Direct Esthetic Adhesive Restorative Materials
Susana Ferreira, DDS; Gerard Kugel, DMD, MS, PhD; Stephen Martin, DMD; Jack Ferracane, PhD
In the past 10 to 20 years, significant advances in restorative materials have revolutionized dentistry.1-5 Esthetic considerations are growing in importance for the restoration of posterior teeth.6 Composite, compomers, glass-ionomers, and resin-modified glass ionomers (RMGIs) have given dentists a viable and esthetic alternative to amalgam restorations. These materials have become more desirable restoratives because they provide patients with a superior esthetic result, with acceptable physical and chemical properties.
In recent years, a growing public interest in cosmetic procedures has had a significant impact on both dentists and patients. A large percentage of articles targeting the general dentist are focused on esthetic procedures and materials. In fact, a multitude of journals entirely devoted to esthetic dentistry have now become part of the regular circulation of publications to arrive in dental offices around the world. With these publications come advertisements for products claiming to be the new breakthrough in esthetic restorative dentistry, sometimes without appropriate clinical evidence and research. Patients are certainly not immune from dental advertising, either. Television, magazines, and other outlets for consumer-directed marketing have created a public demand for bleaching, lasers, and metal-free dentistry. As with all consumer-directed advertising, there has been a great deal of misinformation and unrealistic expectations. Perhaps the greatest example is the misinformation regarding mercury in dental amalgams. An unsupported public fear of metal restorative materials has grown to the point where some patients consider an amalgam restoration unacceptable treatment. The controversial discussions raised about the continued use of amalgam as a contemporary restorative material and its effects7,8 have contributed to the development of alternative restorative materials.
As the dental industry has embraced esthetic restorative materials, the need for improved dental adhesion has become an area of major interest. The roots of adhesive dentistry can be traced to Buonocore in 1955.9 Following the principles of industrial bonding, Buonocore suggested that acids be used on tooth structure as a surface treatment before the application of resins. Nearly a decade later, Buonocore and colleagues10 continued to expand on this, suggesting that resin tag formation was responsible for the adhesion to acid-etched enamel. This laid the groundwork for the current accepted principles of adhesive dentistry.
Since Buonocore’s initial work 50 years ago, adhesive dentistry has evolved from first-generation, enamel-only bonding with little or no dentin adhesion yielding bond strengths of only 1 MPa to 3 MPa with poor clinical results,11 to the present use of sixth- and seventh-generation systems. These new systems rely on use of an acidic primer, eliminating the need of phosphoric acid for etching the tooth surface. The sixth-generation systems can be divided into two types: Type I, adhesive components and self-etch primer that are applied separately to the tooth, and Type II, adhesive components and self-etch primer that are mixed before being applied.
The seventh-generation system is a self-etching adhesive system that does not require mixing. Introduced in the late 2002, these new systems combine etchant, primer, and adhesive in a single bottle.12
The importance of direct-placement, esthetic, tooth-colored restorative materials is still increasing. Direct composite restorations require a time-consuming and more costly treatment procedure, and our laboratories reported the potential benefit of using RMGIs as liners under posterior composites and amalgams as a way to reduce cervical microleakage13,14 and recurrent caries.15,16 This may be because of their fluoride release and/or self-adhesive properties.17,18 Glass-ionomers have also been shown to be antibacterial.19 From an esthetic standpoint, glass-ionomers can only be considered as “long-term” provisional restorations in stress-bearing posterior cavities and will not be addressed in this article.
Direct Adhesive Restorative Materials
Along with enhanced bonding, the clinical performance of direct restoratives materials has also been improved. Today, the most commonly used direct esthetic restorative material is resin composite.
Composite can be defined as any material with two or more distinct components. With most composites, there is either a single or a combination of two or more high-strength materials joined together with an adhesive binder. Typically, there is a recognizable interface between these components. Resin-based composites generally consist of three primary ingredients: an organic resin matrix, inorganic filler particles, and a coupling agent.
The matrix of most composite resins is a combination of dimethacrylate monomers containing bisphenol A glycidyl methacrylate (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA), and/or urethane dimethacrylate (UDMA). These monomers are usually used in combination to maximize the desirable characteristics. For example, EGDMA and TEGDMA are usually added to bis-GMA to reduce viscosity, increase crosslinking, and increase hardness. With activation, the resin matrices undergo a polymerization reaction that causes hardening of the composite. In this process, the resin contracts as the monomers form cross-linked polymer chains, resulting in the undesirable characteristic of polymerization shrinkage.
The fillers in composite resins are added to control most of the physical and mechanical properties of the resin-based composite. The filler increases strength and modulus of elasticity while reducing the coefficient of thermal expansion and polymerization shrinkage. There is more direct control of these mechanical properties as the percentage of filler contained within the composite is increased. It has been demonstrated that filler volume and filler load correlate with the strength of the composite resin. Increased filler volume results in an increase in fracture toughness along with an increased flexure modulus.20 The filler also determines many important physical properties of the composite, such as radiopacity and polishability. While radiopacity is determined directly by the type of filler used, there is an increase in polishability with decreased filler particle size. Additionally, the wear of composite restorations depends on filler particle size, interparticle spacing, and filler loading.21 There is reduced wear with smaller particles due to less filler plucking, which creates surface voids. Smaller interparticle spacing provides matrix protection by having the harder filler particles shield the softer matrix areas between them from abrasive particles. Therefore, developments in strength, shrinkage, radiopacity, polishability, and wear resistance have significantly improved the clinical results of resin-based composites.22
Superiority of any specific filler has not been proven.22 Examples of fillers include: amorphous silica, quartz, radiopaque glasses (barium, strontium, others), sol-gel zirconia-silica, and fluoride containing fluorosilicates, ytterbium, and yttrium trifluoride.
Fillers can be further classified by particle size: macro fillers are 10 µm to 100 µm; midi fillers are 1 µm to 10 µm; mini fillers are 0.1 µm to 1 µm; micro fillers are 0.01 µm to 0.1 µm; and nano fillers are 0.005 µm to 0.1 µm.
Filler silanation is another important aspect of improved resin characteristics. The filler particles of composite resins are silanated in order for the hydrophilic filler to bond to the hydrophobic resin matrix and to protect the hydrophilic filler from water degradation. Silanation is usually accomplished through the use of organosilanes, such as gamma-methacryloxypropyltrimethoxy-silane, as coupling agents between the filler and matrix. Good silanation will result in a stable composite resin that is resistant to wear with a homogenous composition.23
Other ingredients in resin-based composites include polymerization initiators, polymerization inhibitors, color stabilizers, and pigments. Polymerization initiators, like camphoroquinone, are required as catalysts for light activation and curing of the composite. On the other hand, polymerization inhibitors are added to prevent the composite from auto-curing during storage. Color stabilizers help to absorb ultraviolet radiation that would otherwise cause discoloration of the composite over time. Finally, pigments play the obvious role of providing the composite with the appropriate tooth-matching shade.
Composites have been classified based on filler size and volume as previously stated. They also can also be classified depending on consistency or viscosity, ie, packable and flowable. Nevertheless, it can get more difficult for the dentist to decide which composite to choose for posterior and anterior teeth.
In the early 1980s, microfilled composites were introduced. These resin-based composite materials contain submicron inorganic filler particles, averaging 0.04 mm in size, which are added to monomers to make a very viscous blend. This blend is polymerized with heat and subsequently ground into 5-mm to 50-mm particles. These “pre-polymerized” particles are then added to a microfiller-particle-filled resin matrix to form a composite with increased filler loading. Despite this increased loading capability, they still have lower filler content and strength than non-microfill composites and are typically not radiopaque. As a result of the small particle size, they are highly polishable but subject to increased marginal breakdown and fracture.24,25
Microfilled composites are contraindicated for posterior load-bearing restorations because of poor mechanical properties. Microfill composites have shown lower fracture resistance, stiffness, and fatigue in comparison with more heavily filled composites.26-29 On the other hand, it is generally accepted that microfilled resin-based composites polish extremely well and can retain their surface smoothness over time. For these reasons they are indicated for Class V, Class III, small Class I, and composite veneers when bruxism is not present. The classic material for this category is Durafill VS (Heraeus Kulzer, Armonk, NY).
Hybrid and Microhybrid Composites
The majority of the resin-based composites on the market today are “hybrids.”22 Dental manufacturers typically define all resin composites having particles that are a few tenths of a micrometer and larger in combination with fumed silica particles of 0.04 µm as hybrid composite-based resins. Hybrids incorporate fumed silica to help with the handling properties.30 This combination of varying types and sizes of fillers supposedly improves strength and esthetics.
Microhybrids have been called “evolved hybrids,” having a blend of both submicron particles (0.040 µm) and small-micron particles (0.1 µm to 1 µm). However, based on recent classification systems these composites would fall into the minifill category, distinguishing them from more traditional hybrids (or midifills) which can have particles much greater than 1 micrometer. This mix allows finer polish and improved handling characteristics than traditional hybrids. In a recent study, a traditional hybrid and microhybrid composite resins were evaluated in different cavity configurations and the traditional hybrid composite still showed higher mechanical properties than the microhybrid composites.31
Filler size, interparticle spacing, and filler loading also play an important role in wear characteristics.21 Composites have generally shown less wear than compomer and resin-modified glass ionomer.32 Examples of current microhybrids available on the market are: 4 Seasons® (Ivoclar Vivadent®, Inc, Amherst, NY), Esthet-X® (DENTSPLY Caulk, Milford, DE), Vit-l-escence® (Ultradent Products, Inc, South Jordan, UT), Venus™ (Heraeus Kulzer, Inc, Armonk, NY), and Gradia® Direct (GC America, Inc, Alsip, IL) (Figure 1 and Figure 2).
Nanofilled and Nanohybrid Composites
A new generation of resin-based composites have been introduced to improve wear and fatigue resistance while maintaining good esthetics. These materials are called nanofilled and nanohybrid composites. Nanofills only contain nanometer-sized particle throughout the matrix. Nanohybrids have nanometer-sized particles combined with more conventional filler particles.33 Questions have been raised about the stability of the filler matrix interface of these materials on the basis of short-term immersion and in vitro fatigue stress testing. In terms of wear and fatigue resistance, nano-structured composites may perform either similarly or comparatively worse than a microfilled composite.34
On the plus side, nanofills have been called the first truly universal resin-based composites. This new category has shown good esthetics, good mechanical properties, good handling, and good polishability. In a recent study, the surface roughness of microfilled, microhybrid, and nanofilled composites was evaluated after polishing. It showed that some nanofilled composites were not substantially different from the microfilled and microhybrid composites.35
Like nanofills, nanohybrids can be polished to a relatively smooth finish at placement, but develop a slightly frosty appear-ance over a period of time. This would likely go unnoticed by most patients. The most well-known nanofilled resin-based composite is Filtek™ Supreme (3M™ESPE™, St. Paul, MN), which is considered for nearly universal use (Figure 3 and Figure 4).
As the dental profession searched for an amalgam substitute, many limitations in composite materials on the market were found. These limitations included resistance to wear, fracture of the restoration within the body and at the margins, marginal leakage due to polymerization shrinkage, and technical problems including difficulty in obtaining adequate proximal contacts.
Packable composites were thus introduced to the profession as an amalgam substitute. They have a stiffer consistency than more conventional hybrid composites, which is produced by altering the particle size distribution or filler type, and they do not necessarily contain a higher filler loading as studies have shown. For some, this stiffer consistency allows for improved handling characteristics. Another potential advantage of these materials is greater ease in establishing interproximal contacts while placing Class II restorations.
Flowable Composite Resin
Flowable resin-based composites are another category of resins which have a lower filler volume than the conventional direct composite resin restorative material. This lower filler volume results in a decreased viscosity which, in turn, makes them a good choice as pit-and-fissure restoratives, Class V restoratives, or as a liner under posterior composites. However, flowable resins have increased shrinkage, decreased wear resistance, and decreased strength. It should be noted that in vitro evidence does not support their use as a liner to reduce microleakage.36
Compomers, Ceromers, Ormocers, Smart Composites
The term compomers is the result of the hybridization of the words composite and glass ionomer. Compomers are polyacid-modified resins that are basically light-cured, low-fluoride-releasing composite resins. The difference between compomers and composites is that the compomers contain some monomers with acidic functional groups that can potentially participate in an acid/base glass ionomer reaction after polymerization of the dimethacrylate resin molecule. The level of fluoride release from the compomers is significantly lower than what is seen for conventional glass-ionomers or RMGIs.37 Originally, manufacturers said that acid-etching was not required. From a sensitivity standpoint, no etching and fluoride release was seen as an advantage over composites. It was subsequently demonstrated that the use of an acid-etched procedure significantly improves both the retention and resistance to marginal leakage of the compomers.38-40
The term ceromer stands for ceramic-optimized polymer and was introduced by Ivoclar Vivadent to describe their composite Tetric® Ceram. This material consists of a paste containing barium glass (< 1 µm), spheroidal mixed oxide, ytterbium trifluoride, and silicon dioxide (57% volume) in dimethacrylate monomers (Bis-GMA and urethane dimethacrylate). They are set by a polymerization of C=C of the dimethacrylate monomers. They must be bonded to tooth structure. The properties of the ceromers are identical to those of composites and they exhibit fluoride release lower than conventional glass ionomers or compomers.
Ormocer is the acronym for organically modified ceramics. This class of material represents a novel inorganic-organic copolymer in the formulation. The inorganic-organic copolymers are synthesized from multifunctional urethane and thioether(meth)acrylate alkoxysilanes as sol-gel precursors. Alkoxysilyl groups of the silane permit the formation of an inorganic Si-O-Si network by hydrolysis and polycondensation reactions. The methacrylate groups are available for photochemical polymerization.3,41 The filler particles are 1 µm to 1.5 µm in size and the material is 77% filler weight and 61% filler volume.
“Smart” composites are a class of ion-releasing composites. Ivoclar released Ariston pHc in 1998 as a smart composite. It is white in color and indicated for posterior teeth. It releases fluoride, hydroxyl, and calcium ions as the pH drops in the area immediately adjacent to the restorative material. The drop in pH values is the result of active plaque that results in a corresponding increase in the release of functional ions.3 Smart composites work based on the developed alkaline glass filler which was designed to reduce secondary caries formation at the margin of a restoration by inhibiting bacterial growth, resulting in a reduced demineralization and a buffering of the acid produced by caries-forming microorganisms.41 The physical properties of this material are comparable to those of other composites.42 The fluoride release from this material is claimed by the manufacturer to be lower than conventional glass-ionomers but more than that of compomers. Although it had promising properties, this resin composite was not very well accepted in the dental community, it did not meet the esthetic need, and it was difficult to handle.
In hopes of finding a superior alternative to dental amalgam, dental researchers and manufacturers have introduced direct restoratives with different and, in some cases, similar mechanical properties, physical properties, and handling characteristics. Currently, resin-based composites are the most widely used direct restoratives and perhaps the most promising. Further clinical research and material development will certainly be needed as the dental profession continues to search for the perfect direct esthetic restorative material.
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About the Authors
Susana Ferreira, DDS
Department of Prosthodontics and Operative Dentistry
Tufts University School of Dental Medicine
Gerard Kugel, DMD, MS, PhD
Associate Dean for Research
Tufts University School of Dental Medicine
Stephen Martin, DMD
Jack L. Ferracane, PhD
Professor and Chair
Department of Restorative Dentistry
Division Director of Biomaterials and Biomechanics
Oregon Health & Science University