The aim of this study is to compare the effect of passive ultrasonic irrigation versus needle irrigation on post-operative pain and healing in patients with permanent posterior teeth with apical periodontitis treated in a single visit.

Full medical and dental history using a dental chart for all patients treated in this study.

Clinical and radiographic examination will be done for all patients to verify the diagnosis of apical periodontitis with periapical radiolucency at least 2mm in diameter.

The treatment protocol and its associated risks and benefits will be explained to the eligible patients before obtaining informed consent. Endodontic procedures will be done in a single visit as follows: access cavity preparation using sterile round bur and Endo-Z bur, isolation using a rubber dam, canal preparation using a nickel-titanium rotary system then obturation. During mechanical preparation, irrigation will be done using passive ultrasonic irrigation (Experimental group) where ultrasonic activation will be done for irrigant-filled canals using the U- file tip (Varios NSK) in an ultrasonic device (P5 Newtron; Satelec Acteon) 1mm from the working length after each file change for 10 sec, or needle irrigation (Control group) where only the endodontic irrigation needle will be used; the irrigation protocol for each patient will be determined following a random-number sequence.

Inclusion Criteria:

• Age between 18-55 years old.
• Males and females.
• Positive patient's acceptance to participate in the study.
• Mature, permanent, mandibular, posterior teeth with apical periodontitis.

Exclusion Criteria:

• Medically compromised patients so that no other pain source or drug interaction could interfere with pain resulting from the endodontic therapy.
• Patients who had received analgesic treatment recently (past 8 hours) as it can affect the results
• Pregnant females.
• Teeth with previous root canal treatment.