Effect of different surface treatments on the repair bond strength of flowable single-shade bulk-fill composites

To assess how various surface treatment methods affect the repair shear bond strength (SBS) of flowable single-shade bulk-fill composite resins.

200 composite discs were fabricated using a nanohybrid composite (GrandioSo, VOCO) to simulate aged restorations. All specimens underwent 5,000 thermal cycles to mimic oral aging. Five different surface treatment approaches were tested: no treatment (control), diamond bur abrasion, phosphoric acid etching, Er,Cr:YSGG laser irradiation, and silane application. After treatment, three flowable single-shade bulk-fill composites: Omnichroma Flow Bulk (Tokuyama Dental), SimpliShade Bulk Fill Flow (Kerr), and Charisma Bulk Flow ONE (Kulzer) were bonded onto the aged surfaces using a universal adhesive (Tokuyama Universal Bond). SBS was measured with a universal testing machine, and failure modes were examined under a stereomicroscope. Statistical analyses were conducted with robust ANOVA and chi-square tests (α= 0.05).

Both resin composite type and the surface treatment protocol, as well as their interaction, had significant effects on SBS values (P< 0.001). Among the tested groups, the highest SBS was recorded for GrandioSo combined with silane treatment (23.85 MPa), while the lowest was found for Charisma Bulk Flow ONE after acid etching (10.8 MPa). Silane application resulted in significantly greater SBS compared to other treatments (P< 0.001). Adhesive failures were most prevalent in the acid and control groups, whereas laser and silane groups displayed more cohesive and mixed failures.

Effective surface conditioning is essential for durable and predictable composite repairs. Silane application and Er,Cr:YSGG laser irradiation significantly improved the repair bond strength of flowable single-shade bulk-fill composites, whereas phosphoric acid etching alone was insufficient.