Marginal sealability of resin-modified glass ionomers for base/liner: effect of hygroscopic expansion and bond strength

Hygroscopic expansion of a compomer and a composite on artificial gap reduction

OBJECTIVE

This study compared the effect of water sorption on the extent of marginal gap reduction in a compomer (Dyract AP, Dentsply) and a composite (Spectrum, Dentsply) over a 12-week storage period.

MATERIALS AND METHODS

Artificial gaps were created in 40 borosilicate glass cylinders, each 5mm deep and with an internal diameter of 5.5mm. Half of the internal bonding surface of each cylinder was sandblasted, silanised and coated with a dentine adhesive (Prime&Bond NT, Dentsply). Twenty partially bonded cylinders were incrementally filled with Dyract AP (AP) and the rest with Spectrum (S). For each material, 10 specimens were stored in de-ionised water (W), and 10 (control) in non-aqueous silicone fluid (O) at 37 degrees C. The dimension of the same maximum gap created in each specimen was repeatedly measured at 0, 1, 2, 4, 6, 8, 10 and 12 weeks, using a light microscope under incident light at 570x magnification. Gap widths in each of the four groups (n=10) were statistically compared. Correlations between mean gap width reduction and storage time were also examined.

RESULTS

Friedman repeated measures ANOVA on ranks revealed significant differences (p<0.001) among the gap widths measured at different time intervals in groups AP-W and S-W. No significant differences (p>0.05) were found in groups AP-O and S-O. Tukey's multiple comparison test indicated that no significant differences (p>0.05) were detectable beyond the sixth week in AP-W and the fourth week in S-W. Linear regression analyses showed that mean gap widths decreased exponentially with time for AP-W (r=0.97) and S-W (r=0.90). From the slopes of the regression lines, the rate of marginal gap reduction in AP-W was 4.6 times faster than S-W.

CONCLUSION

Marginal gap reduction in both the materials are directly attributed to water sorption and that the reduction is larger and more rapid in Dyract AP.

Marginal adaptation of a new compomer under different conditioning methods

OBJECTIVES

This in vitro study evaluated the marginal adaptation of compomer restorations placed using three different conditioning protocols.

MATERIALS AND METHODS

Thirty extracted caries-free molars with 3mm diameter cylindrical cavity preparations were divided randomly into three groups based upon the conditioning treatment used: (I) 36% phosphoric acid; (II) non-rinse conditioner (NRC, Dentsply DeTrey); and (III) no conditioning. Cavities were restored with Dyract AP using Prime&Bond NT (Dentsply DeTrey) as an adhesive. Silicone impressions of the briefly etched enamel surfaces were taken after finishing the restorations. Each sample was then longitudinally sectioned and impressions were taken. Epoxy resin replicas were prepared for SEM analysis. Qualitative and quantitative assessment were performed separately for the enamel- and dentine-restorative interface.

RESULTS

Enamel fractures and open margins along the enamel-restorative margin were observed in some specimens in each group. No statistically significant difference was found in the percentage of gaps/cohesive failures between specimens prepared using different conditioning methods. For the dentine-restorative interface, uniform hybrid layers and long resin tags were often observed in Groups I and II. The hybrid layer in Group III was irregular and discontinuous along the interface. A significant difference (p<0.01) in the proportion of marginal gap was found between Group I (2%) and Group III (30%).

CONCLUSIONS

Pre-treating the cavity with 36% phosphoric acid significantly improved the adaptation of the compomer and adhesive to dentine compared with no etching. The marginal quality at the enamel-compomer interface was not affected by the conditioning method used.

Marginal gap formation of light-activated base/liner materials: effect of setting shrinkage and bond strength

OBJECTIVES

The purpose of this study was to compare the effect of setting shrinkage and the shear bond strength on the marginal gap formation in dentin cavities of commercially available light-activated base/liner materials during the early stage of setting, with those of chemical-cured base/liner materials.

METHODS

The maximum marginal gap width and the opposing width in the dentin cavity was measured 30 min after the start of mixing. The setting shrinkage and shear bond strength to dentin was measured as the same procedure. To estimate the setting shrinkage, the maximum marginal gap width and the opposing width that occurred with materials placed in a Teflon cavity was measured.

RESULTS

The two light-activated base/liner materials produced a significantly wider marginal gap than the chemical-cured base/liner materials (p < 0.05). There were no significant differences in the width of the marginal gap between the remaining five light-activated base/liner and chemical-cured base/liner materials (p > 0.05). The material that produced the smaller marginal gap width in the dentin cavity had a smaller marginal gap width in the Teflon cavity. There was a highly significant correlation between the two findings (r = 0.93, p < 0.01). However, shear bond strength to dentin had no effect on the marginal gap in the dentin cavity (r = 0.04, p > 0.50).

SIGNIFICANCE

It appeared that the setting shrinkage of light-activated base/liner materials in the early stage of setting had a greater effect on the marginal gap formation in the dentin cavity than the bond strength to the dentin structure.