Microwave Disinfection: Cumulative Effect of Different Power Levels on Physical Properties of Denture Base Resins Plinio M. Senna, DDS, MSc, 1 Wander Jose Da Silva, DDS, MSc, PhD, 1 Fernanda Faot, DDS, MSc, PhD, 2 & Altair Antoninha Del Bel Cury, DDS, MSc, PhD 1 1 Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil 2 Department of Prosthodontics, School of Dentistry, Federal University of Pelotas, Brazil Keywords Microwaves; disinfection; dentures; acrylic resin. Correspondence Plinio M. Senna, Piracicaba Dental School—Prosthodontics and Periodontology, Av. Limeira, 901 PO Box 52, Piracicaba S ˜ ao Paulo 13414-903, Brazil. E-mail: psenna3@fop.unicamp.br The authors would like to thank the National Council for Scientific and Technological Development (CNPq PQ #308141/2006-7) for the research support and S ˜ ao Paulo Research Foundation (FAPESP #2007/06482-6) for a scholarship granted to the first author. Accepted January 24, 2011 doi: 10.1111/j.1532-849X.2011.00770.x Abstract Purpose: This study evaluated the cumulative effects of different microwave power levels on the physical properties of two poly(methylmethacrylate) (PMMA) denture base resins. Materials and Methods: Eight sets of four PMMA specimens each (two polymerized in a water bath and two using microwave energy) were immersed in beakers contain- ing 200 ml of distilled water. Each beaker was subjected to microwave irradiation for 3 minutes at a power level of 450,630, or 900 W. The surface roughness, surface hard- ness, linear stability, flexural strength, elastic modulus, impact strength, and fracto- graphic properties were evaluated after either 6 or 36 simulated disinfection cycles. The data were statistically analyzed using ANOVA and the Tukey post hoc test (α = 0.05). Results: The polymerization method did not influence any property (p > 0.05) ex- cept linear stability. The surface roughness (p < 0.001) and hardness (p = 0.011) increased after 36 irradiation cycles at 630 or 900 W. The resin polymerized us- ing microwave energy exhibited greater linear distortion (p = 0.012), and there was a cumulative effect on linear stability for both resins (p < 0.001). No sig- nificant change (p > 0.05) was observed in flexural strength; however, the elastic modulus decreased (p = 0.008) after 36 disinfection cycles. The impact strength and crack propagation angles displayed no significant differences (p > 0.05). Conclusion: Within the limitations of this study, it can be concluded that microwave disinfection at 450 W to 630 W for 3 minutes is safe for PMMA. Due to the potential for the development of denture stomati- tis, 1 the presence of Candida spp. biofilms on dentures must be controlled. Microwave energy has been proposed as an effec- tive, clean, and low-cost disinfection method. 2,3 However, the effectiveness of this procedure is maximized when the denture is immersed in water, 4 and this may expose the resin to high temperatures. Temperatures exceeding 71 ◦ C may cause distor- tion of the PMMA polymer matrix due to relaxation of internal stresses acquired during polymerization. 5 The effect of microwave irradiation and the resulting increase in temperature on the microstructure and physical properties of PMMA resin is still unclear, since there are no standard irradia- tion protocols and assessment methodologies in the literature. 6 The parameters affecting the final temperature of the water are the water volume, exposure time, and irradiation power. This study employed a 200 ml volume, as this is sufficient to sub- merge a complete denture. 7,8 Exposure time has been defined as the shortest time required for an effective disinfection. A 3 minute exposure at 450 W to 650 W promotes disinfec- tion 4,9 while minimizing the polymer shrinkage observed after 6 minutes of exposure. 6,8 Regarding irradiation power, Thomas and Webb 10 reported that lower power levels should be harmless to the resins due to the exposure to lower final temperatures; however, it has not been conclusively determined whether varying this param- eter produces different effects on the physical properties of the PMMA resin. 9 It has also not yet been determined whether PMMA resin specifically formulated for microwave polymer- ization is more resistant to irradiation than PMMA polymerized using the conventional water-bath method. Furthermore, avail- able studies have not taken into consideration the fact that the disinfection is performed several times, and the cumulative ef- fect of irradiation on the resin properties has been neglected. In light of the need to establish a safe and effective home denture disinfection protocol that does not affect biomechan- ical performance, the aim of this study was to evaluate the 606 Journal of Prosthodontics 20 (2011) 606–612 c  2011 by the American College of Prosthodontists