Hundreds of bacterial species inhabit the mouth, because of bacterial interactions, nutrient availability and low oxygen potentials in root canals with necrotic pulp, the number of bacterial species present in endodontic infections are restricted. These selective conditions lead to the predominance of facultive and strictly anaerobic microorganisms that survive and multiply, causing infections that stimulate local bone resorption, and are more resistant to endodontic treatment. Instrumentation of the root canal alone is not sufficient to remove infected necrotic tissues. The root canal must never be instrumented dry, an irrigating solution is always needed to reduce the amount of friction between the instrument and dentin surface to prevent binding and sticking. An irrigating solution is also needed to improve the cutting effectiveness of the files and instruments, dissolve tissue, cool the file and tooth, wash debris from the root canal, and be bactericidal in areas of the canal which could not be instrumented. Sodium hypochlorite has been the most widely used root canal irrigating solution for several decades, because it is inexpensive, can quickly dissolve infected necrotic tissues and is bactericidal. Sodium hypochlorite is very toxic to tissues when undiluted and so accidental spillage is always a concern among dentists. A severe limitation of Sodium hypochlorite and most other irrigating solutions is that they are unable to dissolve the instrumentation debris on cut dentin surfaces, called smear layer. The smear layer is a 1 to 5 micron thick layer of cut debris created on the surface of instrumented dentin, composed of dentin, odontoblastic processes, non-specific inorganic contaminants and microorganisms. To remove smear layer a chelating agent must be used after the irrigating solution to help clean the instrumented root canal surfaces. The most widely used chelating agent inside the root canal is 17% ethylenediaminetetraacetic acid (EDTA). A solution of 17% EDTA is a very reliable endodontic chelating agent when used fresh and at room temperature, but its chelating effects are time sensitive. Testing and clinical evidence has shown that 17% EDTA needs to be placed inside the root canal for 1 minute to effectively dissolve organic components and smear layer. The EDTA then needs to be suctioned, dried with paper points, and/or rinsed with Sodium hypochlorite to ensure it has been completely removed from the root canal after use. The process of canal preparation with files, instruments, and irrigating solutions is usually sufficient to remove most of the necrotic and infected tissues. Some recent articles suggest that the ultrasonic activation of irrigating solutions by using a high-speed vacuum; the EndoVac system, and that a laser using photon-induced photoacoustic streaming (PIPS) can improve the debridement of root canals.
