Evaluation of flexural strength and shear bond strength of repaired microhybrid composite with varied surface treatments

The aim of this study was to evaluate the effect of three different surface treatments on the flexural strength (FS) and shear bond strength (SBS) of repaired microhybrid composite.

For the FS test composite specimens (n  = 36: 2 mm ´ 2 mm ´ 12.5 mm) used for repair were shaped in a silicone mould. For the SBS test composite specimens (n = 36: diameter =  8 mm, thickness = 3 mm) used for repair were shaped in a metal mould, cured in visible light, and embedded in dental stone. Three different methods of surface preparation were evaluated: group A (control) specimens were treated by etching with 35 % phosphoric acid, group B specimens were air abraded with 50 μm aluminium oxide at 100 psi and group C specimens received two parallel 1 mm deep grooves using a 1/4 round burr before being treated by etching with 35 % phosphoric acid. After surface treatment a bonding agent was placed on each specimen and light-cured. Repairs were accomplished by adding more composite to the FS or SBS specimens. Specimens were thermocycled 500 times between 5 °C and 55 °C with 30 s dwell times. All specimens were tested by loading to failure at a rate of 0.5 mm/min using an Instron universal testing machine. Statistical analysis was done using ANOVA test to compare the means of each group within the SBS or FS tests and Tukey post hoc tests (p < 0.05) were used to detect any differences.

Mean FS values (MPa) were: group A = 60.5 ± 9.9, group B = 73.9 ± 13.2 and group C = 81.3± 14.3. Mean SBS values (MPa) were: group A = 24.5 ± 4.4, group B = 28.5 ± 4.3 and group C = 27.0 ± 2.8. The FSs of groups B and C were significantly greater than in group A but there was no significant difference between group C and group B. For the SBS test group B was significantly greater than in group A but with no significant differences between groups C and B.

Either mechanical retention or air abrasion is recommended prior to repairing an existing composite restoration to achieve the highest bond strength.