ORIGINAL ARTICLE Root groove depth and inter-orifice canal distance as anatomical predictive factors for danger zone in the mesial root of mandibular first molars Gustavo De-Deus 1 & Evaldo Almeida Rodrigues 2 & Jong-Ki Lee 3 & J. Kim 4 & Emmanuel João Nogueira Leal da Silva 1,5 & Felipe Gonçalves Belladonna 1 & Daniele Moreira Cavalcante 1 & Marco Simões-Carvalho 1 & Diogo da Silva Oliveira 1 & Marco Aurélio Versiani 1 & Erick Miranda Souza 6 Received: 18 May 2020 /Accepted: 6 November 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Objectives This study evaluated the danger zone (DZ) in mesial roots of mandibular molars and the correlation between anatomical references of the DZ and some anatomical landmarks including tooth/root length, depth of mesial and distal grooves, and inter-canal orifices distance. Material and methods Twenty-eight mesial roots of mandibular molars with 2 independent canals were scanned and divided into 2 groups according to root length. The anatomical landmarks were correlated (Pearson or Spearman coefficients) with root level, thickness, and position of the DZ and also compared (independent samples t or Mann-Whitney tests) between the 2 groups at α = 5%. Results No statistical difference was observed between groups regarding DZ parameters and depth of mesial and distal grooves (P > 0.05). Orifice distance in group 2 (4.49 ± 0.75 mm) was significantly greater than group 1 (3.76 ± 0.89 mm) (P < 0.05). Significant correlations (P < 0.05) were found between (i) DZ level and root/tooth length (r = 0.54 and 0.49, respectively), (ii) DZ thickness and distal groove depth (r = - 0.45), and orifice distance (r = 0.38), and (iii) DZ position and depth of mesial (r = 0.39) and distal (r = 0.40) grooves. Other variables such as root length and distal groove depth (r = 0.28), and orifice distance and mesial groove depth (r = 0.36) were also correlated (P < 0.05). Conclusions The length of tooth/root, the distance of canal orifices, and the depth of mesial/distal grooves of mesial roots of mandibular molars might be predictive factors for the root level, position, and thickness of the DZ. Clinical relevance The length, distance of mesial canal orifices, and the depth of mesial and distal grooves of the mesial roots of mandibular molars might be moderate predictive factors for the root level, position, and thickness of the DZ. Keywords Danger zone . Dentine thickness . Mandibular molar . Mesial root . Micro-CT Introduction Forty years ago, Abou-Rass et al. [ 1 ] proposed the anticurvature filing technique to prepare curved canals and, simultaneously, introduced the concept of danger zone (DZ). According to these authors, DZ would be a specific region of the root more susceptible to strip perforation in case of exces- sive removal of dentine during mechanical preparation. Since then, different aspects of the DZ have been investigated [2–5] but, in most of the studies, the underlying principle of this concept was associated only to the distal aspect of the mesial root of mandibular molars (furcation area) [5–10]. However, in the last years, this concept was revised and the evaluation of hundreds of cross-sections of mesial roots of mandibular * Gustavo De-Deus endogus@gmail.com 1 Department of Endodontics, Fluminense Federal University, Niterói, Rio de Janeiro, Brazil 2 Department of Endodontics, Feira de Santana State University, Feira de Santana, Bahia, Brazil 3 Private Practice, Endodontics, Changwon, Republic of Korea 4 Research Director, Cybermed Inc., Daejeon, Republic of Korea 5 Department of Endodontics, Grande Rio University, Duque de Caxias, Rio de Janeiro, Brazil 6 Department of Dentistry II, Federal University of Maranhão, São Luís, MA, Brasil Clinical Oral Investigations https://doi.org/10.1007/s00784-020-03686-7