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I 13 research _ mandibular first molars I roots1_2011 chamber, exposing the developmental groove be- tweenthetwomaincanals.Anendodonticexploreris then used, followed by negotiation with small files. Additionally, the use of operating microscopes fur- ther improves the possibility of finding and treating this accessory canal.42 Taking into consideration the distalconcavityofthemesialroot,instrumentationof the third medial canal must be done carefully using small instruments to avoid stripping perforations.27 The middle mesial is an entirely independent canal in up to 25% of cases.17 _Distal root morphology Gulabivala et al. evaluated 139 MFMs and found that 74.8% of the distal roots had a flattened MD morphology.21 Theyalsonotedthatconicaldistalroots frequently presented a single canal, while the vast majorityhadmorecomplexconfigurations.Therefore, routine access openings should be modified in search of a second or a ribbon-shaped canal. The access design has evolved from the classic triangular to a rectangular shape shifted to the MB.27,43 Martinez-Berna and Badanelli were the first to reportathirdcanalinthedistalrootandtermeditthe disto-central(DC)rootcanal.44 Aliteraturereviewsets the incidence of DC at 1%.21,31,39,45–50 _Intercanal communications The morphology and buccolingual width of the mesial root allow for intercanal communications and isthmuses (Fig. 4). An isthmus (anastomosis) is defined as a pulpal passageway that connects two or more canals in the same root.51 In young patients, we should expect to find large canals with wide isthmuses. As secondary dentine is deposited throughout the maturation of the tooth, these large communications are divided into smaller ones and, eventually, its frequency decreases after age 40.52 Of the 1,615 MFMs reviewed, 50% of the mesial and 20% of the distal roots presented isthmuses of type V. Type V is recognised as a true connection or widecorridoroftissuebetweenthetwomaincanals.53 Therefore, the presence of isthmuses should be considered the rule rather than the exception when treating young MFMs. Given the extreme difficulty in disinfecting these inaccessible spaces,54 our efforts should be focused on improving our irrigation protocols with the more efficient systems available today. The clinical impor- tance of recognising, treating and disinfecting isth- museswasrecentlypointedoutbyVonArx,whoiden- tifiedcompletecross-anastomosisin29%ofcasesof failedroot-canaltherapiesrequiringapicalsurgery.55 _Conclusion The following is a summary of the findings of our review: 1. The number of roots in the MFM is directly related to the ethnicity of the population studied. 2. The instrumentation of the third root requires a different access and the use of small and flexible instruments, considering the curvature at the apical third. 3. Mesial roots present two canals on a regular basis, with2-2and2-1themostfrequentconfigurations. A third canal might be present in 2.6% of the pop- ulation. 4. The most common configuration in the distal root is 1-1 (62.7%), followed by 2-1 (14.5%) and 2-2 (12.4%). 5. Access modifications are required in order to find extra roots and/or canals. 6. Thepresenceofisthmusesis55%inthemesialroot and20%inthedistalroot.Thisanatomicalconfig- uration should be taken into consideration during endodontic treatment and peri-apical surgery._ Editorial note: A complete list of references is available fromthepublisher. Fig. 4_Micro-computed tomography of an MFM with 3-D reconstructions on different projections showing the very complex anatomy of the root-canal system (Image courtesy of Prof Marco Versiani and Prof Manoel D. Sousa Neto, Ribeirão Preto Dental School, University of São Paulo). Dr Nestor Cohenca Department of Endodontics University ofWashington Box 357448 Seattle,WA 98195-7448 USA cohenca@uw.edu _contact roots Fig. 4