Dental cements are designed to retain restorations, prefabricated or cast posts and cores, and appliances in a stable, and long-lasting position in the oral environment. Resin-based cements were developed to overcome drawbacks of nonresinous materials, including low strength, high solubility, and opacity. Successful cementation of esthetic restorations depends on appropriate treatment to the tooth substrate and intaglio surface of the restoration, which in turn, depends on the ceramic characteristics. A reliable resin cementation procedure can only be achieved if the operator is aware of the mechanisms involved to perform the cementation and material properties. This article addresses current knowledge of resin cementation concepts, exploring the bonding mechanisms that influence long-term clinical success of all-ceramic systems.
Dental luting cements are designed to retain restorations, appliances, and post and cores in a stable and, presumably, long-lasting position in the oral environment. Retention mechanisms are reported to be chemical, mechanical (friction), and micromechanical (hybridized tissue), but they are usually a combination of two or three mechanisms, depending on the nature of the cement and of the substrate. Among dental luting cements commercially available, there are resin-based and nonresin-based cements. They must have for their acceptable clinical performance adequate resistance to dissolution, strong bond through mechanical interlocking and adhesion, high strength under tension, good manipulation properties, and also be biologically compatible with the substrate.
Traditionally, zinc phosphate cement has been regarded as the most popular material, despite its disadvantages as low hardness, solubility and lack of adhesion. Although the final marginal accuracy of all-ceramic systems processes (computer-aided design [CAD]/computer-aided manufacturing [CAM] especially) have significantly improved, these still result in larger internal gaps than cast crowns, possibly resulting in thicker cement layers and poorer frictional retention to the abutments. These would be critical challenges for zinc phosphate cement because of its solubility and lack of adhesiveness. Although zinc–phosphate has been indicated for some all-ceramic systems (ca In-Ceram [VITA Zahnfabrik H. Rauter GmbH & Co KG, Bad Säckingen, Germany], Procera [Nobel Biocare, Switzerland], Cercon [DeguDent, Hanau, Germany], Lava [3M ESPE, St Paul, MN, USA]), long-term clinical data are yet to be published.
Glass ionomer cements are also of great interest for clinicians. These cements exhibit several clinical advantages, including physicochemical bonding to tooth structures, long-term fluoride release, and low coefficients of thermal expansion. However, their low mechanical strength compromises their use in high-stress-bearing areas. Glass ionomer cements can be used to cement core-reinforced ceramics such as those also allowed to be cemented with zinc phosphate. However, glass ionomer cements would not suit well for ceramics requiring support from the cement. Therefore resin-based cements have become popular, because they have the potential to address the disadvantages of solubility, support, and lack of adhesion described for previous materials.
Restorative dentistry constantly undergoes changes, and no currently available cement is ideal for all situations. The advent of adhesive luting cements has considerably expanded the scope of fixed prosthodontics. This article will focus on resin-based cements as luting agent for all-ceramic systems, due to the important role the cement plays in the final clinical success of such treatment modality.
Resin cements and adhesive interfaces
Resin cements have become popular clinically because of their ability to bond both the tooth structure and restoration. Bonded indirect restorations constitute a substantial part of contemporary dentistry. The successful use of resin cements depends on several aspects related to the bonding mechanisms to both dental tissues and restorations. The scientific knowledge of the materials currently available as well as the acknowledgment of their limitations and indications are key factors for durable restorations. Several new luting cements and ceramic systems have been introduced in the last few years, and their chemistry and structural characteristics are fundamental to produce an optimal and reliable bonding to both tooth and restoration interfaces.
Resin Cements: Chemistry and Properties
Table 1 summarizes various resin-based cements with their respective characteristics and chemical compositions.
|Bistite II DC||Tokuyama (Tokyo, Japan)||Primer 1A+1B/primer 2||Paste/paste||Dual||Silica-zirconia (77% weight) filler, dimethacrylate, MAC-10 (adhesive promoter), initiator|
|Calibra||Dentsply/Caulk (Milford, DE, USA)||Prime and bond NT||Paste/paste||Dual||Base: barium boron fluoralumino silicate glass, bis-phenol A diglycidylmethacrylate, polymerizable dimethacrylate resin, hydrophobic amorphopus silica, titanium dioxide, di-camphoroquinon
Catalyst: barium boron fluoralumino silicate glass, bis-phenol A diglycidylmethacrylate, polymerizable dimethacrylate resin, hydrophobic amorphopus silica, titanium dioxide, benzoyl peroxide
|C&B Cement||Bisco (Schaumburg, IL, USA)||All-bond 2 one step||Paste/paste dual syringe||Self||Base: Bis-GMA, ethoxylated bis-gma, trietheleneglycol dimethacrylate, fused silica, glass filler, sodium fluoride
Catalyst: Bis-GMA, trietheleneglycol dimethacrylate, fused silica
|Choice 2||Bisco||All-bond/one step||Paste||Light||Strontium glass, amorphous silica, Bis-GMA|
|Duo-Link||Bisco||All-bond 2 one step/plus||Paste/paste dual syringe; automix||Dual||Base: Bis-GMA, triethyleneglycol dimethacrylate, urethane dimethacrylate, glass filler Catalyst: Bis-GMA, triethyleneglycol dimethacrylate, glass filler|
|BisCem||Bisco||Self-adhesive||Paste/paste dual syringe; automix||Dual||Bis (hydroxyethyl methacrylate) phosphate (base), tetraethylene glycol dimethacrylate, dental glass|
|NX 3 Nexus||Kerr (Washington, DC, USA)||Optibond solo plus optibond all-in-one||Paste/paste dual syringe; automix||Dual||Uncured methacrylate ester monomers, nonhazardous inert mineral fillers, nonhazardous activators and stabilizers, and radiopaque agent|
|Maxcem||Kerr||Self-adhesive||Paste/paste dual syringe; automix||Dual||GPDM (glycerol dimethacrylate dihydrogen phosphate), comonomers (mono-, di-, and tri-functional methacrylate monomers), stabilizer, barium glass fillers, fluoroaluminosilicate glass filler, fumed silica (filer load 67% weight, particle size 3.6 μm|
|Super Bond C&B||Sun Medical (Grand Prairie, TX, USA)||Monomer/catalyst V/polymer powder||4 drops of monomer/1 drop of catalyst s/1 small cup of standard measuring spoon||Self||MMA (methyl methacrylate), 4-methacryloxyethyl trimellitate anhydride (4-META), tri-nbutylborane (TBB- catalyst)|
|Clearfil Esthetic Cement||Kuraray (Tokyo, Japan)||Self-etch DC bond system||Paste/paste dual syringe; automix||Dual||Paste A: bis-phenol A diglycidylmethacrylate, TEGMA, methacrylate monomers, silanated glass filler, colloidal silica
Paste B: bis-phenol A diglycidylmethacrylate, TEGMA, methacrylate monomers, silanated glass filler, silanated silica, colloidal silica, benzoyl peroxide, CQ, pigments
|Panavia F 2.0||Kuraray||ED primer||Paste/paste||Dual||Paste A: 10-methacryloyloxydecyl, dihydrogen phosphate, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, hydrophilic aliphatic dimethacrylate, silanated silica filler, silanated colloidal silica, dl-camphorquinone, catalysts, initiators
Paste B: hydrophobic aromatic dimethacrylate, hydrophobic aliphatic, dimethacrylate, hydrophilic aliphatic dimethacrylate, silanated barium glass filler, catalysts, accelerators, pigments
|Breeze||Pentron Clinical Tech (Orange, CA, USA)||Self-adhesive||Paste/paste dual syringe; automix||Dual||Mixture of bis-GMA, UDMA, TEG-DMA, HEMA, and 4-MET resins, silane-treated bariumborosilicate glasses, silica initiators, stabilizers and UV absorber, organic and/or inorganic pigments, opacifiers|
|GCem||GC (Tokyo, Japan)||Self-adhesive||Capsules||Dual||Powder: fluoroaminosilicate glass, initiator, pigment
Liquid: 4-MET, phosphoric acid ester monomer, water, UDMA, dimethacrylate, silica powder, initiator, stabilizer
|Embrace WetBond||Pulpdent (Watertown, MA, USA)||Self-adhesive||Standard syringe or automix||Dual||Uncured acrylate resins, amorphous silica, sodium fluride|
|MonoCem||Shofu Dental (San Marcos, CA, USA)||Self-adhesive||Paste/paste dual syringe; automix||Dual||Not available|
|Multilink Sprint||Ivolcar/Vivadent (Schaan, Principality of Liechtenstein)||Self-adhesive||Paste/paste dual syringe; automix||Dual||Dimethacrylates and acidic monomers, barium glass, ytterbium trifluoride, silicon dioxide, mean particle size is 5 μm, total volume of inorganic fillers is ∼48%|
|Multilink||Ivoclar/Vivadent||Primer A + B||Paste/paste dual syringe; automix||Self||Dimethacrylate and HEMA, barium glass filler, silicon dioxide filler, ytterbium trifluoride, catalysts and stabilizers, pigments|
|Variolink II||Ivoclar/Vivadent||Excite adhesive||Paste/paste||Dual||Paste of dimethacrylates, inorganic fillers, ytterbiumtrifluoride, initiators, stabilizers and pigments; (Bis-GMA, triethylene glycoldimethacrylate, urethanedimethacrylate, benzoyl peroxide)|
|Rely X ARC||3M ESPE (St Paul, MN, USA)||Adper single bond||Paste/paste; clicker||Dual||Bisphenol-A-diglycidylether dimethacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA), polymer, zirconia/silica filler, filler loading approximately 67.5% by weight, particle size for the filler is 1.5μm.|
|Rely X Unicem||3M ESPE||Self-adhesive||Capsules (Aplicap: 0.01 mL; Maxicap: 0.36 mL)||Dual||Powder: glass fillers, silica, calcium hydroxide, self-curing initiators, pigments, light-curing initiators (filler load 72% weight, particle size <9.5 μm); Liquid: methacrylated phosphoric esters, dimethacrylates, acetate, stabilizers, self-curing initiators|
|Rely X Unicem||3M ESPE||Self-adhesive||Paste/paste; Clicker||Dual||Glass powder, methacrylated phosphoric acid esters, triethylene glycol dimethacrylate (TEG-DMA), silane treated silica, sodium persulfate|
Resin-based luting cements were primarily based on acrylic resin chemistry. Their subsequent development, however, has been based on the chemistry of resin composites and adhesives. Currently, several new resin cements are available in the market. These are offered in a wide variety of bonding mechanisms, curing modes, indications, and features (see Table 1 ). The choice of a particular resin cement requires understanding of the material’s characteristics as well as how it performs individually and integrated in the restorative system. There are two main categories of resin luting cements: the conventional resin luting cements, which have no inherent adhesion to tooth structure and require a bonding agent, and the self-adhesive resin cements, which do not require a separate bonding treatment to the dental substrate.
Conventional resin luting cements
Since the 1970s, resin cements have been formulated based on dimethacrylate resin chemistry as two-paste systems, which are easy to mix and cure at room temperature. Their bonding to tooth structure relies on the use of etch-and-rinse or self-etch adhesives. Composition is usually a mixture of dimethacrylate monomers, inorganic fillers (60% to 70% by weight), and initiator. Silica or high molecular weight oligomers may also be added to modify the rheological properties and achieve optimum handling characteristics. Examples of their clinical applications include metal-based crowns and bridges, zirconia and alumina framed crowns, fiber posts, and cast metal post and core. Because of the importance of light- and dual-curing systems, a separate topic was created to address their clinical application and polymerization mechanisms.
Curing protocols and its clinical relevance
Conventional resin cements can be exclusively light-cured. When comparing cements, light-cure products offer the clinical advantages of extended working time, setting on demand, and improved color stability. However, the use of light-cure only cements is limited to situations such as cementing veneers or shallow inlays, where the thickness and color of the restoration do not affect the ability of the curing light to polymerize the cement.
Dual-cured resin cements are often provided in two-paste systems (see Table 1 ). Dual-cure resin cements are indicated when delivering restorations where material characteristics may inhibit sufficient light energy from being transmitted to the cement. In these situations, light intensity reaching the cement may be sufficient to trigger the light-activated polymerization process, but a self-polymerized catalyst is needed to ensure maximal cure. The delivery method usually involves mixing of paste to paste (see Table 1 ). One of the pastes contains a reducing amine and a photo initiator. The other paste contains peroxide, usually benzoyl peroxide. In an interesting variation of the initiator system, the anaerobic cements begin polymerization only when the ambient oxygen supply is cut off after placement of the prosthetic device. This feature provides extended working and setting times and offers easy removal of excess materials. Although the dual-curing concept seems to be attractive, several issues have been brought up in the dental literature regarding its performance, and these concerns are going to be addressed throughout the text.
Little has been published on the light-curing potential of dual-cure cements. While earlier research suggests that auto-cure alone is not sufficient to achieve maximum cement hardening, recent literature indicates that the curing kinetics of dual-cure resin cements is more complex than previously thought. Studies indicate that light-activating some dual-cure cements appears to interfere with the self-cure mechanism and restrict the cement from achieving its maximum mechanical properties.
Some dual-cure cements show their self-curing mechanism to be somehow limited when immediately light-activated in the dual-cure mode. This limitation may compromise the final mechanical properties of the resin cements. This information is of great importance for the clinical practice, since light activation is always recommended by the manufacturer. It remains to be demonstrated whether the same phenomenon occurs with all resin-based cement systems. While such information is not available, it is advisable to delay the light-curing procedure of dual-cure cements to the maximum time clinically possible. In this way, self-curing progress will be further along, to a point when light activation no longer interferes with the self-curing kinetics. The ideal time frame between mixing and the light-activation has not yet been determined, but some studies have shown that light-curing 5 to 10 minutes after mixing does not seem to interfere with final cure and properties, at least for most of the cements evaluated.
It is interesting to note that there is no direct correlation between alterations in the degree of conversion (DC) caused by different curing modes and changes in the mechanical properties of the resin cements. The lack of linear correlation between DC, properties, and density of crosslink in the polymer has also been reported elsewhere. However, cements that do not cure properly with light activation or have a compromised self-cure reaction may experience adverse chemical reactions and permeability issues when associated with simplified adhesive systems. This clinically implies that the longer the resin cement takes to set, the greater will be the chance of adverse effects when coupling resin cements to simplified adhesives. To overcome these problems, clinicians have been advised to use three-step etch and rinse or two-step self-etch adhesives. When using these systems, adverse chemical reaction and permeability are prevented by the nonacidic and relatively higher hydrophobic characteristics of the bonding resin that comprise the last step of the application of such systems. Clinical trials are necessary to demonstrate how these issues may affect the long-term performance of different combinations of adhesives and cements.
Concerns on mixing and working time of dual-cure cements
Resin-based cements are formulated to provide the handling characteristics required for particular applications. The setting mechanism of dual-cure resin cements is usually based on redox reaction of benzoyl peroxide with aromatic tertiary amines (represented by catalyst and base paste, respectively). At least one paste contains the light-sensitive compound (camphorquinone [CQ]) responsible for initiating the light-cure setting mechanism. After the pastes are mixed together, and until light is provided, the working time is controlled by the ratio between inhibitors of the self-curing reaction and the amount of peroxide and aromatic tertiary amines. Both inhibitors and peroxides are organic chemical compounds susceptible to degradation upon storage. Therefore, dual-cure resin cements have a limited shelf-life and the setting mechanism of those cements may fluctuate during that time, depending on the conditions of storage that might alter the reactive potential of such components. In vitro evidences indicate that both working time (WT) and setting time (ST) may be significantly altered upon storage, particularly if the storage temperature is far above the recommended (> 18°–22°C). In one study, some cements presented shortened WT/ST, while others presented extended WT/ST after the kits were purposely aged for 12 weeks at 37°C. This occurred because of the instability of the components during storage. Degradation of peroxide would extend the WT/ST, and degradation of inhibitors would shorten them. The implications of such changes on the mechanical properties of the resin cements are yet unclear. However, clinicians handling resin cements with shortened WT may experience some clinical difficulties when luting multiple crowns, for example. Conversely, increased adverse chemical reaction and permeability problems may be expected for resin cements with extended WT and ST used in combination with an acidic and permeable simplified adhesive system. This is mostly because the extended ST allows the uncured cement longer time in contact with the acidic adhesive, thus prolonging the adverse reaction.
Bonding mechanisms and incompatibility issues of dual-cure cements
Most adhesive systems used with resin-based cements are simplified adhesive systems, because of clinical trends for reduced steps during adhesive procedures. These simplified adhesives are basically of two types: the etch-and-rinse, single-bottle systems and the all-in-one self-etch adhesives. They are both somewhat acidic and hydrophilic in nature. During cementation, the acidic groups in the uncured layer of simplified adhesive agents (due to presence of oxygen) compete with peroxides for aromatic tertiary amines of the luting agent, resulting in an acid–base reaction between the adhesive and the resin cement. This reaction minimizes proper copolymerization between the two, and the longer the cement takes to cure, the more extent is the compromising effect. Additionally, the hydrophilic characteristics of such adhesive systems function as permeable membranes. This hydrophilic behavior permits the flux of water through the adhesive after polymerization. The presence of water at the interface between the adhesive and the cement compromises the total bonded area and proper polymerization of the cement. Water droplets may accumulate at the interface and then function as stress raisers, leading to failure of the adhesive–cement interface ( Fig. 1 ). This permeability problem could be partially solved by the application of an intermediate layer of a relatively more hydrophobic, nonacidic low viscosity resin separating the acidic layer of adhesive from the composite resin cement. The water that accumulates at the interface derives from the hydrated dentin underneath. The negative effect of such water permeation on the bond strength of resin cements to dentin could be confirmed in in vitro studies. Those studies demonstrated improved bond strengths when the teeth were purposely dehydrated in ascending ethanol series before bonded. As such dehydration of dentin is impossible to achieve in daily practice, clinicians are advised to use less permeable adhesive systems such as three-step etch and rinse or two-step self-etch when bonding self- or dual-cured resin cements to dentin. The major advantage of these systems is that they include a layer of a relatively more hydrophobic and nonacidic resin as the third or second step. This additional layer will not cause adverse reaction with the basic amines of the cement and will reduce the permeability of the adhesive layer. The incompatibility issue has brought up concerns for several clinical procedures. The worst clinical scenario would occur when luting posts using simplified adhesives associated with dual-cured resin cements (see Fig. 1 ). Proper bonding to the apical portion might be severely compromised by the adverse interactions between adhesive and luting composite due to a lack of light exposure. Without light activation, dual-cure resin cements will actually function as exclusively self-cure cements. In this mode, the cement will take longer to cure, and this allows more time for the adverse reaction and transudation of water from dentin to occur. A similar situation occurs when the cement takes longer to set because of alterations in the WT/ST caused by inadequate storage conditions, as described previously. Based on those limitations, some studies have suggested the development of a specific bonding system for this purpose. Recent studies have shown that the push-out resistance of posts luted with resin cements was similar, regardless of the use of adhesive systems to bond to root dentin. A more predictable, truly adhesive luting procedure can only be achieved when clinicians combine the use of resin cements with three-step etch and rinse or two-step self-etch bonding systems. Self-adhesive resin cements have been strongly recommended to lute posts and crowns as an alternative to conventional luting systems, avoiding incompatibility and permeability issues due to wrong combinations of adhesives and cements.