Stabilizing the Hybrid Layer

Lorenzo Breschi, DDS, PhD (LB): Degradation of the hybrid layer is, indeed, a very critical issue that has been discussed for many years because it is considered to be one the of reasons for the failure of composite restorations. More recently, significant progress has been made in improving its longevity. Research laboratories around the world have managed to create similar enough setups that we can now use the same techniques and compare results and findings with each other. Manufacturers have moved in the direction of simplification, and researchers have been able to test the new developments and either validate them or raise concerns. We have moved from three-step adhesive systems, which have been considered the gold standard, to the newest formulations of universal adhesives, which are not equal but are considered to be more stable than previous alternatives. Degradation still remains a significant issue; however, manufacturers understand that they need to produce materials that not only facilitate simplification but also stability. Consequently, we are getting very close to having dental materials that can create hybrid layers that are stable over time and more resistant to degradation.

ID: Is there long-term clinical evidence that the longevity of the hybrid layer can be improved with the addition of chlorhexidine or other agents?

LB: We have worked extensively on this. There is evidence, but it is adhesive-dependent. In vitro studies have shown that all adhesives seem to activate matrix metalloproteinases (MMPs); therefore, any molecule that decreases the activity of the MMPs is important to maintain the hybrid layer and stabilize the bond over time. Clinically, things change. In particular, it must be considered that because MMPs are active on exposed collagen, the adhesives that are more susceptible to this type of degradation will be the ones that result in larger amounts of exposed collagen in the hybrid layer. These are the two-step, etch-and-rinse materials. Alternatively, three-step materials envelop the collagen very nicely because they have a separate primer. Although we have shown a positive effect even after 10 years of in vitro aging, clinically, it is not easy to demonstrate the benefit of chlorhexidine. Self-etch materials do not fully expose collagen because it remains partially embedded in the mineralized tissue, which protects it. However, for the two-step, etch-and-rinse materials, I am very confident that the use of chlorhexidine as an additional therapeutic primer is beneficial in stabilizing the hybrid layer. Some studies have shown that chlorhexidine may leach out after 5 years, but we have found evidence via a study utilizing Raman spectroscopy that some chlorhexidine remains after 10 years.^1,2 However, again, the benefit depends on the type of adhesive being used. If, for example, a universal adhesive is being utilized with a selective-etch technique, which is probably the most supported method at the moment,³ then the use of chlorhexidine might be irrelevant because it is not necessary on enamel, and on dentin, the collagen remains completely encapsulated within a mineral coating. To summarize, so far, there is no strong scientific evidence from clinical trials that confirms that chlorhexidine pretreatment can improve the longevity of hybrid layers. However, because no side effects have been observed clinically, there is no reason to discourage the use of chlorhexidine.

ID: Although proving the benefits of chlorhexidine on bond strength in vivo is difficult, do you use it in your own practice?

LB: Yes, I do. Even if you remain skeptical about whether it helps to stabilize the hybrid layer,⁴ it still serves to disinfect the preparation. When we perform clinical studies, it is evident that so many different parameters are important in determining outcomes after 5 to 10 years. The most significant and variable is the patient; some restorations in patients with excellent oral hygiene do not last, and some restorations in patients with terrible oral hygiene last 20 years. Another major factor is the operator. Predictable outcomes are dependent on adherence to consistent protocols. Other factors, such as the type of composite used, the patient's occlusion, and more, can affect the outcome of a restoration during the course of 10 years. In that regard, chlorhexidine is just one small factor among many. It is hard to say if it would ever be possible to demonstrate over 10 to 20 years that such a small factor is definitively beneficial. However, if a were a patient, I would be happy to have chlorhexidine applied as an additional therapeutic primer. Again, we believe that chlorhexidine positively impacts the bond, but even if that does not happen, it will disinfect the preparation, so unless the patient has demonstrated an allergy, there is no reason not to use it.

ID: Shifting from materials to technique, what clinical protocol is recommended to improve the longevity of the hybrid layer?

LB: This is a wonderful question with no easy answer. First, we need to define whether an etch-and-rinse or a self-etch adhesive system is being utilized. For an etch-and-rinse system, the most important aspect is the impregnation of the exposed collagen fibers. This means that sufficient time is required when brushing on the primer for it to really penetrate and saturate the interfibrillar spaces that have been saturated with water after etching or have collapsed after air drying. During this time, it is very important for the primer to be applied, reapplied, and reapplied again. Then, air should be blown in order to abruptly evaporate the solvent, which accounts for approximately 40% of the primer. The primer will not make the adhesive layer thicker; it will help to penetrate the surface and change it from hydrophilic to hydrophobic, allowing the bonding agent to get into the network. Without primer, there is no bond. If the priming agent is not adequately applied, then the bonding agent will not sufficiently penetrate the demineralized dentin structure. For the self-etch approach, the protocol is exactly the same regarding the necessity of spending adequate time actively moving the brush on the surface to facilitate proper demineralization and refresh the self-etching agents. Therefore, the technique used to apply the material is crucial in all approaches. Another aspect that is very frequently underestimated is curing. There is no such thing as an overcured material in the mouth. Oftentimes, materials are significantly undercured, and research has indicated that suboptimal polymerization is one of the major issues impacting both bond longevity and restoration longevity. In summary, following a very meticulous protocol is important in order to improve infiltration for etch-and-rinse systems and to improve demineralization for self-etch systems. Solvent must be properly evaporated, especially for the self-etch method, to stop the chemical reaction. Finally, curing for an appropriate length of time is a crucial part of the process because, again, the materials very frequently are not cured properly.

ID: How do universal adhesives fit into the picture?

LB: When the majority of the substrate is enamel, I use a three-step adhesive, and when the majority of the exposed surface is dentin, I use the self-etch approach—either two-step or one-step. The problem with the three-step approach is that, as we discussed earlier, a very meticulous protocol must be followed, particularly for indirect restorations; otherwise, the bond thickness can be impacted, which we know can be as thick as a sealant because these materials are viscous and loaded. Conversely, universal materials have essentially solved that problem because they are much more hydrophilic and have a very small amount of filler, which allows for a very thin layer of adhesive. So, in my view, the substrate drives the choice of adhesive more than the clinical situation. However, I believe that the thickness can also depend on whether or not the bond is cured. If a significant amount of enamel is involved, the three-step approach is still the gold standard. If a significant amount of dentin is exposed, the research tells us that a two-step, self-etch system containing MDP is probably the best approach. The research on the one-step materials containing MDP indicates that these materials are getting very close to the efficacy of the gold-standard three-step, etch-and-rinse and two-step, self-etch adhesives. When the differences in the adhesives are that minor, they matter less than the techniques that we use to apply them, the skill of the laboratory technician, patient characteristics, and other factors. Therefore, I support the use of universal adhesives.

ID: Would you agree that any time a universal one-bottle system is being utilized, if enamel is present, it should first be selectively etched?

LB: Absolutely. Universal adhesives work very well, but when they are used on ename—particularly if it is uncut or superficial enamel—etching is very important. Research has confirmed this. Although many studies have shown that the bond strength is not much different, bond strength is not the primary problem that etching is needed to solve. The problem is the marginal leakage that can occur, which in clinical practice results in marginal staining and can pose significant issues for anterior restorations. Therefore, despite the fact that adequate bond strength could be achieved on enamel that is not etched, etching is very important to prevent the consequences of marginal leakage.

ID: What other developments do you believe will be the most impactful in this area?

LB: The most popular topic now is the use of cross-linkers—agents that are able to stiffen collagen, make it more stable and robust, and allow the bond strength to be more stable over time. This will be one of the most important developments for the future. Regarding the composite involved, the development of antibacterial properties and anything else bioactive will be very important. The challenge is that any material considered to be bioactive releases something, but the ability to release something beneficial means that the material is less stable and potentially weaker from a physical perspective, and it may not work as well as we need it to in the dynamic environment of the mouth. Conversely, if a material is super stable, it will not be bioactive and able to release minerals or any antibacterial agents. The future for both adhesives and composites lies in the development of materials that are both chemically and physically stable but active biologically in terms of reducing bacterial colonization and being self-repairable.