Greciana Bruzi, Larissa Fernanda Pottmaier and Luana Dutra de Carvalho
Statement of Problem: Whether using different methods of pouring and storage will affect to the surface hardness of gypsum Type IV casts.
Aim: To compare the surficial microhardness (Knoop) of gypsum Type IV submitted to different methods of pouring and storage.
Material and Method: Seventy-five specimens were made with gypsum Type IV (Herostone, Coltene) and distributed in 5 groups (n=15). A metal device was fabricated to simulate the tray and standardize the impression procedures. A Polyvinyl Siloxane (PVS. Express XT, 3M ESPE) was manipulated according to the manufacturer's instructions. After the setting time, the impression was removed and cleaned with water, dried, and disinfected with 2% glutaraldehyde. The experimental groups formed were: 1) Control/15–the die was removed from the impression material 15 min after pouring the cast; 2) 4 hour – the die was removed from the impression material 4 h after pouring the cast; 3) 40°C–the die was removed from the impression material, and stored during 30 min at ≅ 37°C; 4) sulfate/K– the impression was immersed in 2% potassium sulfate solution during 5 min after the gypsum pouring and removed from the PVS after 15 min; 5) sulfate/Ca– the impression was immersed in 2% Ca sulfate solution during 5 min after gypsum was poured and removed from the PVS after 15 min. The microhardness test Knoop was performed at 3 different points. The average was used for the statistical analysis. The normality of the data was confirmed by Kolmogorov-Smirnov test (P=0.200). Statistical differences between groups were analyzed by one-way ANOVA. The details of the analysis were performed using the Dunnett test and the Tukey HSD test. All tests were performed with a significance level of 5%.
Results: The lowest microhardness values were found in the Control group/15<4=40°C=sulfate/K= sulfate/Ca
Conclusion: All the experimental treatment methods increased significantly the surface microhardness of the gypsum Type IV when compared to the control.
Clinical relevance: Die treatments can improve the hardness of the gypsum die, preventing cast fracture.