Fracture Strength of Indirect Composite Laminates Cemented on Existing Restorations

Objectives: This study evaluated fracture strength and failure types of indirect laminates adhered to teeth with aged composite restorations. Methods: Preparations were made on intact human maxillary central incisors (N=60). Laminates using a highly-filled polymeric material (Estenia) were produced. Thickness of laminates in the original form of teeth was achieved using impression moulds made before tooth preparation. Class III cavities (3×3 mm) were opened and filled with composite (Quadrant Anterior Shine). The unrestored teeth served as a control group (Group 6). All restored teeth (n=50) were aged in thermocycling and subjected to one of the following protocols: (1) Air-particle abrasion with alumina particles coated with silica (30 µm CoJet) + silanization, (2) Air-particle abrasion with alumina particles (50 µm, Al2O3) + silanization, (3) 9.5% hydrofluoric acid for 90 s (Ultradent) + silanization, (4) Protocol of Clearfil Repair Kit, (5) Adhesive resin (Quadrant Unibond Sealer). A three-step bonding procedure and resin cement (Panavia F 2.0) were employed. The cementation surfaces of laminates were conditioned (CoJet®-Sand, 30 µm SiO2) and silanized (ESPE-Sil). All specimens were cemented ultrasonically (Amdent). The specimens were stored in water for one month prior to fracture test. Failure types were classified as; a) Complete adhesive failure between tooth and laminate (Type A); b) Cohesive failure within laminate (Type B), c) Mixed failure (Type C). Data were analyzed using 1-way ANOVA and Tukey's test. Results: Significant effect of conditioning was observed (p=0.0261). The only significant difference was between Group 2 (299±103 N) and Group 3 (471±126 N) (p=0.0239). The majority of experienced failures were Type C (35/60) followed by Type B (21/60). Conclusions: Fracture strength of laminates did not show significant difference when they were adhered either to existing, aged Class III restorations or to intact teeth but the failure types varied between groups.