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Adhesion
Many current restorative procedures rely on the adhesive properties of resin-based materials to tooth structure and to the restoration. Adhesives are applied to join materials together, defy separation, and transmit loads across the bonded interface.1 Improvements in adhesive systems have been made but the interface between the restoration and tooth structure still presents problems. Marginal discoloration and loss of the restoration occur frequently.2 This may be because immediate bond strength is not always an indicator of long-term bond stability.3 Bonding systems can be classified based on the number of components, their mode of action, and/or the variety of acid used.4 Three-component systems consist of etching, priming, and bonding steps. Examples of these systems include: Adper™ Scotchbond™ Multi-Purpose Plus Adhesive (3M ESPE, St. Paul, MN) and OptiBond FL® (Kerr Corporation, Orange, CA)
Etching is usually done with phosphoric acid (35% to 37%); this step will remove the smear layer, decalcify intertubular dentin, and decalcify portions of the rods in enamel. Primers serve to guarantee that wetting and flow occurs on the surface being bonded. Primers are hydrophilic in nature with low viscosity. A solvent is usually added to the primer to reduce its viscosity and enable better wetting. Multiple-layer application permits the outer dentin surface to be filled so that it will contact the bonding agent. The solvent should be evaporated before the adhesive is placed. The solvents are usually acetone or ethanol/water combinations. A bonding agent is required because it is needed to connect the hydrophilic primer to the overlying composite, which is hydrophobic.4
To expedite adhesive procedures, the number of steps has been reduced. One method combined etching and priming in one step (Clearfil™ SE Bond, [Kuraray America, New York, NY]) and the other combined the primer and bonding agent together (OptiBond® Solo Plus™ [Kerr Corporation]). The latter would be a two-step, total-etch system. The primer-bonding materials have a high percentage of solvent to prevent the two components from separating. Each type has its advantages and disadvantages. Phosphoric acid has many advantages. Because it is only partially dissociated, as it reacts on tooth structure the pH remains buffered and the acid keeps its strength for a long period of time. Organic acids, on the other hand, are not very acidic and because they do not possess the buffering capacity of phosphoric acid, they have to work expeditiously because the pH increases as the etching occurs.4
The two component systems that combine the etching and priming steps contain acidic monomers that dissolve or partially dissolve the smear layer. Decalcification should proceed at this point as a hybrid layer is formed and the acidic monomers are polymerized so that further decalcification does not occur by acidic side groups. Further decalcification would create a weakened layer.5 The effect of these acidic monomers on uncut enamel has also come into question.6
Combining all of these components together creates one-component systems called self-etching adhesives. Some of these components are kept separate until use, such as Adper™ Prompt™ L-Pop™ (3M ESPE) and Touch&Bond® (Parkell, Edgewood, NY). These systems allow final mixing at the time of application. Other systems combine all the components in a single bottle (OptiBond® All-In-One [Kerr Corporation], and Adper™ Easy Bond Self-Etch Adhesive [3M ESPE]). Unfortunately, some of these systems are unstable, many require cold storage, and many have a short shelf life. Because many of these systems contain water, air-drying must be used; however, excessive air-thinning will dislodge unset material from the surface and prevent its interaction with the hydrophobic resin. Because the smear layer must be rapidly dissolved, a microbrush should be used to expedite the dissolution of the smear layer. In addition, these bonding agents are hydrophilic, which permits water to be absorbed from layers beneath the hybrid layer.4
Selection of a bonding agent is dependent on the type of restoration and upon the cementing medium if an indirect restoration is being placed. Some bonding agents, such as Prime & Bond® NT ™ Dual Cure (DENTSPLY Caulk, Milford DE), provide an activator to expand the uses of the material for indirect restorations.
Direct Composites
Composite-resin restorative materials have been used since 1964, when they were first introduced by 3M.7 A composite consists of four basic components bonded together: a resin (organic polymer matrix), filler (inorganic) particles, a coupling agent (silane), and an initiator of polymerization. Composites have often been classified by their filler particle size. A relatively new addition is the nanohybrid. Nanohybrids contain nanometer-sized filler particles (.005 µm to .01 µm) along with conventional filler particles in the resin matrix. These materials can be used for both anterior and posterior restorations because they have improved polishability and handling similar to microfills and, in addition, have the strength and wear resistance of a hybrid. Filtek™ Supreme Plus (3M ESPE), Herculite® Ultra (Kerr Corporation), and Premise™ (Kerr Corporation) are examples. These materials also provide varying degrees of opacity, translucency, fluorescence, and sculptability; in addition, they are non-sticky, do not slump, and are naturally shaded to allow the tooth to be restored by replacing the dentin and enamel in layers corresponding to natural tooth structure.
Direct posterior composites can provide excellent long-term service for small- to medium-sized restorations. The ideal posterior composite should possess the following attributes: high resistance to wear and marginal breakdown; polishability; esthetics; highly filled for strength, stiffness and toughness; sculptability; low sorption of water or solvents to limit staining; non-toxic and non-allergenic; low shrinkage for stress release; antibacterial; curable to any depth with one light application; and self-adhesive. Some of the newer composites have some of these features incorporated in their chemical composition.8 Many improvements have been made to composites usually by optimizing the fillers. Most composites contain dimethacrylates TEGDMA, bis-GMA, or UDMA. However, the main problem with these materials has been their polymerization shrinkage. Shrinkage will result in stress at the bonded interface with tooth structure. High bond strengths are needed to counteract these shrinkage and polymerization stresses. When exposed to a curing light, each resin’s molecules move toward each other and, as this polymer network forms, significant volume contraction occurs.7
Previously, most manufacturers would try to increase the filler load and this would result in less shrinkage because the resin is decreased. However, several manufacturers recently changed the resin. 3M ESPE developed Filtek™ LS Low Shrink Posterior Restorative material that is based on silorane chemistry with no methacrylates. The silorane ring opening monomers result in less polymerization shrinkage. Traditional methacrylates polymerize by means of a radical addition reaction of their double bonds and subsequent polymerization shrinkage.4 The initiator system in Filtek LS requires a curing time of 20 seconds and higher-intensity lights cannot be used to decrease the curing time. A special two-step, self-etch adhesive had to be developed to bridge between the hydrophobic silorane material and the hydrophobic tooth. This adhesive must be used with the Filtek LS restorative. Current adhesives on the market will not work with this restorative material. Polymerization stress is reduced and the stress remains reduced and does not build up, unlike conventional methacrylate-based materials.9 Septodont USA (New Castle, DE) introduced N’Durance®, which uses nano-dimer technology to create a composite that, according to the manufacturer, has a high degree of conversion and low volumetric shrinkage, high compressive strength, and a low modulus of elasticity to resist wear.
Indirect Composite Restoration
Indirect composite materials provide the ability to refine margins, achieve properly contoured contacts, develop a highly esthetic restoration, and reduce wear compared to conventionally placed composite. Some of these indirect composites are processed under pressure, temperature, and nitrogen to reduce air pockets in the resin and increase polymerization when light-cured. Examples of these systems include Premise™ Indirect (Kerr Corporation), Cristobal® (DENTSPLY/Ceramco, Burlington, NJ), and Tescera™ (Bisco, Schaumburg, IL). These materials can be used for inlays, onlays, full crowns, and, sometimes, bridges with or without metal or fiber support. They are more easily repaired intraorally and are kinder to the opposing dentition compared to porcelain, but they do wear more.10
Resin Cements
Cements are a critical link between tooth structure and the restoration. There has been modification of the preference of clinicians toward resin-based cements (eg, resin-modified glass ionomers RelyX™ Luting Plus Cement [3M ESPE] or FujiCEM Automix [GC America, Alsip, IL] and self-etch resin cements, Maxcem Elite™ [Kerr Corporation] and RelyX™ Unicem [3M ESPE]). The resin-modified glass ionomers are good for metal alloy substructures (eg, porcelain-fused-to-metal). For all-ceramic and composite restorations, resin cements are preferred as they are able to transfer stress more efficiently. Except for the self-etch resin cements (universal cements) all resin cements require an adhesive agent.11 These adhesives are often of two basic types: the total-etch adhesives or self-etch adhesives. The problem that may occur with the simplified adhesives is the reaction between the acidic groups in the uncured layer competing with the aromatic tertiary amines of the luting agent (according to the manufacturer [Kerr] NX3 is amine free to prevent this problem and in addition makes the material more color stable). This occurs in the presence of oxygen. This limits the co-polymerization between the two materials and, because for resin cements the adhesive system is hydrophilic in nature, it acts as a permeable membrane.12 The presence of water at the interface between adhesive and cement may compromise the bonded surface and be deleterious to proper polymerization. In an attempt to partially overcome this problem, it has been suggested to place hydrophobic, non-acidic, low-viscosity resin between the adhesive and the composite resin cement.13 All-Bond SE Liner is radiopaque and according to the manufacturer decreases water permeability when placed over All-Bond SE (Bisco, Schaumburg, IL) The use of a simplified adhesive with dual-cured cement allows transudation of water from the dentin and may result in the restoration not remaining in position.14-16 According to the manufacturer (Kerr) the addition of a proprietary redox initiator system makes Maxcem Elite and NX3 acid tolerant and enabling them to be cured in the presence of acidic monomers such as in Optibond All-In-One.
Cementing indirect restorations has become very confusing for many clinicians. In order to achieve the optimal result with an indirect restoration careful consideration has to be given to the best luting material. There are many cases where a variety of material choices would provide adequate performance. However, many times specific materials are needed. This may also affect the selection of the restoration. If an area can not be appropriately isolated to prevent moisture contamination, resin cements may be compromised. A cement that is not as sensitive to moisture such as glass ionomer, resin-modified or self etch resin cements would be preferred. Restorations that require resin cement such as porcelain veneers could not be used. Metal or PFM restorations can be cemented with conventional cements such as glass ionomer or self-cured cements and self-adhesive cements. Zirconia and Alumina restorations can be cemented with dual-cured cements (margins can be light-cured) and self-adhesive cements. Feldspathic porcelain restorations a light-cured or dual-cure cement should be used. For composite restorations light-cured or dual-cured cement can be used. A self-cure cement is recommended for metal posts and dual-cured cements can be use with fiber posts. Self-adhesive cements can be used with both fiber and metal posts.
Impression Material
An ideal impression material should demonstrate certain characteristics and behavior in the clinician’s office and in the laboratory: It should create an accurate impression of hard and soft tissue, be dimensionally stable, set within a realistic time period, be biocompatible, be cost effective, should not have to be poured immediately if time does not allow in order to remain dimensionally stable, should allow multiple pours, remain accurate after disinfection, have a long shelf life and a pleasant odor and taste, and the color should clearly delineate the margins.16
Impression materials currently being used include hydrocolloids, polyvinyl siloxanes (PVS), and polyethers. Impression materials also can be separated according to their viscosity.17 A low-viscosity material is used to capture excellent detail of the preparation and soft tissue. A tapered tip is attached to an automixing syringe to allow accurate placement of the material around the preparation and in the sulcus. A more viscous material is used to thrust the light body in close proximity to the area being impressed. Monophase materials can be used alone or together with a high-viscosity or putty material.
The materials also can be separated according to their hydrophilic vs hydrophobic character.18 One of the limitations of PVS is its hydrophobic nature. PVS is hydrophobic because of the hydrophobic aliphatic hydrocarbons groups around the siloxane bond. The presence of water will create voids or pitted surfaces on the impressions. The incorporation of non-ionic surfactants enhances the hydrophilization. The unpolymerized material has to be able to wet intraoral tissues and the wettablity of the polymerized impression is important in model fabrication with dental gypsum The surfactants create a diffusion transfer of surfactant molecules from the PVS into the aqueous phase that decreases surface tension and increases wettability. Moisture control is extremely important with PVS. The hydrophilic nature of polyethers makes them more tolerant to moisture.18
Polyether impression materials are recommended when the area to be impressed can not be kept dry due to moisture or blood. However, for both materials the best surface details are under dry conditions. PVS materials have wide range of set times. If only several teeth are involved with no moisture problems, a fast-set PVS material would be ideal. The wider range of consistencies available with PVS materials allows easier removal in taking full mouth impressions and if the teeth are periodontally involved. A medium and light-body material can be used.
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Conclusion
The wealth of new materials that have been introduced in the last decade have provided both advantages and disadvantages in restoring the dentition to proper form and function. A basic understanding of the restorative materials available is a prerequisite to their proper selection and placement. These new materials and technique permit the fabrication of restorations that are both esthetic and highly functional. Many of these restorations can be placed with only relatively minor modification of tooth structure.
Acknowledgment
The author wishes to thank Americus Dental Laboratory (especially technician Sandra Marin CDT, MDT) for fabrication of the belleGlass onlay and photographs.
References
1. Stangel I, Ellis TH, Sacher E. Adhesion to tooth structure mediated by contemporary bonding systems. Den Clin N Am. 2007;51:677-694.
2. Breschi L, Mazzoni A, Ruggeri A, et al. Dental adhesion: Aging and stability of the bonded interface. Dent Mater. 2008;24:90-101.
3. DeMunck J, Van Landuyt K, Peumans M, et al. A critical review of the durability of adhesion to tooth tissue: methods and results. J Dent Res. 2005;84:118-132.
4. Bayne SC. Dental biomaterials: Where are we and where are we going? J Dent Educ. 2005;69:571-585.
5. Proença, JP Polido M, Osorio E, Erhardt MCG. Dentin regional bond strength of self-etch and total-etch adhesive systems. Dent Mater. 2007:23:1542-1548.
6. Erickson RL, Barkmeir WW, Kimmes NS. Fatigue of enamel bonds with self-etch adhesives. Dent Mater. 2009;25:716-720.
7. Data on file. Filtek™LS and LS System Adhesive; 3M ESPE Technical Product Profile.
8. Puckett AD, Fitchie JG, Kirk PC, et al. Direct composite restorative materials. Dent Clin N Am. 2007;51:659-675.
9. Ernst CP, Meyer GR, Klöcker K, Willershausen B. Determination of polymerization shrinkage stress by means of photoelastic investigation. Dent Mater. 2004;20:313-321.
10. Brucia JJ. Materials and techniques for achieving excellence with indirect composite restorations. Dent Clin North Am. 2001;45:71-81.
11. Pergoraro TA, da Silva NRFA, Carvalho RM. Cements for use in esthetic dentistry. Dent Clin N Am. 2007:453-471.
12. Tay FR, Pashley DH, Suh BI, et al. Single-step adhesives are permeable membranes. J Dent. 2002;30:371-382.
13. King NM, Tay FR, Pashley DH, et al. Conversion of one-step to two-step self etch adhesives for improved efficacy and extended application. Am J Dent. 2005;18:126-134.
14. Sanares AME, Itthagarun A, King NM, et al. Adverse interactions between one-bottle light cured adhesives and chemical-cured composites. Dent Mater. 2001;17-542-556.
15. Cheong C. King NM. Pashley DH, et al. Incompatibility of self-etch adhesives with chemical/dual-cured composites: two-step vs one-step systems. Oper Dent. 2003;28:747-755.
16. Suh BI, Feng L, Pashley DH, et al. Factors contributing to the incompatibility between simplified-step adhesives and chemically cured or dual-cured composites. Part III. Effect of acidic resin monomers. J Adhes Dent. 2003;5;267-282.
17. Burgess JO. Impression material basics. Inside Dentistry. 2005;1(1):30-33.
18. Rubell BS. Impression materials: A comparative review of impression materials most commonly used in restorative dentistry. Dent Clin N Am. 2007;51:629-642.
19. Sadowsky SD. An overview of treatment considerations for esthetic restorations: A review of the literature J Prosthet Dent. 2006;96:433-442.
20. Dietschi D, Spreafico R. Adhesive Metal-Free Restorations. Berlin, Germany: Quintessence Publishing Co; 1997.
21. Garber DA, Goldstein RE. Porcelain and Composite Inlays and Onlays. Carol Stream, Ill: Quintessence Publishing Co; 1994.
22. Roulet J-F, Degrange M. Inlay restorations. J Calif Dent Assoc. 1996;24:48-62.
23. Touati B, Miara P, Nathanson D. Esthetic Dentistry and Ceramic Restorations. 1st ed. London: Martin Dunitz; 1999.
24. Magne P, So WS, Cascione D. Immediate dentin sealing supports delayed restoration placement. J Prosthet Dent. 2007;98:166-174.
25. Stavridakis MM, Krejci I, Magne P. Immediate dentin sealing of onlay preparations: thickness of pre-cured dentin bonding agent and effect of surface cleaning. Oper Dent. 2005;30(6):747-757.
26. Magne P, Kim TH, Cascione D, Donovan TE. Immediate dentin sealing improves bond strength of indirect restorations. J Prosthet Dent. 2005;94:511-519.
About the Author
Associate Professor and Associate Director of International Aesthetics
New York University College of Dentistry
New York, New York
Private Practice
Staten Island, New York