete dentures when the acrylic resin is being cured resulting in an overall contraction that requires adjustments of both the base and occlusion. The clinical signs of such contraction in maxillary dentures usually observed as a "pulling away" from the mid-palate, with corresponding pressures on the lateral aspects of the tuberosities. [7]

Although standards for processing cycles established to minimize the dimensional changes in dentures, discrepancy is seen in the dentures at the borders. Manifestation of these dimensional changes at the posterior palatal border of the dentures must be compensated by creation of a postpalatal seal. [8]

A highly significant difference existed between the denture bases processed on the stone casts at temperatures more than 72°C. The increased distortion in the denture bases processed at higher temperatures is associated with rapid polymerization. At a higher temperature, the methyl methacrylate polymerizes readily. The polymerization reaction begins rapidly due to the heat liberated from the exothermic heat of polymerization added to the external heat liberated from the water bath. If the external temperature is much above 72°C, internal stresses may be setup in the structure. The strain begins when the material reaches the temperature at which the resin begins to take on physical properties of a solid, the "glass-transition temperature". From this point to room temperature, the change is related to the resin's coefficient of linear expansion (81*10-6/°C). The gypsum products, which form the mold, have a coefficient of linear expansion one-eighth that of acrylic resin. This difference contributes to the dimensional change and induced strain. [1]

Limitations

Even though the literature strongly recommends the importance of long-curing cycle for the processing of complete denture to minimize the monomer content and complete polymerization but when it comes to clinical practice short curing cycle, 2 hrs of curing at 73°C followed 1-hour terminal boiling is commonly followed. So, the study could have been conducted for denture bases cured in short curing cycle and compare the amount of distortion at different curing temperatures.


   Conclusion Top


A highly significant difference existed between the denture bases processed from 60 to 70°C distortion was significantly high from 60 to 68°C in the decreasing order. The reason could be at lower temperatures short molecular chains of the resin formed that are not stable once the resin reaches room temperature.

The study concluded that there was increased amount of distortion in the posterior region as the curing temperature increases with least distortion at 70°C and maximum distortion occurring at 80°C. Distortion was less with the denture bases processed using fiber-reinforced acrylic resin when compared with the simple acrylic resin but the amount of discrepancy was not significant.

The result of this study verified the observation that maxillary complete denture base show the greatest discrepancy at the central portion of the posterior palatal seal region as the processing temperature increases and confirmed reports that were done previously.

 
   References Top

1.Firtell DN, Green AJ, Elahi JM. Posterior peripheral seal distortion related to processing temperature. J Prosthet Dent 1981;45:598-601.  Back to cited text no. 1
[PUBMED]    
2.Craig RG, Berry GC, Peyton FA. Physical factors related to denture retention. J Prosthet Dent 1960;10:459-67.  Back to cited text no. 2
    
3.Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes occurring in dentures during processing. J Am Dent Assoc 1960;61:413-30.  Back to cited text no. 3
[PUBMED]    
4.Consani RLX, Domitti SS, Consani S. Effect of a new tension system, used in acrylic resin flasking, on the dimensional stability of denture bases. J Prosthet Dent 2002;88:285-9.  Back to cited text no. 4
    
5.Glazier S, Firtell DN, Harman LL. Posterior peripheral seal distortion related to height of the maxillary ridge. J Prosthet Dent 1980;43:508-10.  Back to cited text no. 5
[PUBMED]    
6.Vig RG. Method of reducing the shift of teeth in denture processing. J Prosthet Dent 1975;33:80-4.  Back to cited text no. 6
    
7.Lechner SK, Lautenschlager EP. Processing changes in maxillary complete dentures. J Prosthet Dent 1984;52:20-4.  Back to cited text no. 7
[PUBMED]    
8.Hardy IR, Kapur KK. Posterior border seal: Its rationale and its importance. J Prosthet Dent 1958;8:386-97.  Back to cited text no. 8
    

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Correspondence Address:
Nirmala Pasam
Department of Prosthodontics and Crown and Bridge, Gitam Dental College, Vishakapatnam
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.102209

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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    Abstract
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