Abstract
Background: Resin composites as direct posterior restorative material was associated with the polymerization contraction and microleakage. Different methods have been introduced to overcome these drawbacks by increasing the degree of monomer conversion and to minimize the polymerization shrinkage. Composite preheating is an innovative method to improve the handling and physical properties. So this study was done to evaluate the effect of prepolymerization warming of different composites on the marginal adaptation. 
 Materials and Methods: This in vitro study was conducted on 80 extracted human maxillary premolars. Class II cavities on the proximal surface were prepared with the dimensions of 4 mm buccolingual width, 2 mm axial depth, and gingival margin at the cementoenamel junction. They were divided into four groups of 20 teeth each and were restored accordingly: Group I – Bulkfill nanohybrid; Group II – Nanofill; Group III – Ormocer; Group IV – microhybrid composites. Each group was further sub - divided into subgroups of 10 teeth, according to the preheated composites and room temperature composites used. After restorative procedures, samples were sectioned in the mesiodistal direction through the centre of the restoration and analysed using Scanning Electron Microscope at 200x magnification and marginal gap width was measured in three areas at the axial wall using Image Processing and Analysis in Java (ImageJ 1.5 2a) software.
 Statistical Analysis: Data was entered in Statistical Package for Social Sciences version 21 Software for Windows. The comparisons of four different composite materials with two subgroups were analyzed using Kruskal Wallis test. The level of significance was kept at p < 0.05.
 Results: Preheating results in more gap formation with the bulk fill nanohybrid and Ormocer showed higher MQ4 scores than the room temperature composites. No statistically significant results were found. But the percentage of gap formation was comparatively higher in preheated composite group.
 Conclusion: Within the limitations of the present study, it could be concluded that preheated composites showed poor internal marginal adaptation with increased frequency of gap formation.
 Key words: Internal marginal adaptation, Ormocer, Filtek nanofill, Microhybrid, Tetric bulk fill
Highlights
Often clinicians choose composites as an anterior and posterior restorative material, due to its excellent esthetic and mechanical properties
MQ3 and MQ4 scores were categorized in four groups based on the gap width measurements using Image Processing and Analysis in Java (ImageJ 1.5 2a) software
Increased MQ1 scores were observed in room temperature nanohybrid and Ormocer composite group
Summary
Often clinicians choose composites as an anterior and posterior restorative material, due to its excellent esthetic and mechanical properties These materials have its own negative characteristics like polymerization shrinkage and poor marginal adaptation. Posterior composite restorations undergo polymerization shrinkage that results in bulk contraction of the material. Polymerization shrinkage can be minimized by the presence of higher filler loading It improves the wear resistance and mechanical properties of the material. Class II cavities on the proximal surface were prepared with the dimensions of 4 mm buccolingual width, 2 mm axial depth, and gingival margin at the cementoenamel junction They were divided into four groups of 20 teeth each and were restored : Group I – Bulkfill nanohybrid; Group II – Nanofill; Group III – Ormocer; Group IV – microhybrid composites.
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