Shear bond strength of ceramic orthodontic brackets to bulk-fill, packable, and flowable composite restorations under different thermal aging protocol of the substrate: an in vitro study

Long term success of orthodontic treatment depends on the bond strength of orthodontic brackets with tooth and/or restorative materials. This in vitro investigation aimed to assess and compare the shear bond strength (SBS) of ceramic orthodontic brackets bonded to bulk-fill, packable, and flowable composite restorations following various thermal aging protocols and to evaluate associated failure patterns. A total of 108 composite discs (10 mm × 3 mm) were fabricated from three composite types: flowable (Tetric N-Flow), packable (Tetric N-Ceram), and bulk-fill (Tetric EvoCeram Bulk Fill). Each category ( n = 36) was further divided into three subgroups of 12 specimens each ( n = 12): non-aged (control), thermocycled 5,000 times (6 months), and thermocycled 10,000 times (12 months). After surface conditioning of composite and bonding of ceramic premolar brackets using Transbond XT adhesive, specimens were stored in distilled water prior to SBS testing with a universal testing machine. Failure patterns were examined under a stereomicroscope and scored using the Adhesive Remnant Index (ARI). Data were statistically analyzed using two-way ANOVA and Tukey honestly significant difference post hoc tests ( p < 0.05). Two-way ANOVA showed that shear bond strength was significantly affected by both thermocycling and composite type (both p < 0.001), with a statistically significant difference ( p = 0.0175). Bond strength decreased with thermal aging, from 10.25 MPa at baseline to 8.25 MPa after 10,000 cycles, with the flowable composite having maximum SBS. Cohesive (45%) and mixed (41%) failures predominated, while adhesive failure was least frequent (14%). Both composite type and thermal aging of the composite prior to bonding significantly affect bracket adhesion. Non-aged flowable composites achieved the highest shear bond strength compared with packable and bulk-fill composites. The bond to all groups of tested composite materials demonstrated clinically acceptable shear bond strength values.