Factors in polymerization influencing the accuracy of PMMA denture bases

Objectives: Different technologies are available for the manufacturing of denture-base-resins (DBR) with different types of polymerization. Aim of this study was to investigate the dimensional accuracy of conventional and modern DBRs after polymerization depending on type of polymerization and manufacturing technology. Methods: Ten standardized denture bases from 7 polymethyl methacrylate (PMMA) resins (auto-polymerizing PMMA: ProBase Cold, Futura Gen, PalaXpress / heat-polymerizing PMMA: Paladon 65, SR-Ivocap, Futuracryl 2000 / industrially pre-polymerized PMMA: Polyan) were fabricated on identical casts (n=70). Depending on product and manufacturers instructions 4 different manufacturing technologies were applied (conventional-flasking: ProBase Cold, Paladon 65 / pneumatic-pressing: PalaXpress, SR-Ivocap / manual-pressing: Futura Gen, Futuracryl 2000 / injection-moulding: Polyan). The dorsal gap (DG) between resin base and master cast represents a measure of fit and accuracy of a polymerized denture. DG was measured at 5 points (palatal centre, bilateral vertical/horizontal border) using a light-microscope (x500). For each DBR the average DG was calculated. Data were analyzed for statistical differences and correlations (ANOVA, Bonferroni P<0.05). Results: After embedding all products showed different DG (auto-polymerizing PMMA: 196(±46)-256(±83)µm; industrially pre-polymerized PMMA: 301(±116)µm, heat-polymerizing PMMA: 317(±57)-369(±88)µm). After one week storage (22°C, constant humidity) the DG of 5 products increased significantly: 53(±75)-71(±57)µm; p=0.001-0.026. Statistical analysis revealed a significantly higher correlation of DG to the type of polymerization (eta=0.513) than to the applied manufacturing technology (eta=0.145). Conclusions: At different measuring times DG of the auto-polymerizing DBR were smaller than the DG of industrially pre-polymerized DBR followed by the larger DG of heat-polymerizing DBR. 71% of the products had a significant increase of DG after one week. Compared to the manufacturing process the type of polymerization had a greater effect on the dimensional accuracy of the tested denture-base-resins.