Abstract
OBJECTIVES: To evaluate the influence of instruments with identical tip diameters but increasing taper on shaping efficiency and dentin preservation in the root canals of mandibular molars using micro-CT. MATERIALS AND METHODS: Twelve extracted mandibular molars with Vertucci Type III mesial canals and Type I distal canals were scanned using micro-CT before and after each instrumentation step. Mesial canals were sequentially prepared with 25/0.03, 25/0.05, 25/0.06, and 25/.08v instruments, whereas distal canals were prepared with 40/0.03, 40/0.05, and 40/.06v instruments. Co-registered datasets were used to quantify canal volume, surface area, percentage of unprepared canal surface, dentin removal, and minimum dentin thickness. In addition, dentin thickness values were categorized according to clinically relevant thresholds (< 0.5 mm, 0.5-1.0 mm, and > 1.0 mm). Data were analyzed using generalized linear and mixed-effects models (α = 0.05). RESULTS: In mesial canals, unprepared area decreased significantly with larger tapers; reductions were significant with 25/0.06 (-21.9%) and 25/.08v (-37.9%), but not with 25/0.05. Dentin removal increased markedly and progressively with taper, reaching ~ 350% with 25/.08v compared with 25/0.03 (p < 0.001). In distal canals, taper did not significantly affect the unprepared area; however, dentin removal increased significantly with 40/0.05 and 40/.06v. Directional analyses confirmed a stepwise increase in dentin reduction, following the pattern mesiobuccal > mesiolingual > distal at all levels. Threshold analysis showed that the proportion of dentin thickness measurements within the 0.5-1.0 mm range increased with larger tapers, particularly on the distal aspect of the mesial roots. CONCLUSIONS: Increasing instrument taper improves shaping efficiency in mesial canals but substantially increases dentin removal and shifts dentin thickness toward potentially vulnerable ranges. Conservative taper selection may therefore better balance shaping effectiveness and structural preservation in mandibular molars. CLINICAL RELEVANCE: Larger tapers may improve canal wall contact but can substantially reduce dentin thickness in anatomically vulnerable regions, emphasizing the importance of anatomy-guided taper selection during root canal preparation.