Comparison of shear and flexural bond strength tests versus failure modes of dentin bonding systems

To (1) measure the shear and flexural bond strengths of three different bonding agents that use different application techniques, (2) determine whether the shear and flexural bond tests rank the three materials similarly, and (3) determine whether the fractured surface produced with the flexural test, facilitates studies of failures within the adhesive interphase region.

Scotchbond MP (SBMP), Single Bond (SNGB) and Clearfil SE (CLSE) were evaluated. For each material, 16 samples were bonded. Eight of these samples were bonded for shear testing and the other 8 samples for flexural bond strength testing. After the bonding was completed, all samples were stored in water at 37 degrees C for 30 days. Shear bond strength was measured using an orthodontic edge wire loop and flexural strength was measured using a 4-point bending device. The samples used for the flexural test consisted of 3 mm x 3 mm x 20 mm beams in which center a 1 mm thick dentin wafer had been bonded perpendicularly to the length of the beam. An indentation with a microhardness tester was placed at one of the dentin-composite interfaces to serve as an induced flaw. This flaw was placed on the surface in tension during flexural testing. The data were analyzed using ANOVA. Scanning electron microscopy was used to examine each specimen and assign its failure mode. Percent occurrence of failure mode was determined for each material and overall for both test methods. When there was uncertainty regarding failure location, electron dispersive spectroscopy x-ray analysis was used to identify elements present on the fractured surface.

No significant difference (P > 0.05) in bond strength was observed within each test group, while significant differences (P < 0.05) existed between the two test methods (shear: SBMP = 21.2 +/- 4.0 MPa, SNGB= 24.3 +/- 4.7 MPa, CLSE= 24.6 +/- 4.4 MPa; flexural strength: SBMP= 34.6 +/- 9.3 MPa, SNGB = 31.9 +/- 6.9 MPa, CLSE= 34.3 +/- 4.7 MPa). Shear bond test specimens failed mostly within dentin (54.2%), followed by failures within the adhesive interphase (41.6%), and failures in composite (4.2%). Flexural strength specimens failed mostly within the adhesive interphase (83.3%), followed by failure in composite (16.7%). Bond strengths were similar for all three adhesive systems within each test method group. Failure mode analysis revealed significant differences (P < 0.0001) among the two test methods.