Creep and visco-elastic recovery of cured and secondary-cured composites and resin-modified glass-ionomers

Abstract

Objective: The aim of this investigation was to study whether secondary curing had any effect or not on the visco-elastic stability of resin-based restoratives. Materials: Five resin composite restorative materials (Tetric, Z100, Lite-fil, Palfique, Graft LC) and two resin-modified glass-ionomers (RM-GIC) (Fuji II LC&Photac-Bond), were chosen as representative of those clinically available. Methods: Specimens were fabricated as short cylinders (6 mm×4 mm), with and without secondary oven cure (120°C, 7 min). A creep measurement apparatus was used to subject each specimen in turn to a cycle of constant compressive stress of 50 MPa for 6 h followed by 6 h of strain recovery, after load removal. Time-dependent creep and recovery were recorded. Results: The creep curves of these VLC materials during loading and unloading were characterised by an initial rapid elastic deformation, greater than 1%, followed by a visco-elastic response. During unloading there was an initial elastic recovery, followed by the creep recovery, and finally permanent set. Secondary-cure had no statistically significant effect ( p>0.05) on maximum creep strain ( Y 1) which was 1–2% for resin-composites and 3–4% for RM-GIC. Permanent set for the RM-GIC were 0.68–0.77% and for resin-composites ranged from 0.13–0.46%. The influence of secondary-cure on creep recovery and permanent set was limited. However, some materials (Z100, Lite-fil and Photac-Bond) had permanent set significantly reduced ( p<0.05) by application of a second cure. Significance: Visco-elastic creep values of less than 2% are acceptable for composites at the stress-levels applied. However the two-fold increase in creep response by RM-GIC suggests that these materials are unsuitable for stress-bearing areas.