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

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 x 4 mm), with and without secondary oven cure (120 degrees 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 lending 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. (C) 1999 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved.