Objective: The purpose of this study was to investigate how layering techniques affect polymerization shrinkage stresses of high- and low-viscosity bulk-fill resins. Method: Six high-viscosity and six low-viscosity bulk-fill resins were evaluated. Aluminum blocks with a mesial-occlusal-distal (MOD) cavity were machined and randomly divided into groups for different filling techniques (bulk-fill vs horizontal layering vs oblique layering) and further subdivided according to type of resin (high- vs low-viscosity). The cuspal deflection resulting from the polymerization of bulk-fill resin bonded to a MOD cavity within an aluminum block was measured with a digimatic micrometer. Scanning electron microscopy analyses of tested resins were also conducted. Results: In the high-viscosity bulk-fill resins, cuspal deflection of the MOD cavity ranged from 11.2 to 18.2 lm with the bulk-filling technique, from 10.7 to 15.5 lm with the horizontal layering technique, and from 10.9 to 15.2 lm with the oblique layering technique. In the low-viscosity bulk-fill resins, cuspal deflection of the material ranged from 9.2 to 19.8 lm with the bulk-filling technique, from 8.2 to 15.7 lm with the horizontal layering technique, and from 8.4 to 16.4 lm with the oblique layering technique. Conclusion: Cuspal deflections for some high- and low-viscosity bulk-fill resins were significantly reduced by using layering techniques, but the resultant improvement of layering techniques was not applicable to all the bulk-fill resins used in this study.
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