Orthodontic brackets bonding to PMMA-based materials: influence of universal adhesives

Objectives: To assess shear bond strength (SBS) and failure mode (Adhesive Remnant Index, ARI) of orthodontic brackets bonded to polymethylmethacrylate (PMMA) blocks for CAD-CAM fabrication of temporary restorations, following substrate chemical or mechanical treatment.
Methods: Two types of PMMA blocks were tested: CAD-Temp (VITA, C) and Telio^® CAD (Ivoclar-Vivadent, T). The substrate was roughened with 320-grit sandpaper, simulating a fine-grit diamond bur. Two universal adhesives, Scotchbond Universal Adhesive (3M ESPE, S) and Assure Plus (Reliance Orthodontic Products, A), and a conventional adhesive, Transbond XT Primer (3M Unitek, X), were used iin combination with Transbond XT Paste (3M Unitek, P) to bond the brackets. Six experimental groups were formed: 1) C/S/P; 2) C/A/P; 3) C/X/P; 4) T/S/P; 5) T/A/P; 6) T/X/P. SBS and ARI were assessed. On 1 extra block per PMMA-based material surfaces were roughened with 180-grit sandpaper, simulating a normal/medium-grit (100 mm) diamond bur, brackets were bonded with X/P, and SBS/ARI were compared with those of groups 3, 6.
Results: C recorded significantly higher SBSs than T. With the use of X significantly lower levels of adhesion were reached than after the application of S or A. Roughening with a coarser bur resulted in a significant increase in adhesion.
Conclusions: Bracket bonding to CAD-CAM PMMA can be enhanced by grinding the substrate with a normal/medium-grit bur or by coating the intact surface with universal adhesives. By the mentioned pretreatments bracket adhesion to CAD-CAM PMMA temporary restorations can be enhanced to levels compatible with the clinical service.