Methods: From 50 selected first permanent molars, sections of the MB root at 3, 4 and 5 mm from the apex were prepared, acid-etched, washed and dried. The apical side of each section was sputter-coated with gold, examined by a
scanning electron microscope and photographed.

Results: Overall, 18 (36%) of the 50 MB roots had one canal, whereas 32 (64%) had 2 canals. Of the roots with 2 canals, 10 (31.25%) contained either a complete isthmus or accessory canals or both between the 2 main canals. Another 10 (31.25%) showed partial isthmus formation.

Clinical Significance: MB roots exhibit a variety of canal configurations. On the basis of these findings, we propose a classification of the resected root surface of the MB root. Prudent judgement in preparing the canal isthmus,
meticulous skill and comprehensive knowledge of root canal anatomy are essential for successful treatment.