Cusp fracture resistance in composite–amalgam combined restorations

Objective: To evaluate the in vitro resistance to fracture and microleakage in composite–amalgam combined restorations. Methods: Seventeen Class I cavities with unsupported enamel walls prepared in extracted permanent molar teeth were treated with a bonding agent (Scotchbond MP, 3M Dental), the buccal cusps were reinforced with a composite resin (Z-100, 3M Dental) and the cavities were then restored with amalgam (Permite, Oral B). All teeth were load tested using a special fatigue-stress apparatus, immersed in a dye solution and then sectioned for examination by stereomicroscopy and scanning electron microscopy (SEM). Results: Fractures in enamel supported by composite were present in 11 cases while 12 specimens included fractures in enamel supported by bonded amalgam. Microleakage was observed in 16% of the enamel–amalgam interfaces, 10% of the dentin–amalgam interfaces, 7% of the amalgam–composite interfaces, 4% of the composite–enamel interfaces and 11% of the composite–dentin interfaces. Conclusions: Bonded amalgam appears to be as effective as bonded composite in supporting undermined enamel in terms of resistance to fracture, but composite may have better marginal adaptation to enamel compared to bonded amalgam. Good marginal adaptation may be observed between amalgam and composite in composite–amalgam restorations.