Comparative Retreatment Efficacy of Two Multi-File Systems with Different Access Cavity Designs: A Micro-Computed Tomography Study.

: The access cavity design and instrumentation system could affect the remaining root canal obturation materials in root canal retreatment. This study aimed to evaluate the efficiency of two different multi-file systems in removing obturation materials with two different access cavities utilizing micro-CT scanning. : Conservative access cavity (CAC) preparation was performed for 80 mandibular premolars. Then, root canal preparation was employed followed by obturation. The retreatment process began by dividing the samples into two separate groups: conservative (CAC) and traditional (TAC) access cavities. Subsequently, these groups were assigned to eight distinct subgroups (n = 10): Group 1; TAC and ProTaper retreatment system (PTR) with ProTaper Next (PTN); Group 2, TAC and PTR + ProTaper Ultimate (PTUL); Group 3, TAC and PTN; Group 4, TAC and PTUL; Group 5, CAC and PTR + PTN; Group 6, CAC and PTR + PTUL; Group 7, CAC and PTN; and Group 8, CAC and PTUL. The samples underwent micro-CT scans before and after the retreatment process, and the volume and percentages of remaining root canal filling material were calculated. Statistical analysis of the data was performed, and significance was determined at the 5% level. : The influence of the access cavity design ( = 0.500), the ProTaper system ( = 0.138), and the interaction of these variables ( = 0.513) was insignificant. However, group 3 (TAC and PTN) showed the highest percentage of remaining obturation materials at 29.53%, contrasting with group 6 (CAC and PTR + PTUL). : Neither retreatment procedure succeeded in completely removing filling materials. Nevertheless, the impact of access cavity design, different multi-file systems, and their interaction on the remaining root canal obturation materials was deemed insignificant.