The Perfect Match

This month, Inside Dentistry speaks with a panel of clinical opinion leaders to learn what they consider to be the essential elements for predictable success when bonding to an ever-increasing range of substrates. But first, let's take a look at some of the types of restorative materials and cements that are available in the marketplace.

Selection of the most appropriate restorative material is an individualized process, meaning there is no one-size-fits-all solution. In each case, a dentist must decide on a material based on the patient's unique situation and needs. During the last decade, the introduction of a new generation of particle-filled and polycrystalline high strength ceramics, hybrid composites, and technopolymers has offered an extensive palette of dental materials, broadening the clinical indications in fixed prosthodontics. This is due, in part, to an increased patient demand for nonmetallic restorations.

The esthetic and mechanical properties of lithium disilicate and zirconia, along with their versatility, make them the materials of choice for modern prosthetic dentistry. These properties include unparalleled optical and esthetic attributes, high biocompatibility, high mechanical resistance, reduced thickness, and favorable wear behavior.¹

However, at the same time, these monolithic restorative materials have also introduced new challenges in adhesive bonding-many of which are related to understanding the nuances of how a bond is developed to each and how that affects the protocol. For example, whereas lithium disilicate restorations are prepared for adhesion using hydrofluoric acid chemical etching and a silane primer, zirconia restorations require microabrasion with aluminum oxide powder and a methacryloyloxydecyl dihydrogen phosphate (MDP) primer. And due to the high bond strengths achieved by these stronger materials, another challenge involves the development of new instrumentation techniques to conservatively remove them when they require replacement.²

The primary function of a dental cement is to fill the space between a restorative material (eg, definitive or provisional) and a tooth preparation (or implant abutment) to enhance the resistance to restoration dislodgement during function.^3,4 Because loss of retention is one of the most common causes of restoration failure, the long-term success of restorations is heavily dependent on the proper selection and manipulation of dental cements.⁵ With the ongoing development of new material technologies, dental cements have evolved into stronger and more durable materials. Unfortunately, the selection of a suitable dental cement for a specific clinical application has become increasingly complicated as new materials become available and cementation and bonding protocols are being changed accordingly.

Historically, dental cements—or luting agents—were materials used to join a restoration to a prepared tooth and create a seal primarily through mechanical retention. These mostly water-based cements required a retentive preparation design in order to maximize their strength. First introduced in the 1800s, zinc phosphate was once considered the gold standard of permanent dental cements. In 1968, zinc polycarboxylate cement was developed, which was the first cement that exhibited a chemical bond to the tooth structure and was biocompatible with the dental pulp. Later, polycarboxylate cements were hybridized with silicate cements to produce conventional glass-ionomer cements that demonstrated even better biocompatibility through their ability to release fluoride long-term.⁶

Resin cements have been available since 1952 for the cementation of indirect restorations.⁷ When compared with water-based cements, resin-based cements reduce microleakage, have remarkably low solubility, and offer improved strength and retention.^8,9 They are widely used in the fixation of inlays, onlays, crowns, and pins because of their improved mechanical properties, ease of use, and esthetic qualities.¹⁰ Resin cements can be classified based on their method of activation, which can be photoactivation, chemical activation, or dual activation. Light-cured resin cements are often used for thin, highly translucent anterior restorations. Self-cured resin cements do not require light for polymerization and are indicated for placing opaque restorations, such as those made from thick, full-contoured zirconia, metal, and porcelain-fused-to-metal (PFM), as well as endodontic posts. Dual-cured resin cements can be either light cured, self cured, or both.¹¹

Adhesive resin cements require separate bonding steps but offer enhanced bond strength.¹² This makes these cements useful for preparations with nonretentive forms, such as veneers or short clinical crowns. However, one of the commonly noted factors dissuading clinicians from the use of resin cements is the need for multiple steps (eg, etching, drying, priming) to create the bond. Alternatively, self-adhesive resin-based cements are designed to adhere to tooth structure without the need for distinct etching, drying, and priming steps. Although these cements provide a chemical bond and improved bond strength when compared with conventional cements, their properties are inferior to those of adhesive resin cements.

In the 1980s, the desire to improve resistance to fracture and dissolution resulted in the addition of water-soluble polymers to conventional glass ionomers to create a new category of dental cements called resin-modified glass ionomers (RMGIs).^13,14 The resin component provides micromechanical retention to dentin and enamel. Similar to self-adhesive resin cements, RMGIs do not require a bonding agent. "Glass ionomer is more forgiving regarding moisture control than a resin cement, but the preparation needs to be clean and free from saliva, blood, and temporary material," explains James Simon, DDS, MEd, the director of operative dentistry at the University of Tennessee Health Science Center's College of Dentistry.

The hybridization with dentin creates greater bond strength and results in higher fracture resistance when compared with conventional glass ionomers, but this strength and fracture resistance is still less than what can be achieved by standard resin-based cements.^15-17 "We completed in vitro testing at our laboratory that showed that both resin-modified glass-ionomer cements and calcium-releasing cements could help prevent recurrent caries at cementum margins; however, fluoride-releasing materials provided more of a protective effect than calcium-releasing cements," says Nathaniel Lawson, DMD, MA, PhD, the director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry in Birmingham, Alabama. When compared with conventional glass ionomers and zinc phosphate, RMGIs provide approximately double the bond strength to sandblasted, high-noble metal alloys, but the bond strength of resin-based cements is 6- to 10-times higher than that of RMGIs.¹⁸

Universal self-adhesive cements have the potential to significantly simplify and expedite adhesive protocols. Adhesive bonding with composite resins requires multiple steps to adequately prepare the bonding surfaces of both the tooth and the restoration. As these steps are time-consuming and technique sensitive and the restoration is susceptible to contamination, many clinicians prefer conventional cementation protocols and materials that are more efficient. Modern self-adhesive universal resin cements offer a compromise: they are easy to use without additional primers or bonding agents but still offer at least moderate bond strength values.

According to the literature, a universal adhesive is ideally a single-bottle, no-mix system that can be used in total-etch, self-etch, or selective-etch mode, depending on the specific clinical situation and personal preferences of the clinician.¹⁹ They can be used for the placement of both direct and indirect restorations and are compatible with self-cure, light-cure, and dual-cure resin-based cements. Universals can bond to dentin and enamel and are used as adhesive primers on substrates such as zirconia, noble and nonprecious metals, composites, and various silica-based ceramics. However, the manufacturers of some universal adhesives still recommend the use of separate and dedicated primers to optimize the bond strength to substrates such as porcelain and zirconia. Gary Alex, DMD, who maintains a private practice in Huntington, New York, emphasizes that "there are times when the ‘easiest' cement to use may not be the best for the job at hand. This depends on many clinical factors, including the retentiveness of the preparation and the specific type of restoration being placed."

Bioactive cements mimic the physical and chemical properties of teeth, resist fracture, and support remineralization. Once placed, they react to the pH changes of the oral cavity, releasing and recharging ions such as calcium, fluoride, phosphate, and others, which stimulate remineralization and apatite formation. These cements have the ability to either function as a replacement for human tissues or to resorb and encourage the regeneration of natural tissues at the interface between the tooth and restoration.²⁰ Furthermore, they do not form a hybrid layer or require a bonding agent to enable adhesion to the crown or tooth structure.

Of course, like any restorative material, their usage depends on the indication, the product itself, and the needs of the patient. These materials tend to be more hydrophilic, so they're ideal for certain patients such as uncooperative children for whom isolation is challenging or those prone to caries. "If there are deep areas of the preparation, I do use bioactive materials," says Edward A. McLaren, DDS, MDC, a former University of California, Los Angeles School of Dentistry professor who directs the private teaching institute Art Oral America. "Then I make sure that I am able to get an effective seal prior to taking impressions."

Bioactive materials have great potential; however, further in vitro and in vivo studies are needed to determine their ability to demonstrate clinical success long-term. "These materials are very promising, but we at the University of Pennsylvania School of Dental Medicine are working on gathering additional long-term data and scientific evidence before we can include and recommend them without reservation," says Markus Blatz, DMD, PhD, a professor of restorative dentistry in the Department of Preventive and Restorative Sciences at the University of Pennsylvania School of Dental Medicine in Philadelphia, Pennsylvania.

Although there aren't hard-and-fast rules about which materials pair best with which cements for each procedure, both short- and long-term studies have taught us a lot about which combinations can ensure the highest rates of success. Historically, crowns were mainly made from gold and metal alloys, and a traditional cement would be used to fill the space between the metal and tooth structure to provide retention. PFM restorations were the gold standard prior to all-ceramic veneers, onlays, and crowns, the advent of which increased the need for mechanical and chemical adhesion. In some cases, a dentist may still recommend a PFM restoration. "I am more inclined to use PFM for long span bridges if implants are not an option," says Rena Vakay, DDS, a clinical instructor at the Kois Center who maintains private practices in Centreville and Leesburg, Virginia.

It's not about pairing a material and a cement or adhesive together but rather choosing the right combination for a specific clinical situation. "Any type of cement can work for higher strength materials in terms of resistance to fracture," explains McLaren. "But in clinical situations where there is poor retention, it is still ideal to use an adhesive technique-even with higher strength materials." In addition to retention, when assessing a clinical situation, one should also consider the location in the mouth, the amount of tooth structure left, and if a build-up material will be required.

When bonding to dentin, contemporary dental adhesive systems rely on the formation of the hybrid layer, a biocomposite interface formed when a resin-based adhesive infiltrates the collagen fibrils of the dentin. Traditional three-step etch-and-rinse adhesives use primers containing hydrophilic monomers and solvents. They aim to displace water and prepare the collagen scaffold for the infiltration of the solvent-free, hydrophobic bonding resin.^21-23 Simplified two-step etch-and-rinse systems combine the hydrophilic primer and the hydrophobic resin into one solution. Similarly, self-etch adhesives are subdivided into two-step and one-step categories.²⁴ Although the simplified two-step etch-and-rinse and one-step self-etch adhesives may be more user-friendly, many studies show that three-step etch-and-rinse and two-step self-etch systems produce more favorable results when bonding to dentin.^25,26

With so many all-ceramic restorative material alternatives available today, one must strive to choose the most favorable all-ceramic system for each clinical situation. For example, zirconia-based restorations can be a near ideal choice for crowns, fixed partial dentures, and the restoration of implants in esthetic areas. Most zirconia-based ceramics utilize CAD/CAM technology for the fabrication of crowns, bridges, and implant abutments.²⁷ These ceramics have high strength; therefore, the restorations can be either cemented with traditional cements or bonded with resin cements.²⁸ "For high strength ceramics like zirconia or PFM where retention is not an issue, I would use a resin-reinforced glass ionomer," says McLaren.

Universal self-adhesive cements provide efficacy and speed; however, many dentists note that insertion should not be rushed because it is such a crucial step in terms of ensuring long-term success. "Of course, dental materials and protocols should be user-friendly and efficient. However, although saving a few seconds here or there may have a financial impact on a practice, I am more concerned that sometimes the focus on ‘efficacy' and ‘time-saving' can lead to carelessness and sloppiness," says Blatz. "This is especially true for adhesively bonded restorations, which require rubber dam isolation and multiple bonding steps to optimize their success. Although these restorations are more time consuming, they are typically less invasive and thus, a critical part of our core duty as oral healthcare providers: to help our patients keep their teeth for as long as possible."

In addition to choosing the most appropriate material and cement or adhesive for each situation, a dentist must also choose the most appropriate techniques. For example, for glass-based ceramics, McLaren uses an adhesive technique. "First, this allows me to do a minimal preparation, and second, it allows for a stress distribution effect wherein stress is dissipated as it passes through the bonded interface."

Other technique-related challenges include managing shade changes caused by cement, cleaning up residual cured cement consistently, accessing the area, controlling the esthetics, and keeping the preparation dry. "Proper isolation is extremely important if a dentist is using a cement that requires bonding," says Simon.

Lawson agrees. "Probably the most challenging aspect of bonding a crown is maintaining isolation while bonding," he says. "Another challenge with ceramic crowns is that because they are harder than porcelain, they are more difficult to adjust and polish than PFM crowns. If significant adjustments have been made on a high-strength ceramic crown, one helpful trick is to use a laboratory handpiece and polisher."

Properly preparing a restoration is crucial for bonding. "In the case of zirconia restorations, my preference and recommendation is to sandblast the intaglio surface of the zirconia, regardless of the class of cement being used," explains Alex. "This helps to clean away surface impurities, increase surface roughness, raise surface energy, and significantly improve the bond to the zirconia."

Although the selection of the most appropriate materials and techniques will increase the likelihood of restorative success, there are other factors that may affect the survival of restorations, including the patient's caries risk and socioeconomic factors.²⁹ Therefore, the decision of how to restore and the corresponding choice of materials involves a determination of the patient's immediate requirements as well as an understanding of his or her caries risk and what he or she is able and willing to undergo in terms of dental procedures.

Decision-making is a fundamental aspect of clinical dentistry. The trend toward more conservative preparation and the related advances in material science have broadened the options that are available to patients and dentists, increasing the range of protocols that can be used to restore teeth. "When it comes to restorative materials, one of the biggest challenges is the pace of change. New products are emerging quickly, and oftentimes, clinical research can't keep up," says McLaren. With such a broad spectrum of dental materials, there are a number of factors to consider in making an appropriate choice. The selection of an ideal material and cement match for each indication must take into account both the retentiveness of the preparation and the bond strength required by the type of restoration and its location in the mouth. In addition, clinicians should take into consideration biocompatibility, marginal adaptation, color matching, patient selection, and technique sensitivity. With the ever-increasing array of materials available on the market, it's increasingly likely that an ideal match can be found for each procedure. However, newer materials aren't necessarily universally better; there is often a learning curve because they are more nuanced, and some clinicians prefer to wait for an abundance of long-term studies to become available before incorporating a new material into practice. It is up to each practitioner to learn about the properties, benefits, and drawbacks of the material options available and find the perfect match for each patient.