Determination of optimal horizontal beam angulations for canal separation in mandibular molars using cone-beam computed tomography: a retrospective image-based analysis.

Objectives: : Two-dimensional intraoral radiographs often obscure canals due to superimposition, especially in mandibular molars with complex anatomy. This cone-beam computed tomography (CBCT) study identified the horizontal beam angles at which first and second molar canals overlap and derived clinically applicable angulations for enhanced canal separation. Methods: Eighty-five CBCT datasets from 100 patients met the inclusion criteria, yielding 318 mandibular molars (160 first, 158 second). Using ImageJ, absolute horizontal overlap angles (α) were measured to determine the corresponding theoretical separation angles defined as δ* = 90° - α. Separability was modeled across horizontal beam angulation increments from -45° to +45° in five steps, and Wilson's 95% confidence intervals were computed. Group comparisons used the Mann-Whitney U and independent t-tests (p ≤ 0.05). Results: Minimal mesial beam angulations for effective canal separability (δ* = 90° - α) ranged from approximately 7° to 15° for mesial roots and approximately 10° to 13° for distal roots. No significant mesial differences were observed between first and second molars (p > 0.30). Distal roots of second molars exhibited significantly higher angulations (p = 0.003 coronal, p < 0.001 apical). Mesial canals achieved ≥95% separability at approximately 25° and ≥99% at approximately 35°; distal canals required approximately 30° and approximately 40°. Conclusions: A mesial beam angulation of 30° to 35° provides probable canal differentiation in mandibular molars, separating mesial canals in ≥99% and distal canals in ≥95% of cases. This range refines previous recommendations and supports the as low as reasonably achievable (ALARA) principle.