Objectives: The purpose of this study was to compare the internal adaptation of resin composite restorations in Class I cavities with or without an intermediate layer, and to identify the relationships between the internal adaptation and the polymerization shrinkage parameters of the layer material. Methods: Class I cavities prepared in human third molars were treated with a one-step self-etch adhesive (G-Premio Bond, GC). In the control group, Tetric-EvoCeram Bulk-Fill (TBF, Ivoclar Vivadent) was placed using the bulk-fill technique and then light-cured. In the experimental groups, six different layer materials (three flowable resin composites, two bulk-fill resin composites, and a resin-modified glass-ionomer) were applied to a 1.5-mm thickness and light-cured, which was followed by TBF filling. After thermo-cycling, internal adaptation was measured using swept-source optical coherence tomography (SS-OCT) imaging and compared in terms of high brightness percentage (HB%) to represent the microgap. Shrinkage strain (SS), flexural modulus (FM), and polymerization shrinkage stress of the intermediate layer-resin composite complex (PS) were measured for each material. The relationships among HB%, SS, FM, and PS were evaluated statistically. Results: Groups with an intermediate layer showed lower HB% than the control group. The HB% correlated with PS (Pearson's correlation: R2 = 0.883, p < 0.05), and the PS correlated with the product of SS and FM. Conclusion: Internal adaptation depended on the polymerization shrinkage stress of the intermediate layer-resin composite complex. Clinical significance: Bulk-fill resin composite, which can show low polymerization shrinkage stress, may be used as an intermediate layer for better internal adaptation.
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