Influence of Root Fusion on Canal Orifice Configuration, Lateral Anatomy, Isthmus Formation, and Apical Morphology in Maxillary Second Molars: A Micro-CT Analysis

Abstract

This study aimed to evaluate the influence of root fusion on internal root morphology in maxillary second molars by comparing teeth with fused and independent roots using micro-computed tomography (micro-CT), in terms of inter-orifice distances, lateral and accessory canal distribution, apical anatomy, isthmus prevalence, and type. Three hundred extracted maxillary second molars were classified as having fused or independent roots and scanned using micro-CT. Linear distances between canal orifices, vertical position of MB2, isthmus types, and canal ramifications were analyzed. ROC analysis was performed to determine the diagnostic utility of inter-orifice distances in predicting root fusion. Statistical comparisons between groups were made using independent samples t-tests and Chi-square tests with a significance threshold of 5%. Fused roots showed significantly shorter MB1-P and DB-P distances compared to independent roots (p < 0.05). Cutoff values of 3.87 mm (MB1-P) and 3.38 mm (DB-P) demonstrated high sensitivity and specificity, respectively, for predicting fusion. The MB1-MB2 distance was not diagnostically useful (AUC < 0.5). Fused roots exhibited more bifid apices and a significantly higher number of apical foramina in mesiobuccal roots (p < 0.05). Independent roots had a higher number of accessory canals, particularly in MB roots. Type IV (isthmus without boundaries) isthmuses were more prevalent in the fused roots (p < 0.05). Fused roots exhibit distinct anatomical features compared to independent roots. Inter-orifice distances involving the palatal orifice may assist in identifying root fusion and anticipating complex canal configurations during endodontic treatment. Inter-orifice distances involving the palatal orifice (MB1-P, DB-P) were shorter in fused maxillary second molars in this ex vivo sample. Fused molars also tend to have more intricate anatomy with more bifid apices, higher mesiobuccal apical foramina counts, fewer accessory canals, and a greater prevalence of Type IV isthmuses compared with independent-rooted counterparts in the examined population.