An Update on Adhesives and Resin Bonding: Sorting Through the Protocol Options

Adhesive bonding is a critical step in prosthetic and restorative dentistry. Over the years, the evolution of adhesives and bonding systems has focused on simplicity, efficiency, and reliability in clinical practice procedures to achieve less-invasive, long-term, durable direct and indirect restorations.¹ Recently introduced universal adhesives and resin cements offer easy-to-use options that combine the strengths of previous generations of bonding agents into a versatile, user-friendly one-step solution. However, it is important to define what constitutes a “universal” system and to critically assess whether these new systems truly meet the criteria.^1-3

Total-Etch and Self-Etch Adhesives

Adhesive systems can be classified by the applied adhesive strategy: etch-and-rinse (E&R) and self-etch (SE). These two approaches differ in how they interact with the tooth structure. E&R systems involve two or three steps, depending on whether primer and bonding are done separately or combined in a single bottle, and require prior etching with 37% phosphoric acid on the enamel and dentin.⁴ This process removes the smear layer and decalcifies the most superficial 1 µm to 5 µm of dentin, eliminating hydroxyapatite and leaving behind a network of collagen fibers saturated with water from the acid rinsing. The authors consider OptiBond™ FL (Kerr, kerrdental.com) as the material of choice among E&R adhesives.

SE systems utilize an acidic self-etch primer followed by the application of a classic adhesive resin.^5,6 The non-rinsing acidic primer in these systems does not dissolve and remove the smear layer; rather, it integrates it into the adhesive interface while slightly decalcifying superficial hydroxyapatite in dentin and enamel. The depth of decalcification depends on the acidity of the primer: ultra-mild (pH ≥2.5), mild (pH ≈ 2), intermediate (pH between 1 and 2), and strong (pH <1).⁷ In a 13-year randomized controlled clinical trial for noncarious cervical lesions, Clearfil™ SE Bond (Kuraray, kuraraydental.com) showed excellent outcomes.⁸ 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 (SEE) to improve enamel bonding and marginal sealing.

Both E&R and SE systems form a hybrid layer when resin infiltrates the tooth structure, which is vital for effective resin–dentin bonding. Phosphoric acid is known to produce an effective etching pattern in enamel, making E&R bonding systems preferable for indirect restorations when significant enamel is present. SE adhesives are ideal for direct restorations located in dentin. When enamel is also involved, the use of SE adhesives with SEE improves the marginal integrity and retention of composite restorations.^3,9

Universal Adhesives

Universal adhesives were introduced to the market with the prospect of reducing time and facilitating dental procedures. They may be used as E&R adhesives, SE adhesives, or SE adhesives on dentin and E&R adhesives on enamel (SEE).^4,10 Most universal adhesives contain specific monomers, and 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate) is currently 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.^8,11 MDP can also adhere to zirconia via ionic and hydrogen bonding.

Some universal adhesives, such as 3M™ Scotchbond™ Universal (3M Oral Care, 3m.com), Tokuyama Universal Bond (Tokuyama, tokuyama-us.com), and Clearfil™ Universal Bond Quick 2 (Kuraray), have incorporated silane into their formulations to simplify the clinical process of bonding in a single step. The effectiveness of using a combined adhesive–silane solution for bonding silica-based ceramic restorations, however, has been questioned because the presence of a bis-GMA (bisphenol A-glycidyl methacrylate) monomer could decrease surface wettability.^12-14 It is, therefore, advised to use a separate silane solution to ensure optimal adhesion performance to silica-based ceramics. Because of the complex chemistry and technique sensitivity of these materials, it is critical to strictly follow the manufacturer’s recommendations as instructions on clinical handling differ significantly among products.

Resin Cements

Adhesive bonding of indirect partial- and full-coverage restorations is a critical step, as it aims to securely bond the restoration to the tooth’s prepared enamel or dentin. Currently, resin cements are considered the materials of choice in these situations and are typically comprised of bis-GMA or urethane dimethacrylate resin matrix and 20% to 80% inorganic filler particles.¹⁵ Resin cements are often categorized into three types based on their polymerization mechanisms: light-cured, dual-cured, and self-cured. Additionally, they are based on the adhesive systems used during bonding procedures. These procedures can involve multi-step adhesive cements that require pretreatment of the tooth surface, such as total-etch and self-etch systems, or one-step adhesives, known as self-adhesive systems, which do not require any pretreatment.^16-18 Each type of resin cement has different indications, which makes their proper selection challenging.

Desired features of a luting material for indirect restorations include optical characteristics that mimic natural dentition, improved mechanical properties to strengthen the final restoration, and the ability to bond effectively to multiple substrates. However, some disadvantages of resin cements include being technique sensitive and having challenges with cleanup. Some of these materials, especially dual-cure cements, can change color over time, which can be particularly significant for all-ceramic restorations.

When using resin-based cements, surface treatment of the restoration and tooth depends on the type of material being used for the restoration, whether metal, silica-based ceramic, hybrid ceramic, zirconia, or composite. The tooth surface may need to be treated with phosphoric acid, while the restoration surface may require treatment with hydrofluoric acid and silane (silica-based and hybrid ceramics), sandblasting and silane (composite-based materials), or sandblasting and MDP primer (zirconia and metal alloys). The cement selection depends on the preparation type and restorative material to be bonded.

Several types of resin cements have been introduced as one-component “universal” self-adhesive cements with the aim of reducing chairside time, decreasing the number of materials needed in the dental office, and increasing the possibility of using them in different clinical scenarios. These cement systems effectively combine the physical and optical benefits of composite materials with a simplified, single-step application process, bonding to an untreated tooth surface without the need for microabrasion, etching, primer, or bonding agent. They contain acidic functional monomers in the luting paste, which help to demineralize the tooth substrate, facilitating the infiltration of resin components into the demineralized structure. Furthermore, self-adhesive resin cements may offer biological advantages over conventional cements by reducing postoperative sensitivity. Current in vitro studies show that the use of an MDP-containing universal self-adhesive resin cement for bonding to ceramics like zirconia provides comparable or even better bond strength than an MDP-containing primer and luting resin.^19,20

Although the simplified application process represents an advantage in clinical settings, the bonding effectiveness of self-adhesive cements to tooth structure is generally considered to be less than that of conventional resin cements.^19,21 Several studies show that self-adhesive cements bond better to dentin than to enamel. However, bonding to enamel can be enhanced by selectively applying an etchant and bonding agent (SEE). This method involves using an etchant or a self-etching primer before applying the self-adhesive resin cement to potentially improve the bond strength. Conversely, when dentin is etched with phosphoric acid and a bonding agent is subsequently applied using these cements, the bond strength decreases. When self-adhesive resin cements are used without pre-etching, they demonstrate fairly strong bonds to dentin. The use of universal adhesives as primers for self-adhesive resin cements may lead to higher etching efficacy and chemical bonding ability compared to self-adhesive cements alone.^17,19 Some examples include Panavia™ SA Cement Universal (Kuraray), RelyX™ Unicem (3M Oral Care), BisCem® (BISCO, bisco.com), Maxcem Elite™ (Kerr), and Speedcem® (Ivoclar, ivoclar.com).

Continued Enhancements

Adhesive techniques, technologies, and clinical concepts are constantly being updated and enhanced. The current market offers a variety of adhesives and resin cements. While this results in many options for clinicians, it can also complicate the decision-making process to identify the most appropriate product for each case scenario. In general, it is critical to strictly follow manufacturer recommendations as new universal bonding agents are greatly reliant on proper clinical handling. The new universal self-adhesive resin cements offer excellent alternatives to traditional multistep protocols, with most recent data indicating they may even supersede them.

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