Morphometric Mapping of Mandibular Incisor Root Canals and Its Implications for Instrument Size-taper Selection.

INTRODUCTION

This study aimed to characterize the morphology of mandibular incisor root canals and to evaluate their correspondence with commonly used nickel-titanium (NiTi) instruments.

METHODS

One hundred extracted human mandibular incisors were scanned using high-resolution micro-computed tomography. Minor and major canal diameters and aspect ratios were measured at 1-mm intervals from the apical foramen to 10 mm coronally. Canal cross-sections were classified as round, long-oval, or flat based on aspect ratio. A supplementary analysis determined the percentage frequency of canals with diameters equal to or smaller than a wide range of size-taper combinations of constant-taper NiTi instruments. An 80% frequency threshold was applied to identify meaningful correspondence between canal dimensions and instrument sizes. Descriptive statistics were calculated, and the association between major and minor diameters was assessed using Pearson's correlation coefficient.

RESULTS

Both minor and major canal diameters increased progressively in the coronal direction, although enlargement of the major diameter was more pronounced from the middle third coronally. Round canals predominated in the apical region, whereas long-oval canals were most prevalent in the middle and coronal thirds; flat canals occurred mainly between 6 and 7 mm from the apex. Correspondence between NiTi instruments and the minor canal diameter depended strongly on instrument taper, with larger tapers required for smaller tip sizes to reach the 80% threshold. In contrast, correspondence with the major canal diameter was markedly limited across all instrument sizes and tapers, with no instrument achieving ≥80% correspondence along the entire root length. The correlation between minor and major canal diameters was weak, indicating disproportionate enlargement of canal dimensions.

CONCLUSIONS

Mandibular incisors present pronounced anatomical asymmetry, particularly in the middle and coronal thirds, resulting in limited correspondence between conventional NiTi instruments and the major canal diameter.