FRACTURE-TOUGHNESS OF POSTERIOR COMPOSITE RESINS FABRICATED BY INCREMENTAL LAYERING

An incremental buildup technique is advocated for placing posterior co mposite resin restorations. This technique is preferred because it ass ures complete polymerization of large restorations and also reduces th e problem of polymerization shrinkage, which can adversely affect the seal at the gingival margin of class II and large class III restoratio ns. Measurements of tensile bond strengths between layers have been pu blished with conflicting results. This study measured the fracture tou ghness of layered posterior composite resin within the interfacial lay er and in a direction perpendicular to the layering. A compact test-sp ecimen geometry conforming to the ASTM standard E399-83 was used. Thre e groups were tested: Group A specimens were bulk-cured in one complet e increment; group B specimens were incrementally cured in 1.2 +/- 0.2 mm layers perpendicular to the plane of fracture; and group C specime ns were incrementally cured with the interface between layers parallel to and in the plane of fracture. The microfilled composite resin show ed no significant difference in the mean fracture toughness from the b ulk-cured control regardless of layering technique. The hybrid composi te resin showed a significant decrease in fracture toughness for fract ures occurring in the interfacial layer, and fractures perpendicular t o layering showed no significant difference in fracture toughness comp ared with the bulk-cured control.