No field is as dynamic and impactful on clinical dentistry as biomaterials science and arguably, the most influenced perhaps, is the field of endodontics. Endodontic therapy involves the use of a variety of chemicals of different nature, following which different types of materials are used for filling root canal systems. Is there a significant correlation between these two arms of treatment? Does one have a significant impact on the other? That is the purpose of this lecture. Hello, I am Prasanna Neelakantan, Clinical Assistant Professor of Endodontology, at the Faculty of Dentistry, the University of Hong Kong. In this session, we will talk about two paradigms from the biomaterials perspective. What kind of materials do we use in endodontics? And, how irrigants influence these materials, and from the endodontics perspective, should you consider specific irrigation protocols for specific sealers in the clinical scenario. While several classifications exist for root canal sealer materials, let us simplify them based on the modern trend. We can term them adhesive and non-adhesive sealers. Another dimension would be to term them bio-active and bio-inert sealers. There has been considerable confusion and controversy in categorising what adhesive and non-adhesive sealers are and research groups across the world do not appear to reach a consensus. For the purposes of this lecture, we will follow the bio-active and bio-inert sealer categorization. What are bio-active and bio-inert sealers? What you see is a section of root canal dentin with a root filling material. If the biomaterial does not bring about any change, that is, any mineral deposition at the interface with the dentin, it is termed a bio-inert material. Common examples will include zinc oxide eugenol and resin based sealers. If the biomaterial brings about mineral deposition at the interface, it is termed a bio-active material. Such a process is called biomineralisation or bioactivity, include examples of materials including tricalcium silicate based materials. Whether biomineralisation results in bonding of the cement to dentin or not remains a controversy, but endodontic research continues to test the adhesion of these materials to dentin as an indirect function of biomineralisation. For epoxy resin sealers, adhesion has been shown to correlate the sealing ability, whereas, methacrylate resin based sealers such a correlation does not appear to exist. From the perspective of root canal irrigation, we use proteolytic irritants such as sodium hypochlorite, demineralizing agents such as ethylene diamine tetra acetic acid and antiseptics such as chlorhexidine. Since 2009, our group has been actively studying a new irrigation model called continuous chelation wherein sodium hypochlorite is mixed with a weak chelator called etidronic acid and this is used as a 2-in-1 solution. In general, following root canal irrigation with these active chemicals, a final rinse of saline is used. Nevertheless, some traces of the chemicals may still remain and the substrate dentin has been altered in its characteristics to influence the adhesion of root filling materials, either positively, or negatively and that is what our research has shown us as you will see later. There are two ways to test if an irrigating solution has an effect on these root canal sealers. The first, expose the sealer material to the chemicals and characterise them using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Fourier Transform InfraRed Spectroscopy. Or, evaluate the dislocation resistance of these materials to dentin treated with the irrigating agents. Let us first see the impact of irrigating solutions on bio-inert sealers. The most advocated sealers in this category are epoxy resin based sealers and methacrylate resin based sealers. For the purposes of this collection, we will only focus on the push out bond strength of root filling materials after irrigation of root canals with different chemicals. With respect to epoxy resins, a final rinse of sodium hypochlorite decreases the bond strength whereas a final rinse of EDTA or chlorhexidine significantly improves the bond strength of an epoxy resin sealer, AH Plus. The continuous chelation protocol followed by EDTA appears to demonstrate the highest bond strength values for AH Plus. Why does this occur? When you have a final flush of hypochlorite after using EDTA, the naked collagen gets broken down and does not support adhesion. This is because the epoxy resin AH Plus binds chemically to the collagen of dentin. The polymerization of methacrylate resins is inhibited by the presence of oxygen. Hence, it is only logical that a final rinse of sodium hypochlorite will inhibit the adhesion of these materials. Final rinse of acids such as EDTA improves the bond strength of methacrylate resin based sealers. This irrigation protocol of sodium hypochlorite followed by decalcifying agents such as EDTA has been termed soft chelation. Final chlorhexidine rinses can also preserve this bond strength for epoxy, as well as, methacrylate resin sealers as it appears to enhance the durability of resin-dentin bonds by protecting it from the action of collagenolytic enzymes. The effect of irrigants on tricalcium silicates is interesting. Endodontic researchers would be well aware of the body of work done by Professor Josette Camilleri from Malta. Her contribution to our understanding of the behaviour of tricalcium silicate materials is immense. It has been reported that decalcifying agents interfere with the hydration of mineral trioxide aggregate. We also know that sodium hypochlorite causes discoloration of the cement and interferes with the hydration. This means that the irrigants interfere with the biomineralization process. So, we tested the effect of irrigation protocols on the push out bond strength of three bioceramic materials MTA Plus, Endosequence BC sealer and Tech Sealer Endo. While MTA Plus and Endosequence BC are composed primarily of tricalcium silicate, Tech Sealer Endo is said to be based on phyllosilicate. While testing these cements, our group realised one important effect. We know that biomineralization of these materials happens with time. Why is it that no paper evaluated the bond strength of these cements after a period of storage? Hence, bond strength testing of these materials were performed after two time periods, seven days and three months after storage in prospect of a silane. Our results showed that improvement of bond strength to the materials with time was related to the irrigating protocol used. The push out bond strength did not increase with time when the final irrigant was sodium hypochlorite or EDTA. The MTA based sealer, MTA Plus showed the highest bond strength values followed by Endosequence BC sealer and then Tech Sealer Endo. It is possible to correlate these results to what happens when this category of materials are exposed to irrigating solutions. So, we exposed specimens of mineral trioxide aggregate, with a type of tricalcium silicate to sodium hypochlorite, EDTA, and a mixture of sodium hypochlorite and etidronic acid, as well as, a mixture of chlorhexidine and EDTA. The MTA was characterised using SEM-EDS, XRD and FTIR. And our results were astounding. Chlorhexidine, sodium hypochlorite and EDTA interfere with the hydration of MTA whereas a mixture of sodium hypochlorite with etidronic acid, which is commercially available as a continuous chelating 2-in-1 solution and a commercial brand called QMix which is a mixture of chlorhexidine and EDTA, enhance the hydration and the calcium hydroxide release from MTA. Now, the most important considerations while performing these kind of studies are the variables that can influence this push-out bond strength, some authors may also term dislocation resistance. There are three main factors that need to be considered when one performs push-out bond strength tests. First, do we fill the root canals with just the sealer material or, a core-material such as gutta-percha along with the sealer? Second, do we do the bond strength testing corono-apical or apico-coronal? and third, and probably the most important the size of the plunger. The first question, an important and interesting one probably, also the most controversial one. One may argue that the clinical scenario demands the use of gutta-percha along with sealer in agreement. But could the gutta-percha and sealer have different magnitude of forces to bring about deformation or dislodgement? Yes, that is highly likely. We are testing the bond strength of the sealer material and I repeat, we are testing the bond strength of the sealer material and not the core material to dentin. Hence, it is the belief of our research group that to test the adhesion of a root canal sealer, it is essential to remove the confounding factor, which is the core material. Further, newer sealers such as tricalcium silicates have been suggested even as sole root filling materials without a core material. Hence, it is a valid design to fill root canals only with the sealer for these kind of experiments. Second, do we use the push-out bond strength force, corono-apical or apico-coronal? Remember that the canal, and even when prepared, does have a continuous taper. Even at very thin sections, the canals are by no way parallel-walled. So, we must over-ride the possible restriction that this taper gives, by inverting our samples and applying load on the apical surface of the sections so as to extrude the materials in the coronal direction. It is our belief that pushing in the corono-apical direction may require higher loads. So, one could say that in a way, this may be seen as a modified pull-out bond strength testing method. The third and the most important, the size of the plunger. Does it matter? Most definitely. An excellent paper published recently showed us that the size of the plunger is an extremely important variable in this method. Now, let's see why. We take sections of root canals from all the thirds of the tooth. The root canal diameter before or after preparation is not the same at all levels. Hence, the plunger dimensions must be suited to approximate the diameters of the root canal at that level. An approximate guide to this measurements is represented here. So, in summary, the body of evidence revolving around push-out bond strength of root filling materials suggests us the following. A final rinse of sodium hypochlorite is detrimental to both epoxy resin sealers and methacrylate resin based sealers. A final rise of EDTA enhances the bond strength of both these materials. The bond strength of tricalcium silicates is negatively influenced by sodium hypochlorite, EDTA and chlorhexidine. A mixture of sodium hypochlorite and etidronic acid improves the bond strength of epoxy resin and tricalcium silicate based sealers. Thank you for your attention.
