Evaluation of shear bond strength of orthodontic molar tubes bonded using hydrophilic primers: An in vitro study

To evaluate and compare the shear bond strength (SBS) of orthodontic molar tubes bonded using two hydrophilic primers along with a moisture tolerant adhesive system to dry and saliva-contaminated enamel surfaces; and to assess the mode of their bond failure.

A total of 60 extracted human mandibular molars were randomly divided into three major groups according to the primer used, each consisting of 20 molars: XT group acts as a control and bonded with the conventional hydrophobic Transbond XT primer, OS group bonded with the hydrophilic Ortho Solo primer, AP group bonded with the hydrophilic Assure Plus all surface bonding resin. Each major group was further divided into two subgroups, of 10 molars each, according to presence or absence of saliva. All the specimens were thermocycled 500 cycles between 5̊ and 55°C. Shear forces were applied to the specimens with a universal testing machine at a crosshead speed of 1mm/min and SBS was measured in megapascals (MPa). The mode of failure was determined using the adhesive remnant index (ARI). Data were analysed using two-way analysis of variance (ANOVA) followed by univariate analysis and Bonferroni post hoc tests.

The three tested primers did not show a significant difference in the mean SBS in dry conditions (P=0.137); the mean SBS of OS and AP primers were 15.60±5.879MPa and 12.51±2.583MPa respectively which were comparable to that of the hydrophobic XT primer (12.76±2.952MPa). In saliva-contaminated conditions, the mean SBS values were 10.41±4.457MPa and 9.22±3.422MPa for OS and AP primers respectively, which were significantly higher than that of XT primer (4.82±2.050MPa) (P=0.004). When comparing the mean SBS for each group according to the bonding condition, it was significantly higher in dry bonding compared to saliva-contaminated bonding for the three primers; XT (P<0.001), OS (P=0.003) and AP (P=0.011). In the dry field, most of the bond failures of the three primers were adhesive (score 3), whereas in the saliva-contaminated field, most of the failures were cohesive (score 1).

Dry bonding yielded the highest SBS for the three primers. Saliva contamination significantly decreased the bond strength of both hydrophilic primers; however, the values were above the clinically acceptable limit. The hydrophilic primers tested in the present study can be successfully used for bonding orthodontic molar tubes under dry and saliva-contaminated enamel surface conditions.