Evaluation of porosity in microwave-processed acrylic resin using a photographic method.

Statement Of Problem: The effect of microwave polymerization on the porosity of denture base resin has not been fully determined.

Purpose: This study investigated the effect of microwave energy on the porosity of 2 heat-activated denture base resins.

Material And Methods: Two heat-activated denture base resins, one conventional (Paladon 65) and one designed for microwave polymerization (Acron MC), were used to prepare 50 test specimens. Five groups of 10 specimens each were established: Group A (Paladon 65, water bath); Group B(1) (Paladon 65, short microwave cycle); Group B(2) (Paladon 65, long microwave cycle); Group C(1) (Acron MC, short microwave cycle); and Group C(2) (Acron MC, long microwave cycle). Half of the specimens in each group were 3 mm thick, the other half 6 mm thick. After being polymerized, specimens were cut so that 3 cross-sectional areas were formed (S(1), S(2), and S(3)). These surfaces were polished and photographed under a microscope at x100 magnification. On the developed photographs, the area of each pore was measured with a digital planimeter, and the total area of pores per surface was calculated in percentage form. The total number of pores on each surface and the topographical distribution of the pores also were recorded. A 3-factor repeated-measures analysis of variance was used to compare porosity data, and a 1-way analysis of variance was performed to determine possible interactions between groups based on material and specimen thickness (P<.05). The effect of surface area on porosity data was analyzed with the use of contrasts.

Results: Group A specimens exhibited no pores. In the thicker specimens of Groups B(1) and B(2), giant pores (area as great as 3.69 mm(2)) and small, gaseous pores of almost uniform shape and size were found. In Groups C(1) and C(2), only the smaller pores were found; these were not clinically significant. Of the observed surfaces, 75.3% were free of pores and 24.7% contained at least one pore. In a selected group of pore-bearing surfaces, the majority (81%) had pores located near the center. The thicker specimens in Group B exhibited the greatest amount of porosity (P<.0001); Group C specimens exhibited the least porosity. Repeated-measures analysis of variance showed that polymerization cycle had no effect on porosity (P=.19). The 3 other factors (material, specimen thickness, and surface) and all possible interactions among them were significant (P<.05). Among the surfaces, S(1) and S(2) exhibited the highest total mean value of porosity (0.71% and 0.74%, respectively) and S(3) the lowest (0.025%). S(3) showed a different pattern of porosity than S(1) and S(2).

Conclusion: Within the limitations of this in vitro study, minor porosity was identified in thin and more severe porosity in thicker areas of conventional resin specimens that underwent microwave polymerization. The resin designed specifically for microwave polymerization exhibited no clinically significant porosity.