It is well known that the decontamination of the root canal system from pathological microflora is a prerequisite for the success of endodontic treatment. In addition to mechanical instrumentation, various irrigation solutions and intracanal medications are used to decontaminate the root canal from microorganisms. The combined use of mechanical instrumentation and irrigation solutions is referred to as the chemomechanical process. However, due to the complex anatomy of the tooth structure, such as lateral canals, isthmuses, dentinal tubules, and accessory canals, residual microorganisms may remain despite the completion of the chemomechanical process. This can negatively affect the success of the treatment. Additionally, it is noted that the dentin walls may weaken as a result of chemomechanical preparation. Therefore, researchers often emphasize the need for more effective irrigation and disinfection systems.

Recently, the application of antimicrobial photodynamic therapy (aPDT) has been shown to be a promising addition to traditional methods. Today, aPDT is used to eliminate resistant microorganisms in cancer treatment, macular degeneration, arterial plaque, arthritis treatment, and dentistry. aPDT has been reported to effectively destroy oral bacteria present in both planktonic and biofilm forms. However, researchers state that aPDT cannot replace traditional endodontic procedures but can be used as an adjunct in root canal disinfection.

The working principle of aPDT is based on the presence of a specific photosensitizer, a radiation source with a sufficient wavelength, and the presence of oxygen in the environment. As a result of the reaction caused by aPDT, reactive oxygen species (ROS) such as singlet oxygen and free radicals are produced, leading to a bactericidal effect. Thus, a wide range of microorganisms can be eliminated without encountering the resistance problem of current bacterial strains. Additionally, aPDT offers various advantages due to its repeatability, low toxicity, and minimally invasive approach.

Various photosensitizers and light sources with different wavelengths are available for use in aPDT. One of the most widely used photosensitizers in the literature is methylene blue (MB) from the phenothiazine salts group. However, it is known that MB is difficult to remove after application and can cause tooth discoloration. Therefore, alternative photosensitizers to MB are being sought. Today, the antimicrobial efficacy of MB is well recognized. Due to the low absorption exhibited by phenothiazine dyes, which are among the most commonly used photosensitizers, multiple applications are required for a high drug dose and effective treatment outcome. The effectiveness of solutions is not solely dependent on the properties of the solution but also varies with the method used. Although the traditional needle irrigation method (GII) is still in use, its effectiveness is limited. Another method, manual dynamic activation (MDA), involves creating a hydrodynamic force in the irrigation solutions by moving a gutta-percha that fits well into a shaped root canal in a corono-apical direction at the working length. This method is frequently used as it does not require additional materials.

With technological advancements, different methods have been developed to enhance the effectiveness of solutions. The passive ultrasonic irrigation (PUI) method shows effectiveness by causing acoustic streaming through the vibration it generates. The solution is activated during the application without any contact with the root canal walls. Another method, the sonic activation system (SAS), has been used in endodontics for years and has proven its effectiveness. Several SAS devices from different manufacturers have been developed. One commonly used device, Endoactivator, has a polymer tip and works through sinusoidal oscillation. Another widely used device, EDDY (VDW, Munich, Germany), oscillates at 5000-6000 Hz, causing three-dimensional movement in the root canal and triggering cavitation and acoustic streaming. Studies have shown that the effectiveness of PUI and SAS on traditional endodontic irrigation solutions is similar. Enterococcus faecalis (E. faecalis) is a gram-positive facultative bacterium. It is among the most resistant strains involved in the development of anaerobic and endodontic infections. Although detailed literature reviews have shown studies proving the effectiveness of MB on E. faecalis bacteria, there is no study comparing the effectiveness of MB solution in necrotic and periapical lesion teeth using ultrasonic and sonic systems. It has been found that there is no comprehensive study in the literature comparing the discoloration potential and postoperative pain of MB applied with various activation systems. The null hypothesis of the study is that there will be no significant difference in lesion healing, postoperative pain, and discoloration potential with different activation systems using aPDT.