Comparison of immediate and delayed light-curing on nano-indentation creep and contraction stress of dual-cured resin cements.

STATEMENT OF PROBLEM: Polymerization protocol of dual-cured resin cements plays a great role in the success of restorations. Knowledge about the effect of light-curing protocol on the contraction stress value and the mechanical properties would assist in the development, selection and handling of the resin cement material. PURPOSE: This study was undertaken to assess the effects of two different light-curing protocols on the polymerization shrinkage stress and nano-creep of two current dual-cured resin cements. MATERIALS AND METHODS: Twenty-four soda-lime glass disks, measuring 1mm in thickness, with a perforation in their center that measured 3mm in diameter, received four Vickers indentations at a distance of 500micro from the margin. The indent cracks were measured before and at 15-, 30- and 60-min intervals after restoration of the cavity with either CLEARFIL SA or RelyX Ultimate cement. Twelve specimens were prepared from each resin cement and divided into two groups according to the curing protocol used: immediate and delayed light-curing. Stresses at the indent impression were calculated by considering glass fracture toughness and the increase in crack length. Stress at the bonded interface was calculated using the Lame equation for thick-walled cylinder under an internal pressure. Nano-indentation hardness, modulus of elasticity and creep of each cement were measured under a 10-mN load for 20s holding time. The specimens were stored in distilled water at 37 degrees C for 24h before the nano-indentation tests. Data were analyzed with two-way and repeated-measures ANOVA (alpha</=0.05). RESULTS: Curing protocol had a significant effect on both resin cements; higher stress values were recorded with the immediate curing mode (P=0.033). Curing mode did not exert a significant effect on the modulus of elasticity (P=0.595) and hardness (P=0.454) of resin cements. However, in relation to creep, it exhibited significant effects (P<0.0001). CONCLUSIONS: Immediate light-curing resulted in higher polymerization stress and lower nano-creep values compared to the delayed protocol with the resin cements assessed.