Cutting-Edge Developments in Adhesive Dentistry

A key aspect in improving the long-term durability of bonds lies in improving their resistance to breakdown. Undoubtedly, moisture plays a key role in bond degradation. "The presence of moisture in dentin is unavoidable; it is intrinsically there," says Nunez. "But what some manufacturers are doing is formulating adhesive systems with less water content in order to avoid trapping the water in the adhesive layer after polymerization. In general, adhesives need water, especially self-etching or universal adhesives. But too much water can lead to adhesive layer degradation by hydrolysis."

"One compositional detail that is sometimes overlooked is the presence of water in all recent universal adhesives," says Jorge Perdigão, DMD, MS, PhD, a professor of operative dentistry at the University of Minnesota School of Dentistry. "Although water is essential for its ability to ionize dentin hydroxyapatite during the self-etch reaction, studies have shown that when residual water is not consumed in the reaction or eliminated by gently air drying the adhesive for at least 15 seconds, it will cause degradation of the interface." Applications in water-abundant environments should be avoided because additional water molecules can act as catalysts, resulting in significantly decreased hydrolytic stability.³ To mitigate this issue, researchers are developing hydrophobic adhesive formulations that repel water and enhance hydrolytic stability. In addition, the use of water-resistant primers and surface treatments can create barriers that prevent moisture ingress, thereby preserving the integrity of the bond.

Water channels or "water trees" are caused by fluid from dentinal tubules entering into the hybrid layer, explains Nate Lawson, DMD, MA, PhD, associate professor and director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry. "Two-bottle bonding systems that have a hydrophilic primer separated from the hydrophobic adhesive are becoming popular again," he says. "The separation of components in the two-bottle systems is credited for creating a hydrophobic hybrid layer that can prevent absorption of dentinal fluid and hydrolytic degradation."

Collagen fibers become vulnerable to mechanical and hydraulic fatigue as well as degradation by host-derived proteases with collagenolytic activity.⁴ Such enzymatic activity, particularly from matrix metalloproteinases (MMPs), contributes to the degradation of bonding systems. MMPs can break down the collagen matrix within the dentin, compromising the bond.⁵ Research is focused on developing MMP inhibitors that can be incorporated into adhesive systems. These inhibitors aim to neutralize the enzymatic activity, thereby protecting the bond from degradation. "To improve bond durability, current approaches include using 2% chlorhexidine to deactivate the MMPs, using a glutaraldehyde-based desensitizing agent to crosslink the collagen network, or bonding to dentin in a self-etch mode that minimizes the amount of demineralized, exposed dentinal collagen," says Lawson.

Advances in Adhesives and Composites

Recent research in universal adhesives and self-adhesive systems has focused on streamlining the application process and improving overall efficacy. Innovations include formulations that enhance bond strength while reducing technique sensitivity. Research indicates that these advanced bonding agents can achieve reliable adhesion with fewer steps, ensuring strong, durable bonds without compromising the quality of restorations. The addition of phosphate monomers, such as 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), to single-bottle universal adhesives has been a game changer. Laboratory and clinical studies generally demonstrate that these systems perform well if they are used according to the manufacturer's instructions.⁶ "A somewhat new concept in adhesive dentistry is the use of cements that can be used in a self-adhesive mode or with a bonding agent," says Lawson. "There are some companies that have new cements that are marketed to work in a self-adhesive mode or with an adhesive, and there are other companies that have just suggested using a universal adhesive on the tooth to improve retention with their self-adhesive cement." Another significant development is the introduction of bonding agents that require less waiting time or minimal scrubbing during the application process.

Advances in light-cure and dual-cure systems have facilitated more effective bonding for deep restorations or those involving opaque materials. Light-cure systems have been optimized to ensure thorough curing, even in challenging cases, with studies confirming their effectiveness in achieving secure bonds if the manufacturers' instructions are carefully followed. Alternatively, dual-cure systems, which combine light curing and chemical curing mechanisms, offer increased flexibility and improved polymerization in areas where light may not penetrate adequately. There have been many solutions developed to ensure complete polymerization; however, this has made it increasingly important for clinicians to ensure that the adhesives that they are using are compatible with their composite resins and resin cements. "We have been conducting research in our laboratory that has determined that there are many incompatibilities between self-etching and universal bonding agents and dual-cure buildup materials," says Lawson. "There are some dual-cure materials that are compatible with self-etch adhesives, but many are not."

The mechanism of curing can be particularly important for bulk-fill materials, which have undergone developments aimed at enhancing their handling characteristics, adaptation to cavity walls, and wear resistance. Research has also focused on new formulations that allow for the placement of larger increments without compromising the material's integrity or increasing the risk of voids and gaps. "Light-cure systems have limitations when it comes to the bulk-fill technique due to the depth of cure, which is relative to each of the different formulas developed by the different manufacturers," says Nunez. "I strongly believe that if someone wants to save time by using a bulk-fill technique, they should rely on dual-cure systems that are not light-dependent."

Another area that has seen significant improvement is the color stability of light-cure materials. "One drawback of using the classical camphorquinone photoinitiator is its intensive yellow color, which may cause discoloration of the restoration, even at a low concentration," says Perdigão. "The introduction of alternative photoinitiators to replace or enhance the properties of camphorquinone has resulted in the improved color stability of newer materials due to much lower yellow saturation of the initiator group."

Developments in Zirconia Bonding

In recent years, research has enabled the refinement of bonding protocols for zirconia, and there have been significant developments in primers and surface treatments aimed at enhancing adhesion to zirconia. Although 10-MDP is still the gold standard monomer for zirconia bonding, the advent of new or modified monomers has contributed to improving bonding efficacy. Studies have emphasized the importance of mechanical surface treatments, such as air-particle abrasion and tribochemical silica-coating, which significantly improve micromechanical retention.⁷ "A topic of debate with zirconia bonding is whether zirconia should be sandblasted immediately prior to bonding or if it can be sandblasted in the laboratory prior to being sent to the dentist," says Lawson. "In our laboratory, we conducted a pilot test, and it didn't indicate a large difference in bond strength between zirconia that was bonded immediately after sandblasting or seven days after sandblasting."

According to Perdigão, there have been some interesting developments in surface treatments for zirconia. "Besides the micromechanical retention provided by sandblasting, other pretreatments that have been investigated include laser, non-thermal argon plasma (NTAP), ceramic coating, and hot etching with hydrofluoric acid," he says. "NTAP is produced at atmospheric pressure with a gas phase at room temperature. Research has confirmed that adhesion to zirconia improves after treatment with NTAP.⁸ However, the increase in bond strength between zirconia and self-adhesive resin cements is significantly more pronounced if the restoration is cemented 8 hours after the NTAP treatment."

Dentin Bonding and Postoperative Sensitivity

One of the key factors in optimizing the strength of bonds to dentin is moisture control. To address this challenge, some manufacturers have adjusted the formulations of adhesives to make them more moisture tolerant. These adhesives contain hydrophilic monomers that can bond effectively in the presence of moisture, which helps to ensure a durable bond in challenging conditions.

Another challenge in dentin bonding is postoperative sensitivity. Current research focuses on enhancing the penetration and sealing capabilities of adhesives to ensure thorough blockage of the dentinal tubules and prevent the fluid movement that causes sensitivity. The addition of glutaraldehyde-based desensitizing agents to bonding protocols can reduce sensitivity and even provide an antimicrobial effect; however, they must be applied appropriately and carefully to avoid damaging the soft tissues. "Some clinicians choose to use glutaraldehyde-based desensitizing agents, such as GLUMA, prior to bonding to dentin in order to occlude dentinal tubules and decrease sensitivity," says Lawson. "The application of GLUMA prior to bonding is not detrimental to the dentin bond. In fact, it may prolong dentin bonds by strengthening the collagen network." Research has shown that glutaraldehyde-based desensitizers both increase bond strength and help to stabilize the adhesive interface over time.⁹ Although those benefits were unrelated to the inhibition of MMPs, MMPs do play a significant role in the degradation of the dentin-collagen matrix, which could lead to increased sensitivity. "It is possible that MMP inhibitors could reduce sensitivity by protecting the integrity of the dentin bond," suggests Lawson.

The shrinkage stress resulting from the polymerization of composite resins is another factor in the development of postoperative sensitivity. "We teach the use of a glass-ionomer cement-based material to replace dentin in wide/deep preparations in posterior teeth-a technique that has been called the ‘super-closed sandwich technique,'" says Perdigão. "With this technique, the volume of composite resin inserted into the preparation is greatly reduced, which leads to lower shrinkage stress."

The Bioactive Focus on Remineralization

An area of particular focus is on restorative materials that can release ions, such as calcium or phosphate, to aid in remineralizing enamel or dentin, thereby enhancing the long-term bond strength and preventing secondary caries.¹⁰ "One concept for these materials is that they can release ions into the local environment near a tooth-restoration interface to prevent demineralization of the neighboring tooth structure," says Lawson. "However, these ion-releasing materials may have another potential application-to remineralize the neighboring dentin if the restoration is placed on a preparation with caries-affected dentin."

Although numerous restorative materials and procedures have been developed with the aim to remineralize dentin lesions, recently, polymer-induced liquid precursor (PILP) approaches have gained momentum as a means to facilitate the restoration of the mechanical properties of the tissue, which is critical for regaining its function.¹¹ "An interesting concept that may be closer to clinical use than many other self-healing techniques, is the PILP biomineralization concept," says Perdigão. "It involves the addition of charged polymers, like polyaspartic acid, to mineralizing solutions. This allows for the transport of calcium and phosphate ions into the collagen fibrils and crystallization of apatite, inducing intrafibrillar and extrafibrillar biomimetic remineralization of demineralized dentin collagen and resulting in higher concentration of calcium with enhanced remineralization capability. PILP also induces biomimetic remineralization of dentin carious lesions and improves their mechanical properties underneath adhesive restorations. In fact, the bonding interfacial integrity of the remineralized dentin carious lesions is improved and the bonding strength to dentin is significantly increased."

Incorporation of Nanotechnology

The integration of nanoparticles has revolutionized the development of modified nanocomposites, ushering in a new era of enhanced physical and mechanical properties.¹² Nanotechnology has also contributed to improvements in the esthetics of these materials. In addition, the steric attributes of nanoparticles, such as hollow spheres, offer heightened selectivity for the precise transport and controlled release of compounds. "The idea is to use nanoencapsulation as a delivery system of different compounds that could have an effect on remineralization, bacterial growth, and caries prevention," says Nunez.

The use of nanotechnology is also being studied to improve the mechanical properties of adhesives. This has the potential to improve the strength and durability of bonds. "The incorporation of bioactive glasses into dentin adhesives has been studied in vitro for several years to enhance remineralization and improve bonding durability," says Perdigão. "A concentration of 10% bioactive glass applied to normal dentin and demineralized dentin (ie, affected dentin) prior to the dentin adhesive results in higher bond strengths when compared with controls without the application of bioactive glasses. More recently, the addition of sol-gel-derived bioactive glasses to dentin adhesives has been shown to result in enhanced remineralization and superior mechanical properties of adhesive-dentin interfaces."

Perdigão also notes that there has been some research on nanostructured zirconium dioxide (or nanostructured zirconia) incorporated in an experimental adhesive. "It promotes mineral deposition on the adhesive interface (hybrid layer) and an increase in the degree of conversion of the adhesive," he says. "This has the potential to inhibit degradation of the dentin-adhesive interface."

Future Research in Adhesion

In the future, research into dental adhesives, resin cements, resin composites, and other materials related to adhesive dentistry will continue; however, the methods used are likely to receive a boost from artificial intelligence (AI). The traditional approach for developing dental adhesives involves repetitive laboratory measurements, which consumes enormous time and resources. Machine learning, a type of AI, is a promising tool for accelerating this process. By analyzing patterns and correlations within a network of datasets, artificial intelligence may help identify optimal combinations of ingredients that achieve desired adhesive properties, such as strength, durability, and biocompatibility. Furthermore, machine learning is showing promising results in predicting the microtensile bond strength and long-term performance of proposed adhesive formulations using their chemical features.¹³ Advanced computational models can also simulate the interaction between the adhesive layer and the enamel or dentin, which may help provide insights into novel adhesive technologies so that researchers can save time and resources when testing new formulations before physical prototypes are created.

As the materials and protocols of adhesive dentistry continue to evolve, restorations will continue to become more durable, predictable, and minimally invasive as well as more esthetic and efficient. The focus on improving bond strength and longevity, optimizing adhesive systems for a variety of clinical situations, reducing postoperative sensitivity, and developing more bioactive materials will undoubtedly lead to improved outcomes for patients and clinicians alike.

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