Effects of Different Percentages of Microhydroxyapatite on Microhardness of Resin-modified Glass-ionomer and Zirconomer

Background: Hydroxyapatite (HA) is the main mineral component of the tooth structure, which exhibits considerable biological behavior and its incorporation might improve microhardness of dental materials.Microhardness of restora-tive materials, like glass-ionomer, is critical for the clinical longevity of restorations.The aim of this study was to eva-luate the microhardness of two glass-ionomers types by incorporating different percentages of microhydroxyapatite.Material and Methods: In this study, 80 disc-shaped experimental specimens (6 mm in diameter, 2 mm in height) were prepared in 8 groups, including resin-modified glass-ionomer (RMGI, GC, Gold Label, Japan), zirconiareinforced glass-ionomer (Zirconomer, Shofu, Kyoto, Japan), and their mixture with 0, 5, 15 and 25 wt% of microhydroxyapatite (Sigma-Aldrich, Germany).All the specimens were stored in deionized water at 37ºC for 24 hours.Then Vickers mi-crohardness test was carried out on the both sides of specimens and data were analyzed using two-way ANOVA and paired t-test (P<0.05).Results: Microhardness of Zirconomer and RMGI increased significantly due to adding 5 and 15 wt% of microhydrox-yapatite (P<0.001).The highest Vickers hardness number (VHN) was recorded in the RMGI group with 5 wt% of mi-crohydroxyapatite.In addition, in all the study groups RMGI exhibited higher microhardness values than Zirconomer (P<0.001).However, microhardness values decreased significantly after adding 25 wt% of microhydroxyapatite to Zirconomer (P<0.001).Similarly, VHN decreased in RMGI groups containing 25 wt% of HA compared to control groups (without HA) (P<0.001).Conclusions: Incorporation of 5 and 15 wt% of microhydroxyapatite to RMGI and Zirconomer improved microhard-ness, while adding 25 wt% of HA decreased hardness with both experimental materials compared to the control groups (without HA).