Abstract
PURPOSE
A new light curing urethane dimethacrylate and a cold curing resin with simpler and faster laboratory procedures may have even improved flexural properties. This study investigated the 3-point flexural strengths and flexural moduli of two alternate base materials.
MATERIALS AND METHODS
A cold curing resin (Weropress) and a light curing urethane dimethacrylate base material (Eclipse). Along with Eclipse and Weropress, a high impact resin (Lucitone199) and three conventional base materials (QC 20, Meliodent and Paladent 20) were tested. A 3-point bending test was used to determine the flexural strengths and flexural moduli. The mean displacement, maximum load, flexural modulus and flexural strength values and standard deviations for each group were analyzed by means of one-way analysis of variance (ANOVA) (with mean difference significant at the 0.05 level). Post hoc analyses (Scheffe test) were carried out to determine the differences between the groups at a confidence level of 95%.
RESULTS
Flexural strength, displacement and force maximum load values of Eclipse were significantly different from other base materials. Displacement values of QC 20 were significantly different from Lucitone 199 and Weropress.
CONCLUSION
The flexural properties and simpler processing technique of Eclipse system presents an advantageous alternative to conventional base resins and Weropress offers another simple laboratory technique.
Keywords: Denture polymers, Flexural strength, Flexural modulus, Eclipse, Weropress
INTRODUCTION
Polymethyl methacrylate (PMMA) polymers have been referred as conventional base materials and there is an ongoing effort to improve the properties of denture base materials. The goal is to have longer lasting and more biocompatible denture bases with better mechanical properties and simpler processing techniques that require less time to construct denture.1 Mechanical properties of denture base resins are crucial for the clinical success of removable prosthesis. Fractures and cracks are the most common types of failures related to base material.2,3
High impact acrylics were developed by the insertion of a rubber compound into the resin to improve impact resistance and strength properties of PMMA materials.4,5 On the other hand, the long and rather difficult laboratory procedures of both conventional and high impact resins led to studies about self-cure and light-cure resins.6 Self-cure resins offered shorter laboratory procedures but the residual monomer caused an increased risk of tissue reactions and decreased fracture resistance.7 On the other hand, light-curing systems offered simpler laboratory procedures and less risk of allergic reactions as they do not contain methyl metacrylate monomers.8 Furthermore, poor fracture resistance of early light-cure systems was improved by the entrance of a new material on the market.9 However, to our knowledge these new self- and light-cure materials have not been comparatively evaluated yet.
Therefore, the aim of this study was to investigate the 3-point flexural strengths and flexural moduli of these two alternate base materials (Eclipse and Weropress) and compare them to three conventional heat-cure acrylic resins QC 20, Meliodent and Paladent and one high impact heat-cure acrylic Lucitone 199.
MATERIALS AND METHODS
All materials used in this study are listed in Table 1.
Table 1.
Denture base materials used in the study
Specimen Preparation
A total of 48 specimens were prepared according to ISO 1567 standard with 6 different resins. Dimensions of the master patterns were 65×10×3 mm according to specified standards. These patterns were covered with high viscosity polyvinylsiloxane (Silagum Putty, DMG, Hamburg, Germany) before being invested in conventional flasks with Type 3 dental stone (Moldano; Heraeus Kulzer, Hanau, Germany). Acrylic resins were mixed and packed in accordance with the manufacturers' instructions. Conventional acrylics (Meliodent, QC-20, Paladent 20) and high impact Lucitone 199 were polymerized at 74℃ for 9 hours in an automated polymerization unit (Kavo EWL 5501, Kavo Electrotechnisches Werk GmbH, Germany). Weropress specimens were polymerized in a pressure pot heat cure unit (Ivomat IP3, Ivoclar Vivadent AG, Schaan, Lichtenstein) under 2 bars of pressure at 45℃ for 12 minutes. A teflon mold with a transparent plexyglass lid was designed to prepare the Eclipse specimens. The Eclipse dough was sandwiched in this mold and cured in its specific unit (Enterra VLC Curing Unit; DeguDent GmbH, Hanau, Germany) using the recommended 15 minute polymerization cycle. After polymerization the excess materials were removed by trimming with tungsten carbide burs using a handpiece at low speed. Both sides of the specimens were polished under running water with #320-, 400-, and 600-grit silicon carbide papers respectively. Before transverse strength test the specimens were immersed in distilled water at room temperature (20 ± 2℃) for 15 days.
Transverse Strength Testing
A 3-point bending test device (MTS Mini-Bionics, model 858, MTS Corporation, Eden Prairie, MN, USA) was used to determine the flexural strengths and flexural moduli. The device consisted of a loading wedge and a pair of adjustable supporting wedges placed 50 mm apart. The specimens were centered on the supporting wedges and the loading wedge was set to travel at a crosshead speed of 5 mm/min engaged at the center of the upper surface of the specimens. Specimens were loaded until fracture occurred. Transverse strengths were calculated using the following equation;
S = 3PI / 2bd2
Where: S = transverse strength (N/mm2), P = load at fracture (N), I = distance between the supporting wedges (mm), b = width of the specimen (mm), and d = thickness of the specimen (mm).
The mean displacement, maximum load, flexural modulus and flexural strength values and standard deviations were calculated for each group, and the data were analyzed by means of one way analysis of variance (ANOVA) (with mean difference significant at the 0.05 level). Post hoc analyses (Scheffe test) were carried out to determine the differences between the groups by using SPSS statistical software version 11.5 (SPSS Inc., Chicago, IL, USA) for Windows at a 95% confidence level.
RESULTS
The mean flexural strength, flexural modulus, force maximum load and displacement values of Eclipse, Meliodent, Lucitone 199, Weropress, QC 20 and Paladent 20 are given in Table 2. One-way ANOVA indicated that there were statistically significant differences in the flexural strengths (P<.001), flexural moduli (P<.05), force maximum load (P<.001) and displacement values (P<.001) among the six denture base materials (Table 3). Post hoc Scheffé test indicated that for flexural strength, displacement and force maximum load, the values of Eclipse are significantly different from other base materials. Only for displacement, the values of QC 20 are significantly different from Lucitone 199 and Weropres. Besides, there were no significant differences between the values of six denture base materials for flexural modulus (Table 2).
Table 2.
Scheffe multiple comparison test results expressed in mean values and standard deviations
Values having same superscript letter are not significantly different (P<.05)
P: Paladent, Q: QC 20, L: Lucitone 199, M: Meliodent, W: Weropress, E: Eclipse
Table 3.
One-Way ANOVA test results for various parameters between and within groups of six different materials
One-Way ANOVA test and set the level of significance at P<.05
DISCUSSION
Flexural strength of a material is accepted to determine the fracture resistance. The loading characteristics of fracture strength tests imitate clinical situations which a denture base undergoes in the oral environment, we preferred to stick to ISO 1567:1999 standard for transverse strength testing.5 Even though fatigue fracture is clinically more relevant, as it simulates the clinical failure mechanism more closely, the assessment of flexural strength is easier and thus has been used by other researchers.10-12 However, the lack of a thermo-cycling process and loading phase without immersing in artificial saliva could be stated as limitations of this study.
It is well known that self-cure systems offer simpler laboratory procedures. However, besides the other mechanical and chemical properties, these materials should have sufficient fracture resistance. To our knowledge, Weropress has not been investigated on the basis of flexural properties yet. According to the results of present study, Weropress specimens exhibited the lowest transverse strength among all groups but this finding was not statistically significant except for the Eclipse group. This finding indicates that flexural properties of Weropress may be considered as acceptable.
The use of light-polymerized denture base resins has been supported by several authors, not only for exhibiting acceptable strength and dimensional stability, but also for their relatively complete polymerization without free monomerin addition to their ease of manipulation and elimination of the usual denture processing armamentarium.13-17 Earlier studies have investigated the mechanical properties of Triad (light-cured resin) comparing to conventional and high impact resins. The overall results of these studies indicate that the transverse strength of Triad is higher than conventional polymethylmethacrylate but lower than high impact resins.18 However, one of the two different base resins we examined, the light-activated urethane dimethacrylate (UDMA) (Eclipse), revealed significantly higher flexural strength values where the cold-curing Weropress remained within the range of other materials.
High impact resins were developed to overcome the need for higher impact resistance. Uzun and Hersek19 investigated conventional, hi-impact and strengthened injection mold resins and reported that the transverse strength of six different base materials did not exhibit significant differences which are supported by the findings of similar materials in this study.
Our results, supporting previous studies, revealed that Lucitone 199 has better flexural properties and higher displacement values when compared to conventional resins. However, Weropress showed similar displacement and flexural strength values where Eclipse specimens exhibited even improved values. Therefore, both materials must also be evaluated on the basis of their impact resistance.
Newly introduced Eclipse has been reported to have superior flexural strength and higher flexural modulus when compared to high impact (Lucitone 199), conventional base (Meliodent) resins and triad resin.6,18 The results of this study are in good agreement with the findings of previous studies. Machado et al.18 reported that Eclipse exhibited less porosity after polymerization when compared to early UDMA system (Triad). Diaz-Arnold et al.20 reported that Eclipse specimens showed the highest standard deviations in mean flexural strength values (±15.83) in comparison to "so called" high-impact resin groups (Lucitone 199, Diamond D, HI-I, Nature-Cryl Hi-Plus other groups). They suggested that the high standard deviaton indicated variations in specimen fabrication. According to their comment, as the specimens were not flasked and packed they were not dense enough and this led to the high standard deviation in eclipse group. The Eclipse system, as presented by the manufacturer, is not designed for polymerization under pressure. However in the present study the corresponding standard deviation is approximately 35% lower. This might be due to the use of semi-transparent white teflon mold instead of the aluminum mold for Eclipse group. A possible higher quality of polymerization might have taken place.
As the results indicate Eclipse resin to have better and Weropress to have acceptable flexural properties when compared to well known alternatives, these materials deserve further investigation over their physical properties like impact and fatigue strength, water sorption and solubility and surface characteristics like hardness and staining, to clear out any possible advantages for clinical use.
CONCLUSION
Within the limitations of this study following conclusions can be drown;
1. Regarding the flexural strength properties and simpler processing technique, Eclipse system owns a potential as an advantageous alternative to conventional base resins.
2. Weropress system may have similar advantages of easy processing when compared to conventional resins.
References
- 1.Phoenix RD. Denture base materials. Dent Clin North Am. 1996;40:113–120. [PubMed] [Google Scholar]
- 2.Darbar UR, Huggett R, Harrison A. Denture fracture-a survey. Br Dent J. 1994;176:342–345. doi: 10.1038/sj.bdj.4808449. [DOI] [PubMed] [Google Scholar]
- 3.Beyli MS, von Fraunhofer JA. An analysis of causes of fracture of acrylic resin dentures. J Prosthet Dent. 1981;46:238–241. doi: 10.1016/0022-3913(81)90206-7. [DOI] [PubMed] [Google Scholar]
- 4.Rodford RA, Braden M. Further observations on high impact strength denture-base materials. Biomaterials. 1992;13:726–728. doi: 10.1016/0142-9612(92)90135-b. [DOI] [PubMed] [Google Scholar]
- 5.Jagger DC, Jagger RG, Allen SM, Harrison A. An investigation into the transverse and impact strength of "high strength" denture base acrylic resins. J Oral Rehabil. 2002;29:263–267. doi: 10.1046/j.1365-2842.2002.00830.x. [DOI] [PubMed] [Google Scholar]
- 6.Khan Z, von Fraunhofer JA, Razavi R. The staining characteristics, transverse strength, and microhardness of a visible light-cured denture base material. J Prosthet Dent. 1987;57:384–386. doi: 10.1016/0022-3913(87)90319-2. [DOI] [PubMed] [Google Scholar]
- 7.Pfeiffer P, Rosenbauer EU. Residual methyl methacrylate monomer, water sorption, and water solubility of hypoallergenic denture base materials. J Prosthet Dent. 2004;92:72–78. doi: 10.1016/j.prosdent.2004.04.003. [DOI] [PubMed] [Google Scholar]
- 8.Pfeiffer P, Rolleke C, Sherif L. Flexural strength and moduli of hypoallergenic denture base materials. J Prosthet Dent. 2005;93:372–377. doi: 10.1016/j.prosdent.2005.01.011. [DOI] [PubMed] [Google Scholar]
- 9.Jagger DC, Alshumailin YR, Harrison A, Rees JS. The effect of the addition of poly (methyl methacrylate) fibres on the transverse strength of repaired heat-cured acrylic resin. J Oral Rehabil. 2003;30:903–908. doi: 10.1046/j.1365-2842.2003.01078.x. [DOI] [PubMed] [Google Scholar]
- 10.Ali IL, Yunus N, Abu-Hassan MI. Hardness, flexural strength, and flexural modulus comparisons of three differently cured denture base systems. J Prosthodont. 2008;17:545–549. doi: 10.1111/j.1532-849X.2008.00357.x. [DOI] [PubMed] [Google Scholar]
- 11.Memon MS, Yunus N, Razak AA. Some mechanical properties of a highly cross-linked, microwave-polymerized, injection-molded denture base polymer. Int J Prosthodont. 2001;14:214–218. [PubMed] [Google Scholar]
- 12.Vallittu PK. The effect of void space and polymerization time on transverse strength of acrylic-glass fibre composite. J Oral Rehabil. 1995;22:257–261. doi: 10.1111/j.1365-2842.1995.tb00083.x. [DOI] [PubMed] [Google Scholar]
- 13.Ogle RE, Sorensen SE, Lewis EA. A new visible light-cured resin system applied to removable prosthodontics. J Prosthet Dent. 1986;56:497–506. doi: 10.1016/0022-3913(86)90397-5. [DOI] [PubMed] [Google Scholar]
- 14.Fellman S. Visible light-cured denture base resin used in making dentures with conventional teeth. J Prosthet Dent. 1989;62:356–359. doi: 10.1016/0022-3913(89)90350-8. [DOI] [PubMed] [Google Scholar]
- 15.Khan Z, Haeberle CB. One-appointment construction of an immediate transitional complete denture using visible light-cured resin. J Prosthet Dent. 1992;68:500–502. doi: 10.1016/0022-3913(92)90418-a. [DOI] [PubMed] [Google Scholar]
- 16.Johnston EP, Nicholls JI, Smith DE. Flexure fatigue of 10 commonly used denture base resins. J Prosthet Dent. 1981;46:478–483. doi: 10.1016/0022-3913(81)90232-8. [DOI] [PubMed] [Google Scholar]
- 17.Dixon DL, Breeding LC. The transverse strengths of three denture base resins reinforced with polyethylene fibers. J Prosthet Dent. 1992;67:417–419. doi: 10.1016/0022-3913(92)90261-8. [DOI] [PubMed] [Google Scholar]
- 18.Machado C, Sanchez E, Azer SS, Uribe JM. Comparative study of the transverse strength of three denture base materials. J Dent. 2007;35:930–933. doi: 10.1016/j.jdent.2007.09.006. [DOI] [PubMed] [Google Scholar]
- 19.Uzun G, Hersek N. Comparison of the fracture resistance of six denture base acrylic resins. J Biomater Appl. 2002;17:19–29. doi: 10.1177/0885328202017001597. [DOI] [PubMed] [Google Scholar]
- 20.Diaz-Arnold AM, Vargas MA, Shaull KL, Laffoon JE, Qian F. Flexural and fatigue strengths of denture base resin. J Prosthet Dent. 2008;100:47–51. doi: 10.1016/S0022-3913(08)60136-5. [DOI] [PubMed] [Google Scholar]