Factors influencing the outcome of root canal treatment of teeth with apical periodontitis as related to molecular microbiological findings.

INTRODUCTION: This study evaluated the influence of microbiological, clinical and therapeutic factors on the outcome of root canal treatment/retreatment of teeth with apical periodontitis using a sensitive molecular microbiology approach for bacterial quantification in canal samples. METHODS: Root canals of infected single-rooted teeth with apical periodontitis were treated or retreated using different instrument systems, irrigants, and intracanal medications. Root canal samples were taken before (S1) and after chemomechanical preparation (S2), and immediately before obturation (S3). Bacterial levels were quantified by using a quantitative real-time polymerase chain reaction assay. Patients were followed up for 1 to 2 years and the treatment/retreatment outcome was evaluated by clinical and radiographic criteria and categorized as healed, healing, or diseased. Lesions that were decreasing in size (healing) were classified as failure under a strict criterion and success under a lenient one. RESULTS: One hundred sixteen patients returned for follow-up evaluation. Diseased cases exhibited significantly higher bacterial counts both at baseline and at the time of filling compared to healing and healed cases (p<0.01). Canals of teeth with large lesions had 15 times higher bacterial counts at S1 than those with small lesions. Overall, 56/116 (48%) cases were classified as healed, whereas 14/116 (12%) were diseased. Treatment was successful in 51% (strict criterion) and 94% (lenient criterion) of the cases, while retreatment succeeded in 42% and 75%, respectively. Patients with large/very large lesions were 91% less likely to exhibit healing or complete healing compared to those with small lesions. Cases with baseline periapical index (PAI) scores of 4 or 5 showed lower healing/healed rates than those with a PAI score of 3. Use of 1% NaOCl was associated with lower odds of healing or complete healing compared with 2% chlorhexidine. The probability of success increased as a direct function of the magnitude of bacterial reduction from S1 to S2/S3. CONCLUSION: Treatment outcome was influenced by multiple factors, including initial bacterial load, lesion size, PAI score, irrigant type, and retreatment. Higher bacterial counts at the time of filling were directly associated with persistent apical periodontitis. Bacterial reduction had a significant impact on the outcome - for each 1% decrease in bacterial counts from baseline, the odds of a favorable outcome significantly increased by 10%.