Fluoride releasing restorative materials: Effects of pH on mechanical properties and ion release

ObjectivesSecondary caries and restorative fracture are the two main reasons for restoration failures. Fluoride ion (F) release can help inhibit caries. Plaque pH after a sucrose rinse can decrease to a cariogenic pH of 4–4.5. The objective of this study was to investigate the effects of solution pH and immersion time on the mechanical properties and F release of restorative materials.MethodsThree resin-modified glass ionomers (Viremer, Fuji II LC, Ketac Nano), one compomer (Dyract Flow), and one composite (Heliomolar), were tested. Flexural strength and elastic modulus were measured before and after 84 d of immersion in solutions of pH 4, 5.5, and 7. F release was measured as a function of pH and immersion time.ResultsImmersion and material type had significant effects on mechanical properties. Vitremer had a flexural strength (mean ± sd; n = 6) of 99 ± 25 MPa before immersion; it decreased to 32 ± 9 MPa after 84 d of immersion (p < 0.05). In comparison, Heliomolar had a smaller strength loss, decreasing from 99 ± 9 MPa to 65 ± 7 MPa (p < 0.05). Solution pH had little effect on mechanical properties. For example, Fuji II LC had a strength of 63 ± 15 MPa at pH 4, similar to 61 ± 30 MPa at pH 5.5, and 56 ± 22 MPa at pH 7 (p > 0.1). In contrast, solution pH had a significant effect on F release. F release at 84 d for Fuji was 609 ± 25 μg/cm2 at pH 4, much higher than 258 ± 36 μg/cm2 at pH 5.5, and 188 ± 9 μg/cm2 at pH 7.SignificanceThe restoratives tested were able to greatly increase the F release at acidic, cariogenic pH, when these ions are most needed to inhibit caries. However, mechanical properties of these F-releasing restoratives degraded significantly in immersion. Efforts are needed to develop F-releasing restoratives with high levels of sustained F release, as well as improved durability of mechanical properties for large stress-bearing restorations.