Effects of various storage media on surface hardness and structure of four polyacid-modified composite resins ("compomers").

It was the purpose of this study to determine surface microhardness and structure of four poly-acid modified composite resins ("compomers") after 1-year storage in water and various solutions. Specimens of the compomers Dyract (DY), Compoglass F (CG), an experimental compomer (EC), and of F2000 (F) were stored at 37 degrees C for 1 year in distilled water, acidic buffer (pH 4.2), neutral buffer (pH 7.0), and neutral buffer supplemented with 1.6 u/ml porcine liver esterase. Then the specimens were kept dry for 3 days further. Continuously dry-stored samples (37 degrees C) served as controls. Surface microhardness (Vickers) was read at baseline (prior to wet storage), after 24 h and 144 h, then every 30 days up to 1 year and after the further 3 days of dry storage. Surface roughness (Ra) was determined at baseline, after 3, 6, 9, and 12 months and, finally, after the further 3 days. In addition, randomly selected control and experimental specimens were investigated by SEM. Data were statistically analyzed by a three-factor ANOVA using Scheffé tests (P<0.05). Vickers hardness of all experimental samples dropped significantly due to wet storage (P<0.000) during the first month. However, the type of storage medium did not influence microhardness significantly. Surface roughness was not altered by wet storage except for compomer F, which revealed pronounced surface cracks in the SEM. Altogether, our findings point out that surface microhardness and structure of the investigated compomers were not significantly deteriorated by the tested intraoral conditions, such as humidity, pH-fluctuation, or the action of hydrolytic salivary enzymes.