Effect of Incorporation of Various Amounts of Nano-sized Hydroxyapatite on the Mechanical Properties of a Resin Modified Glass Ionomer

Nano-sized hydroxyapatite nano particles (nHA) have optimal biological properties and by incorporating them into the restorative materials, we can benefit from these properties. The present study sought to assess the effect of incorporation of various amounts of nHA on the mechanical properties (compressive and flexural strengths) of a resin modified glass ionomer.

In this experimental study, a total of 252Fuji II LC improved GI samples were divided into 6 groups including a control group (0%) and the nHA groups (1%, 2%, 5%, 7% and 10% based on mass percent). Of the samples, 108 were fabricated for the flexural strength testing using a two-piece aluminum mold (2x2x25 mm) according to ISO standard 4049. In each group, a total of 18 samples in 3 subgroups (6 samples each) were fabricated. All samples were removed from the mold after 80 seconds of light irradiation. A total of 144 samples made for compressive strength testing using a two-piece brass mold (4x6 mm) according to ISO standard 9917. Thus, in each group, a total of 24 samples in three subgroups (8 samples each) were fabricated and removed from the mold after 80 seconds of light irradiation. After removal from the mold, all samples were stored in an incubator at 37˚C and 100% moisture and underwent flexural and compressive strength testing with the primary load of 2 N and crosshead speed of 0.5 mm/min-1in a Zwick testing machine after one day, one week and one month. Data were analyzed using normal statistical tests of one-way ANOVA and Tukey’s HSD (Post Hoc). In order to assess the correlation of time (in groups with various mass percentages of HAP) with understudy outcomes, one way ANOVA was employed and P<0.05 was considered statistically significant.

Study results showed that incorporation of 5% nHA resulted in a significant increase in flexural strength after 30 days (17.4 MPa increase)(P<0.05). Also, if the nHA weight percent exceeded 5%, flexural strength suffered a dramatic drop after one month(28.9 MPa reduction). This study also showed that addition of 5% NHAP significantly increased flexural elastic modulus after one month (2.2 GPa increase). Addition of NHAP to resin modified glass ionomer caused a small increase in compressive strength and compressive elastic modulus which was not statistically significant (about 1MPa increase).

Addition of nHA to resin modified glass ionomers (Fuji II LC improved) not only does not reduce compressive strength, but also can increase flexural strength which would be the greatest if nHA is added in an amount of 5%.