The Evolution of Adhesives Leads to Current Innovations

Following the expiration of Kuraray Noritake Dental's patent for 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate) in 2003, other manufacturers began to introduce this monomer and other phosphoric acid esters in their new adhesive formulations. Scotchbond^™ Universal Adhesive (3M Oral Care, 3m.com) was the first universal adhesive to be launched in 2012. Subsequently, various universal adhesives reached the market, with the capability of reducing treatment times and offering compatibility with enamel or dentin without any surface treatment or the need for resin-luting cements.⁴ Despite this promising versatility, some studies indicate that the bond durability of these multimode adhesive systems over time is not as robust as that of previous generations. Thus, while there has been no "golden" protocol for achieving a stable and optimal adhesion to dentin,⁵ nevertheless, due to their flexibility, universal adhesives have gained significant clinical popularity, accentuating the need for continued research into optimizing their use.

Categorizing adhesives from first to eighth generation, as has been done over the years, provides little or no information about the adhesive strategy employed, and thus can lead to confusion. Classifying adhesive systems by the applied adhesive strategy makes more sense: the terms "etch-and-rinse" (E&R) and "self-etch" (SE) contrast how these systems interact with tooth structures.

E&R systems involve two or three steps, depending on whether primer and bonding are separate or combined in a single bottle, and require prior etching with 37% phosphoric acid on the enamel and dentin.⁶ Besides removing the smear layer, the acid also decalcifies the most superficial 1 μm to 5 μm of dentin to remove hydroxyapatite and leave behind a network of collagen fibers soaked in water left from rinsing the acid. In the authors' view, OptiBond^™ FL (Kerr, kerrdental.com), a three-step E&R adhesive, is still the reference for all other E&R adhesives.

SE systems include an acidic self-etch primer followed by the application of a classic adhesive resin.^7,8 Their non-rinsing acidic primer does not dissolve and remove the smear layer. Instead, it integrates it into the adhesive interface while slightly decalcifying superficial hydroxyapatite in dentin and enamel. The depth of this decalcification depends on the acidity of the primer: ultra-mild (pH ≥2.5), mild (pH ≈2), intermediately strong (pH between 1 and 2), and strong (pH <1).⁹ In a 13-year randomized controlled clinical trial for noncarious cervical lesions,¹⁰ Clearfil^™ SE Bond (Kuraray Noritake Dental, kuraraydental.com) showed excellent outcomes. The retention rate was 93% when enamel was selectively etched. When the adhesive was applied to enamel and dentin in SE mode, the retention rate was 86%. Consequently, the clinical recommendation today is to use selective enamel etching to improve enamel bonding and marginal sealing.

E&R and SE systems form a hybrid layer when resins impregnate the tooth structure. The creation and quality of this hybrid layer are crucial for effective resin-dentin bonding. It is well-accepted that phosphoric acid creates a more pronounced and retentive etching pattern in enamel. Therefore, E&R bonding systems are often preferred for indirect restorations and when large areas of enamel are still present. Because of their superior bond strength to dentin, SE adhesives are ideal for direct restorations and cavities primarily located in dentin. When enamel is present, the combination of SE adhesives with a "selective enamel etching" (SEE) improves the marginal integrity and retention rate of composite restorations.^3,11

Despite current trends toward fewer and simpler clinical application steps, one-step dentin bonding systems exhibit lower bond strengths and seem less predictable than multi-step etch-and-rinse and self-etch systems.

Adhesive systems are among the few biomaterials in dentistry that have undergone numerous evolutionary changes and are frequently adjusting their commercial names, making it difficult for clinicians to stay updated and determine which adhesive to use in their daily practices.¹² From the clinical perspective of reducing time and facilitating dental procedures, combining multiple components into a single bottle is challenging while maintaining effective bonding performance across various clinical scenarios.

These newer "universal" adhesives may be used as E&R adhesives, SE adhesives, or SE adhesives on dentin and E&R adhesives on enamel (SEE). ^6,13Most universal adhesives contain specific monomers that bond ionically to calcium in hydroxyapatite, and of the many functional monomers investigated, 10-MDP is presently the most effective. Although product indications may vary, these adhesives are also designed to promote adhesion to other restorative substrates, such as resin composites, ceramics, and metal alloys. MDP can also adhere to zirconia via ionic and hydrogen bonding. Several studies suggest that the application of MDP-based universal adhesives improves the immediate bond strength of zirconia but results in a significant drop in bond strength after 6 months.¹²

Discussion has continued over whether these universal adhesives should be applied as E&R adhesives to both enamel and dentin. Twenty-four-hour bond strengths to dentin are comparable between E&R and SE approaches. However, the etching process removes calcium from dentin, leaving a superficial layer of collagen fibers surrounded by water. This may delay any potential bonding between the adhesive and the calcium, phosphate, and/or carboxylate groups.¹² Furthermore, the effectiveness of universal adhesives diminishes when they are used as E&R adhesives on dentin.

In a study by Cardoso et al, universal adhesive systems had different bonding abilities to dentin, and the bonding strategy (E&R or SE approach) did influence the failure modes but did not have a significant effect on the bond strength of the adhesives.¹³ In contrast, a systematic review of in vitro studies reported that the enamel bond strength of universal adhesives is improved with prior phosphoric acid-etching, whereas the same effect was not evident for dentin when mild universal adhesives were used.¹⁴ In general, it appears that applying universal adhesives to dentin should not be preceded by phosphoric acid-etching, although this may vary depending on each case.

Some universal adhesives, such as Scotchbond Universal, Tokuyama Universal Bond (Tokuyama, tokuyama-us.com), and Clearfil Universal Bond (Kuraray Noritake Dental), have incorporated silane into their formulations to simplify the clinical process of luting in a single step. In theory, clinicians would not need to apply a separate silane solution after etching the ceramic restoration intaglio with hydrofluoric acid. However, recent studies have questioned the effectiveness of using a combined adhesive-silane solution for bonding ceramic restorations since the presence of a bisphenol A-glycidyl methacrylate (bis-GMA) monomer significantly lowers the contact angle of the solution, decreasing surface wettability.¹² Consequently, it is advised to use a separate silane solution or a freshly mixed silane with the adhesive to ensure optimal adhesion performance.

The clinical success of resin composite restorations depends on the effectiveness and durability of the adhesive interface. A new technique called "selective dentin etching" has been developed to improve resin-dentin bonding.¹⁵ This method consists of applying phosphoric acid to the dentin for 3 seconds, which, after rinsing and drying, results in a partially demineralized substrate. The remaining calcium content is usually linked to the etching duration, and comparable residual calcium levels are found between self-etch mode and 3 seconds of selective etching. This finding shows that this technique could be an alternative to improve the long-term bond strength to dentin. Likewise, a scrubbing technique with active application of universal adhesives for a period longer than 10 seconds increases dentin bond durability by penetrating the adhesive into the dentin tubules and enhancing solvent evaporation. Other alternatives like double adhesive application, prolonged curing time to 40 seconds, use of an additional hydrophobic resin layer, and prior application of matrix metalloproteinase (MMP) inhibitors could benefit the bonding performance to dentin.¹⁵ Conversely, reducing the application time and applying desensitizers to the dentin should be avoided, as these practices may compromise bond strength.

It is critical to understand that these protocols are strictly related to the exact products evaluated in the respective studies. Chemical compositions and instructions for these procedures vary significantly among different products. In addition, due to their complex chemistry, they are extremely technique-sensitive. Therefore, it is vital to exactly follow manufacturer instructions, as these also significantly differ.

The current market offers a variety of universal adhesives. While this affords clinicians many options, it can also complicate the decision-making process of identifying the most appropriate adhesive for each case scenario. However, patient- and operator-related factors may have a greater impact on restoration longevity than the specific adhesive materials used.

Macarena Rivera, DMD, MSc
Assistant Professor, Department of Prosthodontics, University of Chile, Santiago, Chile; Adjunct Professor, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Private Practice, Santiago, Chile

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry, Chair, Department of Preventive and Restorative Sciences, and Assistant Dean, Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania

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