الفهرس 
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Abstract
Cementation is the most important process in fixed prosthodontics as cements are used to bond a pros¬thesis to a substrate with a luting agent to keep a seal between the prosthesis and the tooth. Cements are classified into definitive and temporary and both types must give adequate prosthetic retention and an effective seal.
Glass ionomer cement has unique properties such as tooth-like appearance, fluoride release, adhesion to tooth structure and base metals, favourable coefficient of thermal expansion, and biocompatibility. On the other hand, GIC suffers from low mechanical properties, brittleness, undesirable appearance, and moisture sensitivity in the early stages of its placement.
Resin-modified glass ionomer cement was introduced to overcome the limitations of GIC but it cannot be used in areas under high occlusal stresses. Some strategies have been applied to enhance the cross-linking and physical properties of RMGIC such as addition of hydroxyapatite (HA), metal particles, zirconia, alumina and glass to their composition.
Nanoparticles have been used to enhance the characteristics of the materials without compromising their optimal characteristics. Hydroxyapatite nanoparticles (HANPS) have high solubility and can result in deposition of mineral ions such as calcium and phosphate into enamel defects.
The current study was conducted to evaluate the effect of addition of hydroxyapatite nanoparticles to resin modified glass ionomer on setting time, degree of conversion, bond strength and fluoride release.
A total of 80 specimens were used in this study. The specimens were divided into two main groups (n=40) according to the type of glass ionomer used: Conventional RMGIC (group I) and RMGIC incorporated with HANPs (group II). Each group was then divided into four subgroups (n=10); According to the type of the test performed: samples for setting time test (A), samples for degree of conversion test (B), samples for bond strength test (C) and samples for fluoride release test (D).
Formulation of RMGIC with hydroxyapatite nanoparticles as an additive material was done with a ratio of RMGIC powder / HANPs powder; 3/ 1 by weight using a digital sensitive balance accurate to 0.0001 g. producing a mixture of RMGIC and HANPs (25% by weight). To have a uniform mixture, an amalgamator was used to mix the powders of RMGIC and HANPs in clean amalgam capsules for 20 seconds. The experimental mixture was then compared to conventional resin modified glass ionomer.
Characterization of specimens was done for HAPNs, conventional RMGIC and the experimental cement by Scanning electron microscope (SEM) and Energy dispersive X-ray analysis (EDAX).
For setting time test, samples of group I(A) (n=10) and the samples of group II(A) (n=10) were prepared according to the manufacturer’s instructions. The mix was transferred to a teflon mold and placed in custom-made setting time machine with a Gillmore needle (weighed 100 grams untill the moment when the needle fails to make complete circular indentation in the cement. The results revealed statistically significantly lower mean setting time of group I(A) than group II(A) (P-value <0.05).
Regarding degree of conversion test, samples of group I(B) (n=10) and samples of group II(B) (n=10) were prepared according to the manufacturer’s instructions. The samples were examined by FTIR to be analysed at different intervals: soft, after 24hrs and after 72hrs from mix and compare that after 24hrs and after 72hrs to soft mix. After 24 hours as well as 72 hours, he results showed that the degree of conversion of group I(B) was significantly higher than group II(B) (P-value = 0.028) and (P-value = 0.036), respectively.
To investigate shear bond strength, human-extracted lower third molars (n=20) were collected and stored in 0.1% Thymol for a period not more than 2 months. The teeth were imbedded in self‑cure acrylic resin and the occlusal surfaces of the molars were prepared by removing the enamel and exposing the dentin without exposing the dental pulp. Prepared teeth were randomly divided into two groups, group I(C) & II(C) and a teflon mold was held on the dentin surface using a custom made shear bond strength fixation device. The cements of both groups were prepared as mentioned before and after setting the mold was removed. All the samples were placed in a universal testing machine. The results revealed that group I(C) has significantly higher mean shear bond strength than group II(C) (P-value = 0.009).
For fluoride release test, specimens of group I(D) & II(D) were prepared as mentioned before. Disc-shaped specimens of each material were prepared in split teflon molds and a nylon thread was incorporated into each cement disc during setting. After 24hrs from mixing, each disc individually was immersed and hanged vertically by nylon thread in a polyethylene vial filled with 10 mL of deionized water. Then the immersed deionized water was collected periodically every 24hrs for the first week, then subsequently at 14, 21, 28 and 35 days. The collected deionized water was buffered by equal volumes of SPADNS reagent then the amount of Fluoride released in deionized water was measured by using a Spectrophotometer which was
connected to an ion digital analyzer. The results showed that group I(D) has significantly lower mean daily and weekly Fluoride release than group II(D) (P-value <0.05).
Conclusion:
Within the limitations of this study:
- The addition of 25% of hydroxyapatite nanoparticles to RMGIC has a great promising effect on fluoride release property which enhances the remineralization effect and anti-cariogenic potential of RMGIC.
- Addition of a high percentage of hydroxyapatite nanoparticles to RMGIC counteracted the expectations for setting time, degree of conversion and shear bond strength of the cement which limits its use as a permanent cement.