[The effect of left bacteria in the root canal on prognosis of the root canal therapy].

Objective: To study the effect of the left bacteria on the root canal therapy.
Methods: 50 single-rooted teeth with chronic apical periodontitis were divided into two groups, one was instrumented with step-back technique and 2.5%NaOCl ultrasonic irrigation for 3 min, then filled with Thermafil. Samples were taken after instrumentation to culture. The other was treated with traditional RCT at three visits.
Results: In 24 months the apical radiolucency were greatly reduced in all cases. There weren't significant relationship among the postoperative pain and the left bacteria, the degree of the obturation or the pre-operative symptoms (P > 0.05).
Conclusion: The effect of left bacteria in root canal filled with Thermafil wasn't observed.