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12 I I research _ mandibular first molars laris. Its shape and curvature are highly variable (Fig. 1b).23–25 Typically, the axis of the root faces the buccal aspect of the molar. Therefore, it could be easier to select the disto-buccal cusp as a reference point, instead of the typical DL. The combination of the slope present at the orifice and the buccal curva- ture at the apical third results in a highly complex canal to be instrumented and irrigated. To prevent mishaps, it is advisable to choose a small and highly flexible instrument when treating the apical portion. Diagnosis, access and proper treatment of the thirdrootwithinthecomplexcanalsystemareessen- tial in order to achieve successful endodontic treat- ment.Incasesofendodonticsurgicalprocedures,the third root will be a significant challenge.19 In a recent publication, Tu et al. report high DL root prevalence amongst the Taiwanese population.26 The authors found that the inability to recognise and treat this extrarootwasdirectlycorrelatedtotreatmentfailure, leading to tooth extraction. Table I summarises the findings of a systematic review compiling data on 4,745 MFMs.17 On average, threecanalswerepresentin61.3%ofcases,followed by four canals in 35.7% of cases and five canals in almost 1% of cases. In vivo studies performed by endodontists demonstrated the presence of four canals in 45% of the treated cases.27–30 Five canals werefoundin0.8%ofthesamples,whilecasereports have demonstrated the possibility of six- and even seven-root canals.31,32 _Mesial root morphology A systematic literature review of studies concern- ing more than 4,000 mesial roots confirmed the presence of two root canals in 94.2%.17 These canals merge in a common apical foramen (type II) in 35% of cases or remain independent with separate apical foramina in 52.3% of cases (type IV of Vertucci’s classification;TableI).Aclinicalapproachtoidentify- ing the internal canal configuration should include evaluation of the distance between the main orifices. The short distance between mesiobuccal (MB) and mesiolingual (ML) orifices often leads to confluence and termination in a common foramen. An increased distanceisdirectlycorrelatedtotypeIVconfiguration with two separate foramina.33 When facing a type IV configuration (2-2), the clinician should treat the canals independently. For merging canals, Castellucci explains that initially the canalsshouldnotbeinstrumentedtoworkinglength, thus preventing unnecessary removal of dentine.34 In addition, full instrumentation of both canals to workinglengthwillcreateanhour-glasspreparation, with the narrowed area at the junction and widening canalspaceapicaltothejunction.The3-Dobturation inthiscaseismuchmorecomplicatedandposesarisk of extrusion, as well as leaving some empty space in the most apical divergent zone.29 It is clinically safer and easier to instrument the ML canal to working length and the MB to the level of the confluence, since the latter is the closest to the outer surface of the root and also presents more severe curvatures than the ML.35,36 Marroquin et al. report that the average size of the maximum diameter is 0.31mm when the apical foramen is common.37 In contrast, the average maxi- mum diameter does not exceed 0.25mm when two separate foramina are present. This data suggests that treating a type IV configuration could allow a more conservative apical preparation. Nevertheless, canal preparation must always be correlated to the anatomy and the microbiological status of the canal. While vital cases should be treated more conser- vatively, infected canals may require larger apical preparations to allow efficient irrigation and disin- fection.38,39 Several publications report the presence of three canals in the mesial root.40,41 Our systematic review reportsanincidenceof2.6%(Figs.2&3).17 Inorderto localise it, access modifications are required. Briefly, once the main canals have been localised and their accessinstrumented,smallbursorultrasonictipsare used to remove the dentinal bridge that connects both entries, providing a direct view of the angle formed by the mesial wall and the floor of the pulp Figs. 3a–f_Root-canal treatment on an MFM: pre-op radiograph (Fig. 3a); working length radiograph (Fig. 3b); working length off-angle radiograph after location of three canals on mesial root (Fig. 3c); post-op, off-angle radiograph demonstrating three canals treated on mesial root (Fig. 3d); post-op, ortho-radial radiograph (Fig. 3e); final restoration control (Fig. 3f). roots1_2011 Fig. 3a Fig. 3b Fig. 3c Fig. 3d Fig. 3e Fig. 3f