The Search for a Low-Shrinkage Direct Composite
Decreasing polymerization shrinkage stress is a critical determinant on the affects of composite curing on the adhesive interface.
Robert A. Lowe, DDS
Since the advent of light-cured direct composite restorations in the early 1980s, the search for the “tooth-colored amalgam replacement” has continued. Every dentist who places posterior composite resins has at the top of his or her “wish list” a composite material that can be placed using a bulk-fill technique similar to that of dental amalgam. Two primary reasons this has not occurred are polymerization shrinkage stress during the curing process and a limited depth of cure for composite materials. Traditional placement techniques for composite resins include incremental placement mainly for these reasons. The effect of polymerization shrinkage stress is greater on larger increments of composites than on smaller increments. Most clinicians recommend placing composites in 2-mm increments. Depth of cure is also critical. If the curing light does not cure the material in the deeper areas of a cavity because of the proximity to the light source or the inability of the light to penetrate the restorative material, the resultant uncured material can adversely affect the bond to tooth structure and, hence, the quality and longevity of the restoration.
Material science has been focused on creating a low-shrinkage composite (most of the current composites on the market shrink about 2.5% to 3.5%) to increase the durability of the composite bond to tooth structure and reduce the possibility of microleakage, one of the main causes of recurrent decay and ultimate restoration failure. While progress has been made in this area, as evidenced by the introduction of composite materials with lower shrinkage values, lower shrinkage alone cannot justify bulk placement. It is the stress created at bonded interfaces that must be lowered and stress can be independent of shrinkage. That is to say, two materials with the same amount of shrinkage can create different levels of stress on bonded interfaces depending on their polymerization dynamics.1-11
Two of the approaches that have been looked at as a potential solution to this problem are the development of a low-stress composite filling material using a different monomer system other than bis-GMA, and development of a low-shrinkage flowable material to use as a dentin replacement under conventional posterior composite resins.
Low-Shrinkage Direct Composites
A new monomer system described by Weinmann et al,12 called silorane, is obtained from the reaction of oxirane and siloxane molecules. The mechanism of compensating stress in this system is achieved by opening the oxirane ring during the polymerization process. Filtek™ LS (3M ESPE, http://www.3mespe.com) is a silorane-based composite material that has been developed out of this research. As an alternative to conventional composite resins, the main advantage of silorane is its low shrinkage. Filtek LS also requires a dedicated adhesive bonding resin, LS Bond (3M ESPE) to achieve similar bond strengths to enamel and dentin as seen in conventional adhesive systems using bis-GMA-based composite resins. The volumetric shrinkage of Filtek LS has been reported at 1.7%. Most conventional composites shrink between 3% and 5% during polymerization when volumetric shrinkage is measured. Conventional composites such as Aelite™ LS (Bisco, http://www.bisco.com), Kalore™ (GC America, http://www.gcamerica.com), N’Durance™ (Septodont, http://www.septodontusa.com), and Grandio® (VOCO America, http://www.vocoamerica.com) are advertised as “low-shrinkage composites” and have volumetric shrinkages of less than 3% (Aelite LS: 1.39%, Kalore: 1.72%, N’Durance: 1.4%, and Grandio: 2.4%, respectively).13-16 It has been shown that these low-shrinkage composite materials do tend to have significantly less microleakage after mechanical load cycling.17Some clinical studies do, however, raise the question on whether this translates into a clinically significant difference as to the long-term durability of the restoration.18
Bulk Fill Flowable Base for Posterior Restorations
Recently, a unique type of flowable composite resin has been developed that is intended to be used as a base beneath posterior composite resin restorations. Certainly, the use of a flowable composite as a liner or base beneath posterior composite restorations is not a new concept. Such use has been claimed to increase marginal adaptation in the gingival marginal area of Class II composite restorations, thereby reducing microleakage. It has also been claimed to counter the polymerization shrinkage stress of overlying composite resins because of the more elastic nature of flowable composites. Neither of these perceived advantages has been validated, but there is relatively broad consensus that the use of flowable composites does help to achieve optimal adaptation of overlying composite to the intricacies of cavity preparations.
A new flowable composite (SureFil® SDR Flow, DENTSPLY Caulk, http://www.caulk.com) is indicated for use as a bulk-fill base beneath posterior composite restorations and can be bulk filled in layers up to 4 mm in depth. Being able to place that amount of material in a single increment is a significant time saver, and while the concept sounds quite simple, there are several important requirements a material must meet for this particular indication. According to the manufacturer, these include the following.
Increased Depth of Cure
This is perhaps the most obvious requirement for the material. It is essential that the flowable composite cure from top to bottom to a minimal depth of 4 mm. The manufacturer reports that Surefil SDR meets this requirement because of its polymerization initiating process and its optical properties that enhance light transmission. It should be noted that while this material is radiopaque, it would appear more translucent in color than many composite “dentin replacements.” This is to allow for light penetration and a greater depth of cure. It is important to pay attention to what manufacturers claim regarding the depth of cure of their materials and to have an idea as to how those claims were substantiated. It may be possible to realize a certain depth of cure in the laboratory where the light can be positioned only a millimeter from the surface of the material being cured but not in a clinical situation where the light can be several millimeters removed.
This requirement is essential if the material is to offer true convenience and performance. Surefil SDR, being a flowable material, can be placed in bulk quantities very quickly because it readily adapts to the internal configuration of cavities without the need for manipulation after dispensing (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11). In addition, the material levels itself (self-leveling) after only seconds to form a uniform base for subsequent placement of composite, again obviating the need for further manipulation. Without the rheological properties this material possesses, it would not be possible to bulk-fill while ensuring optimal adaptation to all aspects of the cavity preparation (Figure 12, Figure 13, Figure 14).
Low Polymerization Shrinkage Stress
Composites resins all shrink to some extent upon photopolymerization. Flowable composites shrink to a greater extent because of a lower filler loading. If not allowed to shrink, as when bonded to tooth surfaces, stress will be created on the bonded surfaces which can lead to marginal defects, and is thought to lead to postoperative sensitivity. The challenges facing a bulk-fill base with respect to polymerization shrinkage stress are large. The volume of material being placed can be relatively large, and the larger the volume of material, the greater the shrinkage stress. In addition, the C factor of posterior cavity preparations is large—a Class I restoration, in fact, has the largest C factor of all cavity classifications. The C factor is the ratio of bonded to non-bonded surfaces and would compute to a 5 for Class I restorations and a 2 for a Class II restoration. Reduction of polymerization shrinkage stress, as might be expected, is the most difficult requirement to meet for a bulk fill material. The manufacturer reports that Surefil SDR meets this requirement by incorporating a unique curing process that builds very little stress as the material is forming the bonds of polymerization. The net effect of this is very little stress being created on bonded surfaces after the material has polymerized.19-28
Composite shrinkage will continue to be a problem that is looked at by scientists and clinicians alike to see if it is possible to create a material that has no shrinkage, better marginal integrity, but yet has satisfactory physical properties and handling characteristics that make significant improvements in today’s available technologies. Decreasing the polymerization shrinkage stress seems to be a critical determinant on the affects of composite curing on the adhesive interface and does not necessarily translate to the volumetric shrinkage of the restorative material.
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About the Author
Robert A. Lowe, DDS
Diplomat, American Board Of Aesthetic Dentistry
Charlotte, North Carolina