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.