[The tensile strength of bracket adhesives depending on the adhesive layer thickness--an in-vitro study]

The purpose of this study was to determine the influence of the thickness of the adhesive layer between bracket base and enamel surface on tensile bond strength. Stainless steel brackets with retentive net bases were bonded to bovine incisors with adhesive layers varying in thickness from a minimum of 0.0 mm to a maximum of 0.8 mm. Eight different orthodontic adhesives were investigated; in order to simulate the procedure of indirect bonding, combinations of two different adhesives were also tested. It was found that the highly filled composites provided greatest bond strength. Increasing the thickness of the adhesive layer had no significant influence on tensile bond strength. In the case of light-cured composites, maximum tensile bond strength was achieved at a thickness of the adhesive layer of 0.2 mm. This is probably due to better penetration of light at this thickness. In the case of chemically curing no-mix adhesives, it was impossible to produce effective adhesive layers thicker than 0.2 mm, presumably because curing at the primer paste interface becomes a problem at greater thickness. The influence of cement thickness on tensile bond strength was basically identical whether brackets were bonded with one or two adhesives. Thus, an additional adhesive-adhesive interface has no influence on bond strength. If adhesive layer thickness of more than 0.2 mm are required, a chemically cured, highly filled paste-paste system should be used.