Thesis of Dr. Chafic Safi – one of our distinguished faculty – Abstract: Introduction: The purpose of this study was to compare PA and CBCT healing after EMS using MTA or ERRM as root-end filling materials in a randomized clinical trial. Materials and Methods: EMS was performed on patients with persistent or recurrent AP. The root-end filling material was randomly assigned. Regular follow-up visits were scheduled 6 months, 1 and 2 years. PA and CBCT were taken and the tooth was investigated clinically. Results: Regardless of the material, the overall PA and CBCT success rates were 95.6% and 89.1% respectively with no statistical significant difference. Evaluating each material separately, MTA success rate was 95.5% and 89.3% as assessed by PA and CBCT respectively. ERRM success rate was 95.8% and 88.7% as assessed by PA and CBCT respectively. Fisher exact test showed no statistical significant difference between the 2 materials. Pre-operative prognostic factors such as microsurgical classification, presence of cortical, size of lesion and root canal filling quality had significant effect on outcome. Follow-up factors such as alignment and depth of filling material and root fractures also had significant effect on outcome. Conclusion: The null hypothesis is accepted: there is no statistical significant difference in EMS outcome using RRM or MTA as retro-filling material. However, certain factors were found to have a significant association with outcome.