Effect of Different Surface Treatments on the Shear and Flexural Re-bond Strengths of a Micro-hybrid Composite

Abstract

This study compared the shear bond strength (SBS) and flexural strength (FS) of repaired interfaces using three techniques of surface preparation and repair of a micro-hybrid composite. Composite specimens for SBS tests (n=36: diameter=8 mm, thickness=3 mm) for repair were shaped in a metal mold, visible-light cured, and embedded in dental stone. Composite specimens for FS tests (n=36: 2 mm x 2 mm x 12.5 mm) for repair were shaped in a silicone mold. 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 microm aluminum oxide at 100 psi; and Group C specimens received two parallel, 1 mm-deep grooves using a (1/4)-round bur before being treated by etching with 35% phosphoric acid. After surface treatment, a bonding agent was placed on each specimen, which was then light cured. Repairs were accomplished by adding more composite to the SBS or FS specimens. Specimens were thermocycled 500 times between 5 degrees C and 55 degrees C with 30-second dwell times. All specimens were tested by loading to failure at a rate of 0.5 mm/min using an Instron Universal Testing Machine. Mean SBSs in MPa were: Group A=24.5 +4.4, Group B=28.5 +4.3, and Group C=27.0 +2.8. Mean FSs (MPa) were: Group A=60.5 +9.9, Group B=73.9 +13.2 and Group C=81.3 +14.3. For the FS tests, Group B and C were significantly different than Group A, but Group C was not significantly different than group B. For the SBS test, Group B was significantly different than Group A, but Group C was not significantly different. Acid-etching alone was not very effective in producing well-bonded composite repairs (only 55% of the FS of normal micro-hybrid composite). Mechanical retention was more effective (SBS, FS) than acid-etching and as effective (SBS) or more effective (FS) than air-abrasion (p<0.05) for repaired composite specimens. Either mechanical retention or air abrasion is recommended prior to repairing an existing composite restoration to achieve the highest bond strength.