Comparing SDR® Bulk Fill Flowable Composite:
In-vitro Studies on Polymerization Shrinkage and Marginal Adaptation
For in-vitro studies, new technologies have been introduced to assess marginal adaptation of different types of composites in Class I and II posterior composite restorations. Microcomputed tomography (μ-CT) allows for non-destructive evaluation of the restorative interface in two and three dimensions and can measure volumetrically in microns the marginal integrity and internal adaptation of the restorative materials studied.8,19,27
A recent study28 evaluated three bulk-fill composite resins, SDR® Bulk Fill Flowable, 3M™ Filtek™ One Bulk Fill† and Tetric®N-Ceram Bulk Fill† against a conventional composite resin Filtek™ Z350† in caries free and crack free human molars prepared with a MOD restoration. The teeth were thermocyled and cyclic loaded to mimic aging and stresses in the oral cavity during function. Micro-CT was performed on each sample to measure the interfacial gap at the restorative interface.
The results demonstrated that SDR® Bulk Fill Flowable had the smallest gap volume of all tested composite resins. An additional in-vitro Micro-CT study19 on Class I restorations concluded SDR® Bulk Fill Flowable exhibited less volumetric polymerization shrinkage compared with the other flowable groups, Permaflo™†, Filtek™ Bulk-fill flow†, and Vertise™ flow†.
Swept-Source Optical Coherence Tomography (SS-OCT) is another in-vitro technology developed to visualize how composites behave while undergoing light polymerization using real-time video display. Two studies29,30 concluded SDR® Bulk Fill Flowable Composite had a slower polymerization rate and a delayed gel point of resin matrix increasing its flow capacity as demonstrated by a lower incidence of voids and better internal dentin adaptation at both 2 mm and 4 mm depths compared to a conventional flowable composite. Another SS-OCT study31 demonstrated that SDR® Bulk Fill Flowable Composite had a low shrinkage stress making it ideal as an intermediate dentin replacement layer for better internal adaptation compared to other bulk-fill composites.
A biocompatibility study32 found that only SDR® Bulk Fill Flowable and one other bulk fill composite demonstrated cell viability above 70% at a 4mm increment, whereas two standard composites, two bulk-fill pre-reacted glass ionomer composites and one other bulk-fill composite showed lower cell viability.
In studies assessing the degree of conversion and depth of cure when light curing bulk fill composites, SDR® Bulk Fill Flowable material showed significantly higher conversion degrees at the bottom and top surface than the other composites tested. Shorter curing times (10 seconds) significantly reduced the polymerization of Filtek™ Bulk Fill† and x-tra base†. SDR® Bulk Fill Flowable demonstrated high polymerization in 4 mm thick bulks reaching the conversion degree of 2 mm thick composite layers. SDR® Bulk Fill Flowable, in the manufacturer’s light-curing time, cured properly in 4 mm. Higher radiant exposure of SDR® Bulk Fill Flowable had no adverse effect on its polymerization shrinkage. Flowable bulk-fill composites like SDR® Bulk Fill Flowable performed better regarding polymerization efficiency at 10 seconds compared to high viscosity bulk-fill composites like SonicFill™† that required 20 seconds.33,34,35,36
The Benefits of SDR® Bulk Fill Flowable Composite for Class II Restorations
At the onset of research on SDR® Bulk Fill Flowable Composite, several studies demonstrated its ability to bond well to the cavity floor in high C-factor restorations at 4 mm increments.24,25,26 In the direct Class II posterior composite restoration, the goal for the clinician is to apply materials that reduce polymerization shrinkage and shrinkage stress and work effectively on enamel, as well as in the proximal box of deep restorations where dentin and cementum become primary concerns for long-term success. Therefore, the type of composite resin used in the proximal box may play a critical role in the marginal adaptation of a Class II posterior composite restoration.
Using SDR® Bulk Fill Flowable Material in the open-sandwich technique for placement of a Class II posterior composite restoration allows a layer placed as dentin replacement extending from the gingival box floor up to 4 mm. With the remaining top or enamel layer of at least 2 mm all proximal margins exposed to the oral cavity are sealed, which is of primary concern for long-term clinical success.