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. 2020 Jan 27;10:1247. doi: 10.1038/s41598-020-58364-1

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Measurement parameters in each type of isthmus according to the classification reported by Hsu & Kim (5). (A) Type I, (B) type II, (C) type III, (D) type IV, (E) type V, and (F) one canal. L1: The shortest line between the mesial and distal root surfaces. L2: A line between the external lingual limit of the mesiobuccal (MB) canal and the external buccal limit of the mesiolingual (ML) canal. L3: The shortest line between the mesial and distal root surfaces passing through the centre of the canal. MBB: The shortest linear distance between the MB canal wall and the buccal root surface perpendicular to L1. MLL: The shortest linear distance between the ML canal wall and the lingual root surface perpendicular to L1. MBM: The shortest distance between the MB canal wall and the mesial root surface. MBD: The shortest distance between the MB canal wall and the distal root surface. MLM: The shortest distance between the ML canal wall and the mesial root surface. MLD: The shortest distance between the ML canal wall and the distal root surface. IM: Distance from the intersection of L1 and L2 to the mesial root surface. ID: Distance from the intersection of L1 and L2 to the distal root surface.