- Email: [email protected]
Flexural strength and moduli of hypoallergenic denture base materials
Flexural strength and moduli of hypoallergenic denture base materials Peter Pfeiffer, Prof Dr Med Dent,a Christian Rolleke, Dr Med Dent,b and Lamia Sherifc School of Oral Medicine, University of Cologne, Cologne, Germany Statement of problem. Hypoallergenic denture base materials show no residual methyl methacrylate (MMA) or significantly lower residual MMA monomer content compared to polymethyl methacrylate–based (PMMA) heat-polymerizing acrylic resin. There is insufficient knowledge of the mechanical properties of hypoallergenic denture base materials to warrant their use in place of PMMA–based acrylic resins for patients with allergic reaction to MMA. Purpose. This in vitro study compared flexural strength and flexural modulus of 4 hypoallergenic denture base materials with flexural strength/modulus of a PMMA heat-polymerizing acrylic resin. Material and methods. The following denture base resins were examined: Sinomer (heat-polymerized, modified methacrylate), Polyan (thermoplastic, modified methacrylate), Promysan (thermoplastic, enterephthalate-based), Microbase (microwave-polymerized, polyurethane-based), and Paladon 65 (heat-polymerized, methacrylate, control group). Specimens of each material were tested for flexural strength and flexural modulus (MPa, n = 5) according to ISO 1567:1999. The data were analyzed with 1-way analysis of variance and the Bonferroni-Dunn multiple comparisons post hoc analysis for each test variable (a=.05). Results. Flexural strength of Microbase (67.2 6 5.3 MPa) was significantly lower than Paladon 65 (78.6 6 5.5 MPa, P,.0001). Flexural strength of Polyan (79.7 6 4.2 MPa, P=.599), Promysan (83.5 6 3.8 MPa, P=.412), and Sinomer (72.3 6 2.1 MPa, P=.015) did not differ significantly from the control group. Significantly lower flexural modulus was obtained from Sinomer (1720 6 30 MPa, P=.0007) compared to the PMMA control group (2050 6 40 MPa), whereas the flexural modulus of Promysan (2350 6 170 MPa, P=.0005) was significantly higher than the PMMA material. Microbase (2100 6 210 MPa, P=.373) and Polyan (2070 6 60 MPa, P=.577) exhibited flexural modulus similar to the PMMA material. The tested denture base materials fulfilled the requirements regarding flexural strength (.65 MPa). With the exception of Sinomer, the tested denture base resins passed the requirements of ISO 1567 regarding flexural modulus (.2000 MPa). Conclusion. Flexural modulus of Promysan was significantly higher than the PMMA material. Microbase and Sinomer exhibited significantly lower flexural strength and flexural modulus, respectively, than PMMA. The other groups did not differ significantly from the control group. (J Prosthet Dent 2005;93:372-7.)
CLINICAL IMPLICATIONS Flexural strength and flexural modulus of denture base materials Polyan and Promysan were in the same range as or higher than the PMMA material tested. When mechanical properties are examined, these hypoallergenic denture base materials appear to represent an alternative to conventional polymethyl methacrylate (PMMA) to minimize the risk of adverse reactions induced by residual methyl methacrylate monomer in denture-wearing patients with potential allergy susceptibility. Other clinical properties of the hypoallergenic materials, however, require further investigation.
A
dverse reactions (gingival/mucosal ulcerations, systemic/urticarial and skin reactions) associated with the use of local anesthetics and dental materials have been reported at a frequency of about 1 per 2600 treated patients.1 Due to the general increase in patients with allergies, dentists are confronted with more patients with allergic reactions to the classic polymethyl methac-
a
Associate Professor, Department of Prosthetic Dentistry. Research Assistant, Department of Prosthetic Dentistry. c Research Assistant, Department of Prosthetic Dentistry. b
372 THE JOURNAL OF PROSTHETIC DENTISTRY
rylate (PMMA) denture base materials.2-5 However, epidemiologic data of the prevalence of contact allergy to acrylic resins in patients could not be identified by the authors after a search of the literature using Medline and a hand search. For denture-wearing patients with potential allergy susceptibility, hypoallergenic denture base materials represent an alternative to conventional PMMA to minimize the risk of adverse reactions induced by residual methyl methacrylate monomer (MMA). The MMA has been replaced by presumably hypoallergenic resins, such as diurethane dimethacrylate, polyurethane, polyethylenterephthalate and VOLUME 93 NUMBER 4
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Table I. Denture base materials tested Material
Batch No.
Polymerization mode
Composition
Microbase Polyan Promysan
2/1 D 980730 99-05/R
Microwave Thermoplastic Thermoplastic
Sinomer
IPMF 103
Heat
Paladon 65 (control group)
9044072
Heat
Highly cross-linked polyurethane Modified methyl methacrylate Polyethylenterephthalate, polybutylenterephthalate Acrylic polymers of methyl methacrylate, urethane, and acrylate- based oligomers Polymethyl methacrylate
polybutylenterephthalate.6,7 Modified methacrylatebased denture base resins Polyan (thermoplastic) and Sinomer (heat-polymerized) exhibited a significantly lower residual monomer content than the heatpolymerized PMMA material Paladon 65.8 The enterephthalate-based Promysan (thermoplastic) and the polyurethane-based denture base material Microbase (microwave-polymerized) did not contain any detectable residual MMA monomer. Flexural failure of denture base materials is considered the primary mode of clinical failure.9 However, there are several methods of conducting flexural tests, and none has been demonstrated to be the most appropriate. Chitchumnong et al9 compared 3- and 4-point bendtesting for 4 different dental polymers. The values for modulus of elasticity determined by the 2 tests were not statistically different. Clinical studies have shown midline fracture to be a common problem in dentures.10,11 Zappini et al10 stated that to evaluate the resistance of denture base resins against fracture, fracture toughness tests as well as impact strength measurements should be performed. The fracture toughness method seems to be more suitable than impact strength measurements to demonstrate the effects of resin modifications. In terms of the flexural strength, the microwavepolymerized, injection-molded, polyurethane-based polymer offered no advantage over the existing heatand microwave-polymerized PMMA–based denture base polymers.10 Mechanical properties of denture base materials varied depending on the polymerization method.12-26 Ilbay et al18 showed that acrylic resin polymerized by microwave energy was more resistant to mechanical failure than conventionally polymerized acrylic resin, whereas Smith et al13 found that microwave polymerization improved the modulus of elasticity of 2 denture base resins but had little effect on the mechanical properties of 2 other resins. One of the properties of acrylates is water sorption and release, which causes dimensional instability, thus subjecting the material to internal stresses that may result in crack formation and, eventually, fracture of the denture.27-31 Takahashi et al30 found that water molecules diffused between the macromolecules of APRIL 2005
Manufacturer
DeguDent, Hanau, Germany Polyapress, Altkirchen, Germany Pedrazzini Dentaltechnologie, Munich, Germany Alldent, Rugell, Liechtenstein Heraeus Kulzer, Hanau, Germany
the material and thus forced them apart. This behavior affects dimensional behavior and denture stability; therefore, water sorption and solubility of these materials should be as low as possible.29-32 The mechanical properties of the hypoallergenic materials, however, require further investigation. This in vitro study aimed to evaluate the fractural strength and the fractural modulus of hypoallergenic denture base materials in comparison to a conventional heat-polymerized PMMA material, based on the hypothesis that recently introduced denture base materials exhibit fractural strength and fractural modulus in the same range as PMMA materials.
MATERIAL AND METHODS The denture base resins examined are listed in Table I. Each denture base material was tested for flexural strength and flexural modulus according to ISO 1567:1999 (E).33 For flexural strength testing, specimens (65 3 40 3 5 mm) of each material from 2 different mixes were prepared according to manufacturers’ instructions. Each plate was cut lengthwise into 3 equal strips. The strips were ground to the required length, width, and height of 64 mm 310 mm (60.2 mm) 3 3.3 mm (60.2 mm) with an automatic grinding and polishing unit (Phoenix Beta; Buehler, Lake Bluff, Ill) under water cooling, using water-resistant, metallographic grinding paper (1200 FEPA; Buehler) with a grain size of 15 mm. Five specimens, free of porosity, were used for the 3-point bending tests. Specimen height and width were recorded. The 5 specimens were stored in water at a temperature of 37°C 6 1°C for 50 hours prior to flexural testing. The specimen strips were removed from water storage, and the flat surface was immediately laid on the supports of the flexural testing device (Zwick, Ulm, Germany) immersed in a water bath (37°C 6 1°C). The flexural testing device consisted of a central loading plunger and 2 polished cylindrical supports, 3.2 mm in diameter and 10.5 mm long. The distance between the centers of the supports was 50 mm. The force was applied perpendicular to the center of the specimen strips. The specimens were continuously 373
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Table II. Mean flexural strength and flexural modulus of denture base materials Flexural strength MPa Acrylic resin
Microbase Polyan Promysan Sinomer Paladon 65 (control group)
Table III. ANOVA for flexural strength and flexural modulus of denture base materials
Flexural modulus MPa
Mean
SD
Mean
SD
67.2a 79.7b 83.5b 72.3ac 78.6bc
5.3 4.2 3.8 2.1 5.5
2100d 2070d 2350e 1720f 2050d
210 60 170 30 40
df
Sum of squares
Mean square
F
P
Flexural strength Material 4 833.912 208.478 15.515 ,.0001 Residual 20 268.748 13.437 Flexural modulus Material 4 1011634.566 252908.641 16.808 ,.0001 Residual 20 300945.496 15047.275
Groups with same superscripted letters are not significantly different (P ..05).
loaded using a servohydraulic universal testing machine (Type 1445; Zwick, Ulm, Germany) at a crosshead speed of 5 mm/min. The recording of the measuring results ended when the specimens fractured. The maximum force [N] upon fracture was recorded. The flexural strength (s) was calculated from the equation33: s¼
3Fl 2 b h2
where F is the maximum load (N) exerted on the specimen, l is the distance (mm) between the supports, b is the width (mm) of the specimen measured prior to water storage, and h is the height (mm) of the specimen measured prior to water storage. According to ISO 1567,33 the minimum flexure strength of denture base materials of Type 1, Type 3, Type 4, and Type 5 (heat-polymerized polymers, thermoplastic blank or powder, light-polymerized materials, and microwave polymerized materials, respectively) should not be less than 65 MPa. If results obtained for at least 4 out of 5 specimens comply with this requirement, the material passes. If only 1 or 2 of the specimens comply with the requirement, the material fails. If 3 of the specimens comply with ISO 1567,33 6 new specimen strips must be prepared and the test must be repeated. If at least 5 of the second series of specimens comply with the requirements, the material passes. Flexural modulus was calculated from the flexural strength tests. Additionally, the deflection of the specimens (mm) and the corresponding forces (N) were determined. The flexural modulus (E) was calculated from the equation33: E¼
F1 l3 4 b h3 d
where F1 is the load (N) at a convenient point in the straight-line portion of the trace; d is the deflection (mm) at load F1; l is the distance (mm) between the supports, b is the width (mm) of the specimen measured prior to water storage, and h is the height (mm) of the specimen measured prior to water storage. 374
If at least 4 of the results passed the requirements of the flexural strength on the first series, the flexural modulus was calculated for each of the 5 specimens. If a second series was tested for the flexural strength, the flexural modulus for 5 out of the 6 specimens was calculated. According to ISO 1567, the minimum flexural modulus of denture base materials of Type 1, Type 3, Type 4, and Type 5 should not be less than 2000 MPa. The requirements are fulfilled if at least 4 out of 5 results exceed 2000 MPa. If at least 3 of the results do not comply with the requirement, the material fails. If 3 of the flexural modulus results comply with the requirements, 6 new specimens should be prepared. If at least 5 results of the second series comply with the requirement, the material passes. Statistical analysis of the data was performed with 1-way analysis of variance (ANOVA) and the Bonferroni-Dunn multiple comparisons post hoc analysis for each test variable (a=.05).
RESULTS Table II summarizes the results of the determined fractural strength and flexural modulus. One-way ANOVA revealed a significant effect of brand of polymer on the fractural strength (Table III). Flexural strength of Microbase (67.2 6 5.3 MPa) was significantly lower than Paladon 65 (78.6 6 5.5 MPa, P,.0001) (Tables II and III). Flexural strength of Polyan (79.7 6 4.2 MPa, P=.599), Promysan (83.5 6 3.8 MPa, P=.412), and Sinomer (72.3 6 2.1 MPa, P=.015) did not differ significantly from the control group. The tested denture base materials fulfilled the requirements regarding flexural strength (.65 MPa). One-way ANOVA revealed a significant effect of brand of polymer on the flexural modulus (Table III). Significantly lower flexural modulus was obtained from Sinomer (1720 6 30 MPa, P=.0007) compared to the PMMA control group (2050 6 40 MPa), whereas the flexural modulus of Promysan (2350 6 170 MPa, P=.0005) was significantly higher than the PMMA material (Tables II and III). Microbase (2100 6 210 MPa, P=.373) and Polyan (2070 6 60 MPa, P=.577) exhibited flexural modulus similar to the PMMA material. VOLUME 93 NUMBER 4
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With the exception of Sinomer, the tested denture base resins passed the requirements of ISO 1567 regarding flexural modulus (.2000 MPa).
DISCUSSION Based on the results, the hypothesis that recently introduced denture base materials exhibit flexural strength and flexural modulus in the same range as PMMA materials is accepted for Polyan only. Microbase showed significantly lower flexural strength than the PMMA material. Flexural modulus of the PMMA material was significantly higher than the results obtained for Sinomer. The flexural modulus of Promysan was higher than the PMMA base material. Darbar et al11 reported that the fracture of dentures is an unresolved problem. The authors distributed questionnaires to 3 different dental laboratories to determine the prevalence of types of fracture. Results obtained showed that 33% of the repairs performed were due to debonded/detached teeth. Twenty-nine percent were repairs to midline fractures, more commonly seen in maxillary complete dentures. The remaining 38% were other types of fractures, involving detachment of acrylic resin bases from the metal in metal-based dentures and the fractures of connectors in the all-acrylic resin removable partial dentures. Some authors stated that the mechanical properties of denture base materials varied depending on the polymerization method.12-26 Blagojevic and Murphy20 compared mechanical properties of denture base polymers processed by both microwave and water-bath methods and demonstrated that, in general, water-bath polymerization with a long polymerizing cycle and a 3-hour terminal boil produced superior properties. Moreover, these authors showed that microwave polymerization of autopolymerizing resin improved mechanical properties and reduced residual monomer. Alkhatib et al15 compared 3 denture base resins, 2 designed for microwave polymerization and 1 for hot-water-bath processing. Results indicated that all 4 tested thicknesses (3, 6, 11.6, and 17.7 mm) of the water bath–polymerized specimens and 1 of the microwave-polymerized resins were porosity-free. When the water bath–polymerized material was microwave processed, porosity was found when resin thickness exceeded 3 mm. When 1 resin designed for microwave polymerization was irradiated using the high-wattage (513 W), shorter cycle (4 minutes, 52 seconds), porosity also occurred when the thickness exceeded 3 mm. Porosity did not occur until there was 9 mm of thickness when the lower-wattage (75.9 W) and longer cycle (15 minutes 26 seconds) were used. No significant differences in transverse strength or hardness were noted between the materials. Bafile et al16 compared the porosity of denture base material polymerized by microwave energy to denture resin polymerAPRIL 2005
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ized by the conventional heat method. No significant differences were found in mean porosity between the control group and the 4 groups of microwave-processed specimens that used Micro Liquid monomer (H.D. Justi Co, Oxnard, Calif). The 2 groups of microwaveprocessed specimens of MMA monomer showed a significantly higher mean porosity. A study has shown that water-bath polymerization resulted in enhanced mechanical properties.32 If the temperature cycles and polymerization time are changed when using PMMA materials, the material properties can be influenced positively. Extended polymerization time results in longer polymers, so that reduced water sorption and solubility and lower residual monomer content are obtained.32 Takahashi et al30 showed that the flexural strength of denture base materials decreased if the water sorption increased. A study has shown that the water sorption of Promysan was significantly lower compared to Paladon 65, whereas water sorption of other hypoallergenic denture base materials (Polyan, Sinomer) was not significantly lower than that of PMMA base material.8 The water sorption of Microbase was significantly higher than the PMMA group.8 In the present study, Microbase showed significantly lower flexural strength than the PMMA material. The results of the present study confirm the findings of Takahashi et al,30 with the exception of Promysan.8 Promysan showed flexural strength in the same range as the PMMA material, but water sorption was significantly lower than that of Paladon. Lassila and Vallittu21 indicated that the denture base material Sinomer passed the requirements of the ISO 1567 standards and that its mechanical properties can be increased considerably by an incorporation of glass fiber reinforcement. The flexural strength of Sinomer was 85.8 MPa, and the flexural modulus was 2730 MPa.21 The authors also showed that storing of test specimens in water reduced the flexural strength and the modulus. In the present study, flexural strength and flexural modulus of Sinomer after water storage were 72.3 MPa and 1720 MPa, respectively. Polymethyl methacrylates (PMMA) dominate the market of denture base resin materials. New products must not only equal the approved PMMA products but also offer improved properties and advantages to be competitive.8 Promysan showed the highest flexural modulus of the tested materials. Though the tested Microbase material passed the flexural strength test according to ISO 1567, it was still inferior to Paladon 65 in this respect. For Polyan, the flexural strength and modulus were similar to those of the heat-polymerized resin. The flexural modulus is important because it reflects the rigidity of the material, which in turn is important for the integrity of the supporting ridge and tissues, along with the fitting accuracy of the denture.9-11 Denture base resin should not deform under loading to permit proper load distribution to the underlying 375
THE JOURNAL OF PROSTHETIC DENTISTRY
structures.11 The experimental design of the present study strictly followed the ISO standards. Modifying parameters, such as polymerization times and temperatures and water immersion, may have effects on the flexural strength and flexural modulus of the tested denture base materials. These effects should be tested in further studies. Currently, there are a range of alternatives to the PMMA–based heat-polymerizing denture base materials available. Mechanical properties of other hypoallergenic denture base resins—for example, Luxene (heatpolymerized, polyvinyl copolymer/acrylate monomer; Astron Dental Corp, Lake Zurich, Ill), Eclipse (lightpolymerized, MMA monomer–free; Dentsply Trubyte, York, Pa), Flexite M.P. (thermoplastic, acrylate-based multi-polymer; Rapid Injection Systems, Mineola, NY), and Northerm Denture (thermoplastic, styrene-acrylonitrile copolymer; Rapid Injection Systems)—require further investigation. This in vitro study demonstrated that some of the tested materials represent an alternative to the classic PMMA resins for patients who are allergic to MMA monomer. In this study, significant differences in fractural strength and fractural modulus were obtained between different brands of materials used for denture bases. The study involved a limited analysis of mechanical properties for the resins used. Further investigation regarding other properties of these materials is necessary. To overcome the limitations of the in vitro tests, denture base materials must be evaluated intraorally.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The polyurethane-based denture base material Microbase (microwave-polymerized) exhibited a significantly lower flexural strength than the heat-polymerized PMMA material Paladon 65, whereas the flexural strength of the hypoallergenic denture base materials (Polyan, Promysan, Sinomer) was not significantly different from the results obtained for the PMMA material. 2. The modified methacrylate-based denture base resin Sinomer (heat-polymerized) exhibited a significantly lower flexural modulus than the heat-polymerized PMMA material Paladon 65, whereas the flexural modulus of the enterephthalate-based material, Promysan (thermoplastic), was significantly higher than the results obtained for the PMMA material. 3. Flexural modulus of the polyurethane-based denture base material Microbase (microwave-polymerized) and the modified methacrylate-based denture base resin Polyan (thermoplastic) was not significantly different from the results obtained for the PMMA material. 4. With the exception of Sinomer (flexural modulus), the tested denture base resins passed the require376
PFEIFFER, ROLLEKE, AND SHERIF
ments of ISO 1567 regarding flexural strength and flexural modulus.
REFERENCES 1. Jacobsen N, Aasenden R, Hensten-Pettersen A. Occupational health complaints and adverse patient reactions as perceived by personnel in public dentistry. Community Dent Oral Epidemiol 1991;19:155-9. 2. Douglas WH, Bates JF. The determination of residual monomer in polymethylmethacrylate denture-base resins. J Mater Sci 1978;13: 2600-4. 3. Vilaplana J, Romaguera C, Cornellana F. Contact dermatitis and adverse oral mucous membrane reactions related to the use of dental prostheses. Contact Dermatitis 1994;30:80-4. 4. Alanko K, Kanerva L, Jolanki R, Kannas L, Estlander T. Oral mucosal diseases investigated by patch testing with a dental screening series. Contact Dermatitis 1996;34:263-7. 5. Kanerva L, Jolanki R, Estlander T. 10 years of patch testing with the (meth) acrylate series. Contact Dermatitis 1997;37:255-8. 6. Murray MD, Darvell BW. The evolution of the complete denture base. Theories of a complete denture retention – a review. Part 1. Aust Dent J 1993;38:216-9. 7. Price CA. A history of dental polymers. Aust Prosthodont J 1994;8:47-54. 8. Pfeiffer P, Rosenbauer EU. Residual methyl methacrylate monomer, water sorption, and water solubility of hypoallergenic denture base materials. J Prosthet Dent 2004;92:72-8. 9. Chitchumnong P, Brooks SC, Stafford GD. Comparison of three- and four-point flexural strength testing of denture-base polymers. Dent Mater 1989;51:2-5. 10. Zappini G, Kammann A, Wachter W. Comparison of fracture tests of denture base materials. J Prosthet Dent 2003;90:578-85. 11. Darbar UR, Huggett R, Harrison A. Denture fracture—a survey. Br Dent J 1994;176:342-5. 12. Jerolimov V, Brooks SC, Huggett R, Bates JF. Rapid curing of acrylic denture-base materials. Dent Mater; 1989:518-22. 13. Smith LT, Powers JM, Ladd D. Mechanical properties of new denture resins polymerized by visible light, heat, and microwave energy. Int J Prosthodont 1992;5:315-20. 14. Kurata S, Yamazaki N. Mechanical properties of poly(alkyl alpha-fluoroacrylate)s as denture-base materials. J Dent Res 1989;68:481-3. 15. Alkhatib MB, Goodacre CJ, Swartz ML, Munoz-Viveros CA, Andres CJ. Comparison of microwave-polymerized denture base resins. Int J Prosthodont 1990;3:249-55. 16. Bafile M, Graser GN, Myers ML, Li EK. Porosity of denture resin cured by microwave energy. J Prosthet Dent 1991;66:269-74. 17. Fitton JS, Davies EH, Howlett JA, Pearson GJ. The physical properties of a polyacetal denture resin. Clin Mater 1994;17:125-9. 18. Ilbay SG, Guvener S, Alkumru HN. Processing dentures using a microwave technique. J Oral Rehabil 1994;21:103-9. 19. Umemoto K, Kurata S. Basic study of a new denture base resin applying hydrophobic methacrylate monomer. Dent Mater J 1997;16:21-30. 20. Blagojevic V, Murphy VM. Microwave polymerization of denture base materials. A comparative study. J Oral Rehabil 1999;26:804-8. 21. Lassila LV, Vallittu PK. Denture base polymer Alldent Sinomer: mechanical properties, water sorption and release of residual compounds. J Oral Rehabil 2001;28:607-13. 22. 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;14:214-8. 23. 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;29:263-7. 24. Uzun G, Hersek N. Comparison of the fracture resistance of six denture base acrylic resins. J Biomater Appl 2002;17:19-29. 25. Lai CP, Tsai MH, Chen M, Chang HS, Tay HH. Morphology and properties of denture acrylic resins cured by microwave energy and conventional water bath. Dent Mater 2004;20:133-41. 26. Phoenix RD, Mansueto MA, Ackerman NA, Jones RE. Evaluation of mechanical and thermal properties of commonly used denture base resins. J Prosthodont 2004;13:17-27. 27. Hiromori K, Fujii K, Inoue K. Viscoelastic properties of denture base resins obtained by underwater test. J Oral Rehabil 2000;27:522-31.
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28. Arima T, Murata H, Hamada T. The effects of cross-linking agents on the water sorption and solubility characteristics of denture base resin. J Oral Rehabil 1996;23:476-80. 29. Cucci AL, Vergani CE, Giampaolo ET, Afonso MC. Water sorption, solubility, and bond strength of two autopolymerizing acrylic resins and one heat-polymerizing acrylic resin. J Prosthet Dent 1998;80:434-8. 30. Takahashi Y, Chai J, Kawaguchi M. Effect of water sorption on the resistance to plastic deformation of a denture base material relined with four different denture reline materials. Int J Prosthodont 1998;11: 49-54. 31. Wong DM, Cheng LY, Chow TW, Clark RK. Effect of processing method on the dimensional accuracy and water sorption of acrylic resin dentures. J Prosthet Dent 1999;81:300-4. 32. Jagger RG. Effect of the curing cycle on some properties of a polymethylmethacrylate denture base material. J Oral Rehabil 1978;5:151-7. 33. Dentistry–denture base polymers. ISO 1567:1999. Available at: www.iso. ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=20266 &ICS1=11&ICS2=60&ICS3=10.
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Reprint requests to: DR PETER PFEIFFER DEPARTMENT OF PROSTHETIC DENTISTRY SCHOOL OF ORAL MEDICINE UNIVERSITY OF COLOGNE KERPENER STR. 32 50931 COLOGNE GERMANY FAX: 49-221-478-6722 E-MAIL: [email protected] 0022-3913/$30.00 Copyright Ó 2005 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2005.01.011
377
CLINICAL IMPLICATIONS Flexural strength and flexural modulus of denture base materials Polyan and Promysan were in the same range as or higher than the PMMA material tested. When mechanical properties are examined, these hypoallergenic denture base materials appear to represent an alternative to conventional polymethyl methacrylate (PMMA) to minimize the risk of adverse reactions induced by residual methyl methacrylate monomer in denture-wearing patients with potential allergy susceptibility. Other clinical properties of the hypoallergenic materials, however, require further investigation.
A
dverse reactions (gingival/mucosal ulcerations, systemic/urticarial and skin reactions) associated with the use of local anesthetics and dental materials have been reported at a frequency of about 1 per 2600 treated patients.1 Due to the general increase in patients with allergies, dentists are confronted with more patients with allergic reactions to the classic polymethyl methac-
a
Associate Professor, Department of Prosthetic Dentistry. Research Assistant, Department of Prosthetic Dentistry. c Research Assistant, Department of Prosthetic Dentistry. b
372 THE JOURNAL OF PROSTHETIC DENTISTRY
rylate (PMMA) denture base materials.2-5 However, epidemiologic data of the prevalence of contact allergy to acrylic resins in patients could not be identified by the authors after a search of the literature using Medline and a hand search. For denture-wearing patients with potential allergy susceptibility, hypoallergenic denture base materials represent an alternative to conventional PMMA to minimize the risk of adverse reactions induced by residual methyl methacrylate monomer (MMA). The MMA has been replaced by presumably hypoallergenic resins, such as diurethane dimethacrylate, polyurethane, polyethylenterephthalate and VOLUME 93 NUMBER 4
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THE JOURNAL OF PROSTHETIC DENTISTRY
Table I. Denture base materials tested Material
Batch No.
Polymerization mode
Composition
Microbase Polyan Promysan
2/1 D 980730 99-05/R
Microwave Thermoplastic Thermoplastic
Sinomer
IPMF 103
Heat
Paladon 65 (control group)
9044072
Heat
Highly cross-linked polyurethane Modified methyl methacrylate Polyethylenterephthalate, polybutylenterephthalate Acrylic polymers of methyl methacrylate, urethane, and acrylate- based oligomers Polymethyl methacrylate
polybutylenterephthalate.6,7 Modified methacrylatebased denture base resins Polyan (thermoplastic) and Sinomer (heat-polymerized) exhibited a significantly lower residual monomer content than the heatpolymerized PMMA material Paladon 65.8 The enterephthalate-based Promysan (thermoplastic) and the polyurethane-based denture base material Microbase (microwave-polymerized) did not contain any detectable residual MMA monomer. Flexural failure of denture base materials is considered the primary mode of clinical failure.9 However, there are several methods of conducting flexural tests, and none has been demonstrated to be the most appropriate. Chitchumnong et al9 compared 3- and 4-point bendtesting for 4 different dental polymers. The values for modulus of elasticity determined by the 2 tests were not statistically different. Clinical studies have shown midline fracture to be a common problem in dentures.10,11 Zappini et al10 stated that to evaluate the resistance of denture base resins against fracture, fracture toughness tests as well as impact strength measurements should be performed. The fracture toughness method seems to be more suitable than impact strength measurements to demonstrate the effects of resin modifications. In terms of the flexural strength, the microwavepolymerized, injection-molded, polyurethane-based polymer offered no advantage over the existing heatand microwave-polymerized PMMA–based denture base polymers.10 Mechanical properties of denture base materials varied depending on the polymerization method.12-26 Ilbay et al18 showed that acrylic resin polymerized by microwave energy was more resistant to mechanical failure than conventionally polymerized acrylic resin, whereas Smith et al13 found that microwave polymerization improved the modulus of elasticity of 2 denture base resins but had little effect on the mechanical properties of 2 other resins. One of the properties of acrylates is water sorption and release, which causes dimensional instability, thus subjecting the material to internal stresses that may result in crack formation and, eventually, fracture of the denture.27-31 Takahashi et al30 found that water molecules diffused between the macromolecules of APRIL 2005
Manufacturer
DeguDent, Hanau, Germany Polyapress, Altkirchen, Germany Pedrazzini Dentaltechnologie, Munich, Germany Alldent, Rugell, Liechtenstein Heraeus Kulzer, Hanau, Germany
the material and thus forced them apart. This behavior affects dimensional behavior and denture stability; therefore, water sorption and solubility of these materials should be as low as possible.29-32 The mechanical properties of the hypoallergenic materials, however, require further investigation. This in vitro study aimed to evaluate the fractural strength and the fractural modulus of hypoallergenic denture base materials in comparison to a conventional heat-polymerized PMMA material, based on the hypothesis that recently introduced denture base materials exhibit fractural strength and fractural modulus in the same range as PMMA materials.
MATERIAL AND METHODS The denture base resins examined are listed in Table I. Each denture base material was tested for flexural strength and flexural modulus according to ISO 1567:1999 (E).33 For flexural strength testing, specimens (65 3 40 3 5 mm) of each material from 2 different mixes were prepared according to manufacturers’ instructions. Each plate was cut lengthwise into 3 equal strips. The strips were ground to the required length, width, and height of 64 mm 310 mm (60.2 mm) 3 3.3 mm (60.2 mm) with an automatic grinding and polishing unit (Phoenix Beta; Buehler, Lake Bluff, Ill) under water cooling, using water-resistant, metallographic grinding paper (1200 FEPA; Buehler) with a grain size of 15 mm. Five specimens, free of porosity, were used for the 3-point bending tests. Specimen height and width were recorded. The 5 specimens were stored in water at a temperature of 37°C 6 1°C for 50 hours prior to flexural testing. The specimen strips were removed from water storage, and the flat surface was immediately laid on the supports of the flexural testing device (Zwick, Ulm, Germany) immersed in a water bath (37°C 6 1°C). The flexural testing device consisted of a central loading plunger and 2 polished cylindrical supports, 3.2 mm in diameter and 10.5 mm long. The distance between the centers of the supports was 50 mm. The force was applied perpendicular to the center of the specimen strips. The specimens were continuously 373
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Table II. Mean flexural strength and flexural modulus of denture base materials Flexural strength MPa Acrylic resin
Microbase Polyan Promysan Sinomer Paladon 65 (control group)
Table III. ANOVA for flexural strength and flexural modulus of denture base materials
Flexural modulus MPa
Mean
SD
Mean
SD
67.2a 79.7b 83.5b 72.3ac 78.6bc
5.3 4.2 3.8 2.1 5.5
2100d 2070d 2350e 1720f 2050d
210 60 170 30 40
df
Sum of squares
Mean square
F
P
Flexural strength Material 4 833.912 208.478 15.515 ,.0001 Residual 20 268.748 13.437 Flexural modulus Material 4 1011634.566 252908.641 16.808 ,.0001 Residual 20 300945.496 15047.275
Groups with same superscripted letters are not significantly different (P ..05).
loaded using a servohydraulic universal testing machine (Type 1445; Zwick, Ulm, Germany) at a crosshead speed of 5 mm/min. The recording of the measuring results ended when the specimens fractured. The maximum force [N] upon fracture was recorded. The flexural strength (s) was calculated from the equation33: s¼
3Fl 2 b h2
where F is the maximum load (N) exerted on the specimen, l is the distance (mm) between the supports, b is the width (mm) of the specimen measured prior to water storage, and h is the height (mm) of the specimen measured prior to water storage. According to ISO 1567,33 the minimum flexure strength of denture base materials of Type 1, Type 3, Type 4, and Type 5 (heat-polymerized polymers, thermoplastic blank or powder, light-polymerized materials, and microwave polymerized materials, respectively) should not be less than 65 MPa. If results obtained for at least 4 out of 5 specimens comply with this requirement, the material passes. If only 1 or 2 of the specimens comply with the requirement, the material fails. If 3 of the specimens comply with ISO 1567,33 6 new specimen strips must be prepared and the test must be repeated. If at least 5 of the second series of specimens comply with the requirements, the material passes. Flexural modulus was calculated from the flexural strength tests. Additionally, the deflection of the specimens (mm) and the corresponding forces (N) were determined. The flexural modulus (E) was calculated from the equation33: E¼
F1 l3 4 b h3 d
where F1 is the load (N) at a convenient point in the straight-line portion of the trace; d is the deflection (mm) at load F1; l is the distance (mm) between the supports, b is the width (mm) of the specimen measured prior to water storage, and h is the height (mm) of the specimen measured prior to water storage. 374
If at least 4 of the results passed the requirements of the flexural strength on the first series, the flexural modulus was calculated for each of the 5 specimens. If a second series was tested for the flexural strength, the flexural modulus for 5 out of the 6 specimens was calculated. According to ISO 1567, the minimum flexural modulus of denture base materials of Type 1, Type 3, Type 4, and Type 5 should not be less than 2000 MPa. The requirements are fulfilled if at least 4 out of 5 results exceed 2000 MPa. If at least 3 of the results do not comply with the requirement, the material fails. If 3 of the flexural modulus results comply with the requirements, 6 new specimens should be prepared. If at least 5 results of the second series comply with the requirement, the material passes. Statistical analysis of the data was performed with 1-way analysis of variance (ANOVA) and the Bonferroni-Dunn multiple comparisons post hoc analysis for each test variable (a=.05).
RESULTS Table II summarizes the results of the determined fractural strength and flexural modulus. One-way ANOVA revealed a significant effect of brand of polymer on the fractural strength (Table III). Flexural strength of Microbase (67.2 6 5.3 MPa) was significantly lower than Paladon 65 (78.6 6 5.5 MPa, P,.0001) (Tables II and III). Flexural strength of Polyan (79.7 6 4.2 MPa, P=.599), Promysan (83.5 6 3.8 MPa, P=.412), and Sinomer (72.3 6 2.1 MPa, P=.015) did not differ significantly from the control group. The tested denture base materials fulfilled the requirements regarding flexural strength (.65 MPa). One-way ANOVA revealed a significant effect of brand of polymer on the flexural modulus (Table III). Significantly lower flexural modulus was obtained from Sinomer (1720 6 30 MPa, P=.0007) compared to the PMMA control group (2050 6 40 MPa), whereas the flexural modulus of Promysan (2350 6 170 MPa, P=.0005) was significantly higher than the PMMA material (Tables II and III). Microbase (2100 6 210 MPa, P=.373) and Polyan (2070 6 60 MPa, P=.577) exhibited flexural modulus similar to the PMMA material. VOLUME 93 NUMBER 4
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With the exception of Sinomer, the tested denture base resins passed the requirements of ISO 1567 regarding flexural modulus (.2000 MPa).
DISCUSSION Based on the results, the hypothesis that recently introduced denture base materials exhibit flexural strength and flexural modulus in the same range as PMMA materials is accepted for Polyan only. Microbase showed significantly lower flexural strength than the PMMA material. Flexural modulus of the PMMA material was significantly higher than the results obtained for Sinomer. The flexural modulus of Promysan was higher than the PMMA base material. Darbar et al11 reported that the fracture of dentures is an unresolved problem. The authors distributed questionnaires to 3 different dental laboratories to determine the prevalence of types of fracture. Results obtained showed that 33% of the repairs performed were due to debonded/detached teeth. Twenty-nine percent were repairs to midline fractures, more commonly seen in maxillary complete dentures. The remaining 38% were other types of fractures, involving detachment of acrylic resin bases from the metal in metal-based dentures and the fractures of connectors in the all-acrylic resin removable partial dentures. Some authors stated that the mechanical properties of denture base materials varied depending on the polymerization method.12-26 Blagojevic and Murphy20 compared mechanical properties of denture base polymers processed by both microwave and water-bath methods and demonstrated that, in general, water-bath polymerization with a long polymerizing cycle and a 3-hour terminal boil produced superior properties. Moreover, these authors showed that microwave polymerization of autopolymerizing resin improved mechanical properties and reduced residual monomer. Alkhatib et al15 compared 3 denture base resins, 2 designed for microwave polymerization and 1 for hot-water-bath processing. Results indicated that all 4 tested thicknesses (3, 6, 11.6, and 17.7 mm) of the water bath–polymerized specimens and 1 of the microwave-polymerized resins were porosity-free. When the water bath–polymerized material was microwave processed, porosity was found when resin thickness exceeded 3 mm. When 1 resin designed for microwave polymerization was irradiated using the high-wattage (513 W), shorter cycle (4 minutes, 52 seconds), porosity also occurred when the thickness exceeded 3 mm. Porosity did not occur until there was 9 mm of thickness when the lower-wattage (75.9 W) and longer cycle (15 minutes 26 seconds) were used. No significant differences in transverse strength or hardness were noted between the materials. Bafile et al16 compared the porosity of denture base material polymerized by microwave energy to denture resin polymerAPRIL 2005
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ized by the conventional heat method. No significant differences were found in mean porosity between the control group and the 4 groups of microwave-processed specimens that used Micro Liquid monomer (H.D. Justi Co, Oxnard, Calif). The 2 groups of microwaveprocessed specimens of MMA monomer showed a significantly higher mean porosity. A study has shown that water-bath polymerization resulted in enhanced mechanical properties.32 If the temperature cycles and polymerization time are changed when using PMMA materials, the material properties can be influenced positively. Extended polymerization time results in longer polymers, so that reduced water sorption and solubility and lower residual monomer content are obtained.32 Takahashi et al30 showed that the flexural strength of denture base materials decreased if the water sorption increased. A study has shown that the water sorption of Promysan was significantly lower compared to Paladon 65, whereas water sorption of other hypoallergenic denture base materials (Polyan, Sinomer) was not significantly lower than that of PMMA base material.8 The water sorption of Microbase was significantly higher than the PMMA group.8 In the present study, Microbase showed significantly lower flexural strength than the PMMA material. The results of the present study confirm the findings of Takahashi et al,30 with the exception of Promysan.8 Promysan showed flexural strength in the same range as the PMMA material, but water sorption was significantly lower than that of Paladon. Lassila and Vallittu21 indicated that the denture base material Sinomer passed the requirements of the ISO 1567 standards and that its mechanical properties can be increased considerably by an incorporation of glass fiber reinforcement. The flexural strength of Sinomer was 85.8 MPa, and the flexural modulus was 2730 MPa.21 The authors also showed that storing of test specimens in water reduced the flexural strength and the modulus. In the present study, flexural strength and flexural modulus of Sinomer after water storage were 72.3 MPa and 1720 MPa, respectively. Polymethyl methacrylates (PMMA) dominate the market of denture base resin materials. New products must not only equal the approved PMMA products but also offer improved properties and advantages to be competitive.8 Promysan showed the highest flexural modulus of the tested materials. Though the tested Microbase material passed the flexural strength test according to ISO 1567, it was still inferior to Paladon 65 in this respect. For Polyan, the flexural strength and modulus were similar to those of the heat-polymerized resin. The flexural modulus is important because it reflects the rigidity of the material, which in turn is important for the integrity of the supporting ridge and tissues, along with the fitting accuracy of the denture.9-11 Denture base resin should not deform under loading to permit proper load distribution to the underlying 375
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structures.11 The experimental design of the present study strictly followed the ISO standards. Modifying parameters, such as polymerization times and temperatures and water immersion, may have effects on the flexural strength and flexural modulus of the tested denture base materials. These effects should be tested in further studies. Currently, there are a range of alternatives to the PMMA–based heat-polymerizing denture base materials available. Mechanical properties of other hypoallergenic denture base resins—for example, Luxene (heatpolymerized, polyvinyl copolymer/acrylate monomer; Astron Dental Corp, Lake Zurich, Ill), Eclipse (lightpolymerized, MMA monomer–free; Dentsply Trubyte, York, Pa), Flexite M.P. (thermoplastic, acrylate-based multi-polymer; Rapid Injection Systems, Mineola, NY), and Northerm Denture (thermoplastic, styrene-acrylonitrile copolymer; Rapid Injection Systems)—require further investigation. This in vitro study demonstrated that some of the tested materials represent an alternative to the classic PMMA resins for patients who are allergic to MMA monomer. In this study, significant differences in fractural strength and fractural modulus were obtained between different brands of materials used for denture bases. The study involved a limited analysis of mechanical properties for the resins used. Further investigation regarding other properties of these materials is necessary. To overcome the limitations of the in vitro tests, denture base materials must be evaluated intraorally.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The polyurethane-based denture base material Microbase (microwave-polymerized) exhibited a significantly lower flexural strength than the heat-polymerized PMMA material Paladon 65, whereas the flexural strength of the hypoallergenic denture base materials (Polyan, Promysan, Sinomer) was not significantly different from the results obtained for the PMMA material. 2. The modified methacrylate-based denture base resin Sinomer (heat-polymerized) exhibited a significantly lower flexural modulus than the heat-polymerized PMMA material Paladon 65, whereas the flexural modulus of the enterephthalate-based material, Promysan (thermoplastic), was significantly higher than the results obtained for the PMMA material. 3. Flexural modulus of the polyurethane-based denture base material Microbase (microwave-polymerized) and the modified methacrylate-based denture base resin Polyan (thermoplastic) was not significantly different from the results obtained for the PMMA material. 4. With the exception of Sinomer (flexural modulus), the tested denture base resins passed the require376
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ments of ISO 1567 regarding flexural strength and flexural modulus.
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28. Arima T, Murata H, Hamada T. The effects of cross-linking agents on the water sorption and solubility characteristics of denture base resin. J Oral Rehabil 1996;23:476-80. 29. Cucci AL, Vergani CE, Giampaolo ET, Afonso MC. Water sorption, solubility, and bond strength of two autopolymerizing acrylic resins and one heat-polymerizing acrylic resin. J Prosthet Dent 1998;80:434-8. 30. Takahashi Y, Chai J, Kawaguchi M. Effect of water sorption on the resistance to plastic deformation of a denture base material relined with four different denture reline materials. Int J Prosthodont 1998;11: 49-54. 31. Wong DM, Cheng LY, Chow TW, Clark RK. Effect of processing method on the dimensional accuracy and water sorption of acrylic resin dentures. J Prosthet Dent 1999;81:300-4. 32. Jagger RG. Effect of the curing cycle on some properties of a polymethylmethacrylate denture base material. J Oral Rehabil 1978;5:151-7. 33. Dentistry–denture base polymers. ISO 1567:1999. Available at: www.iso. ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=20266 &ICS1=11&ICS2=60&ICS3=10.
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Reprint requests to: DR PETER PFEIFFER DEPARTMENT OF PROSTHETIC DENTISTRY SCHOOL OF ORAL MEDICINE UNIVERSITY OF COLOGNE KERPENER STR. 32 50931 COLOGNE GERMANY FAX: 49-221-478-6722 E-MAIL: [email protected] 0022-3913/$30.00 Copyright Ó 2005 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2005.01.011
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