How Reliable a Bonding Strategy Is the Use of Universal Adhesives?

Clinical studies have shown that the performance of UAs is dependent on the bonding strategy used. UAs exhibit satisfactory clinical performance when applied in ER mode or with selective etching of enamel in SE mode.¹ Some commercially available UAs are less susceptible to dentin moisture and provide acceptable bonding effectiveness when applied to wet or dried dentin surface, thus facilitating their clinical application. Ultra-morphological microscopic evaluation performed by our research group at the USC Pincus Laboratory for Biomaterials Research revealed that a UA was capable of promoting re-expansion of collapsed collagen fibrils after the etching of dentin with phosphoric acid for 15 seconds, followed by 10 seconds of air-drying. However, UA applied in SE mode should be used with caution because of a high risk of debonding compared to the clinical performance of UA when ER or selective enamel etching was used. Postoperative sensitivity was rarely observed in clinical studies irrespective of the UA strategy of bonding. Moreover, some additional procedural modifications have been suggested to improve the bonding performance of UA, including active application, application of multiple coats, extending time of evaporation of solvent from 5 seconds to 15 seconds, and increasing the photopolymerization time.

Manufacturers attempted to expand the use of UAs by incorporating silane, aiming to decrease the number of steps needed for bonding to ceramic. Silane-containing UAs are less effective in bonding to lithium-disilicate glass-ceramic or feldspathic ceramic than hydrofluoric acid-etching followed by silanization.² Similarly, the long-term benefit of using MDP-silane-containing UA to bond to zirconia, instead of using airborne particle abrasion followed by the application of a separate 10-MDP-containing silane, is still questionable.

The process of bonding to dental structures and dental materials involves a variety of complexities that are invisible to the naked eye (even using magnification), and it requires powerful ultra-structural microscopic analysis to be understood. Thus, the bonding process can be difficult to comprehend and, consequently, to be executed. Our experience with UAs has been overall positive. For the majority of cases (mostly full-mouth adhesive rehabilitations) performed in USC's Advanced Specialty Clinic, we have used UA in ER mode with the aforementioned modifications, with no evidence of postoperative sensitivity, debonding, or other adhesion issues.

Some might be concerned that the modifications to the original technique for UAs may increase the length of application time. However, I would argue that these modifications can also be applied with most other current dental adhesives, and they represent time well spent since they make bonding more reliable and predictable. All available dental adhesives have advantages and limitations that should be carefully considered before each clinical use, irrespective of their classification and mode of application. In the end, the main goal of dental adhesive technology is to preserve and reinforce dental structures; hopefully, one day a true breakthrough in adhesion technology will happen and the bonding process will be truly simplified.

Universal or multimode adhesives, the latest generation of bonding agents, are intended to simplify clinical procedures by being more user-friendly and less technique-sensitive than past generations. These dental adhesives are called "universal" because they contain functional monomers that provide chemical bonding to dental tissues and indirect restorations, affording clinicians the opportunity to use them in different adhesive approaches, ie, etch-and-rinse (ER), self-etch (SE), or selective enamel etching (SEE).

Although they are referred to as universal, clinicians may ask themselves how "universal" these adhesives really are and whether they should use these bonding agents in a particular adhesive strategy to obtain an optimal clinical outcome when placing a restoration with them.

The evidence concerning the performance of universal adhesives (UAs) that can be found in the literature is referred from both in vitro and clinical trials. It seems that laboratory studies do not support the hypothesis that UAs can be used with any adhesive strategy. In vitro studies report improved bond strength to enamel when an additional etching step with phosphoric acid is carried out before applying a UA. On the other hand, it seems that dentin etching does not improve bonding performance of UAs. Interestingly, some laboratory studies report better longevity when an extra layer of hydrophobic resin is applied after the bonding procedure.

Although in vitro studies provide a solid background for the evaluation of a material's properties, conclusions drawn from carefully planned randomized clinical trials (RCTs) are considered to be more valuable. So far, an RCT with the longest follow-up period (5 years) reported that composite restorations placed in noncarious cervical lesions (NCCLs) were 2.6 times more likely to debond or fracture when using UAs in SE mode compared to ER mode.³

Lastly, our research group at the University of Bologna has conducted a systematic review of RCTs that evaluated clinical performance of composite restorations placed in NCCLs with UAs using different adhesive strategies. The results from our meta-analyses showed the ER mode could lead to better medium-term retention of composite restorations of NCCLs compared to the SE application strategy, while the use of the SE approach could lead to less immediate postoperative sensitivity and, therefore, better short-term patient satisfaction. The SEE approach was comparable with the ER method in terms of retention and postoperative sensitivity.

It remains of great interest to wait for more long-term RCTs on UAs and to see if challenges associated with previous generations of adhesives have been overcome.

Simplification is the motor of success! Since the early 1980s, dental adhesives have evolved from three-step etch-and-rinse (3ER) to simplified one-step self-etch (1SE) systems. For a long time, 3ER adhesives were the "gold standard," with their bonding to dentin being susceptible to operators' errors. Although simplified, 1SE adhesives result in inferior enamel bonding due to their milder acidic monomers. Thus, the use of selective enamel etching (SEE) with phosphoric acid (H₃PO₄) along with self-etching adhesives is recommended as the best of both worlds. The first universal adhesive (UA) that allowed the use of both application techniques with one single product was launched in 2012.

Parallel to this mainly micromechanical approach for adhesion to teeth, scientists have looked into functional monomers able to chemically bond to the calcium in hydroxyapatite. The first synthesized molecule commercialized in the 1980s was 10-MDP, which can form self-assembled MDP-calcium salts in the form of nanolayers, a condition that might not occur if the dentin is etched with H₃PO₄. Once the patents for 10-MDP expired, many companies started to use it and other functional monomers in their dental adhesives, resulting in multimode UAs. Due to 10-MDP's high affinity to oxides, it can be used as a primer for zirconium-oxide ceramics and base metals. Silane was added to UAs to promote adhesion to glass-based ceramics and resin composite glass fillers.

UAs, which have been well researched in the past 20 years, more so in vitro than in vivo, have behaved better than expected when applied to teeth.⁴ Studies have shown better initial bond strength on dentin for the etch-and-rinse technique; however, the self-etch approach in long-term studies (artificial aging) has shown inferior but more stable bond strength results. On caries-affected dentin, the bond stability was not maintained. Selective etching of the enamel with H₃PO₄ definitely leads to better outcomes. In accord with these findings, a 5-year clinical study, using only one UA, reported better results for the etch-and-rinse approach in combination with enamel etching, when compared with the self-etch technique.¹

When incorporated into UAs, the functional monomers (eg, MDP, silane) are less efficient when compared with those applied in a solvent to adequately conditioned surfaces (hydrofluoric-acid-etched or sandblasted), regardless of the targeted material (oxide ceramic, base metals, glass-based ceramics, resin composites).

Dentistry has not yet attained the perfect adhesive, as simplicity still has its price.

Sillas Duarte, Jr., DDS, MS, PhD
Rex Ingraham Chair in Restorative Dentistry, Chair, Division of Restorative Sciences, Director, Advanced Program in Operative and Adhesive Dentistry, Herman Ostrow School of Dentistry of University of Southern California, Los Angeles, California

Lorenzo Breschi, DDS, PhD, FADM
Full Professor of Restorative Dentistry, Department of Biomedical and Neuromotor Sciences, Chair of Restorative Dentistry and Director of Master in Aesthetic Restorative Dentistry, University of Bologna, Bologna, Italy

Jean-François Roulet, DDS, Dr. med. dent.
Professor, Department of Restorative Sciences, University of Florida (UF) College of Dentistry, and Director, Center for Dental Biomaterials, UF, Gainesville, Florida