This study aims to assess the effect of rotary instrumentation using ProTaper Next rotary system versus TruNatomy rotary system on postoperative pain and the bacterial load reduction following single visit root canal treatment in necrotic mandibular molars.

In the last years several rotary systems had launched to the market for mechanical preparation. The new era in the mechanical instrumentation aims to preserve the maximum amount of the peri-cervical dentine without affecting the proper cleaning and bacterial elimination. This will improve the long-term survival of the tooth with appropriate removal of microbes and microbial irritants.

ProTaper Next is a rotary system made from M wire. It is characterized by an innovative off-centered rectangular cross section that is suggested to give the files a snake-like swaggering movement as it advances into the root canal. This movement minimizes the engagement between the file and dentin and improves extruding debris out of the canal.

TruNatomy (Dentsply Sirona), is recently introduced as a new generation of rotary file system. TruNatomy files are designed to shape root canal systems to a continuously tapering preparation with maximum preservation of peri-cervical dentine. TruNatomy system offers a more safety, simplicity, improved cutting efficiency and mechanical properties compared to previous generations of rotating instruments, Many researches had been conducted to address the effect of ProTaper Next on post-operative pain considering it the most commonly used system. Thus, this study will be performed to evaluate and compare the influence of using the two systems on the intensity of postoperative pain and bacterial load reduction.

Inclusion Criteria:

• - Medically free patients.
• Mandibular permanent molar teeth:
• Diagnosed clinically with pulp necrosis.
• Absence of spontaneous pain.
• Slight widening in the periodontal membrane space or with periapical radiolucency
• Patient's age ranges between 22 to 45 years with no sex predilection.
• Patients who can understand Modified Visual Analogue Scale (VAS).
• Patients' acceptance to participate in the trial.
• Patients able to sign informed consent.

Exclusion Criteria:

• - Medically compromised patients: Pain levels and healing following treatment would be compromised as these patients have shown higher incidence of pain and lower healing rate.
• Pregnant women: Avoid radiation exposure, anesthesia, and medication.
• If analgesics or antibiotics have been administrated by the patient during the past 24 hours preoperatively might alter their pain perception.
• Patients reporting bruxism or clenching: Avoid further pressure on an already inflamed tooth inducing subsequent irritation and inflammation.
• Teeth that show association with acute periapical abscess and swelling: Need special treatment steps which could involve additional visits with incision and drainage. Also, it could influence initiation and progression of postoperative pain.
• Greater than grade I mobility or pocket depth greater than 5mm. Need special surgical and/or periodontal therapy.
• No restorability: Hopeless tooth.
• Vital teeth.
• Immature teeth.
• Radiographic evidence of external or internal root resorption vertical root fracture, perforation, calcification.

The assistant supervisor is the one who will generate the random sequence, assign the participants to the intervention or control groups and the only one who knows whether A or B represents the intervention or the control.

The operators will enroll the participants after they find the eligible ones and confirm with the assistant supervisor.