Effect of preheating on the mechanical properties of resin composites IntRoductIno The use of resin composites for restoring increased paste temperature, free radicals and posterior stress-bearing cavities has increased propagating polymer chains become more mobile significantly in recent years. As an alternative as a result of decreased paste viscosity and react to amalgam, the early attempts to apply resin to a greater extent, resulting in a more complete composites in posterior teeth had only limited polymerization reaction and greater cross- success because of insufficient material linking. The increase in polymerization may lead properties.1 Improvements in the properties of to improved mechanical properties and increased the materials together with their positive clinical wear resistance.13,16,17 performances encourage continuously the use of Fractures within the body of restorations posterior resin composites as a viable alternative and at the margins have been cited as a major to amalgam.2 The mechanical properties of problem regarding the failure of posterior resin resin-based composite mainly depend on its composites.18 The fracture related material microstructure and composition. The micro properties, such as fracture resistance, elasticity, structural characteristics involve the type, size and and the marginal degradation of materials quantity of filler particles. These characteristics under stress have usually been evaluated by the are directly related to the composition of the determination of the material parameters such as composite.3 flexural strength and flexural modulus.19 One of the primary concerns is adaptation (both The aim of this in vitro study was to assess internal and marginal) and the resulting interfacial the flexural strength and flexural modulus of two seal of resin composite to the preparation walls. resin composites prepared at room temperature When placing and contouring a composite, a or preheated to a temperature of 40, 45 or 50°C. clinician often has difficulty to adapt the material to the cavity preparation because of the paste’s MAteRIALs And MetHods high viscosity. Unlike amalgam, highly filled One nanohybrid (Grandio, VOCO, Cuxhaven, composite resins cannot be “condensed” by using Germany), and a microhybrid resin composite a heavier force to reduce porosity or to enhance (Filtek Z250, 3M ESPE, St. Paul, MN, USA) adaptation.3-5 To decrease wear and polymerization were used. The materials were inserted into shrinkage in posterior applications, resin rectangular moulds with the dimensions specified composite manufacturers increased filler content. by the ISO 4049/2000 specification (25 mm x 2 mm However, this modification results in higher paste x 2mm) at room temperature (RT) or preheated viscosity.6,7 Furthermore, many contemporary to a temperature of 40, 45 or 50°C with EASE-IT resin composites are also sticky and difficult composite softener (Ronvig Dental, Daugaard, to manipulate, resulting in greater problems in Denmark). Then the specimens were cured with placement.3Freedman8andFriedman9 claimedthat standard (700 mW/cm2 for 40 sec) or step-cure warming resin based restorative materials prior (150-180 mW/cm2 for 10 sec and 650-700 mW/ to placement and contouring enhances composite cm2 for 30 sec) modes of high intensity halogen adaptation to preparation walls by increasing the (Elipar Highlight, 3M ESPE, St. Paul, MN, USA) viscosity of unpolymerized resin composite paste. light curing unit. Ten specimens were prepared The extent of viscosity change may be attributed for each preheating and light curing protocol. to many factors: resin composition, filler content The mould was positioned over a glass slide and shape.10-12 Thus because of the wide variety and a mylar strip and the resin composite was in chemistry and composition of resin composites inserted as a single increment. Another mylar strip currently used, a great variation in the viscosity was positioned and pressed against it with a glass of these materials in response to evaluated slide for excess removal before polymerization. temperatures may be expected.3 An additional The excess of material in the corner was carefully advantage of heating the resin composite prior to removed with a scalpel blade. A three-point placement and polymerizing is the accompanying bending test was performed using a universal increase in monomer conversion.13-17 With testing machine (Lloyd LRX; Lloyd Instruments 264 European Journal of Dentistry