The current trend in composite development is around bulk fill materials for the posterior. We have both flowable and hybrid formulations from multiple manufacturers available. The depth of cure ranges from 4-7mm for the different products. Why bulk fill? Being able to fill to the top of a prep and cure in one increment is a time saving strategy. If possible without compromising the quality of the restoration bulk fill makes the placement of a posterior composite more cost effective and therefore more profitable. On the clinical side we could argue that filling in one increment will minimize the incorporation of voids between the layers. In addition materials with greater flow are more likely to seal at the margins.
The manufacturers have had several challenges to overcome in creating workable bulk fills. One is polymerization shrinkage and shrinkage stress, which have classically been tied to the size of the bulk of composite. Additionally they have had to conquer depth of cure as leaving uncured material is not an option. Meeting these goals has been accomplished with changing the percentage and type of resin, filler particles and chemicals that catalyze the process. Filler particles tend to reflect light and create opacity that prevents light penetration to depth. The more chromatic or bright the composite the less light penetration we get, even with our traditional composites. Therefore to increase depth of cure the filler particles are reduced in number (flowable materials), or the material is fabricated with increased translucency.The flowable materials have to be capped with a traditional composite as they do not have the wear resistance to withstand occlusal loading. The translucency means that we have limited esthetic capabilities with these materials. Therefore they should be used in clinical situations where close is the clinical outcome for color matching.
Another approach is to create a more effective chemical catalyst that will drive the polymerization further once initiated. The challenge in adding this is handling time that still allows the clinician adequate time to work without ambient light causing polymerization. Lastly is managing shrinkage stress. Shrinkage stress causes marginal failure, post operative sensitivity and a host of other problems when it is too great.
With our currently available bulk fill materials understand how to use them appropriately. If the manufacturer recommends a capping layer, this is a must for longevity int he mouth. Another factor is to be clear on the depth of cure. The average proximal box ranges from 6-10 or more millimeters. In truth none of our materials are one increment yet, but we are getting there, so stay tuned!!