Polymerization contraction stress in dual-cure cements and its effect on interfacial integrity of bonded inlays Roberto R. Braga a,b, * , Jack L. Ferracane b , John R. Condon b a Department of Dental Materials, School of Dentistry, University of Sa ˜o Paulo, Av. Prof. Lineu Prestes, 2227, Sa ˜o Paulo, SP 05508-900, Brazil b Department of Biomaterials and Biomechanics, School of Dentistry, Oregon Health & Science University, 611 SW Campus Drive, Portland, OR 97201, USA Abstract Objectives. To compare the polymerization contraction stress of resin cements in dual-cure and self-cure modes, and verify its effect on microleakage and gap formation in class I cavities restored with porcelain inlays. Methods. Dual-cure cements (Calibra, Choice and RelyX ARC) were monitored for contraction stress during 30 min in the tensilometer, in the presence or absence of photo-activation. Microleakage was evaluated in porcelain inlays bonded to cavities prepared in bovine incisors, combining the same adhesive (One-Step) with each of the cements in both activation modes. Epoxy replicas of the sectioned specimens were analyzed in the SEM to determine the percentage of discontinuous margins. Results. The stress developed in dual-cure mode was higher than in self-cure mode for all cements. In self-cure mode, no significant difference in stress was found among materials. Choice and RelyX ARC showed higher microleakage in dual-cure mode than in self-cure mode. Calibra did not show a significant difference in microleakage between activation modes. Contraction gap results showed wide variations, and no significant differences were detected among the groups. Gaps occurred more often in dentin, with an overall average of 22 ^ 17.5%. Conclusions. The higher stress generated in dual-cure cements subjected to photo-activation may lead to higher marginal leakage. In the confinement condition represented by the class I inlay, even the lower contraction stress of the cements in self-cure mode had enough magnitude to disrupt the bonding to dentin. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Resin cement; Dual-cure composites; Polymerization; Contraction stress; Stress rate; Adhesion; Microleakage; Contraction gaps 1. Introduction The polymerization shrinkage of resin-based composites under confined conditions generates stress at the tooth/ restoration interface, which may lead to gap formation, post- operatory sensitivity and secondary caries [1]. The magni- tude of the stress generated during the polymerization is related to material properties (i.e. volumetric contraction and elastic modulus) and to the degree of confinement of the shrinking mass of the composite [2]. It is currently accepted that when lower amounts of free surface area are available, higher contraction forces will be generated during polym- erization. The contraction stress developed in thin films of restorative composite may reach values up to 25 MPa [3], and from a clinical standpoint the chance of obtaining a gap- free, durable bond to tooth structure under such conditions is small. Based on this rationale, the use of composite-based materials for luting purposes represents a very critical situation. In fact, clinical studies with bonded inlays have reported incidences of post-cementation hypersensitivity as high as 30% [4]. Laboratory studies have shown that interfacial and/or marginal defects are always present under bonded indirect restorations [5,6]. Evidence of insufficient marginal sealing has also been shown in microleakage studies [7,8]. In contrast to the incremental technique suggested for directly-placed restorations, there is no insertion technique that can be used to minimize the contraction stress of the resin cement during the bonding of an indirect restoration. For this reason, a relatively low contraction stress may be an important parameter for the selection of resin cements. One way to achieve lower stress levels in a given composite is to decrease its rate of polymerization. Feilzer et al. have shown a reduced stress generation for self-cure versus light-cure composites [9]. Dual-cure cements may show different cure characteristics, some of them being more dependent on light activation than others [10]. Moreover, the light intensity 0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0300-5712(02)00047-7 Journal of Dentistry 30 (2002) 333–340 www.elsevier.com/locate/jdent * Corresponding author. Address: Department of Dental Materials, School of Dentistry, University of Sa ˜o Paulo, Av. Prof. Lineu Prestes, 2227, Sa ˜o Paulo, SP 05508-900, Brazil. Tel.: þ55-11-30917842 x 224; fax: þ 55-11-30917842 x 224. E-mail address: rrbraga@usp.br (R.R. Braga).