James F. Simon, DDS, MEd, and Waldemar G. de Rijk, BA, MS, PhD, DDS, FADM
The indirect restoration (crown,inlay/onlay, veneer, or fixed partial denture) has been accepted by the dentist as well as the patient for fit, esthetics, and occlusion. Once an indirect restoration is decided upon, which material to use for joining the restoration to the dentition must be selected. This decision is based on consideration of the material from which the restoration is made, the surface it will be placed against, and the characteristics of the joining material. Recent developments have produced a plethora of available cements, making the selection of the correct cement a complicated task. In this brief review, the major classes of cements will be identified as well as what can be expected of these products in clinical performance.
The terms cementing, luting, and bonding are often used as interchangeable terms; however, they each have very specific meanings. The term “luting” refers to a mechanism in which no chemical or physical interaction occurs between the objects to be connected (ie, there is no adhesive interaction between the surfaces). Because micromechanical locking is the mechanism by which the parts are held together, the luting agent depends heavily on the retentive form of the restoration to provide long-term success. Cementing is a generic term for a method of joining materials where the joining medium provides both some form of adhesion as well as micromechanical locking of the parts to be held together. “Bonding” is a specific term which implies that true adhesion of the joining material occurs to both surfaces that are to be connected. True adhesion occurs when the bonding material is in intimate contact with the surface and dipole interactions (van der Waals forces) provide the desired adhesion. Intimate contact means a thorough “wetting” of the surface by the adhesive material. The van der Waals forces are thought to provide the highest adhesive strengths with the most durable bond in adhesion technology. Chemical bonding through ionic forces (electrostatic forces) is believed to be less desirable, because the chemical reaction is usually reversible and subject to hydrolysis.
In dentistry, an extensive evolution has occurred from luting agents, to cements, to adhesive bonding systems. All of the materials used in the placement of indirect restoratives, however, are still generically referred to as “cements.” This review addresses each classification separately.
Dentin consists of both collagen (organic phase) and hydroxyapatite (the inorganic phase). Cements have been developed that bond to the organic phase of dentin via a dentin bonding system, as well as cements that bond to the inorganic phase. Because the bonding mechanisms are quite different for the two materials, they will be discussed separately.
Cements that bond to the organic phase of dentin with the aid of a dentin bonding system are referred to as the “resin cements” or the “composite resin cements.” These bonding systems rely on the formation of a hybrid layer with the collagen fibers of dentin. This process has been abundantly described in the dentin bonding literature.1-3 These resin cements achieve the highest bond strengths of all cementing media and have developed a very good clinical record. The physical properties of these cements fall under the International Organization for Standardization (ISO) standard No 4049, which establishes stringent requirements for these cements. When selecting a cement, demanding that the cement is in compliance with this standard is a good first step in the decision-making process.
Cements that bond to the inorganic phase of dentin are referred to as “cementing media,” and are materials that have some adhesive interaction with dentin (ie, low bond strength) and also act as luting agents.
The Luting Agents
Zinc Phosphate Cement
This cement has a long history of use in dentistry and continues to be used despite physical properties that are less than ideal. Zinc phosphate cements completely lack adhesion to tooth structure and exhibit a high degree of solubility, a low compressive strength (83 to 110 MPa), and a low ensile strength (4 to 5 MPa).4,5
This cement was developed in the late 1960s as an adhesive dental cement. The main advantages of this cement are the low pulpal irritation, some adhesion to tooth and alloys, easy manipulation, and low film thickness. The major disadvantages include low compressive strength (80 MPa),6 critical powder/liquid ratio, and the need for a clean surface for maximum adhesive potential; however, the resultant bond strengths are still low.7
Glass Ionomer Cements
Glass ionomer cements were developed to use the fluoride release of the glass ionomer to reduce the potential for recurrent caries around the restoration. They have a low coefficient of thermal expansion, which allows them to maintain a bond to tooth even though the bond is minimal. This cement takes time to develop maximum strength. Because of the low bond strength, these materials are classified as luting agents. These luting agents are all water based, and therefore display an inherent sensitivity to moisture. The corollary is that the cements need to be chemically active to be able to release fluoride. The release of clinically meaningful amounts of fluoride requires a chemically active (ie, degrading) cement. This is the reason for the swelling of glass ionomer cements after placement, leading to the possible fracture of an all-ceramic restoration. The continued use of these materials as luting agents is not recommended.
Resin-modified Ionomer Cements
The cements that bond to the inorganic phase of dentin are the resin-modified glass ionomers and the phosphate-modified resin cements. For these cements, the bond to dentin is obtained by a link to the calcium ion in dentin. Because calcium is a divalent ion, cross-linking of the acid chains occurs (Figure 1). For all of these cements, the link to calcium is through an acid-base reaction, which takes place in an aqueous environment. For this reason, the bond can be hydrolyzed and reversed in a moist environment.
These cements contain an acid-soluble glass, polyacid polymers (polyacrylic, itaconic, or maleic), and polymerizing dimethacrylates. The polyacid polymers react with the calcium in the glass filler and the dentin, while the dimethacrylates polymerize into a solid resin. This combines the advantages of a conventional glass ionomer and resin technology. They do exhibit some fluoride release, resistance to marginal leakage, some adhesion to enamel and dentin, some moisture resistance, and less solubility than a conventional glass ionomer. Materials in this category include GC Fuji Plus™/GC Fuji Plus™ Capsules (GC America, Inc, Alsip, IL), GC FujiCEM™ (GC America, Inc), RelyX™ Luting Plus Cement (3M™/ESPE™, St. Paul, MN), RelyX™ Luting Cement (3M™/ESPE™), PermaCem® Dual Smartmix™ (Zenith/ DMG, Foremost Dental, LLC, Englewood, NJ). These cements come in capsules, hand-mix, or automix delivery systems (Figure 2). They exhibit low film thickness (especially the FujiCem at 3µm)8 and good handling consistency. Powder and liquid delivery systems cannot always be dispensed in a constant ratio; therefore, the capsule, the paste pak, or the automix delivery systems are much better to ensure a cement with maximum properties. The GC America products come with a conditioner (Fuji Plus™ Conditioner), which can be used to increase the bond strength even though it is listed as an optional step in the literature from the manufacturer. It is of interest that at least one major producer of resin-modified glass ionomer cements has introduced a bonding resin cement that performs very well9 (LinkMax™, GC America, Inc).
The Phosphate-grafted Resin Cements
Recently, some dual-cured, resin-modified cement systems (RelyX™ Unicem, 3M™/ESPE™, and Maxcem™, Kerr Corporation, Orange, CA) (Figure 3) have been introduced which do not use a bonding system before placement. These systems are claimed to be self-etching, self-adhesive, all-in-one cements. These particular systems contain an acid-soluble glass, water, and a polymerizing dimethacrylate, which has phosphoric acid grafted onto the resin (Figure 4). The polymerizing can be described as a “polyphosphoric acid” similar in structure to the polyacrylic acids. The setting mechanism is similar to that of the resin-modified glass ionomers. After mixing the material, the phosphoric acid (in the presence of water) reacts with the filler particles and the dentin, forming a bond to calcium through an acid-base reaction. Simultaneously, the resin polymerizes into a cross-linked polymer. With the bond to the filler particles and to dentin via a salt bridge, the propensity for hydrolysis remains a significant risk for these materials. By using a hydrophobic dimethacrylate, the susceptibility to moisture is anticipated to be less than that of the resin-modified glass ionomers. The reported bond strength to dentin varies among researchers, but most report modest bond strengths.10,11 In our laboratories, we observed shear bond strengths of 4.8 to 7.0 MPa for these systems,9 with resin cements producing shear bond strengths of 15 to 28 MPa.9
Bonding Resin Cements
The bonding resin cements provide the practitioner with a full range of solutions for many clinical situations. The available systems range from very simple to relatively complex kits designed for esthetic restorations (Figure 5 and Figure 6). Specific coupling agents are available to bond to dentin, silica-based substrates, and noble metals. Without the specific bonding agents, the resin cements should be considered very strong cementing media, which have only modest bond strengths to dentin. The use of bonding agents and primers is therefore essential in obtaining optimal results.
The resin cements set by the additional polymerization of dimethacrylates, such as BIS-GMA, UDMA, and TEGDMA, in different ratios to achieve the desired viscosity, flow, and film thickness. The polymerization process is initiated by light, chemicals (self-cured), or a combination of the two (dual-cured). The light-cured cements produce the highest bond strengths, are the most stable (as seen by their long shelf life), and obtain the highest degree of conversion. Because of their chemical complexity, the dual-cured systems achieve lower degrees of conversion and have the shortest shelf life. In those cases whenlittle or no light cure can be achieved, a self-cure resin is a better choice than a dual-cured resin. This is reflected in the fact that for many of the cements intended for esthetic veneers, the manufacturers make the material available primarily as a light-cured material, with the option of adding a dual-cure catalyst only if itis really needed.
Because all resin cements use dimethacrylates, the materials polymerize into densely cross-linked polymers, which are highly resistant to moisture absorption and have a high durability. Resin cements are similar to composite restorative materials, but with lower concentrations of filler particles (50% to 70% by weight with glass or silica).12 The three different curing methods, which define their handling characteristics and potential uses, are:
1. Self-cure resin cements. These cements do not react to light because they cure by a chemical reaction, which requires the mixing of two materials. These cements are useful in areas where light does not penetrate, such as for metal or metal/ceramic restorations, endodontic posts, and ceramic restorations with a thickness that places the tip of the light curing unit >3 mm from the cement, resulting in a lack of adequate light penetration.13 Cements in this group include Panavia™ 21 (Kuraray America, Inc, New York, NY), MultiLink® (Ivoclar Vivadent, Inc, Amherst, NY), C&B Metabond® (Parkell, Inc, Farmingdale, NY), and C&B™ Cement (Bisco, Inc, Schaumburg, IL) (Figure 7).
2. Light-cure resin cements. These cements require that the beam of the light must reach every part of the cement. The photoinitiators must have light to activate them or the cement will not be completely activated, which could lead to eventual failure of the restoration. Compliance with ISO standard 4049 provides a depth of cure of at least 1.5 mm and high-intensity curing lights will enhance the depth of cure, making these cements well suited for esthetic restorations. Cements in this group include Variolink® Veneer (Ivoclar Vivadent, Inc) or the base portion of most of the light-cured/dual cured cements such as Variolink® II (Ivoclar Vivadent, Inc), Calibra® (Dentsply Caulk, Milford, DE), Nexus®2 (Kerr Corporation, Orange, CA), Illusion™ (Bisco, Inc), or Insure® (Cosmedent, Inc, Chicago, IL) (Figure 8). These cements can be used for metal-free restorations <1.5-mm thick, metal-free orthodontic retainers, or metal-free periodontal splints.
3. Dual-cure resin cements. These cements have the capacity to cure with or without activation from a curing light. There are enough self-cure initiators to cure the cement with the addition of the curing light to help in the process and to seal the margins. Dual-cure-only cements within this group include Panavia™ F 2.0 (Kuraray America, Inc), Bistite II DC (J. Morita USA, Inc, Irvine, CA), Duo-link™ (Bisco, Inc), and RelyX™ ARC (3M™/ESPE™). Light-cured/dual-cured cements include VariolinkII, Insure, and Nexus 2. These cements can be used for any metal-free restoration in which there is some question as to the ability of complete light penetration from the curing light and to seal the margins quickly.
High-strength ceramic restorations with zirconia or alumina cores are not typically etchable and therefore cannot be bonded. All-ceramic restorations are etchable with hydrofluoric acid and should always be bonded with silane primer, bonding agent, and resin cement.14
The physical properties required for the resin cements are given in the ISO standard 4049. This standard requires a minimum transverse flexural strengthof 50 MPa, a radiopacity equivalent toaluminum, and water sorption no greater than 40 µg/mm.3 At this time, no other cement can pass the requirements imposed on the resin cements. There is evidence from clinical studies that crowns luted with resin cement and a placement procedure incorporating a dentin bonding stage have enhanced rates of survival.15
Some conclusions can be set forth from the information on cements. If the restoration has been designed with conservation of tooth structure in mind and retention is to be derived from the adhesive properties of the cement, then:
- For optimal bond strength, a resin cement in conjunction with a dentin bonding agent will provide the highest retention.
- The best choices for resin cements are (in this specific order):
- Light-cured cements (if clinically feasible);
- Self-cured cements;
- Dual-cured cements.
An ideal luting material should satisfy many requirements, including a good seal, a durable bond to both the tooth andthe restoration, sufficient mechanical strength, low viscosity, resistance against disintegration, tissue compatibility, and ease of handling.16 It is important that any decision to switch to a new cement be based on scientific evidence and a thorough understanding of the chemical and physical mechanisms of the cement. This must be followed by a study of the manufacturers’ recommendations for use and following the steps as published by the manufacturer because new products will often require procedures that are unfamiliar to the practitioner. These new cements are technique sensitive and incorrect application of the cement may lead to failed restorations and unhappy patients.
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About the Authors
James F. Simon, DDS, MEd
Director, Division of Clinical Research, Director, Division of Esthetic Dentistry
University of Tennessee
College of Dentistry
Department of Restorative Dentistry
Waldemar G. de Rijk, BA, MS,PhD, DDS, FADM
Director, Division of Biomaterials
University of Tennessee
College of Dentistry
Department of Restorative Dentistry