Can a Flowable be Cured Beneath an Incremental/Bulk-fill Resin-based Composite?
Objectives: To achieve better adaptation of a resin-based composite (RBC) to a cavity preparation, some clinicians apply a flowable RBC, then apply an incremental or bulk-fill RBC on top of the flowable, and cure both together. Such procedures are known as “snowplow” or “injection-molding” techniques. This study used microhardness testing to determine the thickness of sufficiently cured flowable when cured beneath a RBC.
Methods: In the first system (EXP/FSUF), 2mm of an experimental incremental RBC were placed on Filtek™ Supreme Ultra Flowable in a 4mm diameter stainless-steel mold. In the second (FO/FBFF), 4mm of Filtek™ One Bulk Fill were placed on Filtek™ Bulk Fill Flowable in a 4mm diameter stainless-steel mold. Two conditions were used: “worst case”, where the shades and light intensity were selected to minimize depth-of-cure; and “common use”, where commonly used shades and curing light were used. Samples were cured according to the incremental/bulk-fill’s instructions for use (IFU). Samples were removed, potted in epoxy, and ground and polished such that a plane through the axis of the cylinder was revealed. Knoop microhardness testing was then performed along that axis.
Samples of flowable alone were cured according to their IFU and prepared as above. The maximum hardness of the flowable was determined. In “snowplow” samples, the depth at which the flowable was sufficiently cured was defined as when its hardness equaled 80% of the maximum of the flowable when cured alone, abbreviated as D(80-flow).
Results: For each system, D(80-flow) is greater than the layer thickness of the incremental/bulk-fill (2-sample t-test, p<0.05), showing that measurable flowable beneath the incremental/bulk-fill is sufficiently cured.
Conclusions: Within the limitations of this study, these results show that a flowable can be cured sufficiently beneath an incremental/bulk-fill if the incremental/bulk-fill is placed in a layer thickness specified by, and cured according to, the incremental/bulk-fill’s IFU.