Combining Technologies to Improve Aesthetics

DIRECT COMPOSITE RESTORATIONS: MARRYING OLD AND NEW TECHNOLOGIES
As clinicians, we are continually searching for the perfect combination of aesthetics, longevity, and ease of placement in the world of direct composite restorative dentistry. Until we have a “magic bullet” which covers all of the aforementioned requirements, we should consider which combination of materials currently at our disposal gives us the best chance of achieving this lofty goal.

A lot of research and discussion revolves around the quality of the adhesive bond to dentin as one of the keys to longevity and clinical success. How many megapascals (MPa) of bond strength are really needed to offset the polymerization stress created while curing composite resins that are affixed to these adhesives? Can these “numbers” be sufficiently achieved when using self-etching systems, as compared with total-etch systems? Is the problem really in the polymerization stress that composite resins place on this interface during the curing process?

Most clinicians and researchers agree that the quality of the bond between composite resin restorative materials and etched enamel, as compared to dentin, is the most predictable bond in dentistry. However, we do not always have enamel available to bond to in all clinical situations. So at this point in time, how can we maximize our efforts to provide the best clinical results for our patients?

GLASS IONOMER CEMENTS AS A UNIVERSAL DENTIN REPLACEMENT
One of the “best kept secrets” in this era of dentin adhesive dentistry is the use of glass ionomer (GI) cements as a useful adjunct in the placement of direct composite resin restorations. Since the first GI cements were introduced, they have been one of the most widely researched of all dental materials. In many countries of the world, GIs are the restorative material of choice, due to their low cost and anticariogenic properties.

According to Dr. Hien Ngo (professor, University of Queensland, Brisbane, Australia), GI cement forms a “chemically fused seal to dentin” which many argue is superior to dentin adhesive technology. What most clinicians don’t understand is that there is some truth to that statement. GIs offer not only mechanical, but chemical bonding, due to the chemistry of GIs. That is why it’s a more leak-resistant seal than traditional adhesive bonding.

It has been argued that the dentin bond hydrolyzes with time allowing microleakage to occur. Some even say that “the dentin bond doesn’t work.” Bonded restorative material to enamel has to be cut off, while material bonded to dentin can be “popped off.” Again, the author would agree that this is partly true. However, an important clinical distinction that needs clarification is that there is “good” dentin (to bond to) and “bad dentin” as well. Dentin close to the dentinoenamel junction has fewer dentinal tubules per square millimeter; therefore it has more peritubular dentin to demineralize through the acid-etching process. This creates a very good micromechanical surface for adhesive resins to bond to. Areas close to the dental pulp (deep cervical erosions or lesions and proximal boxes of Class II cavities whose gingival floor is on root surface) are areas where the dentin bond does not perform as well. Using traditional adhesives in these areas can lead to failure of the margin, microleakage, and eventually recurrent decay.

It is important as a clinician to differentiate between “good dentin” and “bad dentin” when choosing a material to close this restorative interface. It is well accepted that GI cements can seal “bad dentin” better than dentin adhesive bonding systems. In areas of “good dentin,” dentin adhesives perform very well, especially when used in conjunction with etching the enamel with a 37% phosphoric acid gel. So, the key is to use the appropriate material where and when it performs the best!

Because of the challenges associated with the placement/finishing of GI materials and the resultant aesthetics that are achieved, it is easy to overlook GI as a direct restorative. However, as a liner or base (dentin replacement), it is hard to beat. The coefficient of thermal expansion is the same as dentin, so it expands and contracts at the same rate as dentin.

USE OF GLASS IONOMER CEMENTS AS LINERS AND BASES
GI cements make good bases under restorations because they have a proven ability to remineralize any demineralized tooth structure present. GI cements will bond to dentin (about 6 to 8 MPa of bond strength) without removing the smear layer. Recently, it has been shown that modifying the smear layer with a mild acid and leaving the smear plugs behind (much like the self-etching bonding agents do) improves the seal while helping to limit postoperative sensitivity. This technique of using a GI cement as a base under composite resin materials has been referred to as the “sandwich technique.”

However, the placement of GI cement as a base has always been a little technique sensitive. Finding that “right consistency” to conveniently place the cement base has traditionally been a challenge, since the window of opportunity for handling the material prior to setting has traditionally been very short. Using automix capsules in conjunction with the unique handling of the newer GI cements such as Fuji II LC and Fuji IX (GC America) has simplified the placement of these materials. Resin modified ionomers (RMI) such as Fuji II LC are commonly used as liners at 1.0 mm (or less) in thickness on dentinal surfaces, and not “built-up” for bulk placement. For large areas of dentin replacement, such as after caries excavation or removal of a restoration, a material like Fuji IX will have greater compressive strength than an RMI.1-5

NEW MONOMER TECHNOLOGY
Advances in composite resin technology have been largely in the fillers used—changes in particle size, particle shape, or filler type. These changes were carried out in an attempt to maximize the aesthetic potential of the material, while maintaining the physical properties necessary to enable the material to withstand the stresses of masticatory forces in the oral environment. With a new composite resin called Kalore, the manufacturer (GC America) reports an innovation in monomer technology.

A monomer developed by Dupont that has a long rigid core with flexible arms has been used in this material. With this new monomer formulation, the polymerization shrinkage challenge may be solved by removing the shorter chain methacrylate matrix, providing the potential for reducing such clinical challenges as marginal gap formation, microleakage, stain, and secondary caries; while enhancing aesthetics and wear resistance. By using the Dupont monomer, they are replacing many shorter chain polymers in the polymerization process, resulting in less shrinkage, and more importantly, less polymerization shrinkage stress.

This new composite resin system includes universal, translucent, and opaque shades that allow the dentist to “stack” a composite resin like a dental laboratory technician would layer porcelain, thus providing the ability to create lifelike aesthetics. However, most of the time, the universal shade alone will provide excellent shade blending with natural tooth structure. The simplified shade system offers the dentist the “recipe” to produce beautiful aesthetics in the anterior segments; even in Class IV situations where neutralizing the darkness of the oral cavity presents a challenge for even the most experienced clinician.