SDR is a flowable bulk filler for posterior teeth from Dentsply. It allows light penetration for curing up to 4mm at a time. Polymerisation shrinkage is 3.5%, comparable to other flowables, but less than many hybrid composites. Dentsply claim that the marginal stress caused by polymerisation shrinkage is around 1.5 MPa - substantially less than other composites, so the chance of marginal leakage is greatly reduced.
After bulk filling with SDR, a 2mm "capping layer" of aesthetic composite is placed on top of the SDR.
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Slide Show: Multiple restorations