The goal of this clinical trial was to evaluate the effect of cryotreated sodium hypochlorite solution on the intensity of postoperative pain in root canal treatment procedures in mandibular first molar teeth with symptomatic irreversible pulpitis. The main question it aims to answer is:

- Does controlled irrigation with cold sodium hypochlorite in root canal treatment reduce the severity of postoperative pain? In the cryotreated NaOCl group, unlike the normal NaOCl group, the root canals were irrigated with 2 °C NaOCl solution.

Various irrigation solutions that are effective for organic and inorganic tissues are used during root canal treatment. Sodium hypochlorite (NaOCl) is the most commonly used irrigation solution. It has antibacterial and physicochemical properties 20 and is unique in dissolving organic tissues 21. NaOCl irrigation combined with mechanical instrumentation has been consistently shown to substantially reduce the microorganism load in root canals 22. There have been several attempts to increase the antimicrobial efficacy of NaOCl in the root canal system, such as changing its concentration, increasing the contact time with the dentin walls of the root canal, increasing the total volume of irrigation, increasing its temperature, and using manual or mechanical activation methods 3, 23. These methods to increase the efficacy of NaOCl can also result in heightened toxic effects on periapical tissues owing to the tissue-irritating properties of the solution. This extrusion into the periapical region increases postoperative pain, especially in teeth with open apices and necrotic pulps 21, 24. Therefore, instead of using high concentrations of NaOCl, which could increase its toxic effects, safer means, such as increasing its temperature and using it in large volumes, may increase its efficacy 21, 25.

Therefore, ascertaining the clinical impact of cryotreated NaOCl on postoperative pain is essential. The temperature of the irrigation solution is known to affect postoperative pain, and final irrigation with cold saline significantly reduces postoperative pain levels 19, 26. However, studies investigating the effects of cryotreated NaOCl, which is used in all stages of root canal treatment, and using NaOCl at concentrations of 2.5% and 5% on postoperative pain are lacking. This randomized clinical trial investigated the effects of cryotreated 2.5% and 5% NaOCl on postoperative pain during endodontic treatment procedures in mandibular first molars with symptomatic irreversible pulpitis.

Inclusion Criteria:• Vital mandibular first molars with 3 root canals that responded positively to the cold test and electric pulp test

• Mandibular first molars with a preoperative VAS value of 4 and above (no pain (0 mm), mild pain (1-3 mm), moderate pain (4-6 mm), severe pain (7-10 mm))
• Teeth without periapical radiolucency seen radiographically
• Patients who did not take analgesics or antibiotics in the last 24 hours before the procedure were included in the study.

Exclusion Criteria:

• Devital teeth that responded negatively to cold tests and electric pulp tests,
• Teeth with poor periodontal status (pocket depth > 4 mm),
• Mandibular first molars with 4 or more root canals detected radiographically or after opening the endodontic access cavity,
• Teeth with no preoperative pain or with mild preoperative pain (VAS value below 4) were excluded from the study.