Influence of Composite Type, Surface Treatment, and Thermocycling Aging on the Shear Bond Strength of Repaired Composite Resins.

PURPOSE: This study evaluated the effects of different surface treatment methods and thermocycling on the shear bond strength (SBS) of repaired composite resin restorations across three composite types (nanohybrid, bulk-fill, and microhybrid). METHODS AND MATERIALS: A total of 252 disk-shaped specimens (Ø20 × 5 mm, n = 7 per group) were fabricated from three composite resins: a nanohybrid composite (Filtek Z550; 3M ESPE), a bulk-fill composite (Opus Bulk Fill; FGM), and a microhybrid composite (Gradia Direct Anterior; GC). The specimens were divided into four subgroups according to surface treatment: no surface treatment (ST-C), airborne-particle abrasion (ST-AO), bur roughening with a medium-grit diamond bur under water cooling (ST-B), and phosphoric acid etching followed by application of a universal adhesive (ST-PH). For all groups, the repair procedure was performed using a nanohybrid composite (Filtek Z550; Ø2.38 × 3 mm). Aging protocols included water storage at 37°C for 24 h (Immediate), thermocycling for 10,000 cycles, and 30,000 cycles between 5°C and 55°C. Shear bond strength (SBS) was measured using a universal testing machine at a crosshead speed of 1 mm/min. Data were analyzed using the Shapiro-Wilk and Levene tests, followed by three-way ANOVA (with stratified one-way ANOVAs where interactions were significant) and Tukey post hoc tests (α = 0.05). RESULTS: A three-way ANOVA revealed that composite type, surface treatment, and aging condition significantly affected SBS (P 0.001). SBS differed significantly among composites and surface treatments (P 0.001). For RC-BF, ST-PH provided the highest bond strength after 10,000 and 30,000 cycles. In RC-MH, bur roughening (ST-B) yielded the highest bond strength at 10,000 cycles. For RC-NH, ST-PH initially showed the highest SBS values; however, at 30,000 cycles, ST-B provided the best repair stability for the RC-NH group (12.96 MPa). However, the highest overall SBS value across all tested materials at this final aging stage was achieved by the RC-BF group treated with ST-PH (13.91 MPa).. Thermocycling significantly reduced SBS (P 0.001). Nanohybrid composites exhibited relatively stable SBS values across aging conditions. CONCLUSION: Appropriate surface treatment, particularly phosphoric acid etching combined with universal adhesive application, significantly increased shear bond strength following thermocycling. Nanohybrid composites demonstrated stable SBS trends across aging conditions compared with the other materials.