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DENTAL SCIENCE - ORIGINAL ARTICLE
Year : 2015  |  Volume : 7  |  Issue : 6  |  Page : 461-464  

Comparison of flexural strength in three types of denture base resins: An in vitro study


1 Department of Prosthodointics, Rajah Mutiah Dental College, Chidambaram, Tamil Nadu, India
2 Department of Oral Medicine and Radiology, Indira Gandhi Institute of Dental Sciences, Puducherry, India
3 Department of Oral and Maxillo Facial Surgery, Rajah Mutiah Dental College, Chidambaram, Tamil Nadu, India
4 Department of Orthodontics, Adhiparasakthi Dental College, Melmaruvathur, Tamil Nadu, India

Date of Submission28-Apr-2015
Date of Decision28-Apr-2015
Date of Acceptance22-May-2015
Date of Web Publication1-Sep-2015

Correspondence Address:
Dr. R Arun Jaikumar
Department of Prosthodointics, Rajah Mutiah Dental College, Chidambaram, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0975-7406.163505

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   Abstract 

Aim: The aim of this study was to evaluate whether the flexural strength of a commercially available, heat polymerized acrylic denture base material could be improved using reinforcements. Materials and Methods: A total of 30 specimens (65 mm × 10 mm × 3 mm) were fabricated; the specimens were divided into three groups with 10 specimens each. They were Group 1 - conventional denture base resins, Group 2 - high impact denture base resins, and Group 3 - glass reinforced denture base resins. The specimens were loaded until failure on a three-point bending test machine. An one-way analysis of variance was used to determine statistical differences among the flexural strength of three groups. Data were analyzed by SPSS software version 21.0© (IBM Corporation, Armonk, NY, USA) and the results were obtained. Results: The flexural strength values showed statistically significant differences among experimental groups (P < 0.005). Conclusion: Within the limitations of the study polymethyl methacrylate (PMMA) reinforced with glass fibers showed the highest flexural strength values this was followed by PMMA reinforced with butadiene styrene, and the least strength was observed in the conventional denture base resins.

Keywords: Dies, flexural strength, glass reinforced denture base resins, high impact denture base resins, polymethyl methacrylate, reinforcements, specimen


How to cite this article:
Jaikumar R A, Karthigeyan S, Ali SA, Naidu N M, Kumar R P, Vijayalakshmi K. Comparison of flexural strength in three types of denture base resins: An in vitro study. J Pharm Bioall Sci 2015;7, Suppl S2:461-4

How to cite this URL:
Jaikumar R A, Karthigeyan S, Ali SA, Naidu N M, Kumar R P, Vijayalakshmi K. Comparison of flexural strength in three types of denture base resins: An in vitro study. J Pharm Bioall Sci [serial online] 2015 [cited 2019 Aug 25];7, Suppl S2:461-4. Available from: http://www.jpbsonline.org/text.asp?2015/7/6/461/163505

Polymethyl methacrylate (PMMA) was first introduced in 1937 by Dr. Walter Wright and is currently the material of choice for the fabrication of removable partial denture and a complete denture. [1] PMMA is continued to be used, because of its favorable working characteristics, processing ease, accurate fit, stability in the oral environment, and superior esthetics. Despite these excellent properties, there is a need for improvement especially in terms of strength properties. [2] The denture base resin is subjected to various stresses during the function, these includes, compressive, tensile, shear, and impact stresses. Fractures of denture base resins are still a reported clinical problem. Flexural strength of denture base resin is considered the primary mode of clinical failure. [3],[4] Hence, the aim of this study was to evaluate whether the flexural strength of a commercially available, heat polymerized acrylic denture base material could be improved using reinforcements.


   Materials and Methods Top


In this study, 30 acrylic specimens were prepared. Based on the type of acrylic resin used, the specimens were divided into three groups with 10 specimens each. They were Group 1 - conventional denture base resins, Group 2 - high impact denture base resins, and Group 3 - glass reinforced denture base resins. Each group was subjected to flexural strength evaluation.

Making of acrylic specimen

To make the mold space for the specimens, three stainless steel cuboidal dies were milled measuring 65.5 mm × 10.5 mm × 3.5 mm in length, breadth, and thickness, respectively, [Figure 1]. Thirty acrylic specimens were fabricated using these three stainless steel dies. The metal dies were flasked using type II dental plaster to complete the flasking procedure. The plaster was allowed to set for an hour, and parts of the flask were separated [Figure 2]. The stainless steel dies were retrieved to create the mold space for the acrylic specimen. The separating medium (cold mold seal) was applied to the mold space created and is allowed to set for 20 min. Three different PMMA were evaluated, namely
Figure 1: Milled stainless steel die

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Figure 2: Three stainless steel die were placed in a single flask

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  • Group 1 - conventional denture base resins (Dental Products of India heat cure resins) [Figure 3]
    Figure 3: Ten acrylic specimens taken from each group for testing flexural strength

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  • Group 2 - high impact denture base resins (Trevalon Hi) [Figure 3]
  • Group 3 - glass reinforced denture base resins (acrylic composite) [Figure 3].


The polymer and monomer were proportioned as suggested by the manufacturer in a ratio of 3:1 by volume. All the manipulation was done at the same room temperature. The acrylic resin was packed into the mold, after trial closure the flask was tightened to their final position. The specimens were subjected to curing cycle starting from room temperature to reach 74°C in 30 min and held at this temperature for approximately 2 h and then terminal boiling point was done at 100°C for 1 h. After bench cooling, the acrylic specimens were retrieved, trimmed, finished, and polished to the required dimension measuring 65 mm × 10 mm × 3 mm in length, breadth, and thickness, respectively (according to International Organization for Standardization [ISO] standardization 1567). The polished specimens were measured using a digital vernier caliper. The exclusion criteria for the samples were specimens with smaller dimensions, internal porosity, external porosity, worn out edges, and surface defects. All the thirty specimens were immersed in distilled water for 28 days at room temperature to simulate the oral conditions.

Evaluation of flexural strength

Flexural strength of the samples was accessed using the universal testing machine. The specimens were prepared by marking three lines A, B, and C. The first line A was drawn at a distance of 10 mm from the border of the specimen. The second line B was marked at 45 mm away from line A. These two lines A and B correspond to the location of supporting arm in the universal testing machine. A midline between these lines A and B was marked as line C, and it is the location the striker of the testing machine would come and contact with the specimen [Figure 4]. As the universal testing machine plunges into the specimen, the specimen would fracture at a particular load [Figure 5]. This maximum load before fracture (F) is given in Newtons in the display of the testing machine. The flexural strength of the given sample in megapascals was computed from the maximum load by using the formula S = 3FL/2BD2

S → Flexural strength
F → Maximum load (force) before fracture
L → Length of the support arm (45 mm)
B → Width of the specimen
D → Thickness of the specimen.

Figure 4: Jigs of supporting arm mounted at 45 mm apart

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Figure 5: Fracture occurring at the center of the acrylic specimen

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The mean value of the flexural strength of all the groups was computed and then statistically analyzed using one-way analysis of variance (ANOVA), using SPSS software version 21.0© (IBM Corporation, Armonk, NY, USA).


   Results Top


One-way ANOVA was used to determine statistical differences among the flexural strength of three groups. Data were analyzed by SPSS software, and the results were obtained [Table 1], [Table 2] and [Table 3]. The mean flexural strength of conventional PMMA (Group 1) was 101.18 Mpa, for high impact PMMA (Group 2) it was 121.50 Mpa, and for E - Glass reinforced PMMA (Group 3) it was 144.45 Mpa. All reinforced specimens showed higher flexural strength than the conventional unreinforced acrylic resin. Specimens reinforced with glass fibers showed the highest flexural strength values, followed by a specimen reinforced with butadiene styrene and followed by conventional unreinforced acrylic resin. The results of the statistical analysis for flexural strength were shown in the bar diagram [Statistical Figure 1].
Table 1: Results of flexural strength between three denture base resins measured in terms of megapascals

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Table 2: Descriptive analysis of data on flexural strength and impact strength

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Table 3: ANOVA on flexural strength

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The mean transverse strengths of all test groups have statistically significant differences (P < 0.005). The analysis of data revealed a significant difference between Group 1 and Group 2, and 3 (P < 0.005). Scheffe S post-hoc test was done to substantiate this [Table 4].
Table 4: Scheffé S post-hoc test on flexural strength

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   Discussion Top


The prime and most frequent site of fracture in the upper denture is in the medial line. During chewing, denture base material is subjected to flexural deformation. Flexural strength is a measure to know the resistance of the polymer to flexural deformation. Therefore, in this study, flexural were evaluated for the above three denture base resins. Over the years, there have been various modifications attempted to improve the mechanical properties of PMMA. The modifications include chemical modification of PMMA, through the incorporation of butadiene styrene to produce graft copolymer (high impact denture base resins) and mechanical reinforcement through the inclusion of various fibers (fiber reinforced denture base resins). [5],[6],[7] Therefore, all the three denture base resins (conventional resins, high impact, and glass reinforced) were included in this study. Artificial aging such as underwater storage in thermally controlled condition was simulated in this study. Different authors use different time periods of underwater storage, but the important influence of water on the flexural strength occurs during the first 4 weeks of immersion causing decrease of the flexural strength values. Hence, a 28 days immersion in distilled water at 37°C was used in this study. Regarding the fiber reinforcement in denture base resins, it has long been hypothesized that the addition of synthetic fibers to the monomer-polymer mixture may strengthen the resultant acrylic resin. Different authors used various types of fibers such as carbon, aramid, glass, polyethylene fibers. [6],[7],[8] The result of the present study, showed that silanized (E)-glass fibers could considerably enhance the flexural strength of dental polymers, which could be due to the proper impregnation of fibers with resin polymer. The other factors that are related to the strength of the fiber composite are the type of fibers, percentage of fibers in polymer matrix, the orientation of the fibers, fiber form (chopped, continuous, unidirectional, bidirectional), and the adhesion of the fibers to polymer. [8],[9],[10],[11] The use of carbon and aramid fibers produced an important clinical problem, namely poor esthetics and difficulties in polishing. Highly drawn linear polyethylene fibers appear to be a promising choice. It is biocompatible and possesses high stiffness and strength. Other advantages include its white translucent appearance, negligible water sorption, and excellent polishing characteristics. Glass fiber reinforcement has been the most clinically successful; glass fibers are also esthetically suitable and are reported to have improved flexural strength, flexural modulus, fatigue strength, and impact strength of resin. Therefore out of all the fibers, self-reinforced glass fibers were selected for this study. In the present study, the flexural strength of unreinforced and reinforced acrylic resin specimens were tested according to ISO/Draft International Standard 1567. [12] All reinforced specimens showed higher flexural strength than the conventional unreinforced acrylic resin. Specimens reinforced with glass fibers showed the highest flexural strength values, followed by specimens reinforced with butadiene styrene and followed by conventional unreinforced acrylic resin. The reason behind this would be the presence of glass fibers, which may prevent the propagation of a crack when the stresses are applied. Hence, glass fibers are strongly recommended in patients with heavy occlusal load or when fracture strength of denture base resin is of great concern. This finding is in agreement with results of previous studies. For better understanding, the fracture and deformation mechanisms future researches about the effects of the residual monomer content and the viscoelastic properties on the fracture process and microstructure of acrylic resins could be performed.


   Conclusion Top


Within the limitation of the current study, the following conclusions were drawn

  • The flexural strength values of heat polymerized PMMA were considerably enhanced by reinforcements
  • Polymethyl methacrylate reinforced with glass fibers showed the highest flexural strength values this was followed by PMMA reinforced with butadiene styrene, and the least strength was observed in the conventional denture base resins.


Financial support and sponsorship

Nil.

Conflict of Interest

There are no conflict of interest.

 
   References Top

1.
Eick JD. Biological properties of denture base resins. Dent Clin North Am 1977;21:459-64.  Back to cited text no. 1
[PUBMED]    
2.
Smith DC. The acrylic denture. Mechanical evaluation, mid-line fracture. J Prosthet Dent 1961;110:257-67.  Back to cited text no. 2
    
3.
Chitchumnong P, Brooks SC, Stafford GD. Comparison of three-and four-point flexural strength testing of denture-base polymers. Dent Mater 1989;5:2-5.  Back to cited text no. 3
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4.
Johnston EP, Nicholls JI, Smith DE. Flexure fatigue of 10 commonly used denture base resins. J Prosthet Dent 1981;46:478-83.  Back to cited text no. 4
[PUBMED]    
5.
Saygili G, Sahmali SM, Demirel F. The effect of placement of glass fibers and aramid fibers on the fracture resistance of provisional restorative materials. J Oper Dent 2003;28:80-5.  Back to cited text no. 5
    
6.
Aydin C, Yilmaz H, Caglar A. Effect of glass fiber reinforcement on the flexural strength of different denture base resin. Quintessence J 2002;33:457-63.  Back to cited text no. 6
    
7.
Kim SH, Watts DC. The effect of reinforcement with woven E-glass fibers on the impact strength of complete dentures fabricated with high-impact acrylic resin. J Prosthet Dent 2004;91:274-80.  Back to cited text no. 7
    
8.
Vallittu PK. Flexural properties of acrylic resin polymers reinforced with unidirectional and woven glass fibers. J Prosthet Dent 1999;81:318-26.  Back to cited text no. 8
    
9.
Vallitu PK. Impregnation of glass fiber with polymethyl methacrylate by using a powder coating method. J Dent Mater 1995;2:51-8.  Back to cited text no. 9
    
10.
Vallittu PK, Ruyter IE, Ekstrand K. Effect of water storage on the flexural properties of E-glass and silica fiber acrylic resin composite. Int J Prosthodont 1998;11:340-50.  Back to cited text no. 10
    
11.
Kanie T, Arikawa H, Fujii K, Ban S. Mechanical properties of woven glass fiber-reinforced composites. Dent Mater J 2006;25:377-81.  Back to cited text no. 11
    
12.
Iso/DIS 1567. Dentistry: Denture base polymers. Geneva: International Organization for Standardization; 1998. p. 1-27.  Back to cited text no. 12
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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