Aspects of enamel bonding using experimental silanes for orthodontic adhesion

This in vitro study investigates bonding to enamel using experimental silane-based primers with and without 2-hydroxyethylmethacrylate (HEMA) under various artificial ageing methods. One hundred and fifty sound extracted human premolars were used and randomly assigned to three experimental study groups. They were first acid-etched for 15 s, rinsed with water spray, air dried, and applied 0.3 ml of artificial saliva on the enamel surfaces. Two groups of enamel surfaces were primed using silane-based experimental primers (1.0 vol% of 3-isocyanatopropyltrimethoxysilane and 0.5 vol% of bis-1,2-(triethoxysilylethane) with and without 25% HEMA) while one group was served as control. Then, stainless steel premolar orthodontic brackets were fixed onto teeth with orthodontic resin composite. The specimens from each group (n=10) were stored under different ageing conditions: thermo-cycling (500, 2000, and 6000 cycles), storage in artificial saliva for 24 h, and for one year. The shear adhesion (bond) strength (SBS) was tested by using a universal testing machine at a crosshead speed of 1.0 mm/min. Surface morphology and failure modes at the debonded interfaces were examined using SEM. Two-way ANOVA and post hoc tests were used to compare the SBS (α=0.05). The results suggested that an experimental primer with 25% HEMA, after 24 h storage in artificial saliva, produced the highest mean SBS (22.1 MPa, SD 2.2 MPa). The lowest mean value (5.8 MPa, SD 1.1 MPa) was obtained with the control group thermo-cycled (6000 cycles). There was a significant difference between the experimental primers (p<0.001) and artificial ageing (p<0.001). We conclude that 25% HEMA inclusion in silane primer could provide satisfactory adhesion strength, and 500 cycles of thermo-cycling (ISO TR 11450) does not correlate with 1-year artificial saliva storage for enamel bonding test.