| Abstract|| |
Background: Acrylic resin dentures are susceptible to fracture after clinical use, which is a problem of concern in prosthodontics. Impact failure outside the mouth and flexure fatigue failure in the mouth are two most important causes of fracture of denture base.
Aim: This study evaluated the transverse deflection and transverse strength of four commercial brands of heat cure acrylic resin (Stellon, Acrylin-H, Trevalon and Trevalon-HI).
Materials and Methods: An experimental design was adapted. Twenty-four rectangular strip specimens, six for each group, were prepared. Strips were finished on the edges and equally from the both the molded surfaces to make strips of specific dimensions. The tests were conducted mainly in accordance with the American Dental Association Specification no. 12/ISO: 1567-1981 (ISO: 6887-1986) for denture base polymer. The transverse deflection and transverse strength were measured by Instron testing machine. Intergroup differences were assessed using student "t" test.
Results: The heat cure denture base material D (Trevalon "HI") had the minimum mean value of transverse deflection under different loads. Trevalon "HI" also had minimum value of mean transverse strength among different brands of acrylic resins. There was no statistically significant variation between Stellon, Acrylin-H and Trevalon, but variation was significantly higher with D (Trevalon "HI") vs. Stellon, Acrylin-H and Trevalon.
Conclusion: The heat cure denture base material D (Trevalon "HI") was the strongest and C (Trevalon) was the weakest among all materials used in this study. The study showed that the deflection of various denture base resins (A to D) increases proportionately with the increase in load.
Keywords: Acrylic resins, dental materials, denture, denture fracture, transverse deflection, transverse strength
|How to cite this article:|
Chand P, Patel CB, Singh BP, Singh RD, Singh K. Mechanical properties of denture base resins: An evaluation. Indian J Dent Res 2011;22:180
Loss of teeth is a matter of great concern to a majority of people, and their replacement by artificial substitutes, such as dentures fabricated with acrylic resin, is vital to the continuance of normal life. Denture base acts as an intermediary between teeth and the jaw and has to transfer all or part of the masticatory forces to the subadjacent tissues. One of the problems encountered in the provision of such prosthesis is whether the limitations of strength and design meet the functional demands of the oral cavity.  Acrylic resins are widely used in dentistry. As Dootz et al. have shown that material aging can dramatically affect the physical and mechanical properties of material, prediction of the service life of acrylic resin material is difficult since many extraoral and intraoral factors affect durability. Impact failure outside the mouth and flexure fatigue failure in the mouth are the two most important causes of fracture of denture base. 
|How to cite this URL:|
Chand P, Patel CB, Singh BP, Singh RD, Singh K. Mechanical properties of denture base resins: An evaluation. Indian J Dent Res [serial online] 2011 [cited 2020 Aug 3];22:180. Available from: http://www.ijdr.in/text.asp?2011/22/1/180/79997
Transverse deflection and transverse strength test had been claimed to be one of the most important tests for a denture base material, since it is subjected to a lot of bending forces in the mouth. ,
In India, a number of heat cure acrylic resins have recently been introduced under the trade names Stellon, Acrylin-H, Trevalon, and Trevalon-HI. It is very difficult for one to know whether any of these materials satisfies the critical requirement of properties of various acrylic resins marketed in India in accordance with well-known American Dental Association (ADA) specification no. 12. 
It is felt that the result derived from the present investigation will enable a general dentist to know the properties like transverse deflection, transverse strength and incorporate this result in choosing a resin for denture base fabrication.
It is further expressed that an investigation of this type will lead to awareness for the need of establishing a suitable program for testing of dental materials and the measures adopted to increase the strength.
| Materials and Methods|| |
This experimental study was carried out in the Department of Prosthodontics with the collaboration of Research Design and Standard Organization, Lucknow, to evaluate mechanical properties of denture base resins.
The following standard materials were used for the purpose of study:
- Denture base resins: four brands of denture base resins were included in this study [Table 1], [Figure 1];
- Dental stone plaster;
- Cold mould seal;
- White petroleum jelly I.P.; and
- Cellophane sheet.
The test was conducted mainly in accordance with the ADA Specification no. 12/ISO:1567-1981 (ISO: 6887-1986) for denture base polymer. The room temperature of laboratory varied from 29 to 34°C during the entire procedure of experiments. Methods for detection of transverse deflection and strength test of different acrylic resins were divided into three parts:
- preparation of specimens,
- testing procedure for transverse deflection, and
- testing procedure of transverse strength.
Stainless steel die plate of dimension 65 × 40 × 5 mm (M.A. engineering work, Lucknow, India) was mounted with stone plaster (Kalabhai Karson Pvt. Ltd., Mumbai, India) in a Hanau flask (Hanau Eng. Co. Buffalo, NY, USA) [Figure 2]. The stone plaster in the shallow half of the flask was poured against a glass plate to obtain smooth surface. Dewaxing was done. Separating media (Cold mould seal Dental product of India, Ltd. Mumbai, India) were applied, heat cure resins were packed, processed in acrylizer (Hustman, Bath, UK) and sample plates were recovered [Figure 3]. After processing, the acrylic plate was cut lengthwise into three equal strips with the help of a fret saw. Strips were finished on the edges and equally from the both the moulded surfaces to make strips of specific dimensions of 64 ± 0.001 mm in length, 10 ± 0.01 mm in width, and 2.5 ± 0.01 mm in thickness [Figure 4].
Testing procedure for transverse deflection
The specimen was mounted on the transverse testing ring of Instron machine (Model no. 6027, Instron Ltd., Buckinghamshire, UK) [Figure 5]. The specimen was initially subjected to a load of 1500 g. A load of 500 g was added during the last 30 seconds of every minute and the readings were noted at the end of first 30 seconds of each minute. The transverse deflection readings were observed on the control and printer on Instron machine having an accuracy of 0.01 mm. The difference between the deflection at the initial load and reading at the specific loads (i.e., at 3500 and 5000 g) was taken as the deflections of the specimen. The record deflections were reported to the nearest 0.01 mm.
Testing procedure of transverse strength
The increments of 500 g weight were continued until the test specimens fractured, indicating the breaking load of the material, then the transverse strengths of the materials were calculated.
Transverse strength can be calculated as
S = 3lp/2wt 2
S = transverse strength,
l = distance between the supports,
p = breaking load,
w = specimen width, and
t = thickness of specimen.
The values were entered as mean ± SD/SE and intergroup comparisons were made using student "t"-test. The confidence level of the study was kept at 95%, and a "P" value less than 0.05 indicated statistically significant difference.
| Results|| |
The present study comprised four brands of commercially available heat cured denture base resins. A total number of 24 specimens were prepared, six for each group, for transverse deflection and transverse strength test. Statistical analysis was done on the computer with the help of Epi-Info software (Epi Info™version 3.5.1, Centers for Disease Controls and Prevention. Atlanta, GA, USA).
[Table 2] shows the mean transverse deflection of four types of denture base resins under various loads. No transverse deflection was observed at 1500 g of load with all brands of acrylic. Deflection was increased with increase in load.
|Table 2: Mean of the measurements of transverse deflections (in mm) at various loads of denture base resins from A to D|
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[Table 3] shows the mean deflection of four denture base materials under the load of 3500 and 5000 g. The mean deflection under these loads was maximum with material C and minimum with D.
[Table 4] shows the comparison of transverse deflection among the different materials under the load of 3500 and 5000 g. Since there was no statistically significant difference in "t" values of all the groups of materials, no significant difference was found in all the materials when compared to each other.
|Table 4: Comparison of deflections among the materials under the load of 3500 and 5000 g with t and P values|
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[Table 5] shows the mean analysis of breaking load of various denture base materials. The mean value of C is minimum with 6.08 kg and maximum with 8.44 kg. [Table 5] also shows the mean transverse strength of four denture base materials. The mean value of C is least with 72.93 MN/m 2 and that of D is maximum with 100.24 MN/m 2 among the groups.
|Table 5: Mean analysis of breaking load and transverse strength of various materials|
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[Table 6] shows the comparison of transverse strength among the different materials. There was no statistically significant difference in A vs. B, A vs. C and B vs. C. It was found that the deflection was significant statistically in A vs. D, B vs. D and C vs. D (P < 0.05)
|Table 6: Comparison of transverse strength among the various materials with t and P values|
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| Discussion|| |
In this study, four brands of commercially available heat cure denture base resins (A-D) were selected. Since the commonly available materials are usually used by the general practitioner, this study will be beneficial from a clinical point of view.
It had been suggested that transverse strength and transverse deflection depend upon the method of curing  and curing time  and method of moulding.  Therefore, in this study, all the above factors were kept constant for the materials A-D.
Transverse strength is a collective measurement of tensile, compressive and shear strengths simultaneously. This transverse strength represents the type of loading born by a denture in the mouth. The higher the value of transverse strength of denture base acrylic, superior the clinical performance will be. Likewise, a stronger material is less likely to cause fracture in function.
The result of transverse deflection under various loads as depicted from [Table 2] and [Table 3] shows that the deflection of various denture base resins of A-D increases proportionately with the increase in load. It was observed that there was maximum deflection for C (1.31 mm) and minimum deflection for D. Thus, the various denture base resins were graded in terms of increasing deflection displaying the following profile:
D < A < B < C
Lesser the deflection under load, stronger the material will be. Hence, D was the strongest and C was the weakest.
[Table 4] shows the comparison of transverse deflection of materials from A to D under the load of 3500 and 5000 g. It is seen that there was no statistically significant difference among the groups when compared with each other, but the deflection was maximum (1.31) for C and minimum (1.02) for D.
[Table 5] shows the mean value of breaking load from A to D. The table shows that D broke at the highest load (8.44 kg) and C broke at the lowest load (6.08 kg). This implies that D was the strongest material and C was the weakest. The greater strength might be due to the good bond between the matrix and the bead in two-phased structures and the fracture line might break the beads of the material, whereas in weaker material C, the fracture line progressed through the matrix and passed around the bead. ,
[Table 5] and [Table 6] show the comparison of transverse strengths of A to D. There was no statistically significant variation between A, B, and C, but variation was significantly higher with D vs. A, B, and C. It can be inferred that D exhibited a maximum variation of transverse strength than other groups. It might be due to the presence of microdispersed rubber phase polymers, i.e., methyl methacrylate and butadiene styrene, copolymerized in an emulsion with a second coating of methyl methacrylate to cover the bead. ,
| Conclusions|| |
Based on the above observations, statistically analyzed and discussed, the following conclusions can be drawn.
- The heat cure denture base material Trevalon "HI" was the strongest and Trevalon was the weakest among all materials used in this study. The descending order of transverse strength was Trevalon "HI" > Acrylin "H" > Detrey Stellon > Trevalon
- Under the load of 3500 and 5000 g, Trevalon "HI" showed minimum deflection and Trevalon the maximum. The descending order of transverse deflection wasTrevalon "HI" < Acrylin "H" < Detrey Stellon < Trevalon
- Among the materials Acrylin "H" to Trevalon "HI", Trevalon "HI" was the best of all and Trevalon was worst of all in respect of mechanical properties of transverse deflection and transverse strength.
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Balendra Pratap Singh
Department of Prosthodontics and Dental Material Sciences, Faculty of Dental Sciences, C.S.M. Medical University, Lucknow
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]