Physical Properties of Nanohybrid and Microhybrid Resin Composites Subjected to Acidic Environment

Objectives: To investigate the hardness and colour stability of five resin composites subjected to different polishing methods and after immersion in distilled water or lactic acid for up to three months.

Methods: Three nanohybrid composites (Estelite Sigma Quick/ Tokuyama, Paradigm/3M ESPE and Ice/SDI) and two microhybrid composites (Filtek Z250 and Filtek P60; 3M ESPE) were examined. Disc-shaped specimens (10 mm ø X 1.5 mm thick) were prepared and immersed in distilled water for 24 h, then polished using either silicon carbide paper, Shofu polishing system, or left unpolished (control). The CIE values and microhardness were determined using a spectrophotometer and digital Vickers hardness tester, respectively (n = 10) after one, 45 and 90 days of storage in distilled water or lactic acid. Data were analyzed using ANOVA/Tukey’s HSD tests, and Pearson’s correlation coefficient.

Results: Ice exhibited the greatest colour change and FiltekTM P60 the least. Distilled water immersion led to more colour change than lactic acid and specimens polished with the Shofu system showed more colour change than the other surface finish methods. The greatest hardness value was obtained for ParadigmTM and the lowest for Estelite®. There were no differences between storage media. The specimens cured by Mylar strip had the lowest hardness.

Conclusions: Acidic environment did not affect colour stability and microhardness of the resin composites. There was a significantly mild reverse correlation between hardness and colour change in both storage media.