Hygroscopic expansion behaviour of composite lutes

Objectives: This study was designed to measure the changes in dimension and weight due to hygroscopic expansion that occur in two representative and commercially available resin-based luting materials. Method: The materials tested were: [Nex]=Nexus2™ (Kerr-Sybron Dental Specialities) and [DyrC]=Dyract®Cemplus (Dentsply DeTrey). 7 disks of each test material were prepared, 15mm diameter by 1mm thickness. All specimens were kept dry in a vacuum-desiccating chamber for 48hrs during polymerisation. Two control specimens of each group were maintained in the desiccating chamber and five specimens in sterile de-ionized water at 37°C. Measurements were made using a computer-controlled laser micrometer (resolution of 0.05µm) and a precision microbalance (resolution of 0.1µg). The diameter of each disc was measured automatically across 200 points by rotation through the laser beam. These measurements, together with weight measurements, were carried out periodically for 30 days. Results: At 30 days, the mean % changes in volume after water immersion were: [Nex]=0.45% (sd=0.3); [DyrC]=2.64% (sd=0.37). The mean gravimetric measurements are [Nex]=10.9µg/mm³ (sd=1.65); [DyrC]=50.46µg/mm³ (sd=6.7). Mann Whitney Test and Wilcoxan rank sum demonstrated a statistically significant difference between the two materials (p<0.05) for changes in dimension and mass. Conclusion: Current resin based luting materials vary significantly in the post-placement expansion behaviour.