Question: Are Bulk-Fill Composites a Good Idea?
A characteristic of an ideal dental composite restorative would be that it can be effectively cured in a single increment, facilitating placement. Therefore, producing bulk-fill dental composites is a worthwhile pursuit. Although currently marketed bulk-fill composites are designed to be used either as complete restoratives or as liners requiring an occlusal capping material, the ultimate goal is to produce the former to maximize convenience.
Most manufacturers recommend their bulk-fill composites for depths up to 4 mm, though some suggest 5 mm is acceptable. The production of a composite with this depth of cure is typically accomplished by enhancing its translucency to allow deeper penetration of sufficient photons of light for activating the photoinitiator system. It is possible to produce a composite that is transparent by matching the refractive index of the fillers and polymer matrix, but this approach has obvious esthetic limitations. Depth of cure can also be enhanced through the inclusion of greater amounts of photosensitizers or through the incorporation of more efficient cure promoters. The other important issue for bulk-fill composites is reduction of the potentially deleterious contraction stresses resulting from the polymerization of the composite within the bonded tooth preparation, and various methods have been used by manufacturers to alleviate these concerns.
The question is, are the manufacturer’s claims of providing dental composites with enhanced depth of cure and reduced stress true? Based on the current literature, the answer to this question appears to be affirmative. There are now more than two dozen articles published on the properties and laboratory performance of commercially available bulk-fill composites. While some studies have identified limitations with certain materials under certain conditions, such as typically reduced radiant exposure (J/cm2), the majority suggest that current materials do achieve the manufacturer’s claims regarding depth of cure of 4 to 5 mm when reasonable light curing conditions are met (approximately 20 J/cm2 when applied very close to the surface of the material). When the distance between the light source and the material is increased to 2 to 3 mm, increasing curing time by perhaps 50% should provide sufficient light energy to affect adequate bulk curing.
The literature also suggests that current bulk-fill composites do produce reduced contraction stress compared to most composites designed for incremental fill, though the clinical outcome of this reduced stress is not completely known. The bulk-fill composites have not to this point shown improved marginal adaptation or reduced marginal leakage in laboratory studies, but have shown similar results to incremental-fill composites. Current studies generally show that bulk-fill composites have slightly to significantly lower mechanical properties, such as hardness, stiffness, and strength, compared to composites designed for incremental placement, and this is likely due to the materials having lower filler contents to make them flowable. At this time, the clinical evaluation of these bulk-fill materials is not sufficient to state whether they will have equivalent, poorer, or improved performance over composites placed and cured incrementally.
I think the concept of light-cured “bulk-fill” composites is a very good one, but I urge dentists to be wary of all the hype and aggressive marketing surrounding these products. Light-cured bulk-fill composites are certainly not new and were never particularly popular. It is the emergence of a new generation of bulk-fill composites—along with a “face-lift” of some of the older ones—coupled with good marketing that has rekindled interest in these materials.
As Dr. Ferracane noted, bulk-fill light-cure composites are designed to be placed in 4- to 5-mm increments, which means they have the potential to save considerable time when placing direct composite restorations. Placing and light curing multiple layers of composite in traditional 2-mm increments when restoring large and deep cavity preparations can take considerable time and can also be very technique sensitive. It makes sense that if same results can be achieved without any adverse effects by placing just one or two large increments of a bulk-filled composite, then why not do that? But are the results really the same?
The literature is equivocal in regards to depth of cure, degree of conversion, physical characteristics, and stress induction at the tooth/restoration interface when comparing bulk-fill composites with conventional composites placed in layers. For example, independent testing performed by the ADA Professional Product Review in 2013 found that some bulk-fill composites did not pass the ISO 4049 standard depth of cure test (meaning the depth of cure was not within 0.5 mm of that stated by the manufacturer).1 These findings have been corroborated in other independent studies in which these same products, as well as other bulk-fill composites, failed to pass the ISO 4049 depth-of-cure test.2-4 However, some of these same products tested more favorably when top and bottom Knoop hardness tests were performed as an indirect way of assessing depth of cure.1
A major concern I have regarding depth-of-cure testing for all dental composites, but particularly for bulk-fill composites because I think the room for clinical error is much greater, is that the laboratory methodology typically used by manufacturers is often far from the clinical reality. For example, in the testing lab it is common to use optimized, high-intensity lights (energy levels typically >1,000 mW/cm2 and sometimes approaching 2,000 mW/cm2) that are fixed in place on a rigid apparatus that holds the light tip at the perfect angle in almost direct contact with the composite material, which is then light-cured for recommended time frames. Is that really the clinical reality? The clinical reality is that light tips are seldom placed directly on the composite surface (sometimes tooth anatomy alone prevents this), the tip is often moving, the angle at which the light is held is almost never ideal, light output is frequently less than optimal for any number of reasons, and recommended curing times are often cut short. Using the MARC (Managing Accurate Resin Curing) system he developed, Dr. Richard Price has clearly demonstrated that clinical technique even with experienced dentists is often less than ideal.5 In addition, the curing lights used in a typical dental office (both LED and QTH) frequently don’t measure up to ones used in laboratory testing.6,7
Some manufacturers claim their bulk-fill composites induce less stress at the adhesive interface during and after polymerization because of low volumetric shrinkage. Indeed, reported volumetric shrinkage values for some bulk-fill composites are fairly low (some around 2%). I think it is important for dentists to understand that low shrinkage in a composite does not necessarily mean less stress is being transferred to the adhesive interface. Photo polymerization is complex and other factors such as modulus of elasticity, rate of polymerization, polymerization kinetics, degree of conversion, initiator chemistry, gel point, and type of filler and monomer all have an effect on stress development and intensity. It is very possible to have a composite that shrinks far less than another but still induce far more stress at the adhesive interface because of the factors mentioned above. I advise dentists to be very wary of claims made by manufactures in terms of “low stress.” It is very easy to make any composite “low stress”—just don’t cure it very well. The degree of conversion will have a direct effect on polymerization shrinkage stress. So when manufacturers report low shrinkage stress values for their composite it is also important to ask them what is the degree of conversion?
Perhaps the biggest concern I have with bulk-fill composites is the potential for abuse of these systems. Let’s assume that a bulk-fill composite works exactly as advertised and is able to predictably cure to a depth of 5 mm. What happens if the preparation is 5.5 or 6 mm deep? How many dentists are actually going to measure the depth of their cavity preparation before using these materials?
My opinion (at this point) regarding bulk-fill light-cured composites is that while I really like the concept, I am not convinced they are where we need them to be just yet. I do feel current products can be used successfully in some clinical situations by responsible dentists employing good technique, materials, and curing lights. However, I also feel the potential for abuse and misuse is very real and that bulk-fill composites may give dentists a false sense of security that could result in an under-polymerized composite. And I already believe that under-polymerization is one of the biggest reasons for composite failure. Although bulk-fill composites have improved, and I suspect will continue to improve, caution is urged. The latest generation of products has not been around very long and at this point there are no clinical studies demonstrating the long-term efficacy of these materials. Results obtained from often unrealistic laboratory studies may be quite different from the reality of clinical situations.
A number of factors determine if a material is adequately cured to the depth its manufacturer states. First of all, as Dr. Alex stated, you have to have an adequate curing light for sufficient polymerization to occur. The three most important characteristics of the light are appropriate wavelength, adequate light output, and exposure time. If the light is not powerful enough and it is too far away from the material, it does not matter what the depth of cure is. Different photoinitiators besides camphorquinone are being added to these composites that absorb visible light at 410 nm. If the curing light used has an output in the range of 450 to 490 nm, the material is not being cured adequately. Polywave lights with different LEDs are being manufactured to cover the wider range of photoinitiators. It is important for the manufacturer to state the wavelength of the photoiniator used. Exposure time is more important than irradiance of the light. We need to have adequate exposure for the material to polymerize. We cannot over-cure our composites but we can under-cure them. Polymerization of resin composites continues at a slow rate after curing and may reach a termination point at almost 24 hours,8 although other studies9 show some surface hardness increases up to 1 month later.
Filler content plays a significant role in the depth of cure possible with these bulk-fill materials. In some studies, it is true that the higher the filler content, the greater the depth of cure. Increased filler loading reduces the volume of resin matrix for polymerization and intrinsically increases hardness. Due to the amount of filler in a material like Sonic Fill (Kerr Dental, www.kerrdental.com), sonic energy is applied through a special handpiece to increase the flowability and to further ease the packing of the composite. The good degree of conversion may be due to refractive index matching between the resin and filler, which enhances light transmission. Reduction in refractive index differences between resin and filler improved degree of conversion10 and increased depth of cure and also improved color shade matching.11
Variations in the depth of cure between bulk-fill resin composites may be ascribed initially to light scattering at particle interfaces12 and light absorbance by photoinitiators and any pigments.13 Both of these factors reduce the light penetration and thus also the degree of conversion of matrix monomers, which are determined by the light irradiance at depth. Degree of conversion is significantly linked with the values of mechanical properties, biocompatibility, and color stability. Therefore, it may also correlate to the clinical success of the restoration. When more extensive polymerization and cross-linking occurs, greater hardness results.8
If the material can be placed in one increment and not have to be incrementally placed, this can potentially reduce the amount of voids present in the restoration. Using these materials in my clinical practice creates a restoration that is much more translucent then some of the non-bulk-fill materials but is satisfactory to the patient.
About the Authors
Jack L. Ferracane, PhD, is a professor in and chair of the department of restorative dentistry and division director of biomaterials and biomechanics at Oregon Health & Science University in Portland, Oregon.
Gary Alex, DMD, has written numerous scientific articles and is involved with a number of continuing education programs. He is in private practice in Huntington, New York.
Robert Margeas, DDS, is an adjunct professor in the department of operative dentistry at the University of Iowa College of Dentistry in Iowa City, Iowa. He is also in private practice in Des Moines, Iowa.
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