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ORIGINAL RESEARCH Table of Contents   
Year : 2010  |  Volume : 21  |  Issue : 3  |  Page : 391-395
Influence of the processing technique on the flexural fatigue strength of denture base resins: An in vitro investigation


1 Department of Prosthodontics, VSPM Dental College and Research Centre, Nagpur, India
2 Department of Orthodontics , VSPM Dental College and Research Centre, Nagpur, India
3 Department of Orthodontics , BVU Dental College and Hospital, Pune, India

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Date of Submission25-Jul-2009
Date of Decision09-Sep-2009
Date of Acceptance04-Mar-2010
Date of Web Publication29-Sep-2010
 

   Abstract 

Background : Though acrylic resins possess many desirable properties, denture fracture due to flexural fatigue or impact failure is a common problem. One major factor influencing the flexural fatigue strength of denture base resins is the processing technique used.
Aim: To measure the flexural fatigue strength of denture base resins polymerized using short and long curing cycles using water bath, pressure cooker, and microwave polymerization techniques.
Materials and Methods: Flexural fatigue strength of 60 samples (n=10) were measured using a cyclic 3-point loading method on a dynamic universal testing machine. Data were analyzed using a Student 't' test.
Results : Comparative evaluation using Student's 't' test of mean flexural fatigue strength of samples processed by water bath processing (660.6) and the microwave technique (893.6) showed statistically significant (P <0.01) result with microwave processing being higher. Comparison of water bath (660.6) and pressure cooker (740.6) processing and microwave (893.6) and pressure cooker (740.6) processing using Student's 't' test was not statistically significant (P >0.05). In the intra-group analysis, it was found that there was statistically significant difference in samples processed using the short and long curing cycle, the latter being better in all groups, P-values being <0.05, <0.001, and <0.001 for water bath, microwave, and pressure cooker polymerization techniques, respectively.
Conclusion : The polymerization procedure plays an important role in influencing the flexural fatigue strength of denture base resins, and the microwave long curing processing technique produced denture bases with highest flexural fatigue strength.

Keywords: Denture base polymerization, flexural fatigue strength, microwave polymerization

How to cite this article:
Banerjee R, Banerjee S, Prabhudesai P S, Bhide S V. Influence of the processing technique on the flexural fatigue strength of denture base resins: An in vitro investigation. Indian J Dent Res 2010;21:391-5

How to cite this URL:
Banerjee R, Banerjee S, Prabhudesai P S, Bhide S V. Influence of the processing technique on the flexural fatigue strength of denture base resins: An in vitro investigation. Indian J Dent Res [serial online] 2010 [cited 2019 Sep 18];21:391-5. Available from: http://www.ijdr.in/text.asp?2010/21/3/391/70810
The introduction of acrylic resin material in 1937 by Wright WH [1] revolutionized the discipline of dental prosthesis to a great extent. By 1946, acrylic resin became the most popular material for fabricating denture base. The reason for the wide acceptance of this material has been a combination of many desirable properties [2] accredited to it, which compensated for many of the shortcomings of the materials previously used.

The denture fracture has been attributed to either impact failure or fatigue failure. Though impact failure is a frequent occurrence, it occurs only when the dentures are dropped. According to Smith, [3] Kelly EK, [4] Beyli MS and Frauntofer A, [5] a more clinically significant cause for denture fracture is fatigue under masticatory loads as a result of the repetitive flexing of the denture bases. Kelly [6] accounted many factors for influencing flexural fatigue strength, some of them being frenum notches, surface irregularities, and foreign body inclusions. Porosities and residual monomer content have been shown as important factors influencing the flexural fatigue strength. The processing technique used to polymerize the denture base resin has been found to be an important factor, which can induce stress into the denture base during processing and finally lead to fatigue failure.

Out of the three processing techniques used in this study, the water bath processing technique has been the most conventionally used polymerization technique. In spite of the advantages provided by this technique like the ease, simplicity and cost-effectiveness, a major disadvantage has been the long processing time required. The microwave polymerization technique introduced by Nishii et al, [7] has in various previous studies produced denture bases with comparable physical and mechanical properties, [7] lesser porosity, [8] and better dimensional accuracy [8] when compared to the conventional water bath processing technique. Indian researchers like Muley, [9] Sidhaye AB, and Undurwade JH [10] extensively investigated the pressure cooker polymerization technique. Conventional acrylic resin material can be used for this technique and requires less than 1 h for polymerization and utilizes conventional equipment. Previous studies of pressure cooker polymerization have shown comparable physical and mechanical properties to the water bath technique. Though many properties [11],[12],[13] such as impact strength, transverse strength, porosity, residual monomer content, and dimensional accuracy have been evaluated extensively, very few studies have reported study of the flexural fatigue strength [14] of denture base resins. Furthermore, flexural fatigue strength of denture base resins polymerized by the microwave and pressure cooker techniques has not been evaluated. Therefore, a need was felt to design an investigation to evaluate and compare the flexural fatigue strength of denture base resins polymerized by the conventional water bath, microwave, and pressure cooker polymerization techniques. The hypothesis established on the basis of the previous studies on the strength properties of denture base resins was that microwave polymerization could produce denture bases with better flexural fatigue strength than the other two techniques due lesser porosities, [15] complete polymerization, [16] and least residual monomer content. [17]


   Materials and Methods Top


Testing method followed those standardized by the American National Bureau of standards [5] for transverse deflection and stiffness tests. Materials and methods used have been summarized in [Table 1].
Table 1 :Materials and methods


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Specimen fabrication began with making of metal strips for the preparation of 60 samples measuring 65Χ10Χ3 mm, one for each specimen. The metal strips were then invested into metal or non-metal fiber flasks depending on the method of polymerization, using type III dental stone (Kalstone, Kalabhai, India). All three types of acrylic resins were mixed according to the manufacturer's direction and packed into the molds at the dough stage. For the water bath polymerization, [18] the polymer:monomer ratio of the material was 3:1, and the dough time was 3 min. Specimens were polymerized using a long curing cycle of 74°C for8 h with no terminal boiling and a short curing cycle of 74°C for 2 h followed by 1 h at 100°C. For the microwave polymerization [19] technique, the polymer:monomer ratio was 3:1 19 ml (4 g) of polymer and 6.5 ml of monomer as specified by the manufacturer. The curing cycles that were used were a long curing cycle of 10 min at 540 watts and a short curing cycle of 7 min at 600 watts. For the pressure cooker polymerization, [9],[10] technique the polymer and monomer was mixed according to dose recommended by the manufacturer i.e. 1:3 by volume. The pressure cooker was filled with 250 ml of water and kept on the gas flame to boil. The lid was placed without locking. As the water started boiling at 100 o C, the clamped flasks with acrylic resin packed into it was placed into the boiling water and the lid was locked into position. When the pressure inside the cooker reached 760 mm of mercury, which was indicated by a low, continuous whistle, the flame was reduced and the pressure was maintained. The curing cycles that were used were a long curing cycle of 30 min from the time the lid is closed and a short curing cycle of 20 min from the time the lid is closed. The samples produced by all the three techniques were finished and polished and were stored in water at room temperature for 7 days till the testing. The flexural fatigue strength was measured using a cyclic (repetitive) one-way flexure 3-point bending test, [20] with a dynamic universal testing machine (Instron testing machine-8502). The one-way flexure test was selected, as Hargreaves [21] suggested it to be more clinically relevant. The samples were horizontally positioned on a metal fixture [Figure 1] with a distance of 36 mm between the two fixed supports (each 3 mm in diameter). A vertical load with a peak force of 16 kg and amplitude of 1.5 mm was applied midway between the supports to better simulate the maximum masticatory force in an edentulous individual and the average deflection of a denture base, respectively, as previously described elsewhere. [22],[23]
Figure 1 :Fixture with samples attached to the Instron testing machine

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The number of cycles to fracture each sample was measured and tabulated to calculate the mean, standard deviation, and significance for the results obtained for each group using the following statistical formulas:

a) Arithmetic mean X= X=εXn

N

where ε=sum of, Xn=individual value,

N=total number of values

b) Standard deviation (SD)=SD=√ε(Xn-X) 2

N-1

where ε=sum of, X=arithmetic mean,

N=total number of observations.

c) 't' value=X-μ

SD-√N

where X=arithmetic mean, μ=expected value, SD=standard deviation, N=total number of observations.


   Results Top


The number of cycles to fracture each sample was measured and tabulated to calculate the mean flexural fatigue strength which was then comparatively evaluated for the three techniques and the observations were statistically evaluated to derive the following results.

In the intra-group analysis, comparative evaluation of the flexural fatigue strength of the samples cured by the short curing cycle to the long curing cycle was done in each group applying the Student 't' test [Table 2]. It was found that there is a statistically significant difference in the flexural fatigue strength values of specimens processed using the short and long curing cycle, the latter being better in each group. In the water bath processing group, the statistical analysis of the samples cured using short cure and long cure cycles showed statistically significant difference as P<0.05 (t =2.29, df=18). In the microwave processing group, the statistical analysis of the samples cured using short cure and long cure cycles showed statistically significant difference as P<0.001 (t =5.43, df=18). In the pressure cooker processing group, the statistical analysis of the samples cured using short cure and long cure cycles showed statistically significant difference as P<0.001 (t =5.98, df=18).
Table 2 :Mean flexural fatigue strength and standard deviation values for each group


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In the inter group analysis the Student 't' test was applied and the following results were obtained. A comparative evaluation of samples processed by water bath processing and the microwave technique showed that the latter produced flexural fatigue strength values higher than the former and the difference was statistically significant (t=2.98, df=18, P<0.01). A comparative evaluation between the flexural fatigue strength values of water bath and pressure cooker curing showed that the difference was not statistically significant (t=1.26, df=18, P>0.05). A comparative evaluation between the flexural fatigue strength values of samples processed by microwave and the pressure cooker polymerization technique showed that the difference was not statistically significant (t=2.09, df=18, P>0.05). Through the results obtained it can be derived that the microwave long curing processing technique produced samples with highest flexural fatigue strength [Figure 2].
Figure 2 :Graphical representation of the result

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   Discussion Top


Through years the water-bath processing technique has been the most widely used due to its ease of handling and cost effectiveness, but the residual monomer content [24] and porosities produced have been suggested as the most significant reasons for the reduced flexural fatigue strength. [25] This has been accounted to the unfavorable thermal gradient produced during the processing technique. [26] In the water-bath processing technique, the benzoyl peroxide is activated by heating the water bath to very high temperatures thus initiating the polymerization reaction by crosslinking methyl methacrylate molecules. At this point, the methyl methacrylate boils creating porosities in the denture base resin. [27] As the reaction progresses heat is liberated and cannot escape easily as the water surrounding the flasks is being heated as well, thus an unfavorable thermal gradient is created. To control the detrimental effects of this unfavorable thermal gradient, it is necessary to polymerize the denture base resin at a much slower rate for complete polymerization, which represents a significant limitation of this polymerization technique. In this study the water-bath polymerization yielded the lowest values of flexural fatigue strength using the long-curing cycle. Therefore, this study support the hypothesis of Kelly, [6] Reitz et al [7] and De Clerck [8] that the residual monomer content and porosities in denture base resins might be the reasons behind such a decrease. The evaporation of the residual monomer leaves porosities in the denture base resin and these imperfections lead to formation of stress and cause propagation of cracks within the acrylic which makes the denture base prone to fatigue failure. [28] Thus, residual monomer content can be directly related to the flexural fatigue strength of acrylic resins.

The microwave polymerization technique takes less than 10 min, which has been claimed as its major advantage. Studies also have reported better dimensional accuracy, [8] transverse strength, [7],[8] lesser residual monomer content [8] and porosities [7] of microwave polymerized denture base resin as compared to other polymerization methods. In this technique, electromagnetic waves produced in the microwave oven are used to generate heat inside the resin. During the polymerization, the methyl methacrylate molecules orient themselves in the electromagnetic field of the microwave and their direction changes nearly 5 billion times in a second. The numerous collisions that occur cause rapid heating, and therefore, a less time-consuming procedure is required. As the heat required to break the benzoyl peroxide molecules into free radicals is created inside the resin, the temperature outside the flask remains cool. As a consequence of this rapid reaction, the polymerization heat is dissipated more effectively and the polymerization has lesser risk of porosity. Moreover, as the temperature increases, the number of monomer molecules decrease and the residual monomer content is reduced to minimum. Therefore, it is suggested that the favorable thermal gradient and reduced residual monomer content, and thereby porosities, seem to be the reasons for the highest flexural fatigue strength with this polymerization technique. The use of pressure cooker for denture polymerization was first reported by Muley [9] in 1976. Various physical properties such as dimensional accuracy, impact strength, polymerization shrinkage, and residual monomer content were extensively reported. [9],[10],[29] Accordingly, the pressure plays an important role in speeding up the initial polymerization and reducing the porosity and residual monomer content, thus increasing the flexural fatigue strength. The advantages of using pressure can be obtained either with an autoclave or a domestic pressure cooker. Yet, an early pilot study by S.V. Bhide [30] suggested that only the pressure cooker was consistent in producing non-porous dentures. In this former study, another proposed advantage of using pressure cooker over an autoclave was also that the required 760 mm of mercury above atmospheric pressure was achieved in less than 5 min, while an autoclave required 20 to 30 min to obtain pressure values at the same level. An important role played by rising steam pressure is that it is instantly transmitted to the resin dough, possibly accelerating the initial polymerization and reducing the boiling of the monomer and thus preventing the residual monomer and the porosities as described by Sidhaye and Undurwade. [10] The faster polymerization due to the steam pressure along with reduction in the residual monomer content and porosities seems to be the reason for the higher flexural fatigue strength of the samples cured using this technique in the present study as compared to the water bath technique. However, when compared to the microwave technique, the thermal gradient in the latter is more favorable than the pressure cooker technique and, although the samples showed better fatigue strength as compared to the water bath technique, showed lower flexural fatigue strength when compared to the microwave technique

The study also evaluated whether there was a difference in the flexural fatigue strength of samples in each group using the short or long curing cycles. The shorter curing cycles provided faster polymerization but as found by previous studies [6],[7],[8] it is suggested here that the incomplete polymerization due to the rapidity of heating resulted in high level of residual monomer content and porosity in denture base resins. This clearly accounts for the significantly higher properties of long curing cycles for all the groups in the present study.

Although the current investigation was done under controlled laboratory conditions and the values give a good estimate of the flexural fatigue strength of the denture base resins polymerized by the three different processing techniques used, the study may not be entirely simulative of the intra-oral conditions and may require further clinical studies to arrive at more clinically significant results. It has been principally postulated that the difference in flexural fatigue strength found was due to differences in porosity and residual monomer caused due to the method of polymerization, but measurement of porosity and residual monomer was not in the scope of this study which has been another limitation of this study. Thereby further research to correlate flexural fatigue strength to porosity and residual monomer should form the basis for future studies.


   Conclusion Top


The polymerization procedure plays an important role in influencing the flexural fatigue strength of denture base resins. The established hypothesis was proved correct as the microwave polymerization technique produced denture base samples with the highest flexural fatigue strength, possibly due to the complete polymerization and reduced residual monomer content and thereby minimum porosities. The pressure cooker polymerization technique might also be a good alternative to the conventional water bath processing technique and should be the preferred method of polymerization after the microwave technique. The water bath technique produced the lowest flexural fatigue strength. In all the techniques, the long curing cycle proved to be better in producing denture bases with higher flexural fatigue strength as compared to the short curing cycle.

 
   References Top

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4.Kelly EK. Flexure Fatigue rιsistance of heat curing and cold curing polymethyl methacrylate. J Am Dent Assoc 1967;74:1273-6.  Back to cited text no. 4      
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7.Reitz PV, Sanders JL, Levin B. The curing of denture acrylic resins by microwave energy: Physical properties. Quintessence Int 1985;8:547-51.  Back to cited text no. 7      
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9.Sidhaye AB. Polymerization shrinkage of heat cured acrylic resins processed under steam pressure. Indian Dent Assoc 1981;53:49-51.  Back to cited text no. 9      
10.Undurwade JH, Sidhaye AB. Curing acrylic resin in a domestic pressure cooker: A study of residual monomer content. Quintessence Int 1989;20:123-9.  Back to cited text no. 10      
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Correspondence Address:
Rajlakshmi Banerjee
Department of Prosthodontics, VSPM Dental College and Research Centre, Nagpur
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.70810

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]

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