The Latest Developments in Universal Adhesives
Inside Dentistry (ID): What, if any, noteworthy developments have occurred in the formulations of universal adhesives during the past few years?
Gary Alex, DMD (GA): Although there have not been any major breakthroughs (that I am aware of) regarding the development of new or better structural and functional monomers, which form the backbones of universal adhesives, many companies have tweaked their adhesive formulations in one way or another. This can involve adjusting the solvent types and/or concentrations, the pH levels, and the monomers used as well as adding or removing various ingredients. For example, one well-known manufacturer that sells a popular universal adhesive system recently altered its formulation so that it would bond to self- and dual-cure composites and cement materials without the use of a separate activator that had previously been required. Other companies have added antimicrobials and other useful adjuncts into their formulations.
ID: Do two-bottle adhesive systems (assuming that they are still considered universal) generally perform better than single-bottle systems?
GA: Getting all of the chemistry of a universal adhesive into a single bottle and having it remain stable over a reasonable period of time and actually do what it's supposed to do can be challenging. In this regard, there can be certain advantages to two-bottle systems in which the chemistry is separated and applied in a sequential fashion. Indeed, some of the two-bottle self-etching systems from years ago, where the hydrophilic resins in the first bottle were placed followed by the hydrophobic resins in the second bottle, proved to be very successful. Most dentists, however, prefer the simpler to use single-bottle universal systems. To that end, the use of phosphate monomers, such as 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), in single-bottle universal adhesives has been a game changer. Laboratory and clinical studies generally demonstrate that these systems perform well if they are used correctly. Personally, I used a single bottle universal adhesive in my practice for more than 10 years and achieved excellent results.
ID: What do you advise regarding the use of universal adhesives with self-etch procedures versus total-etch and selective-etch ones and why?
GA: The pH of current universal adhesives varies from approximately 2.2 to 3.2, depending on the product. Generally, universal adhesives are considered to have "mild" (ie, pH ≥ 2) or "extra-mild" (ie, pH ≥ 2.5) etching capabilities. The universal adhesives in this pH range can be very effective when bonding to dentin; however, the concern is that they may not be as effective when it comes to bonding to enamel, especially uncut enamel. Although the initial bond strength to enamel with a universal adhesive in the self-etch mode can be quite good, many studies show that the bond strength can decrease significantly over time. My personal preference, and recommendation, is to only use universal adhesives in the selective- or total-etching modes when enamel is present in order to ensure predictable long-term bonding to this substrate. In cases in which there is little or no enamel remaining, and a full-coverage restoration will be bonded, using a universal adhesive in the self-etching mode is a viable, and perhaps even preferred, option.
ID: Can universal adhesives be used to replace primers, or should they be used in conjunction with primers in certain cases?
GA: The manufacturers of most universal adhesives typically state they can be used not only for bonding to dentin and enamel but also as adhesive primers on substrates such as zirconia, noble and non-precious metals, composites, and silica-based ceramics. The question isn't whether or not universal adhesives are capable of bonding to these substrates (they are) but whether or not they are as effective-both initially and, more importantly, over time—as separately placed dedicated primers. Regarding bonding to zirconia, there are studies that have demonstrated that some universal adhesives are very effective as zirconia primers. However, there are also studies that have found that the bond strength values of some universal adhesives to zirconia decreased significantly after thermocycling or 6 months of water storage. Many respected researchers believe that the most durable bond to zirconia is attained when separate and dedicated primers are employed.
ID: What is your opinion regarding the incorporation of silane into universal adhesives?
GA: The manufacturers of many universal adhesives have incorporated silane directly into their formulations with the idea that they can now be used without a separately applied dedicated silane solution. This makes sense as long as all of the other chemistry found in these universal adhesives does not interfere with the stability and performance of the silane. Indeed, some contact angle studies have found that the incorporation of silane into universal adhesives reduced the priming efficiency and chemical interaction of the silane with the porcelain surface when compared with pure silane controls. I have long advocated that silane that is free of any added monomers or resins (ie, pure silane) should be used if the goal is to optimize the bond to porcelain, especially over time.
ID: Do most of today's universal adhesives succeed in effectively maximizing the benefits of hydroxyethylmethacrylate (HEMA) while minimizing its negative effects?
GA: I think that some companies are better than others in achieving the delicate balance between HEMA, water, and ethanol in their universal adhesives. Almost all adhesive systems utilize HEMA in their formulations. It is a versatile, low-molecular-weight hydrophilic monomer that is particularly adept at infiltrating and "wetting" dentin substrates. In addition, HEMA is frequently added to adhesives not only to ensure good wetting but also because of its solvent-like nature. This improves the stability of adhesives and helps keep the hydrophobic and hydrophilic monomers in solution by minimizing phase separation in the presence of water. HEMA-free adhesives can have issues with phase separation. Although HEMA has many positive attributes, there are also downsides to its inclusion. In both the uncured and polymerized state, HEMA readily absorbs water. Once polymerized, it can swell, discolor, and contribute to hydrolysis of the adhesive interface. High amounts of HEMA can also decrease the mechanical properties of the resulting polymer, and uncured HEMA has the potential to lower the vapor pressure of water, which may make it more difficult to evaporate during the air-drying step. The concentration of HEMA used in universal adhesives varies from manufacturer to manufacturer; however, the goal should be to optimize it, using the least amount possible to take advantage of its benefits while concurrently minimizing its undesirable properties.
ID: When you are educating other dentists about universal adhesives, what is the single most important thing that you tell them?
GA: The most important thing that I tell them is that although we now have many excellent and chemically sound adhesive systems to choose from, good chemistry will never be able to overcome poor clinical technique. When it comes to adhesive dentistry, the biggest variable at this point is not the adhesive system used, it's the dentist. If you want to achieve better adhesion, then become a better dentist. Clinicians must pay meticulous attention to the details. Details such as control and isolation of the working area, cleaning and disinfection of the preparation, proper application of the adhesive system, complete evaporation of the solvent, and proper light curing of the adhesive, when indicated, are critical to long-term success.
ID: Do you have any predictions as to what developments we will see in the formulations of universal adhesives during the next 5 to 10 years?
GA: One of the most interesting and potentially important areas in adhesive research today focuses on the use of various chemical agents that are able to inhibit proteolytic enzymes, such as matrix metalloproteinases (MMPs) and cysteine cathepsins. Both of these enzymes, which are inherent in dentin and activated when acidic primers and conditioners are used during the bonding protocol, can break down the supportive collagen scaffolding of the hybrid layer and weaken the adhesive interface. Many of these agents are also antimicrobial in nature. In the future, it is likely that MMP inhibitors and antimicrobials will be incorporated, either directly or indirectly, into adhesive, conditioner, and restorative formulations and protocols. Furthermore, manufacturers will continue to attempt to develop self-adhesive restorative materials that may not require the use of a separately placed adhesive.
About the Expert
Gary Alex, DMD, is a private practitioner in Huntington, New York, who lectures extensively on adhesion and authored the frequently cited 2015 Compendium of Continuing Education in Dentistry article, "Universal Adhesives: The Next Evolution in Adhesive Dentistry?"
