title: 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.
  
  
  METHODS
  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 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 (Y1) 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Biomaterials Science Unit, University of Manchester Dental School, UK.
authors.forename: A A
authors.initials: AA
authors.lastname: el Hejazi
authors.forename: D C
authors.initials: DC
authors.lastname: Watts
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D004548:Elasticity; D005899:Glass Ionomer Cements / Q000737:chemistry*; D008422:Materials Testing; D019279:Resin Cements / Q000737:chemistry*; D012212:Rheology; D013997:Time Factors; D014783:Viscosity
chemical_list: D003188:Composite Resins; D005899:Glass Ionomer Cements; D019279:Resin Cements