لعیا زهرایی

Composite Resins with different chemical structure are affected by lots of stimulus within mouth's environment that make them degenerated by the time, change their characteristics. The purpose of this research is to evaluate the effect of five different Food-Stimulating liquids on water sorption and the solubility of Methacrylate and Silorane based Composite resins.
Methods and materials: In this in vitro study, twenty five discs with 1mm thickness and 6mm diameter were prepared by filling aluminum molds with two kinds of composites, Filtek Z350 and P90. Samples were light- activated from top surface, then stored in coded dishes with silica gel, and transferred to the oven with 37c̊ to desiccate them for 24h. Samples were weighed by digital balance (M1) after 24h, and then stored in five Food-Stimulating liquids (Acid citric %2 N, distilled water, Ethanol %75, Heptan, and artificial saliva). After one week, samples were weighed again and the result (M2) was recorded. Samples were placed in silica gel containers until the mass variation became less than ±0.1mg (M3). After last desiccation to get the V (body mass) of samples, the diameter and the thickness of discs were calculated. Results were analyzed with Tukey test.
Results In this research, the solubility of composite Z350 in heptan was the highest (0.040 µg/ mm³) and in distilled water had the lowest solubility (0.009 µg/ mm³).Differences between distilled water, alcohol and heptan were statistically significant (p0.05). The amount of solubility of P90 in different solutions had no statistically significant difference (p>0.05). But, water sorption in artificial saliva was higher than other solutions (P The results showed that solubility of P90 was lower than Z350 but there is no statistical difference between water sorption of two materials.

Composite resins require time to complete their polymerization. This process usually reaches its maximum rate after 24 hours. On the other hand، immediately after restoration، water sorption results in hygroscopic expansion of composite resins and the mentioned two factors usually reachequilibrium after a week. Considering the effect of mentioned processes on the microleakage of composite restorations، the purpose of the present study was to evaluate the effect of finishing and polishing time on the mean microleakage of composite restorations. In this in-vitro experimental study، 60 sound human premolar teeth were selected. A standard Class V cavity was prepared measuring 1. 5 mm in depth، 3 mm in width and 2 mm in length on the buccal surface of each tooth and incrementally restored with composite resin. Next، the teeth were randomly divided into 4 groups. The teeth in groups 1، 2، 3 and 4 were finished and polished immediately، 15 min، 24 h and one week after storage in distilled water at 37°C، respectively. Specimens in each group were subjected to 500 thermal cycles. The teeth were then coated with adhesive wax and nail varnish and immersed in 2% fuchsin solution for 24h. The specimens were washed and each sample was longitudinally sectioned in half by a diamond saw. Both halves were evaluated in terms of dye penetration under a stereomicroscope at 28X magnification. For each sample، the section with greater degree of microleakage was selected. Kruskal Wallis test and Mann Whitney U test were used for data analysis with a 99. 1% confidence interval. The lowest mean microleakage was observed in the occlusal margin of group 3 teeth; while the highest mean microleakage belonged to the occlusal and gingival margins of teeth in group 2. A significant difference was noted between the 4 groups in occlusal margin microleakage (P<0. 009). However، no such difference was found in gingival margin. This study showed that the finishing and polishing timewas effective on the mean microleakage in the enamel margin of composite restorations. The best time of finishing and polishing was 24h after the restoration. Time of finishing and polishing had no effect on microleakage in dentin margins of restorations.