The critical bond strength of orthodontic brackets bonded to dental glass-ceramics

Objectives To evaluate the critical bond strength (sigma) of ceramic and metal brackets to a lithium disilicate-based glass-ceramic. Materials and methods Two hundred and forty ceramic specimens (IPS e-max CAD) were randomly distributed in 12 experimental groups (n = 20). Two ceramic brackets (monocrystalline, BCm; and polycrystalline, BCp) and a metal bracket (BM) were bonded to glass-ceramic specimens after one of the following surface treatments: HF-hydrofluoric acid applied for 60 s; S-silane applied for 3 min; HFS-HF followed by S; and MDP-application of an adhesive containing a phosphate monomer (MDP). All brackets were bonded to the treated glass-ceramic using a resin cement, stored in 37 degrees C water for 48 h before shear bond strength testing. Optical (OM) and scanning electron (SEM) microscopies were used for fractographic analysis. Data was statistically analyzed using Kruskal-Wallis and Student-Newman-Keuls (alpha = 0.05). Results BCm bonded to glass-ceramic treated with either HFS or HF showed the highest median sigma values, respectively, 10.5 MPa and 8.5 MPa. In contrast, the BCp bonded to glass-ceramic treated with MDP showed the lowest median sigma value (0.8 MPa), which was not statistically different from other MDP-treated groups. Conclusions The failure mode was governed by the glass-ceramic surface treatment, not by the bracket type. Quantitative (sigma values) and qualitative (fracture mode) data suggested a minimum of 5 MPa for brackets bonded to glass-ceramic, which is the lower critical limit bond strength for a comprehensive orthodontic treatment.