By John O. Burgess, DDS, MS, and Jessica M. Davidson, BS
Esthetic restorations have been used since the early 1970s with increasing success. This article will discuss the proper placement of the bonding agent used to bond composite resin into the tooth as well as describe newer anterior and posterior composite resin kits, their selection, and use. Although the use of composite resin as a posterior restorative material is increasing, composite resin restorations are difficult and require more placement time than a similar-sized amalgam restoration. Proper interproximal contacts are difficult to obtain, and isolation from contamination, especially blood, is essential. Postoperative sensitivity with Class II composite restorations has been reported and is due primarily to polymerization shrinkage of the composite resin, which causes cracking and crazing at enamel margins and gap formation at the resin-tooth interface.
Bonding agents
Bonding agents are the first and perhaps most important component of a resin-bonded esthetic restoration. Bonding agents can be divided into six generations depending upon the number of components they use. Current generation materials are classified as fourth-, fifth-, or sixth-generation materials.
Fourth-generation materials are composed of three bottles and use three separate steps for placement (etch and rinse, prime, and bond). Examples of this category of bonding agents are Scotchbond Multipurpose, Optibond FL, GLUMA, and Tenure. Generally thick hybrid layers are produced with these bonding agents. Bond strengths of fourth-generation materials range from 16-30 MPa.
Fifth-generation materials are two-step, two-component systems — the etch/rinse step and the bond, dry, and cure step. In these systems the primer and adhesive are combined into a single component, which is applied to properly etched and rinsed tooth structure. Bond strengths for fifth-generation materials are generally lower than bond strengths for fourth-generation materials. Fourth- and fifth-generation materials etch enamel and dentin simultaneously and are often referred to as "total etch systems." Phosphoric acid is used in fourth- and fifth-generation bonding agents to etch enamel and dentin surfaces simultaneously.
The etching step removes the smear layer, opens dentinal tubules, and exposes collagen fibers in dentin. In enamel, etching produces a microscopic etch pattern which provides mechanical retention to bond resin to tooth structure. Etchants have antimicrobial activity and, if proper isolation is maintained, it is redundant and unnecessary to apply a separate antimicrobial agent prior to etching. Etching is not passive, and the etchant should be stirred during application to ensure that fresh etchant is available at the surface being etched and any entrapped air bubbles are moved from the surface being etched to produce a consistent etch pattern. Unetched areas cannot be sealed and ultimately provide a pathway for leakage and recurrent decay. Recommended etch times range from 15-20 seconds but, if no mat pattern appears on enamel after the etchant is rinsed and dried, the etch time should be increased since enamel with high fluoride content is resistant to etching. The etchant should be rinsed until the etchant color is gone and longer rinse times do not improve bond strength. After rinsing, the high-volume suction is placed next to the preparation. This removes excess water, allows you to see the enamel etch pattern, and does not over-dry the dentin. A further benefit is that it does not contaminate the surface as oil or debris in an air spray might. If the air syringe or a drying device is used and the tooth is over-dried, the dentin is wetted with tap water until the dentin is glistening but no puddles are present.
In fourth- and fifth-generation bonding agents, proper hydration of the dentin surface is essential. Excessive drying lowers bond strengths to dentin since the collagen fibers exposed during etching collapse, blocking penetration of the bonding agent and preventing formation of the hybrid layer. A moist cotton pellet should be used to wet the dentin just before applying primer. Air-drying dentin prior to placing the primer — for only three seconds — decreases bond strength. When collagen fibers are supported by water, the bonding agent easily encases the collagen fibers producing the hybrid layer. During drying, collagen fibers collapse and form a dense layer that prevents the bonding agent from surrounding and encasing the collagen fibers. The primer of fourth-generation adhesives or the combined primer and adhesive combination of fifth-generation adhesives must be applied and dried until the preparation has a shiny surface appearance. If the shiny appearance is not apparent, another coat must be applied and dried. Air-dry the primer or primer/adhesive combination thoroughly to evaporate the acetone, ethanol, or water solvents within the adhesive. In most direct restorations, a fourth-generation adhesive (with a separate primer and adhesive) is a better choice. These materials form thicker layers than one-bottle bonding agents. Adhesives containing acetone evaporate, rapidly producing lower bond strengths as the material becomes more viscous and more difficult to apply. Any adhesive containing acetone should be dispensed from the bottle directly to a brush tip and applied immediately to the tooth.
The bonding agent must be light-cured before composite resin is placed over it to optimize bond strength. When the adhesive is not separately light-cured, bond strengths are reduced. There is a direct relationship between bond strength and the distance the light is from the adhesive. Light-curing also affects bond strength. Light intensity decreases as the distance of the light guide increases from the surface being cured. The decrease in intensity also depends on the type of light guide used. When the curing light guide cannot be brought close to the floor of the gingival box, the adhesive is poorly polymerized and a poor bond and a poor seal results (2 mm tips should be used to allow placement into the gingival box). The light guide should be as close as possible to the surface being cured to produce the highest bonds to tooth and the best seal.
Sixth-generation or self-etching bonding materials eliminate the etch rinse step and may have one, two, or three components. Materials classified in this generation are Clearfil SE Bond, Prompt L-Pop, One-Up Bond F, and Optibond Solo Plus. Bond strengths are lower with this generation of bonding agent compared to all other generations. Self-etching materials leave the smear layer but partially dissolve and penetrate it. Because there is no rinse step, contaminants remain on the tooth and reduce bond strength. Additionally, sixth-generation bonding agents do not etch uncut enamel or sclerotic dentin. In those clinical situations, phosphoric acid should be used. Sixth-generation bonding agents are clinically unproven and few studies are available to evaluate their long-term clinical performance. Lack of postoperative sensitivity is often associated with the clinical use of these materials. Only clinical testing will determine the success of these bonding agents and which ones will remain.
Composite resin
Resin-based composite (RBC) consists primarily of a resin matrix surrounding organic or inorganic filler particles. The resin matrix or monomer contains an initiator/catalyst system for polymerization. The first dental RBC monomer based on bis-GMA is a bulky monomer with methacrylate groups at each end of the molecule (dimethacrylate). Polymer-ization occurs through a free-radical addition reaction. Since bis-GMA is viscous, it is thinned to improve handling with diacrylate monomers, such as ethyleneglycol dimethacrylate and triethyleneglycol dimethacrylate. As the monomer cross links, vibration of the molecule decreases and shrinkage occurs. Later, another diacrylate monomer, urethane dimethacrylate (UDMA), was developed for dental use. This monomer is more flexible with a molecular weight that is similar to bis-GMA. UDMA may be used alone or blended with other diacrylate monomers.
After these basic monomers were developed, 30 years passed until the next major development in composite resins occurred. In 1998 the first RBC was introduced based on the ormocer chemistry (Definate, Degussa). Multi-functional urethane- and thioether(meth)acrylate alkoxysilanes as sol-gel precursors have been developed for the synthesis of inorganic-organic copolymers ormocer composites. Ormocers (organically modified ceramics) have inorganic-organic copolymers in the blend that allow the RBC to be manipulated like any other RBC. More recently another ormocer, Admira, was introduced by VOCO (Germany) with claims of decreased polymerization shrinkage. Although this material has promising mechanical properties, its clinical success will have to be established.
Visible light-cured composites are single-paste materials photo-polymerized by free-radical polymerization. Using a diketone photo-initiator like camphoroquinone and an accelerator/catalyst system like dimethylamino ethylmethacrylate, light-cured composites are polymerized when the photoinitiator absorbs light energy (photons) emitted from the curing light and directly or indirectly initiates polymerization. The activated diketone/amine complex initiates polymerization of the dimethacrylate resin monomers. Since visible light-cured RBC contains a lower concentration of amine accelerators than chemical-cured RBC, light-cured composites are more color stable than chemically activated RBC. Camphoroquinone is a commonly used photoinitiator with major absorption of visible light wavelengths in the 460-480 nm (blue) range. RBCs may contain a combination of photoinitiators, each requiring its own specific wavelength for maximum reactivity. Camphorquinone has a maximum absorption at 468 nm, which is close to the peak spectral output of LED curing lights.
During polymerization, the resin begins to cross link and shrink. With present day RBCs, this shrinkage ranges from 1.5 to 3.0 percent per volume. When composite resin is placed in a cavity preparation, it is confined by the preparation. If bonded to the preparation wall, shrinkage of the composite resin transfers stress to the cavity walls. Polymerization shrinkage can tear the adhesive bond of the tooth, deform the tooth, or fracture the marginal tooth structure. Increasing filler content of RBC minimizes resin content, reduces polymerization shrinkage, and increases the stiffness (or modulus of elasticity) of the RBC. The magnitude of the contraction stress is related to the cavity configuration, the flow of the composite, the cavity shape, the degree of conversion, and the modulus of the composite.
Posterior composite resin restorations
Preparations — Posterior composite resin preparations are similar to amalgam preparations, but composite resin preparations can be more conservative, narrower, and shallower than amalgam preparations. Composite resin preparations are essentially preparations in which the caries are removed and the tooth is restored. Since the composite restoration is more conservative than amalgam, ideally the initial restoration in a tooth should be composite resin. The isthmus width for composite resin preparations should range from no isthmus to no greater than the intercuspal distance. The size restriction is not based on poor wear performance of composite, but on the thickness of the remaining tooth structure which might crack during polymerization shrinkage of the composite. Retention has been advocated for slot preparations but with larger preparations no auxiliary retention, such as grooves, are needed. One in vivo study has shown that an occlusal bevel on posterior composite preparations enlarges the preparation width and increases wear of the composite resin restoration.
Matrix systems — Matrices provide the form which will shape the composite that replaces the missing tooth structure. These plastic or metal matrices range from sectional to circumferential. Metal matrices are easiest to place since they pass easily through unbroken contact areas. Toffelmire retainers and Dixieland bands (Teledyne-Getz) are recommended since they are thin and contoured, which provides an anatomical shape for the contact forming a contact area rather than a contact point. Wooden interproximal wedges separate the teeth and compensate for the thickness of the matrix and the shrinkage of the composite resin when it is polymerized. Greater separation is required when a two-surface restoration is fabricated with circumference matrices since the separation must compensate for two thicknesses of matrix. When sectional matrices are used, only one thickness of matrix must be regained to achieve an adequate separation since sectional matrices do not surround the tooth being restored and separate it from the adjacent tooth. Sectional matrices like Palodent matrices (DENTSPLY Caulk) or Composi-Tight (Garrison Dental Solutions) are placed interproximally, and a wooden wedge is inserted against the matrix and the adjacent unprepared tooth. A separating ring is placed in the gingival area between the wedge and the matrix. The spring steel separating ring attempts to close, pushing the teeth apart and separating them. This system helps develop tighter proximal contacts. The Composi-Tight matrix system has rings that are better retained on short bicuspids or mandibular molars. Sectional matrix systems provide good contacts in most posterior restorations but the contact area must be further shaped to provide all the embrasure space needed.
Forming the contact area — Perhaps the most difficult area to develop with posterior composite resins is the contact area. The Contact Pro is placed into unpolymerized composite resin and is used to push the resin against the adjacent tooth. The instrument helps to form a curved contact area. This instrument is useful when the contact is stretched to the adjacent tooth. It provides a snug contact and a nice gingival embrasure.
Materials used — Wear-resistant, low-shrinkage composite resins should be selected for posterior use. Recommended specific materials for posterior use are Z-100, P-60, Pyramid, Heliomolar HB, or SureFil. Only a few composite resins — such as Miris (Coltène/Whaledent) and Filtek Supreme (3M ESPE) — fill all the requirements for anterior and posterior restorations. Condensable or packable RBC have higher filler loading to reduce slumping, allow the composite to be sculpted, and decrease stickiness to placement instruments. While their handling properties are an improvement for larger Class I and Class II restorations, packable resin-based composites do not eliminate the difficulty in producing tight interproximal contact. In an in vitro Class II study, Bagby and others reported that packable RBC had smaller interproximal gaps than one hybrid, but were not comparable to amalgam (Tytin, SDS Kerr) tested. Peumans et al also reported that the packability of the composite resin did not influence the tightness of the contact area. Leinfelder and others reported that overall, the mechanical properties of packable composites are not substantially better than most conventional minifilled hybrids. Although they have not eliminated all the technical problems associated with posterior composite resin restorations, one two-year clinical report of 25 Class II restorations reported that Surefil posterior restorations were clinically acceptable in all categories. The data currently available on condensable RBC as a posterior restorative material is encouraging. Once longer-term clinical trials are reported, a clearer picture will emerge. Because packable RBC was designed for posterior use, handling properties dominate over esthetics. Packables generally exhibit good handling properties — low stickiness and slump, but with limited shade selection and increased opacity.
Editor's Note: The second part of this article will appear in the May/ June issue of Dental Equipment & Materials.
Dr. John O. Burgess is chairman of the operative dentistry and biomaterials department and director of clinical research at Louisiana State University Health Sciences Center's School of Dentistry in New Orleans. Jessica Davidson is clinical research coordinator at Louisiana State University School of Dentistry. They may be reached at (504) 670-2730.
