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Dental product manufacturers have provided the profession with an array of materials and techniques at an ever-increasing rate (Table). Once a restoration has returned from the laboratory or been removed from the in-office milling unit, the dentist decides which material will best attach the restoration to the patient’s tooth. This decision must be based on many factors, including the type of restoration material used, the degree of the preparation’s retention, the ability to isolate the area from the oral fluids, and the esthetic requirements of the patient.
Cementing techniques are strict but their nomenclature is confused. Different descriptors are used for similar techniques and similar cements. “Luting,” “cementing,” and “bonding” have specific meanings, yet dentists and manufacturers alike use these terms interchangeably (and perhaps indiscriminately). The first term, luting, describes a process of joining an indirect restoration to a tooth preparation with no chemical or physical interaction between either surface. Absent any adhesion, the luting agent relies on micromechanical retention to attach the restoration to the tooth.
Cementing is a rather broad and vague term. When we “cement,” we depend on micromechanical locking and some degree of adhesion to join the restoration to the preparation.
Bonding is a deceptively simple term. True adhesion occurs when the bonding material thoroughly wets the joined surfaces, allowing attractive forces between molecules to provide the adhesion. These forces provide the highest adhesive strengths and most durable bonds in adhesion technology. Chemical bonding through electrostatic or ionic forces is less desirable because the chemical reaction is more prone to debonding from hydrolysis.
Evidence proves the sometimes transitory and ever-increasing number of dental materials. The profession is enjoying the conflicting benefits of a hailstorm of new products. Rosenstiel et al published a list of resin cements in 1998.1 Fifteen of the 23 cements listed are no longer available. Others have undergone formula or name changes.
Dentists want simplicity, so for simplicity’s sake this article will use the most-popular term, cements, and primarily consider the latter of the three cements mentioned: bonded cements.
WHAT DO WE WANT IN A CEMENT?
1. Impermeable margins, adequate retention and resistance, and excellent esthetics
2. Manageable working time, short cure time
3. Easy clean up
4. Low cost, long shelf life
5. Accurate mixing (remember the “three hanging drops” used to measure the polycarboxylate liquid?
6. Simplified infection control (unit doses)
7. Ease of use or minimal technique sensitivity
8. Minimal post-cementation sensitivity
9. One component, one bottle
10. Ability to cement in presence of moisture
PAST
Dentistry has gone through numerous cements and techniques searching for this ideal cement. Zinc-phosphate cement was the primary cement used for many years. This required a retentive preparation because it had no bonding capacity. It was also difficult to mix properly. This cement served dentistry well for many years and is still used even though it has less than ideal physical properties.
Polycarboxylate cement was introduced in the late 1960s to overcome some of the problems associated with zinc-phosphate cement. It reduced sensitivity because of change in the acid; however, researchers at the University of Michigan reported that this cement exhibited shorter longevity in terms of retention.2
Glass-ionomer cement was then developed and marketed. It has the potential for a fluoride release that may reduce the potential for recurrent caries around the restoration. It has a low thermal coefficient of expansion and contraction, which allows it to maintain a bond. It does have the undesirable potential to adsorb water and swell, which is an undesirable characteristic under all-ceramic restorations.
PRESENT
An improvement on the glass-ionomer cements has been the introduction of the resin-modified ionomer cements (RMGI). This cement combines some of the characteristics of glass-ionomer and resin technology. RMGIs exhibit some fluoride release, resistance to marginal leakage, some adhesion to enamel and dentin, some moisture resistance, and less solubility in the oral environment compared with conventional glass-ionomer cements. The delivery systems ensure a cement mixture with maximum properties. RMGIs have become quite popular due to their easier handling characteristics.
Many of dentistry’s newest restorations require bonding. This is true because of the material (porcelain) of the restoration and the trend toward minimally invasive, non-retentive preparations. This has led to the introduction of the adhesive resin cements.
Adhesive resin systems are classified according to the etching strategy that is used. The systems have varying techniques, components, shelf lives, nomenclature, and handling characteristics. They also have their own indications.
Craig’s Restorative Dental Materials says adhesion should:(1) reduce secondary or recurrent decay, and (2) reduce the need for retention and its resulting excessive tooth preparation.3 Craig’s also reminds us that adhesion is difficult to achieve. Two complicating factors are moisture contamination and dentin’s complex composition. Proper adhesion requires removal of biofilm and debris from the preparation after removal of the temporary restoration. This is the most difficult factor to control, and is another important principle.
Contaminants increase the cement’s contact angle, limiting its wettability. Trapping microbial detritus under the restoration has its own obvious negative sequelae.
There are essentially three ways for a cement to cure: self-cure, light-cure, and dual-cure. The decision to use one over the other is dependent on the type of restoration that is being placed. Self-cure resin cements (eg, Panavia, Kuraray Noritake, www.kuraraynoritake.com; BisCem®, Bisco, www.bisco.com) do not react to light and require two materials (an acid and a base) to be mixed prior to use. These cements are useful in areas where the light does not penetrate, such as for metal or metal/ceramic restorations and endodontic posts. Ceramics thicker than 1.5 mm are not amenable to light-curing.4
Light-cure resin cements (eg, Variolink® Esthetic LC, Ivoclar Vivadent, www.ivoclarvivadent.com; RelyX™ Veneer, 3M, www.3m.com) require that the light must be able to reach all of the cement. If the light does not reach the photoinitiators in the cement, then eventual restoration failure will occur. Light-cure cement is used for thin all-ceramic restorations (veneers) and other restorations less than 1.5 mm thick.
Dual-cure resin cements (eg, Multilink®, Ivoclar Vivadent; Variolink Esthetic® DC, Ivoclar Vivadent; Calibra®, DENTSPLY Caulk, www.calibracement.com; NX3 Nexus™, KerrDental, www.kerrdental.com) have the ability to cure with or without light activation. These cements are used for thicker all-ceramic restorations. Dual-cure cements proved easier clean up as the marginal excess can be lightly tacked and removed.
Enamel and dentin bonding have allowed the adoption of more minimally invasive procedures. Tooth preparations today require far less preparation than in the days of luting cements. Low-retention preparations require the strong bond strengths that resin cement provides. Zidan and Ferguson found that the retentive values of adhesive resin at a 24º taper for crown preparations was 20% greater than conventional cements at a 6º taper.5 Another study found that some resin cements can create bond strengths to non-retentive preparations that are greater than the strength of ceramic material; however, these bonds strengths cannot be consistently achieved.6
The tooth must be prepared with a bonding agent prior to the placement of the resin cement. This usually consists of treatment with an etchant followed by the placement of a dentin primer. The etchant can yield varying degrees and depths of etch. Primers have different evaporation rates, drying patterns, and penetration characteristics. More confusion arises when we consider that “self-etch” eliminates the possibility of excessive dentin drying, yet the bond degrades faster. “Total-etch” requires especially judicious air-drying to prevent collapse of the dentin matrix. The addition of the new universal bonding agents has made it easier because they can work either with or without an etch; however, it is still recommended that the enamel be etched with phosphoric acid for 15 seconds before the use of these bonding systems (selective etching).
Before this etching and priming of the tooth takes place, it is important that the tooth structure is clean and ready for the placement of the etching material. There does not seem to be a consensus of opinion on the effect of temporary cement, especially those containing eugenol, on adhesive cementation. Galazi et al7 found that provisional cement containing eugenol had a negative influence on the tensile strength of full crowns luted with resin cements, and that frictional retention was not able to suppress this interference even after the teeth were cleaned with pumice stone and water and acid-etched for 15 seconds. They found that Dycal® cement (DENTSPLY International, www.dentsply.co.uk) presented the best results for tensile strength.
A new category of resin cements has been added: self-adhesive resin cements (SARCs) (eg, RelyX™ Unicem, 3M; Maxcem Elite™, KerrDental; Calibra® Universal, DENTSPLY Caulk; SpeedCEM®, Ivoclar Vivadent). These cements are based on filled polymers designed to adhere to tooth structure without the requirement of a separate adhesive or etchant.8 They have become popular because of their simplicity in handling. They do not need a separate bonding step; however, they nearly always show lower bond strengths to enamel. Pre-etching enamel before the use of Unicem showed a higher bond strength (35 MPa vs 19 MPa); however, a lower bond strength to dentin (5 MPa vs 15 MPa) occurred when the phosphoric acid was placed on dentin.9 A separate etching step on enamel should be beneficial for the long-term success of the self-etching cements.
The use of these self-adhesive resin cements should be done with some caution. Weiser and Behr reviewed the literature to evaluate the clinical success of these newer cements.10 They concluded that because of the low number of studies available, the clinical evidence of self-adhesive luting agents cannot be assessed in a sufficient manner. SARCs, which are dual-/self-cure, are generally not recommended for non-retentive, esthetic preparations because of their flexural bond strengths, which are relatively lower than those of light-cured materials.11
Resin cement systems usually require altering the crown’s internal surfaces for maximum bond strengths. All-ceramic restorations are etchable with hydrofluoric acid and should always be bonded with silane primer, bonding agent, and resin cement. This only works on materials containing silica, such as feldspathic porcelain, leucite-reinforced porcelain, and lithium-disilicate porcelains. The etching times vary depending on the material (60 seconds for feldspathic porcelain vs 20 seconds for lithium disilicate) and overetching can reduce the bond strength. Recently, a new product has been introduced (Monobond® Etch & Prime, Ivoclar Vivadent) which may have eliminated the need for separate use of hydrofluoric acid on certain materials.
Air abrasion, with small particles and low air pressure, can be used to prepare zirconia crowns for cementation. A zirconia-specific primer can be substituted. Air abrasion is performed using 50-µm 110-µm grain-sized aluminum trioxide powder under 0.2 MPa pressure from a distance of 10 mm to 20 mm for 13 to 20 seconds until a white opaque color appears.12 The traditional silane chemistry is not applicable to zirconia unless it is silicoated.13In recent years, manufacturers have developed several commercial zirconia primers (eg, Z-PRIME™ Plus, Bisco; Clearfil™ Ceramic Primer, Kuraray Dental, www.kuraraydental.com; and Monobond® Plus, Ivoclar Vivadent) that may help in achieving a strong, durable bond to zirconia. The addition of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) to the bonding system seems to help in establishing a more stable chemical bond to zirconia.14
It is also important that the internal surface of the crown is cleaned before cementation. During the try-in phase of the crown, it is common for the internal surface to be contaminated with saliva, blood, or temporary material. Rinsing with water is not sufficient to remove all of this contamination. A new product (Ivoclean, Ivoclar Vivadent) claims to be a “universal cleaning paste to effectively clean the bonding surfaces after intraoral try-in of restorations.”
FUTURE
The shelves are full of literature on dental resins. The dental adhesive model uses biologically inert materials to attach a tooth to a crown. One investigator, Imazato, called for a viable alternative to the adhesive model, one that would depart from existing technology.15 He hypothesized that cements could have unresearched, novel properties that could contribute to improved clinical successes.
New materials with responsive, interactive, or inductive properties might mitigate restorative failure due to cavosurface margin degradation. A cement that could seal and reseal the margins with apatite would offer increased safety against secondary caries.
Thus, we have a new class of dental cement, the hybrid bioactive cements. They are a mixture of glass ionomer and Portland cement. They act directly on vital tissue to induce healing and repair.16 These cements share three characteristics. They contain high levels of calcium, have an alkaline pH, and they form surface apatite.
Mineral trioxide aggregate (MTA) is 75% Portland cement. It has osteogenic, cementogenic, and odontogenic potential. MTA is used for pulp capping, pulpotomies, and endodontic root perforation repairs. The components in MTA induce biologic processes such as cell adherence and growth, periodontal ligament attachment, cementum growth, and dentinal bridge formation.17
In turn, Portland cement is made largely of calcium silicates and is a “hydraulic” cement. This term is new in dentistry and refers to a material that hardens with water but after setting is impervious to water. Dentistry has not had a cement with this superior capability.
When mixed with water, the bioactive cements release calcium and silicon ions into underlying dentin. Silica is a strong inducer of dentin matrix remineralization. It has been proposed that completely demineralized dentin can be hypomineralized with sound organic matrix, which could in turn be repaired. Niu et al18 explains this potential remineralization as “backfilling the demineralized collagen with amorphous calcium phosphate.” Bioactivity is a “specific biological response which results in the formation of a bond” between tooth preparation and restoration.
In addition to regeneration, a cement that is comprised of calcium silicate (or calcium aluminate) and glass ionomer would be: free of bis-phenol A; moisture tolerant; and chemically bonded to tooth.
Bioactive cements are here. NuSmile® BioCem Universal BioActive Cement (NuSmile, www.nusmilecrowns.com) and Ceramir® C&B (Doxa, www.CeramirUS.com) represent the new class of bioactive cements. NuSmile is marketed for use in pedodontics while Ceramir C&B is for use with porcelain and zirconia crowns.
In addition to the tissue-level bioactivity of these cements, some research has involved adding polymers to resin cements. One would express antimicrobial activity; another would induce tissue growth in endodontic perforations. Strontium release would decrease the demineralization of acid exposure. These approaches have the potential to improve clinical results, yet none have reached the dental market.
CONCLUSION
Dentistry enjoys a constantly changing and ever-increasing array of cements and techniques from which to choose. Years of evidence-based practice and research have elevated adhesive resin cements to a place where they are primary choice for many restorations.
The practical aspect of using resin cements involves navigating through these many old and new cement systems. In addition, there have been astounding advances in indirect restorations. The appropriate cementing media and the necessary cementation protocols are evolving slower than the restorations themselves. A prudent approach would be to select the restoration and the cement first, based on the patient’s needs, and then chose the appropriate preparation design.
Self-adhesive resin cements are simpler to handle and have become quite popular. However, more research is indicated because the evidence does not support a strong clinical recommendation. Selective enamel etching should be beneficial for success of the SARCs.
Biologically active cements are becoming available and they represent a new focus in research. Rather than continue developing the status quo, researchers are adding active polymers to resins in vitro.
It seems that the new bioactive non-resin cements hold out greater promise. These cements have been shown to bond to tooth structure because of their glass-ionomer component. In addition, they repair damaged and affected dentin and can even close very narrow open margins. This new class of hybrid bioactive cements offers additional features that should be taken into account when choosing a cement.
ABOUT THE AUTHORS
Dr. Simon is a professor in the Department of Restorative Dentistry at the University of Tennessee Health Sciences Center College of Dentistry.
Dr. Hamilton is an assistant professor in the Department of Restorative Dentistry at University of Tennessee Health Sciences Center College of Dentistry.
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