Fracture Strength of Indirect Resin Composite Laminates to Teeth with Existing Restorations: An Evaluation of Conditioning Protocols

Purpose: This study evaluated the fracture strength and failure types of indirect resin-based composite laminates bonded to teeth with aged Class III composite restorations that were conditioned according to various protocols. Materials and Methods: Maxillary central incisors (N = 60) with window-type preparations received laminates made of a highly-filled resin composite material (Estenia) (10 per group).On the mesial and distal side, Class III cavities (3 x 3 mm) were prepared using ultrasonic burs and filled with resin composite (Quadrant Anterior Shine). The unrestored teeth served as a control group (group 6). All restored teeth (n=50) were thermocycled (5°C to 55°C, 6000X) and subjected to one of the conditioning protocols: (1) air-particle abrasion with alumina particles coated with silica (30-µm SiO2, CoJet)+silanization, (2) air-particle abrasion with alumina particles (50 µm, Al2O3)+silanization, (3) 9.5% hydrofluoric acid (HF) for 90 s (Ultradent)+silanization and (4) protocol of Clearfil Repair Kit, (5) adhesive resin (Quadrant Unibond Sealer). A three-step bonding procedure and dual-polymerizing resin cement (Panavia F 2.0) were employed. The inner surfaces of the laminates were conditioned (CoJet-Sand, 30 µm SiO2) and silanized (ESPE-Sil). All specimens were stored in water at 37°C for one month prior to the fracture test. Results: A significant difference was observed in fracture strength values between the groups (ANOVA, p = 0.0261). The only significant difference was between group 2 (299 ± 103 N) and group 3 (471 ± 126 N) (p = 0.0239) (Tukey's test, α = 0.05). The majority of failures were type C (35/60) (chipping of the laminate with enamel exposure), followed by type B (21/60) (cohesive failure within the composite laminate). Conclusion: The fracture strengths of the laminates tested did not show significant differences, whether they were bonded to existing, aged Class III composite restorations or to intact teeth. The failure types, however, varied between the groups. The lowest strengths were obtained from the air-particle abraded (50 µm, Al2O3) and silanized group. Keywords: indirect composite, fracture strength, laminates, surface conditioning