Long-Term Bonding Performance of New Universal Adhesives and Resin Cements

Adhesive dentistry is an essential part of daily clinical practice, and the evolution of adhesive systems and resin cements has been driven by a quest for simplicity, efficiency, and long-term clinical reliability.¹ Universal adhesives and resin cements now dominate the market. They are designed to work with multiple etching techniques and bond effectively to various dental materials and tooth substrates, optimizing adhesive protocols.^2,3 However, it is important to define what makes a system “universal” and critically evaluate whether these newer systems deliver durable long-term bond strengths, which should ultimately guide material selection.

Universal Adhesives

The term “universal” refers to the application options. Manufacturers also claim that these adhesives bond effectively to not only enamel and dentin, but also metal alloys, composites, glass-rich ceramics, and zirconia by incorporating silanes and special adhesive monomers like 10-MDP, thereby making them suitable for indirect restorations.^4,5

Universal adhesives may be used in etch-and-rinse (E&R), self-etch (SE), or selective-etch (SE on dentin and E&R on enamel – SEE) modes.^6-8 Etching enamel with phosphoric acid for about 30 seconds, followed by water spray rinsing, has been reported to improve the bond strength of resins to enamel. For universal adhesives, however, shorter etching times of 3 to 15 seconds appear sufficient (see reference below).^9,10

Most universal adhesives contain specific monomers that bond ionically to calcium in hydroxyapatite, with 10-MDP currently the most effective at promoting adhesion to not only tooth structure but also restorative substrates, such as resin composites, ceramics, and metal alloys.^5,11,12 MDP can also adhere to zirconia via ionic and hydrogen bonding. Some universal adhesives include silane in their formulations to simplify the clinical bonding process. This negates the need to apply a separate silane solution after etching ceramic restorations with hydrofluoric acid. However, some research questions the effectiveness of a combined adhesive-silane solution for ceramic bonding, as the bis-GMA monomer can significantly reduce surface wettability.^2,13

Whether universal adhesives should be applied in E&R mode to both enamel and dentin remains a subject of ongoing debate. At the 24-hour interval, bond strength values to dentin are comparable between the E&R and SE approaches. Phosphoric acid etching of dentin, however, removes calcium, exposing a superficial layer of collagen fibers surrounded by residual water, which may delay bond development between the adhesive and calcium, phosphate, and/or carboxylate groups.¹³ Consequently, the bonding effectiveness of universal adhesives is inferior when applied in E&R mode to dentin. Phosphoric acid etching, even at a reduced application time of 3 to 15 seconds, is essential for durable bonding, regardless of the preparation technique employed. These findings support SEE as a durability-based recommendation for clinical use.¹⁴

Long-term bonding differences become even more evident among universal adhesive-cement combinations used for indirect ceramic restorations. An in vitro study comparing four cement systems bonded to lithium disilicate, each with its recommended primer or with silane alone, found that the use of a pure silane primer, as opposed to the system’s recommended priming protocol, may be more beneficial for adhesive bonding to lithium-disilicate ceramic, as some adhesive systems are hydrolytically stable after long-term artificial aging, while others are significantly affected.¹⁵ When bonding to zirconia, the pattern is similar. A study assessing bond strength of resin cement to different translucent zirconia ceramics and its durability influenced by the yttria content (3Y-PSZ, 4Y-PSZ, and 5Y-PSZ) found that artificial aging with thermocycling significantly affected the tensile bond strength of luting resin to translucent zirconia ceramics; however, yttria content had no statistically significant effect. This suggests that surface treatment and bonding chemistry, rather than the specific zirconia generation selected, remain the major factors for long-term retention.¹⁶

Universal Resin Cements

Resin cements can be classified as multi-step adhesive cements, which require pretreatment of the tooth surface (total and self-etch adhesive systems), self-adhesive systems, and “universal” self-adhesive resin cements, which represent the latest novelty in the dental adhesive industry.^12,17

Multi-step adhesive cements continue to show higher bond-strength values after aging compared with self-adhesive systems in in-vitro studies, though clinical relevance of these is difficult to determine. Self-adhesive cements include functional acidic monomers that can demineralize and chemically bond with dentin. However, they do not create a traditional hybrid layer, as their resin-dentin interdiffusion zone is less than 1 μm thick. This raises questions about their mechanical stability compared to hybrid layers formed by standard adhesive systems.¹⁸

A new manufacturer-specific concept is “touch-cure chemistry”, a technology incorporated into some universal adhesive and resin-cement systems. Such systems feature self-adhesive resin cements that contain chemical initiators and amine activators to help a light-cure bonding agent polymerize when the two materials come in contact. This feature allows the application of light-cure universal adhesives for indirect restorations without a separate light-curing step. However, it is critical to ensure compatibility of the materials used, to not interchange materials from different manufacturers or product lines, and to strictly follow manufacturers’ recommendations as these materials are highly sensitive to their specific application protocols. Combining products from different product lines bypasses this mechanism and may compromise the resulting bond strength.^19,20,21

Across enamel, silica-based ceramic, and zirconia substrates, durability depends on the specific adhesive-cement combination, choice of primer, and selected protocol. Etching enamel regardless of preparation, favoring MDP-containing systems with proper surface conditioning for zirconia, and verifying whether a given universal cement benefits from its own primer or from separate silane are material-level decisions available today. For long-term clinical success of any adhesive procedure, a contamination-free environment through rubber dam isolation remains essential. Without it, bond strengths are reduced by more than 50%.²² In clinically challenging situations where rubber dam isolation is difficult, for example, when inserting full-coverage crowns, a hybrid “selective adhesive luting” (SAL) technique has been proposed.⁴ The adhesive system is applied only to the accessible, contamination-free portion of the crown preparation, while the rest relies on self-adhesive cementation, offering a practical, durability-oriented compromise when a complete adhesive protocol is not feasible.

Despite these advantages, evidence supporting the long-term bonding performance of this new generation of universal adhesives and cements to dentin remains limited, and several shortcomings have been identified. Because their film thickness is frequently below 10 μm, a substantial portion of the adhesive layer is susceptible to oxygen inhibition during polymerization. Another limitation refers to the silane incorporated into universal formulations to enable direct chemical bonding to silica-based ceramics without a separate ceramic primer. To preserve silane stability in an aqueous acidic environment, these adhesives must be formulated at a relatively high pH (>2.5), consequently reducing their etching potential and bonding efficacy. Additionally, although most universal adhesives contain 10-MDP, differences in performance among them may still exist, as the 10-MDP concentration and quality (purity) have been shown to significantly affect bonding effectiveness.^5,23

ABOUT THE AUTHORS

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