Influence of Microwave Polymerization Method and Thickness on Porosity of Acrylic Resin Ana Carolina Pero, DDS, MSc, 1 D´ ebora Barros Barbosa, DDS, PhD, 2 Juli ˆ e Marra, DDS, MSc, 1 Adhemar Colla Ruvolo-Filho, PhD, 3 & Marco Antonio Compagnoni, DDS, PhD 4 1 Graduate Student, Department of Dental Materials and Prosthodontics, Araraquara Dental School, S ˜ ao Paulo State University, Araraquara, S ˜ ao Paulo, Brazil 2 Assistant Professor, Department of Dental Materials and Prosthodontics, Arac ¸ atuba Dental School, S ˜ ao Paulo State University, Arac ¸ atuba, S˜ ao Paulo, Brazil 3 Associate Professor, Department of Dental Materials and Prosthodontics, Araraquara Dental School, S ˜ ao Paulo State University, Araraquara, S ˜ ao Paulo, Brazil 4 Associate Professor, Department of Chemistry, S ˜ ao Carlos Federal University, S ˜ ao Carlos, Brazil Keywords Complete denture; porosity; acrylic resin; polymerization method. Correspondence Marco A. Compagnoni, Department of Dental Materials and Prosthodontics, S ˜ ao Paulo State University, Rua Humait ´ a 1680, Araraquara, S ˜ ao Paulo, 14801-903, Brazil. E-mail: compagno@foar.unesp.br Supported by grant No. 03/02036-0 from Fundac ¸˜ ao de Apoio ` a Pesquisa do Estado de S˜ ao Paulo (FAPESP) and Coordenac ¸˜ ao de Aperfeic ¸oamento de Pessoal de N´ ıvel Superior (CAPES). Accepted 28 August 2006 doi: 10.1111/j.1532-849X.2007.00264.x Abstract Purpose: This study evaluated the influence of polymerization cycle and thickness of maxillary complete denture bases on the porosity of acrylic resin. Materials and Methods: Two heat-activated denture base resins—one conventional (Cl´ assico) and one designed for microwave polymerization (Onda-Cryl)—were used. Four groups were established, according to polymerization cycles: A (Onda-Cryl, short microwave cycle), B (Onda-Cryl, long microwave cycle), C (Onda-Cryl, manu- facturing microwave cycle), and T (Cl´ assico, water bath). Porosity was evaluated for different thicknesses (2.0, 3.5, and 5.0 mm; thicknesses I, II, and III, respectively) by measurement of the specimen volume before and after its immersion in water. The percent porosity data were submitted to Kruskal–Wallis for comparison among the groups. Results: The Kruskal–Wallis test detected that the combinations of the different cycles and thicknesses showed significant differences, and the mean ranks of percent porosity showed differences only in the thinnest (2.0 mm) microwave-polymerized specimens (A = 53.55, B = 40.80, and C = 90.70). Thickness did not affect the results for cycle T (I = 96.15, II = 70.20, and III = 82.70), because porosity values were similar in the three thicknesses. Conclusions: Microwave polymerization cycles and the specimen thickness of acrylic resin influenced porosity. Porosity differences were not observed in the polymerized resin bases in the water bath cycle for any thickness. Poly(methyl methacrylate) has been widely used as a denture base material since the late 1930s. It is by no means a completely ideal material, and its advantages and disadvantages are well established; however, it is important to select an appropriate resin for the chosen method of processing to obtain the best results. 1 The polymerization of denture base resin by microwave en- ergy has been studied for more than three decades. The advan- tages of polymerizing denture base resin by microwave energy are a greatly reduced polymerization time, 2 a cleaner method of processing, 3 and a denture base with superior adaptation to the dental cast. 4 Several reports evaluating the effect of mi- crowave energy on the porosity of denture base resin have been published. 5-11 Porosity of denture base resin continues to be one of the un- desirable characteristics of poly(methyl methacrylate). This has been attributed to a variety of factors including the following: air entrapped during mixing, monomer contraction during poly- merization, monomer vaporization associated with the exother- mic reaction, and the presence of residual monomer. 12,13 Ilbay et al 14 reported that the porosity was superior in acrylic resin cured by microwave energy at a higher power. Com- pagnoni et al 6 found no significant differences in porosity among denture base resin specimens polymerized with three different cycles of microwave energy. They also reported the porosity found in the microwave-polymerized denture base resin tested was similar in porosity to the heat-polymerized resin tested. In addition, it is well documented that porosities Journal of Prosthodontics 17 (2008) 125–129 c  2007 by The American College of Prosthodontists 125