Aim: To compare the clinical antimicrobial and adverse effects of irrigation with Sodium Hypochlorite (NaOCl) 0.5% versus NaOCl 3% during root canal treatment.

Methodology: Consecutive patients referred to a specialist clinic of endodontics are randomly assigned to have the root canal treatment performed with a concomitant irrigation either with a buffered 0.5 % NaOCl (Dakin's solution) or 3 % NaOCl. Teeth with vital or necrotic pulps and retreatment cases were included. Root canal treatment procedures varied but followed the routines of the Specialist Clinic of Endodontics, Public Dental Health, Gothenburg, Sweden. Immediately before the root canal filling a microbial sample is taken from the operative field and the root canal. The bacterial samples were processed at the laboratory as outlined by Möller (1966). After each visit each patient was instructed to fill in a questionnaire and assess his or her post-operative pain on a visual analogue scale (VAS), with endpoints 0= "no pain" and 10= "worst imaginable pain", for seven consecutive days. The Fischer's exact test was used for statistical analyses of the differences in outcome between the groups.

The aim of the present study is to compare the antimicrobial and immediate clinical effect of NaOCl 0.5% with the effect of NaOCl 3% in the endodontic treatment of teeth referred to the Specialist Clinic of Endodontics, Göteborg, Public dental Health, Västra Götaland. The efficiency of the treatment will be evaluated with sampling and cultivation methods and the assessment of patients' symptoms with a visual analogue scale.

The sample size calculation was performed with a type I error of 0.05 and statistical power of 80 %. The investigators expected 70 % of the samples to be free from bacteria growth in the control group and the investigators considered clinically relevant a difference of 15 % between the groups in order to demonstrate significant differences attributable to the experimental therapy used. The investigators want our sample size to be two hundred sixty four patients, 132 to each group.

A detailed explanation of the purpose of the study was given. Patients were also informed about the confidentiality of the data to be collected and about the voluntary participation. Every patient gets assigned a code that indicates which group they belong so the same irrigant is used in case of more than one visit.

Root canal treatment followed the standard principles of the Specialist Clinic of Endodontics, Göteborg, Public dental Health, Västra Götaland but the treatments were not restricted to a single protocol.

The recommended minimum or optimal apical size of canal preparation is size 25. All the operators are informed to perform the endodontic treatment the way operators normally do. The only variation during treatment was the concentration of the sodium hypochlorite solution for irrigation of the root canal. One group was irrigated with Sodium Hypochlorite 0.5 % (Dakin's solution) and the other group with Sodium Hypochlorite 3 %.

Operators were free to choose any additional irrigant as clinically perceived necessary. The irrigants available at the clinic were 17% ethylene-diamine-tetra-acetic acid (EDTA) and iodine-potassium-iodide (IPI) 5% used as a short duration antimicrobial agent as intracanal medication for 10 minutes.

Canal irrigation was carried out using 27 gauge side-cut open end needles (Monoject Luer lock syringe) with supplementation by ultrasonic agitation. Calcium hydroxide was the standard inter-appointment medicament. After sampling procedures gutta-percha and AH-plus sealer were used for root filling.

The access cavity was filled with Intermediate Restorative Material (IRM) or a permanent filling with composite after each appointment and after finished root canal treatment.

After bacterial sampling of the root canal, the samples were processed at the laboratory as outlined by Möller. Anaerobic incubation was done in a semi liquid medium (Hunton medium) inoculated under flow of oxygen-free gas. The mediums are checked daily for 14 days or until there is signs of microbial growth.

Fischer's exact test will be used for comparisons between groups regarding growth / no growth. For assessments of pain on the VAS t-test and 95% confidence intervals will be used. All hypothesis tests will be conducted at the 0.05 level of significance.

The outcomes will be analysed in term of positive Cultures and post operative pain. This is discussed further elsewhere in this application.

Inclusion Criteria:

• Patients in need of and willing to have a root canal treatment
• Diagnosis Pulpitis
• Diagnosis Apical periodontitis.
• If a patient was referred for more than one tooth just the first treated tooth was included in the study.
• The teeth had to be restorable and placement of rubber dam must be possible.

Exclusion Criteria:

• patient does not understand Swedish
• the tooth will be treated with a retrograde approach

The data is going to be published at a Group level