SciELO - Scientific Electronic Library Online

vol.36 número3Lesiones bucales de Histiocitosis de Células de Langerhans en pacientes pediátricosFluoruro en el agua subterránea potable y prevalencia de fluorosis en niños y adolescentes: Una revisión sistemática índice de autoresíndice de assuntospesquisa de artigos
Home Pagelista alfabética de periódicos  

Serviços Personalizados




  • Não possue artigos citadosCitado por SciELO

Links relacionados

  • Não possue artigos similaresSimilares em SciELO


Acta Odontológica Latinoamericana

versão impressa ISSN 0326-4815versão On-line ISSN 1852-4834

Acta odontol. latinoam. vol.36 no.3 Buenos Aires dez. 2023  Epub 31-Dez-2023 


Morphological assessment of the isthmus in mesial root canals of first mandibular molars

Avaliação morfológica do istmo nos canais radiculares mesiais dos primeiros molares inferiores

1Universidade Estácio de Sá, Departamento de Endodontia, Rio de Janeiro, Brasil

2Universidade Nova Iguaçu, Programa de Pós-Graduação em Odontologia (Endodontia), Nova Iguaçu, Brasil

3Faculdade São Leopoldo Mandic, Campinas, São Paulo, Brasil


Root canal morphology and its anatomical variations pose a great challenge to endodontists


The aim of this in silico study was to perform a qualitative and quantitative analysis of the threedimensional morphological characteristics of the isthmus in the mesial root canals of mandibular molars using microcomputed tomography (micro-CT)

Material and Method

Six hundred first mandibular molars were selected, including 317 with two mesial canals with isthmuses between the canals, and fully formed root. Isthmus morphology was determined in 3D longitudinal sections using Fan et al. (2010) classification. Root length, and the volume and area of apical and coronal level were measured. Additionally, the structural model index (SMI) of the canals were also assessed


The prevalence of isthmuses in the mesial root canals was 32% type II, 29% type III, 22% type IV, and 17% type I. The root length was found to be 9.1±0.5 mm, the volume and area, of all root canal system, were 41.8±40.1 mm3 and 63.6±24.2 mm2 respectively. The isthmi volume and area alone were 11.06±9.03 mm3 and 30.02±11.02 mm2. The study confirmed that isthmuses are present in mesial canals of mandibular first molars, being more frequent in the apical third


The high prevalence of isthmuses with complex morphological features underscores the importance of using intracanal medications to disinfect areas unprepared by instruments.

Keywords: endodontics; root canal therapy; X ray microtomography


A morfologia do canal radicular e suas variações anatômicas representam um grande desafio para os endodontistas. O objetivo deste estudo ex vivo foi realizar uma análise qualitativa e quantitativa das características morfológicas tridimensionais do istmo nos canais mesiais de molares inferiores por meio de microtomografia computadorizada (micro-CT)

Material e Método

Foram selecionados 600 primeiros molares inferiores, incluindo 317 com dois canais mesiais com istmos e raiz totalmente formada. A morfologia do istmo foi determinada em cortes longitudinais 3D usando a classificação Fan et al. (2010). Foram mensurados o comprimento da raiz, o volume e a área apical e coronal e da cavidade pulpar. Adicionalmente, também foram avaliados o structure model index (SMI) dos canais


A prevalência de istmos nos canais mesiais foi de 32% tipo II, 29% tipo III, 22% tipo IV e 17% tipo I. O comprimento da raiz foi de 9,1±0,5 mm, o volume e a área de todo o sistema de canais radiculares foram de 41.8±40.1 mm3 e 63.6±24.2 mm2, respectivamente. O volume e área do istmo isoladamente foram 11.06±9,03 mm3 e 30.02±11.02 mm2. O estudo confirmou que os istmos estão presentes em canais mesiais dos primeiros molares inferiores, sendo mais frequentes no terço apical


A alta prevalência de istmos com características morfológicas complexas ressalta a importância do uso de medicação intracanal para desinfecção de áreas não tocadas por instrumentos.

Palavras-chave: endodontia; tratamento de canal radicular; microtomografia por Raio X


Root canal morphology and its anatomical variations pose a great challenge to endodontists 1 . Although success rates for endodontic treatments are generally high, cases of failure have been attributed to lack of knowledge of root canal internal anatomy 2 , the understanding of which is essential for performing effective endodontic treatments and ensuring proper cleaning and disinfection. Some of the most common teeth requiring endodontic treatment are mandibular first molars because emerge at about six years of age in the oral cavity and are more exposed to endodontic treatment 3 . They present diverse anatomy and configuration, with the mesial root being particularly complex due to the presence of isthmuses 4 . Isthmuses are anatomical variations found in roots with two canals, primarily in the mesial roots of mandibular and maxillary first molars. The incidence of isthmuses is particularly high in mesial roots of mandibular molars 5 . Larger isthmuses, especially in mandibular molars, may harbor necrotic debris, pulp remnants, or organic matter that promotes microbial growth. This may interfere with proper cleaning and disinfection of the root canal system 6 and lead to endodontic treatment failure 7 .

Accurate morphological mapping of the root canal system is essential to achieve the best possible outcome during endodontic therapy. Micro-CT has been used for in vitro and ex vivo morphological evaluations based on qualitative and quantitative three-dimensional images, and is considered the gold standard in endodontics 8, 9 . It is an advanced method that reveals the internal anatomy of root canal systems through high-resolution images 10 . Numerous studies have been performed to classify the morphology of mesial root canals. A classic study that has undergone modifications over the years is Vertucci’s classification 11-15 . The classification of isthmuses was established by Fan et al. 5 , who identified different shapes such as foliar, separated, mixed, and cannular. In addition, a ribbon-shaped isthmus was described as an oval junction formed between two merging and then dividing root canals, with its own isthmus roof and floor 16. The current study adopted Fan’s classification for isthmuses.

The aim ofthis ex vivo study was to analyze the three-dimensional (3D) morphological characteristics of isthmuses in mesial root canals of mandibular molars using micro-CT. This approach enabled us to make both qualitative and quantitative assessments of the isthmus structures.


This project was approved by the local research ethics committee (Igua^u University Research Ethics Committee’s approval N° 3.422.230). Six hundred extracted mandibular first molars with straight or curved mesial roots were selected from the Igua^u University’s institutional tooth bank. Criteria were met in 317 teeth that had two single mesial canals with isthmuses present between them and complete root formation. Isthmus morphology was determined in 3D longitudinal images. Root length, volume and total area of the root canal, as well as of the apical and coronal level, volume and area of the isthmus, and structural model index (SMI) were analyzed.

Morphometric analysis

Samples were scanned using a micro-scanner CT (Skyscan 1174v2; Bruker micro-CT, Kontich, Belgium), and reconstructed for 3D evaluation of the area, volume, and surface of the root canals using the software CTan V. (Bruker micro-CT, Kontich, Belgium) for qualitative evaluation of the canal configuration analyses. The first assessment is the isthmus morphology according Fan’s 5 classification, in which the Type I, is the sheet connection: narrow sheet and complete connection existing between 2 canals from the top to bottom of the isthmus. In some cases, 1 or more small dentin fusions were discerned in the isthmus area. Type II, separate is the narrow but incomplete connection existing between 2 canals from the top to bottom of the isthmus. Type III, mixed: incomplete isthmus existing above and/or below a complete isthmus. Type IV, cannular connection: narrow cannular communication between 2 canals 5 .

The specimens were analyzed through the longitudinal aspect using the CTan V. sotware (Bruker micro-CT) to assess the following aspects of the mesial roots: root length, apical level, coronal level, volume and total area of the pulp cavity, volume and area of the isthmus, and structural model index (SMI) analysis.

On the longitudinal sections, the measure tool option was used to measure from the cervical and apical portions to determine root length, distance from the isthmus to the apex of the mesial roots and also to determine the distance from the isthmus (mm) to the cervical part of the mesial roots, and also to identify the presence of an isthmus.

Volume and total area of the pulp cavity and isthmus

With the top and bottom root dimensions already determined, the volume and total area of the pulp cavity were evaluated using the Region of Interest tool (ROI). Then, the binary selection tool was used to preview the selected area, observing the histogram bar to obtain two colors: white for solid parts and black for empty parts. Then, the Morphometry tool was used to perform a 3D analysis. The Data were calculated, and the values were automatically obtained by the software.

SMI analysis

The SMI indicates the relative prevalence of regions of trabecular bone and plaques in a 3D model. Total volume and area as well as the volume and area of the isthmuses were obtained from 3D analyses. These measurements were made separately by two calibrated evaluators. 5 Any cases in which measurements differed between evaluators were analyzed by a third evaluator. At the end of the evaluation, the kappa coefficient (k) was used to analyze the agreement between them.


The morphological analysis data of mesial roots in first mandibular first molars showed that root mean root length was 9.1±0.5 mm. The volume and area, of all root canal system, were 41.8±40.1 mm3 and 63.6±24.2 mm2 respectively. The isthmi volume and area alone were 11,0.6±9,03 mm3 and 30,02±11,02 mm2 (Table 1).

Table 1 Morphometric analysis of mesial roots of mandibular first molars in the study. 

Parameter Mean ± SD Range
Total Root Length 9.1±0.5 14.3-5.3
Total Volume of Pulp Cavity 41.8±40.1 74.9-20.7
Total Area of Pulp Cavity 63.6±24.2 141-35.7
Isthmus Volume 11.6±9.3 17.3-0.96
Isthmus Area 30.2±11.2 124-2.7
Canal SMI 1.7±0.7 2.9-1.55
Isthmus SMI 2.2±0.8 2.8-0.3

Micro-CT analysis of the 317 cases identified isthmuses showed higher prevalence in the apical third, especially between 3 and 5 mm from the apex. The distribution of isthmus types and their respective prevalence rates were as follows: Type I - 17%, Type II - 32%, Type III - 29%, and Type IV - 22%, showing separate type and mixed types (Figs. 1 and 2).

Fig. 1 Isthmus prevalence found in mesial roots of mandibular molars according to Fan et al. 5 classification.  

Fig. 2 Representative images of Fan ’s isthmus types found in mandibular molars assessed in the present study. 

Mean SMI of the studied isthmuses was 1.7 mm, close to 0, which is a flat format. A value of 2.2 mm was determined for canal SMI, indicating a more cylindrical shape (values close to 3 indicate cylindrical) (Table 1)


The mandibular first molars assessed in this study show the expected dominance of two mesial and one distal canal, which is consistent with previous reports 4, 17 . The mesial roots were used in this study because of their complexity and frequent connections between canals 7 .

The apical third of the root is a critical area in endodontic treatment because of the presence of ramifications that can serve as bacterial reservoirs 18 . Other complexities along the canal may also exist in this risk area, contributing to treatment failure 19 . Therefore, the aim of this study was to evalúate complexities of the root canal isthmi from mandibular molars, using micro-CT.

Isthmus prevalence ranges from 10% 20 to 100% 21 , and may be age-related, as it seems to be higher in the younger than in elderly age groups 7 . An isthmus may occur anywhere along the root, though they are more common in the middle and apical thirds of the mesial roots of mandibular molars 22-24 . The current study found high incidence of isthmuses along the root canal system, especially in the apical third 4 , which is consistent with a study that found isthmuses within 3-5 mm from the apical foramen of maxillary and mandibular molars 25 .

In line with other studies 5,7, 26 , our findings showed that Type II was slightly more common than other types in mandibular first molars. This highlights the importance of using intracanal medications for disinfection in areas not reached by the instruments. These findings are consistent with studies conducted on Turks 27 and Iranians 28 , but contradicts results reported on Yemenis 29 and Venezuelans 30 .

A systematic review has shown that anatomical variability in mandibular molars can be attributed to population differences, different methods of analysis, inclusion and exclusion criteria, and sample sizes 8 . The most frequently found root canal isthmus configuration is Vertucci type IV 23, 26 , which is described as two independent canals. This configuration is the most common in Egyptians 31 . Many methods such as clearing techniques, micro-CT and CBCT can be employed to assess root canal anatomy 32 . Micro-CT has been used as an advanced research tool to assess the ex vivo anatomy of root canals due to its high-resolution and ability to preserve the specimens 33 . Despite its limited clinical applicability, micro-CT has become established as the reference method for ex vivo studies of root canal anatomy. Its adoption facilitates future comparisons and continuous improvements of the exploration of root canal structures 33.

Regarding morphological analysis of mesial roots in first mandibular first molars, the results showed that root mean root length was 9.1±0.5 mm, similar

to other studies 5,7,23,24, 34 . The volume and area, of

all root canal system, were 41.8±40.1 mm3 and 63.6±24.2 mm2 respectively, in accordance to previous studies 12,13,20, 23 . The isthmi volume and area alone were 11.6±9,.3 mm3 and 30.2±11.2 mm 2 22, 24 (Table 1).

This study identified some limitations, including the lack of information on age and gender of the patients from whom teeth had been extracted, potentially affecting the generalizability of the findings.

In conclusion, this study confirms the presence of isthmuses in the mesial canals of mandibular first molars, with higher frequency in the apical third. Although the present study was conducted ex vivo, its clinical implications highlight the significance of carefully selecting an appropriate root canal disinfection protocol to achieve a higher success rate in the treatment of mandibular first molars. Based on that findings, important to consider their complex anatomy and difficult-to-access areas during endodontic treatment 34, 39-41 .

It has been shown that there is an association between untreated canals and isthmuses and apical periodontitis 34-37 , so, regardless of whether an isthmus is detected, it is recommended to improve the irrigation of all root canal system in these areas as well 36,38,40, 41 . Micro-computed tomography is the gold standard for research of root canal anatomy because it provides nondestructive three-dimensional microscopy data with high spatial resolution that enables accurate assessment of morphological features. Knowledge of anatomical features such as lateral canals and isthmuses is important to guide clinical protocols to ensure adequate disinfection and contribute to the healing of periradicular tissues 34, 38 . However, micro-computed tomography has hardly any clinical value and can only be used for in vitro and ex vivo studies 35-37 .

For clinical practice, preoperative Cone Beam Computed Tomography (CBCT) scanning may play a significant role in revealing complex anatomy 39-41 . CBCT settings include multiple FOVs and voxel sizes that can better address a variety of specific tasks 40, 41 . However, CBCT imaging cannot detect and measure apical isthmus length accurately, even with the highest-resolution settings. This highlights the importance of efficient, rigorous endodontic cleaning, even when such variations are not detected in CBCT exams 39 .


Pineda F, Kuttler Y. Mesiodistal and buccolingual roent-genographic investigation of 7,275 root canals. Oral Surg Oral Med Oral Pathol 1972;33:101-10. [ Links ]

Seelig A, Gillis R. Preparation of cleared specimens for pulp cavity studies. J Dent Res 1973;52:1154. [ Links ]

Hull TE, Robertson PB, Steiner JC, del Aguila MA. Pat-terns of endodontic care for a Washington state population. J Endod 2003;29:553-6. [ Links ]

Vertucci F, Seelig A, Gillis R. Root canal morphology of the human maxillary second premolar. Oral Surg Oral Med Oral Pathol 1974;38:456-64. [ Links ]

Fan B, Pan Y, Gao Y, Fang F, et al. Three-dimensional morphologic analysis of isthmuses in the mesial roots of mandibular molars. J Endod 2010;36:1866-9. [ Links ]

Urban K, Donnermeyer D, Schafer E, Bürklein S. Canal cleanliness using different irrigation activation systems: a SEM evaluation. Clin Oral Investig 2017;21:2681-7. [ Links ]

Gu L, Wei X, Ling J, Huang X.A microcomputed tomograph-ic study of canal isthmuses in the mesial root of mandibular first molars in a Chinese population. J Endod 2009;35:353-6. [ Links ]

Al-Rammahi HM, Chai WL, Nabhan MS, Ahmed HMA. Root and canal anatomy of mandibular first molars using micro-computed tomography: a systematic review. BMC Oral Health 2023;29;23:339. [ Links ]

Piasecki L, José Dos Reis P, Jussiani EI, Andrello AC. A Micro-computed Tomographic Evaluation of the Accu-racy of 3 Electronic Apex Locators in Curved Canals of Mandibular Molars. J Endod 2018;44:1872-7. [ Links ]

Kele§ A, Keskin C. A micro-computed tomographic study of band-shaped root canal isthmuses, having their floor in the apical third of mesial roots of mandibular first mo-lars. Int Endod J 2018;51:240-6. [ Links ]

Gulabivala K, Aung TH, Alavi A, Ng YL. Root and canal morphology of Burmese mandibular molars. Int Endod J 2001;34:359-70. [ Links ]

Peiris R, Takahashi M, Sasaki K, Kanazawa E. Root and canal morphology of permanent mandibular molars in a Sri Lankan population. Odontology 2007;95:16-23. [ Links ]

Sert S, Aslanalp V, Tanalp J. Investigation of the root canal configurations of mandibular permanent teeth in the Turkish population. Int Endod J 2004;37:494-9. [ Links ]

Kartal N, Cimilli HK. The degrees and configurations of mesial canal curvatures of mandibular first molars. J Endod 1997;23:358-62. [ Links ]

Al-Qudah AA, Awawdeh LA. Root and canal morpholo-gy of mandibular first and second molar teeth in a Jorda-nian population. Int Endod J 2009;42:775-84. [ Links ]

Kele§ A, Keskin C, Qiftfioglu E, Alak G. Evaluation of the and-shaped isthmuses in the mesiobuccal root canal sys-tem using micro-computed tomography. Clin Oral Investig 2022;26:5909-14. [ Links ]

Gulabivala K, Aung TH, Alavi A, Ng YL. Root and canal morphology of Burmese mandibular molars. Int En-dod J 2001;34:359-70. [ Links ]

Nair PNR, Henry S, Cano V, Vera J. Microbial status of apical root canal system of human mandibular first molars with primary apical periodontitis after “one-visit” end-odontic treatment. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:231-52. [ Links ]

Carr GB, Schwartz RS, Schaudinn C, Gorur A, et al. Ul-trastructural Examination of Failed Molar Retreatment with Secondary Apical Periodontitis: An Examination of Endodontic Biofilms in an Endodontic Retreatment Fail-ure. J Endod 2009;35:1303-9. [ Links ]

Kele§ A, Keskin C, Alqawasmi R, Versiani MA. Mi-cro-computed tomographic analysis of the mesial root of mandibular first molars with bifid apex. Arch Oral Biol 2020 Sep;117:104792. [ Links ]

Harris SP, Bowles WR, Fok A, McClanahan SB. An anatomic investigation of the mandibular first molar using micro-computed tomography. J Endod [Internet]. 2013;39:1374-8. [ Links ]

Hsu Y Y, Kim S. The resected root surface. The issue of canal isthmuses. Dent Clin North Am. 1997;41:529-40. [ Links ]

Marceliano-Alves MF, Lima CO, Bastos LG do PMN, Bruno AMV, et al. Mandibular mesial root canal morphology using micro-computed tomography in a Brazilian popula-tion. Aust Endod J 2019;45:51-6. [ Links ]

Mannocci F, Peru M, Sherriff M, Cook R, et al. The isthmus-es of the mesial root of mandibular molars: a micro-comput-ed tomographic study. Int Endod J 2005;38:558-63. [ Links ]

Teixeira FB, Sano CL, Gomes BPFA, Zaia AA, et al. A pre-liminary in vitro study of the incidence and position of the root canal isthmus in maxillary and mandibular first molars. Int Endod J 2003;36:276-80. [ Links ]

Martins JNR, Marques D, Silva EJNL, Carames J, et al. Prevalence Studies on Root Canal Anatomy Using Cone-beam Computed Tomographic Imaging: A Systematic Re-view. J Endod [Internet]. 2019;45:372-386.e4. [ Links ]

Demirbuga S, Sekerci AE, Dinjer AN, Cayabatmaz M, et al. Use of cone-beam computed tomography to evaluate root and canal morphology of mandibular first and sec-ond molars in Turkish individuals. Med Oral Patol Oral Cir Bucal 2013;18:e737-44. [ Links ]

Janani M, Rahimi S, Jafari F, Johari M, et al. Anatomic Fea-tures of C-shaped Mandibular Second Molars in a Selected Iranian Population Using CBCT. Iran Endod J [Internet]. 2018;13(1):120-5 [ Links ]

Senan E, Alhadainy H, Madfa AA. Root and Canal Morphology of Mandibular Second Molars in a Yemeni Popu-lation: A Cone-beam Computed Tomography. Eur Endod J 2021;6:72-81. [ Links ]

Gomez F, Brea G, Gomez-Sosa JF. Root canal morpholo-gy and variations in mandibular second molars: an in vivo cone-beam computed tomography analysis. BMC Oral Health 2021;21:424. [ Links ]

Saber SM, Seoud MA El, Sadat SMA El, Nawar NN. Root and canal morphology of mandibular second molars in an Egyptian subpopulation: a cone-beam computed tomog-raphy study. BMC Oral Health 2023;23:217. [ Links ]

Ahmed HMA, Rossi-Fedele G. Preferred Reporting Items for Root and Canal Anatomy in the Human Dentition (PROUD 2020) - A Systematic Review and a Proposal for a Standardized Protocol. Eur Endod J 2020;5:159-76. [ Links ]

Ordinola-Zapata R, Bramante CM, Versiani MA, Moldauer BI, et al. Comparative accuracy of the Clearing Technique, CBCT and Micro-CT methods in studying the mesial root canal configuration of mandibular first molars. Int Endod J 2017;50:90-6. [ Links ]

Susin L, Liu Y, Yoon JC, Parente JM, et al. Canal and isthmus debridement efficacies of two irrigant agitation tech-niques in a closed system. Int Endod J 2010;43:1077-90. [ Links ]

Ricucci D, Siqueira JF Jr. Biofilms and apical periodontitis: study of prevalence and association with clinical and histo-pathologic findings. J Endod 2010;36:1277-88. [ Links ]

Paque F, Laib A, Gautschi H, et al. Hard-tissue debris accu-mulation analysis by highresolution computed tomography scans. J Endod 2009;35:1044-7. [ Links ]

Karabucak B, Bunes A, Chehoud C, et al. Prevalence of apical periodontitis in endodontically treated premolars and molars with untreated canal: a cone-beam computed tomography study. J Endod 2016;42:538-41. https://doi.or-g/10.1016/j.joen.2015.12.026 [ Links ]

Tahmasbi M, Jalali P, Nair MK, et al. Prevalence of mid-dle mesial canals and isthmi in the mesial root of mandibular molars: an in vivo cone-beam computed tomographic study. J Endod 2017;43:1080-3. [ Links ]

Siqueira JF Jr, Rojas IN. Clinical implications and mi-crobiology of bacterial persistence after treatment proce-dures. J Endod. 2008 Nov;34(11):1291-1301.e3. [ Links ]

Tolentino ES, Amoroso-Silva PA, Alcalde MP, Honorio HM, Iwaki LCV, Rubira-Bullen IRF, Húngaro-Duarte MA. Accuracy of High-resolution Small-volume Cone-beam Computed Tomography in Detecting Complex Anatomy of the Apical Isthmi: Ex Vivo Analysis. J Endod. 2018 Dec;44( 12): 1862-1866. [ Links ]

Kamburoglu K, Onder B, Murat S, et al. Radiographic de-tection of artificially created horizontal root fracture using different cone beam CT units with small fields of view. Dentomaxillofac Radiol 2013;42:20120261. [ Links ]

FUNDING This study was supported by grants from Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) (E-26/200.184/2023) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazilian Governmental Institutions (200280/2022-8).

Received: September 2023; Accepted: November 2023

Corresponding Author: Marília F. Marceliano-Alves

DECLARATION OF CONFLICTING INTERESTS The authors declare no potential conflicts of interest regarding the research, authorship, and/or publication of this article.

Creative Commons License This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License