The primary objective of root canal treatment is to clean, disinfect, and hermetically seal the root canal system to prevent bacterial penetration; however, postoperative pain is frequently encountered, particularly in patients with symptomatic irreversible pulpitis. This condition, defined by the American Association of Endodontists as an irreversible inflammatory state of the pulp, is commonly associated with spontaneous pain, lingering discomfort, and heightened thermal sensitivity, and often represents a significant challenge in endodontics. Postoperative pain is usually linked to extrusion of microorganisms and debris into the periapical region during chemo-mechanical preparation. Bacteria remain the most persistent pathogens in root canal infections, and although sodium hypochlorite (NaOCl) is considered the most effective irrigant due to its antibacterial and tissue-dissolving properties, complete bacterial elimination before root filling is not always achieved. To enhance its antimicrobial efficacy, various adjunctive approaches have been suggested, including passive ultrasonic activation (PUA) and modifying NaOCl temperature, either by heating or cooling. Cryotherapy has gained increasing attention in endodontics for its potential to decrease postoperative pain and bacterial load, with additional reported benefits in vital pulp therapy, instrumentation, and anesthesia. Previous studies have shown that cryo-irrigation after NaOCl significantly reduces bacterial counts compared to NaOCl alone. Considering the potential benefits of these different irrigation protocols, the present study aims to evaluate and compare their effectiveness in reducing postoperative pain and bacterial load following single-visit root canal treatment of mandibular first molars diagnosed with symptomatic irreversible pulpitis.

The aim of the study will be directed to evaluate postoperative pain and bacterial load reduction after application of different final irrigation protocols in single visit root canal treatment in patients with symptomatic irreversible pulpitis in first molar teeth. this study consists of 60 patients divided into 4 groups each group having 15 patients.

control: Final irrigation by NaOcl (2.6%) at room temperature 24 °C for 1 min interventions: I1: Final irrigation by cryotreated NaOcl (2.6%) at 2-4 °C for 1 min . I2: Final irrigation by preheated 60°C(15) NaOcl (2.6%) for 1 min. I3:Final irrigation with ultrasonic activation of NaOcl(2.6%) for 1 min

Inclusion Criteria:

• Patients with age range between 18 to 40.
• No sex predilection.
• Patients with symptomatic irreversible pulpitis.
• Patients able to sign informed consent.
• Restorable teeth
• Positive patient's acceptance for participating in the study.
• Type III distal and mesial canal of first mandibular molar.
• Mild to moderate (5-20) ° curvature according to Schneider

Exclusion Criteria:

• Pregnant females
• Medically compromised patients.
• Patients having analgesics before the treatment
• If antibiotic have been administrated during the past two weeks preoperatively.
• Patients having bruxism or clenching
• Patients having sever malocclusion associated with traumatic occlusion.
• Teeth that have:

Peri-apical periodontitis Root resorption Pain on percussion Pulp stones or calcifications Previous endodontic treatment.

-Non-patent canals