Contraction Stresses in Surrounding Tooth Immediately after Adhesive Restorations

Abstract

Polymerization shrinkage of dental resin composite generates stresses that are capable of deforming the composite and tooth, debonding the tooth-resin interface and cracking the surrounding tooth structure. Contraction stress is unfavorable for obtaining good prognosis of composite restorations and often induces postoperative discomfort. The objective of this study was to investigate the contraction stresses in bovine enamel generated by adhesive restorations using the indentation fracture method. Six kinds of materials for the adhesive restoration were used in this study: three resin composites (Z100™: 3M ESPE, Herculite XRV: sds kerr and Heliomolar: Ivoclar Vivadent), a conventional glass ionomer cement (Fuji IX GP: GC), a resin-modified glass ionomer cement (Fuji II LC: GC) and a resin composite cement (Panavia®F2.0: Kuraray Medical) for resin composite inlay. Volumetric polymerization shrinkage of the materials except the conventional glass ionomer cement and the resin composite cement was measured using a volume measuring device (AcuVol™, Bisco). The polymerization shrinkage was recorded for 60 minutes from the start of light irradiation for polymerization. Elastic moduli of the six materials were measured using a nano-indentation device (ENT-1100a, Elionix). Contraction stresses generated by the six restorations were calculated. Cracks were introduced with a Vickers indenter near a tapered cylindrical cavity in flat bovine enamel. The ratio of bonded to unbonded surface area (C-factor) was 2. Lengths of the radial indentation cracks were measured before and after the restoration. The contraction stress in bovine enamel was calculated from the crack length and the fracture toughness of the enamel. Following the second measurement of the crack length, interfacial gaps between the enamel and the restorative and crevices in the enamel around the restoration were observed to calculate the ratio of the gap length to the cavity perimeter and to count the number of specimens with an enamel crevice, respectively. The radial cracks were lengthened in all restorations due to the contraction stresses. The calculated contraction stresses were lower than the previously reported values and not significantly different among the restorations, though the polymerization shrinkage and the elastic moduli varied. The gap ratios were more than 90% for the restorations except the resin composite inlay. The resin composite restorations demonstrated more enamel crevices than the other restorations. The contraction stresses generated by polymerization shrinkage propagated the radial cracks, the enamel crevices and the contraction gaps. The formation of enamel crevices and contraction gaps released the contraction stresses in the enamel and at the interface, thus reducing the stresses at the indentation cracks.