Shear bond strength of orthodontic brackets and cement removal techniques

Objective: To evaluate the shear bond strength (SBS) of different cementation techniques for luting orthodontic brackets. Additionally, the amount remaining resin cement after testing and different material removal techniques were compared by the superimposition of three-dimensional (3D) tooth scans. Methods: Orthodontic brackets were cemented on 40 freshly extracted human premolars according to 4 different protocols (n=10): (G1): 37% H3PO4 etching (E&R) and Transbond XT adhesive primer and cement (TB; 3M); (G2): E&R +Scotchbond Universal Plus adhesive (SBU; 3M) +RelyX Universal cement (RXU; 3M); (G3): SBU in self-etch mode (SE) +RXU; (G4): RXU in self-adhesive mode (RXUSA). SBS was conducted with an Instron Universal Testing machine. Afterwards, teeth were randomly allocated into subgroups to evaluate two adhesive removal techniques: SL-Sof-Lex on low-speed handpiece; TC-tungsten-carbide multi-laminated high-rotation drill. A 3D scan (3shape) of each model was taken at the beginning (T0), after the SBS (T1) and after cement removal (T2) to assess volumetric differences. The data were analyzed in Sigmaplot 14.0 (Systat Software Inc), with the significance level p<0.05. Results: Brackets cemented with E&R+SBU+RXU demonstrated bond strength similar to those cemented with TB (p>0.05), and significantly higher than those cemented with SE+SBU+RXU or RXUSA (p<0.05). After the SDS test, there were more cement remnants in the E&R+SBU+RXU group compared to the SE and self-adhesive RXU techniques (p<0.05), while differences were not noted between other groups (p>0.05). Both adhesive removal techniques successfully cleaned the tooth surface, but removed also a portion of the enamel, particularly SL (p<0.05). Conclusions: Brackets cementation with E&R+SBU+RXU seems to offer the highest bond strength, and also leaves more cement remnants after the bracket removal. Cleaning of the remaining cement from the enamel with tungsten-carbide burs seems to be as efficient and less aggressive in terms of healthy tooth tissue preservation compared to the Sof-Lex.