## 1 Introduction  
Tooth fracture is one of the most common reasons for tooth loss, after caries and periodontal disease. Additionally, fractures of teeth with root canal treatment are common occurrences in clinical practice [1]. Although the risk factors have not been fully defined [2], it was reported that the microstructure of dentin affects how the fracture line extends [3]. Vertical root fractures, described as longitudinal fractures extending parallel to the axis of the root of the tooth, have received attention due to their irremediable prognosis and significantly increased incidence rate after root canal treatment [4]. Several studies have highlighted root canal treatment as an important etiological factor in the vertical fracture of teeth [1, 4, 5, 6]. The main factor is loss of vitality, which highly relevant due to its biochemical effects on dentin-pulp bonding, which provides a natural monoblock structure of the tooth, the prevalence of which is significantly higher in endodontically treated than in vital teeth,resulting in a loss of structural integrity [5]. Although the loss of vitality is considered as the main predisposing factor, root canal treatment procedures like the removal of excess hard tissue during access cavity preparation, especially from pericervical dentin, as well as extensive preparation for root canal instrumentation, long-term exposure to medication, and chelator irrigation solutions, are the other predisposing factors for the formation of microcracks/distortions of structure corresponding to vertical root fractures [2; 5; 7; 8]. Calcium hydroxide (Ca(OH)\({}_{2}\), CH) is widely used in endodontics as an intracanal medication, a pulp capping, or as a component of some root-canal-filling materials [9; 10; 11]. It is frequently used to eliminate bacteria and reduce inflammation in cases where the root canal treatment cannot be completed in a single visit [9; 12]. It also supports hard tissue formation by activating root canal mineralization [13]. Despite the benefits of CH, its use between appointments for root canal treatment has some disadvantages. Failure to properly remove the medication from the dentin surface or irregularities before obturation may affect the sealing of the root canal filling [14; 15; 16]. Residual CH not only negatively affects the physical properties of root canal filling materials, but also blocks the bonding of the sealers to the dentinal walls [16; 17]. These occurrences may reduce the success of the root canal filling and reduce the resistance of the tooth to forces. Therefore, CH should be effectively removed from the root canal system before obturation. Different irrigation solutions, such as saline, sodium hypochlorite (NaOCI), ethylene diamine tetraacetic acid (EDTA), and citric acid and their combinations, have been used to remove CH from the root canal system [16]. However, using the fundamental solution of root canal treatment (NaOCI) alone is insufficient to remove CH [18]. The combined use of EDTA and NaOCI for CH removal is more effective than using the solutions separately. In addition, by increasing the effectiveness of the solution by applying it with an activation method, the solution can reach intracanal irregularities and remove the medication more effectively [19; 20]. The removal of the medicated dressing and both components of the smear layer and irrigation of the root canal system using 17% EDTA followed by NaOCI are recommended [21; 22]. The effectiveness of EDTA is directly proportional to the duration of its use. However, as the contact time with the root denitinal walls increases, the selator solution becomes more likely to cause erosion of the organic-inorganic component of the dentin [22]. The deterioration of dentin may weaken the structure of the tooth and affect its fracture resistance and strength. Due to being a calcium-binding chelating agent, EDTA dissolves the inorganic component of the smear layer [23; 24; 25].