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 5 mm deep and with an internal diameter of 5.5 mm. Half of the internal bonding surface of each cylinder was sandblasted, silanised and coated with a dentine adhesive (Primer&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°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.