Success With Bulk-Fill Composites Requires Understanding, Attention to Detail

Successful adhesive dentistry is more than just choosing a material. It requires attention to detail in all phases of the procedure, particularly when considering the newest trend in composites—bulk-fill. These new composites can be placed in larger increments as long as the increment does not exceed 4 mm to 5 mm, which is their depth of cure.

Care must be taken when placing bulk fills. Scientific studies have shown that the proximal part of up to 90% of cavities in posterior teeth is between 2 mm and 5 mm deep.¹ This means that special attention must be paid to the approximately 10% of cavities that may be deeper than 5 mm, as they will not cure when placed in one increment. This also applies to the pulpal floor areas that must be deepened for decay removal. Most deciduous teeth may be filled with one increment, because the 4 mm to 5 mm depth would rarely, if ever, be encountered.

As opposed to conventional composite materials, most bulk-fill composites, according to manufacturers, generate a lower polymerization shrinkage stress and have higher light transmission properties due to reduction of light scattering at the filler–matrix interface, which is achieved by either decreasing the filler amount or increasing the filler size.² This has changed the esthetic capabilities of these composites, leading to a decrease in the available shades and limiting their use to posterior teeth. It seems that these new bulk-fill composites can be adequately cured at 4-mm depths and that they are similar to current, conventional composites in marginal integrity and in polymerization shrinkage stress they cause in restored teeth.³

Regarding proper use of bulk-fill composites, practitioners need to keep in mind that these materials have been produced in two forms with different recommended uses. Some are indicated for use as a restorative material, and some are indicated as a base material. These two types of composite are a high viscosity and a low viscosity, respectively. High-viscosity composites, which are much more resistant to slumping and contain a greater amount of inorganic fillers, are recommended for direct posterior restorations, preventive resin restorations, core build-ups, Class V restorations, and restorations in deciduous teeth. Low-viscosity (or flowable) composites generally adapt better on the cavity wall, especially in irregular surfaces, and exhibit lower mechanical properties due to a lower filler content.⁴ They are being recommended for dentin build-up, cavity liners, fissure sealants, small Class I restorations, undercut block-out, and small core build-ups. The inferior mechanical properties of the low-viscosity bulk-fill composites may necessitate a 2-mm capping layer with a high-viscosity composite when restoring areas subject to occlusal stress.⁵

While this new category of composites looks very promising from a laboratory standpoint, more long-term clinical data is necessary to validate their use. The bulk-fills do allow a shorter treatment time since they do not need to be placed in multiple layers, making them faster and easier to use. However, it is critical that they be placed correctly to assure a successful, long-term restoration. It is also important to note that not every bulk-fill resin is recommended in the same situation, and clinicians should select the product carefully prior to placement in certain situations. These composites can always be placed in 2-mm increments if there are unfavorable light-curing conditions such as restricted access of the light tip, which could lead to improper placement of the light source.

The use of bulk-fill composites, which cure at 4 mm to 5 mm, has drawn attention to the importance of the light-curing process. The curing light must emit enough energy to cure the composite completely, especially at these increased depths. This is a function of the curing light output, the exposure time, and the proper placement of the curing light on the tooth to make sure that the energy is delivered appropriately. Adequate curing light exposure is essential for success of the final restoration.

Proper light curing of a composite is more complicated than what most dentists’ typically were taught in dental school. It is not just an afterthought to a multiple-step restorative process; rather, it is an extremely critical step that must be well executed to assure long-term success. Studies have found that dentists worldwide are using curing lights that do not have enough light output to cure the composite completely and are unaware that their curing light output is inadequate (most curing lights have a minimum output of 600 mW/cm2 to 1,000 mW/cm2) or that they are using an incorrect wavelength (most composites cure in the 410-nm to 470-nm range) to cure the composite being used.6

The distance between the composite and the tip of the light is also a crucial consideration. The light intensity decreases as the light is moved further away from the composite. This can be especially problematic when restoring the proximal box in a posterior tooth, because the tip of the light can get no closer to the composite than the height of the cusp tips will allow. This means that the light energy to the composite at the gingival margin of the proximal box is limited, especially when the gingival floor is deep. This may necessitate the placement of the composite in multiple layers to ensure that it is completely cured at the area of the restoration that is more prone to failure. Xu et al⁷ concluded that when curing adhesives in deep proximal boxes with the curing light delivering 600 mW/cm2, the exposure time should increase to 40 to 60 seconds to ensure optimal polymerization. This is also recommended for curing lights that deliver more than 1,000 mW/cm2 output.⁸

Depending on the shade and brand of composite resin, the minimum energy requirements to polymerize a composite is in the range between 6 J/cm2 to 24 J/cm2 for a 2-mm increment of composite.⁹ It is important for the clinician to match a commercial composite product and its proprietary photoinitiator with a curing light that is coordinated with producing the photoinitiator’s preferred wavelength range.

Simply put, light curing cannot be taken for granted. Thanks to research by Price et al10 and others, the importance of light curing is much better understood today and is garnering the attention it merits.

Another important consideration for the placement of a clinically successful composite—bulk fill or otherwise—is the choice of matrix system. During the placement of an amalgam restoration, the amalgam can be condensed against the matrix band, allowing for the probability of a tight contact in the final restoration. This is not possible with composite material because it cannot be condensed against the matrix band, which can ultimately lead to poor adaptation of the final restoration to the adjacent tooth. Inadequate contacts can lead to food impaction, periodontal problems, and tooth movement. An additional issue is the possibility of leaving an overhang on the gingival margin, especially when using a low-viscosity material.

Presently, there are two system types that can be used when restoring the interproximal areas of posterior teeth: a circumferential matrix system (Tofflemire) or a newer-generation sectional matrix system. Loomans et al11 found that Class II posterior composite resin restorations placed with a combination of sectional matrices and separation rings resulted in a stronger proximal contact than when a circumferential matrix system was used. In another article,¹² which demonstrated the need for careful operator technique, Loomans found that all matrix systems resulted in a marginal overhang of Class II composite resins and most of these overhangs were located in the cervical region. An overhang in this area is hard to detect and can be extremely difficult to remove. This makes the matrix placement an extremely important step in the composite restorative process.

Besides using the proper materials, successful adhesive dentistry requires attention to detail in all phases of the procedure, starting with proper isolation (rubber dam) to control moisture in the mouth, then proper timing of the etchant procedure (20 seconds on enamel and 10 seconds on dentin), placement of the adhesive and of the composite (layers in deep restorations), proper placement of the curing light, and the adequate energy delivered to the restoration to completely cure the material. With resin-based materials not as forgiving as amalgam, each step of the restorative process is important to a successful, long-lasting result.

About the Author

James F. Simon, DDS, MEd
Professor and Director
Division of Esthetic Dentistry
University of Tennessee College of Dentistry
Memphis, Tennessee