Are Universal Adhesives Really Universal?

Nicholas Marongiu, DDS
Private Practice,
La Jolla, California

Clinicians strive to create the best long-term dental restorations efficiently and cost-effectively. The introduction of universal adhesives represents the leading edge for helping the profession accomplish this goal. Certainly, efficiency of our workflow can be improved by using adhesive protocols that use fewer bottles and steps in mixing, applying, drying, and curing. Also, by decreasing the time needed to execute a procedure and minimizing the amount of inventory required to produce a finished restoration, cost can be reduced. The question is: Are universal adhesives universal enough to be the magic bullet we desire?

Many in the dental profession have used the term universal adhesive. However, the definition is broad and ambiguous. Ideally, a universal adhesive means: one bottle, no mixing, and the ability to use the product with any mode of application from total etch to selective etch on enamel and dentin. Also, we would be able to utilize any self-cure, light-cure and dual-cure cement, as well as a wide range of restorative materials, including precious, semi-precious, and nonprecious metals; zirconia; and silica.

All universal adhesive systems share some components and characteristics that enable them to be so versatile. These are an etchant, primer, and bonding agent. The acidity of the etchant in universal adhesives ranges from a mild level (pH > 2) to an “extra mild” level (pH > 2.5). A universal adhesive also must have a hydrophilic character, usually HEMA, to wet the dentin and a hydrophobic ingredient, usually acetone or ethanol, to limit hydrolysis and water sorption once polymerized.

Functional polymers are a critical part of the formulation. The most common are methacryloyloxydecyl dihydrogen phosphate (10-MDP) and 4-methacryloxyethyl trimellitate anhydride (4-META), which have hydrophilic and hydrophobic properties. Also, water is an important part of most universal adhesives.

Generally, the pH levels of universal adhesives may affect their effectiveness when used with self- and dual-cure resin cements, prohibiting the chemical curing necessary for use with self- and dual-cure cements. Risk is increased when relying solely on the self- and dual-cure components without employing a separately applied activator.

Regarding cementation of silica-based ceramics, metals, composites, and zirconia, universal adhesives may be used as adhesive primers. Some manufacturers have incorporated silane to their formulations. The stability of silane is a concern in an acidic environment for a universal adhesive. Therefore, some manufacturers continue to recommend the use of a dedicated primer.

Due to the diversity of formulations of universal adhesives, the clinician should proceed with caution and diligently review the manufacturers’ protocols and specific guidelines for each material.

Creating a true universal adhesive is an ambitious goal for manufacturers. Combining the chemistry into a single bottle may be more efficient but poses chemical challenges; this requires more research. As clinicians, we do not want to sacrifice long-term stability of our restorations.

In the ever-changing world of adhesives, the definition for the universal adhesive is nebulous. In fact, the meaning varies from manufacturer to manufacturer. In many cases, universal adhesives may be confused with “all-in-one” or “one-step self-etch.” The marketing hype professes that these adhesives can be used in total-etch, self-etch, or selective-etching mode and have an affinity for several substrates such as enamel, dentin, lithium disilicate, zirconia, and metals. While this applies to most universal adhesives, one product categorized as a universal adhesive has a strict two-step self-etching adhesive. Thus, universal adhesives differ considerably in their properties.

Due to variable etching options and the affinity for different substrates, universal adhesives have a much broader application in dentistry than many other adhesive systems. A great amount of confusion exists among clinicians when dental adhesives are brought up for discussion. The actual difference is in the chemistry of universal adhesives. Many contain 10-MDP as the main functional monomer, one with an excellent clinical track record. It can chemically bond to hydroxyapatite and different materials. Some universal adhesives also contain silane, a key chemical for bonding ceramic restorations, which simplified the bonding with glass-based materials and increased the strength bond. The reality is that all the chemistry must be balanced in one bottle, and manufacturers are attempting to combine hydrophilic with hydrophobic monomers.

In 2014, Peumans et al¹ reported in a systematic review that the annual failure rate for one-step self-etch adhesives was 5.4%, significantly lower than that for two-step self-etch (2.5%) and three-step etch-and-rinse adhesives (3.1%). These results correlate with a review² in 2005, in which the authors reported one-step self-etch adhesives had inefficient clinical performance when compared with other groups of adhesives. In 2010, a meta-analysis by Heintze et al³ demonstrated that one-step self-etch adhesives performed poorly compared with three-step etch-and-rinse and two-step self-etch adhesives. No clinical data report bond-strength tests with different materials such as noble metal, zirconia, and lithium disilicate. However, the in vitro studies to date are not promising, and it seems that activators or additional silane applications are still needed for achieving a reliable bond.

Manufacturers are trying to meet a market demand by simplifying procedures and reducing the number of steps needed. So far, this simplification has resulted in poorer clinical performance. For years, the three-step etch-and-rinse and the two-step self-etch products with the selective-etching technique have been the gold standard, and their performance still continues to yield the best results. In both of these approaches, three steps are needed. In reality, the other adhesives systems actually require three steps. With the two-step etch and rinse, the primer and adhesive are in one bottle, and the manufacturer recommends the application of two coats, making it a three-step procedure. In the all-in-one group, selective etching of the enamel and a separate second coat of adhesive or activator are recommended, also making the simplified approach a three-step procedure.

Universal adhesives have been a wonderful addition to the world of adhesive dentistry for both direct and indirect restorations, producing excellent bond strengths, greater ease of use, and less postoperative sensitivity. However, whenever products are considered to be universal, caution must be taken due to our tendency to become complacent. Simply because something is called universal does not suggest it should be used universally across various applications. In addition, several universal adhesives are available on the market, all with different chemistries; each should be considered carefully before making a clinical decision.

Generally speaking, universal adhesives can be universally indicated; however, the way in which they are used is not universal and the risk for lower bond strength must be weighed against the benefit of ease of use. Each tooth should be treated as a unique entity and as ideally as possible to ensure the highest predictability without sensitivity. No two teeth receive identical therapy, and the bonding procedures should reflect those differences to achieve optimal results.

When universal adhesives are employed, the amount of exposed dentin and enamel must be considered. Universal adhesives perform exceptionally well on dentin, but do not bond as well to enamel. When using universal adhesives, self-etch, selective-etch, and total-etch techniques must be considered and executed where necessary to yield the best results. In addition, when using universal adhesives with indirect dentistry, light-cure penetration must also be considered, and, in some cases, a dual-cure activator must be used to ensure a proper cure.

If the bonding takes place entirely in dentin, the self-etching universal adhesives are universally indicated and an excellent option. When bonding both enamel and dentin, the selective-etch technique (pre-etching enamel with phosphoric acid) with placement of a universal adhesive is universally indicated and a great option. When the bonding occurs in pure enamel, universal adhesives can be used in conjunction with the total-etch technique, but do not bond as well as other bonding systems.

While universal adhesives have replaced most of my fifth-generation bonding products, they have not replaced fourth-generation products for me. In my hands, universal adhesives for both direct and indirect bonding are universally indicated when used in conjunction with selective-etch techniques. Universal adhesives have a simplified bonding system with fewer steps, great bond strengths, and decreased postoperative sensitivity, but they have limitations when bonding enamel. When bonding in enamel only, I still perform a total-etch procedure and rely on fourth-generation bonding techniques to get the highest bond strength possible. Although universal adhesives have many applications, they are not universally indicated in my practice.