Bulk-Fill Resin Composites
Bulk-fill resin composites are light-cured resin composites that can be placed in increments or layers of 4 to 5 mm in depth. These materials have been available to the dental profession for several years now. Their adoption has been slow, but they are gaining popularity, and sales are increasing. For a practitioner, the first questions that come to mind are: What are the benefits when compared with incremental 2-mm layering? Can these materials be properly polymerized to a depth of 4 to 5 mm? Can a curing light penetrate 4 to 5 mm of composite? What about shrinkage? These questions come from the knowledge (acquired through dental school and continuing education) that resin composites can only be placed in 2-mm increments due to lack of light penetration and polymerization shrinkage, which may result in a broken or weakened bond to the tooth structure. However, new formulations of materials have allowed bulk placement to become a real option.
At first glance, the most important advantage that these materials provide is time reduction during material placement and polymerization, along with reduced technique sensitivity. However, other advantages such as the elimination of voids between increments are also important.
Regarding light polymerization, several in-vitro studies have confirmed that light will penetrate through 4 to 5 mm of bulk-fill composites, and they can be fully polymerized to those depths. In order to make this possible, the majority of manufacturers, if not all, have formulated materials that are highly translucent. In some cases, these materials are more translucent than enamel. As a result, the practitioner needs to understand that the resulting restoration looks slightly grayer and is discernible from tooth structure. Some practitioners may see this as a disadvantage in terms of esthetics, but many see this as a small price to pay for the convenience and hardly consider it an unesthetic result.
Shrinkage, which is a factor for all composites, does not represent a problem when the material is not constrained by bonded walls, such as in a resin veneer. It becomes an issue when bonded to opposing walls, such as in a Class I, which has been extensively studied and is known as the c-factor. In addition, shrinkage stress is more important than volumetric shrinkage, because shrinkage stress is what produces marginal breakdown and reduces bond strength to tooth structure. Because bulk-fill composites are mostly indicated for Class I and II posterior restorations where the c-factor is higher, shrinkage needs to be considered. Manufacturers have modified the resin formulations of bulk-fill materials, resulting in comparable amounts of shrinkage stress between conventional 2-mm layering and a single increment bulk-fill technique.
Clinical indications for the use of these materials vary based on the individual recommendations by manufacturers. Different materials have different compositions, including the ratio of particles and resin, particle sizes, and particle distribution. One must read the manufacturer's instructions regarding polymerization depth. Some indicate 4 mm, others 5 mm. The same material can require 4-mm depth in a Class I, but 5 mm in the box of a Class II. One must measure the depth of the preparation with a periodontal probe to ensure that the manufacturer indications of 4 to 5 mm are not exceeded. Some instructions also require additional light exposure from the buccal and lingual surfaces after curing from the occlusal aspect.
Light polymerization needs special consideration. Bulk fill does not always mean bulk cure. Make sure that the light tip covers the material. In some instances, such as when the restoration size and bulk increment are wider than the light tip, multiple exposures are necessary to polymerize all of the material properly.
In my clinical experience, it is important to make sure that the material adapts well to the preparation walls, because large increments are slightly more technique sensitive to manage. Compules with long, narrow tips that can be placed all the way to the bottom of the preparation are preferable, then the dentist can backfill while paying attention to the adaptation to the walls.
Another consideration with bulk fill material is its presentation: flowable or normal viscosity. Flowable materials provide ease of adaptation, but unfortunately, have high wear rates; thus, this type needs to be capped with a final increment of resin composite or a normal viscosity bulk-fill material.
About the Author
Marcos Vargas, BDS, DDS, MS, is a professor in the Department of Family Dentistry at the University of Iowa College of Dentistry.