Abstract
This study investigated the effects on gap formation in Class I restorations (observed by vertical and horizontal forms of inspection) and on the mechanical properties of nine resin composite filling materials when the restorations were subject to finishing immediately after setting or after one-day water storage. Class I restorations with resin composite fillings were polished either immediately (3 min) after setting or after one-day water storage. Interfacial gap formation (observed by vertical inspection) was assessed using 14 gap measurement points along the interface between the restoration and cavity walls and floor (n = 10 per resin composite; total points measured per time point = 140). For marginal gaps formed at cavosurface margins in Class I cavities and in Teflon molds, marginal gap formation (observed by horizontal inspection) was assessed by measuring the maximum gap-width and opposing width (if any). Effects on mechanical properties were assessed by measuring shear bond strengths to enamel and dentin, flexural strength and modulus. After one-day storage, marginal gap-widths in Class I restorations were significantly decreased for all composites, alongside a significant increase in shear bond strengths to enamel and dentin, flexural strength and modulus. Resin composite-filled Class I restorations which were polished after one-day delay presented lower gap formation compared with finishing immediately after setting.
Highlights
Polymerization shrinkage occurs during the early stage of polymerization of light-activated dental composites
As for the adhesive systems used to bond the restorative filling materials to tooth structure, their issues of flow ability, polymerization shrinkage and the resulting destructive shrinkage stress further contribute to gap formation in resin composite restorations [1,2,3,4,5,6,7,8]
In butt-joint cavities restored with an adhesive system and a resin composite filling, the magnitude of interfacial gaps formed is dictated by these factors: (1) adhesive forces between the restorative material and cavity walls; (2) degrees of volumetric contraction of filling and luting materials; and (3) flow properties of filling and luting materials [4, 11,12,13]
Summary
Polymerization shrinkage occurs during the early stage of polymerization of light-activated dental composites This phenomenon adversely affects interfacial adaptation and bonding to tooth structure because the shrinkage forces generated can disrupt the bond to cavity walls and result in gap formation. The mechanical properties of both resin composite restorative filling materials and luting agents directly influence the marginal seal or gap formation, which affects the clinical success of dental composite restorations [14, 16, 17]. In our previous studies [14, 17], dental restorations which were polished after one-day storage following light activation had improved shear bond strength and flexural properties, and improved marginal integrity. The hypothesis to be tested in this study was that premature finishing would significantly reduce gap formation as opposed to delayed finishing
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