Basic Research—Technology

Frequency of Root Canal Isthmi in Human Permanent Teeth Determined by Cone-beam Computed Tomography Carlos Estrela, DDS, MSc, PhD,* Luiz Eduardo G. Rabelo, DDS, MSc,* Jo~ ao Batista de Souza, DDS, MSc, PhD,* Ana Helena G. Alencar, DDS, MSc, PhD,* ao Sousa Neto, DDS, MSc, PhD,‡ Cyntia R.A. Estrela, DDS, MSc, PhD,† Manoel Dami~ ‡ and Jesus Djalma Pecora, DDS, MSc, PhD Abstract This study evaluated the frequency of root canal isthmi (RCIs) in human permanent teeth by using cone-beam computed tomography. A sample of 1400 teeth of 618 patients (394 women; mean age, 43.4 years) was selected. RCIs were detected longitudinally on 0.1mm/0.1-mm axial slices of cone-beam computed tomography images of roots scanned from the pulp orifice to the apex, and findings were classified into 7 categories according to RCIs beginning and end: (1) both in the cervical third, (2) begin in the cervical third and end in the middle third, (3) begin in the cervical third and end in the apical third, (4) both in the middle third, (5) begin in the middle third and end in the apical third, (6) both in the apical third, or (7) no isthmus. A c2 test with Yates correction or the Fisher exact test was used to analyze categorical variables, described as frequencies (%). The Student t test was used to compare quantitative variables. RCI is a common anatomic structure in human permanent teeth, except in maxillary anterior teeth. The higher frequencies of RCIs (87.9%) were found in mandibular first molars. The frequencies of RCIs according to mean age and tooth group were not significantly different (P > .05), except in mandibular central incisors. RCIs were less frequent among older patients. (J Endod 2015;-:1–5)

Key Words Apical periodontitis, cone-beam computed tomography, endodontic failure, root canal anatomy, root canal isthmus

From the *Department of Stomatologic Sciences, Federal University of Goias, Goi^ania, Goias, Brazil; †Department of Oral Sciences, University of Cuiaba, Mato Grosso, Brazil; and ‡ Department of Endodontics, University of S~ao Paulo, Ribeir~ao Preto, S~ao Paulo, Brazil. Address requests for reprints to Dr Carlos Estrela, Federal University of Goias, School of Dentistry, Prac¸a Universitaria s/n, Setor Universitario, 74605-220 Goi^ania, GO, Brazil. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2015 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.05.016

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T

he complex shapes of the root canal system, which are responsible for serious difficulties during root canal treatment (1–3), may affect cleaning, shaping, and obturation (4–6). Ramifications in the apical third of root canals have been found in 27.4% of 1140 human permanent teeth, with the greatest variations in premolars and molars (3). Root canal isthmi (RCI), defined as narrow extensions of either 1 or 2 main canals, have been classified as incomplete or complete (4, 7). The unpredictable location and anatomic complexity of RCI make them difficult to clean and disinfect properly, which represents a significant challenge to endodontists (4–9). The highest incidence of RCIs has been found at 3–5 mm from the apex of mesiobuccal roots of maxillary molars, whereas at 4 mm, 12% of the specimens had a complete isthmus, and 88% had a partial isthmus (4). Several methods have been used to describe root canal micromorphology. RCIs have been investigated by using periapical radiography, vertical and cross-sectional cutting, clearing and staining, stereomicroscopy, surgical microscopy, dissecting microscopy, plastic casts, scanning electronic microscopy, cone-beam computed tomography (CBCT), and micro–computed tomography (MCT) (4–19). Periapical radiography, which shows the anatomic structures in only 2 dimensions, is a method that produces the images that are most commonly used in research and clinical endodontics. However, the limitations of periapical radiography are wellknown (9, 20–22), and innovative options such as CBCT and MCT have been used to study internal root canal anatomy (9–19). Pecora et al (9) evaluated the frequency of RCI in maxillary and mandibular molars by using ex vivo and in vivo map-reading dynamics and CBCT images. The frequency of RCI was high in both study models, and map-reading dynamics that used CBCT scans accurately detected RCIs. RCIs may contribute to root canal treatment failure because of the difficulty in reaching infected areas, as demonstrated by outcomes of surgical and nonsurgical root canal treatments (23). Studies about root canal anatomy have described the characteristics of each tooth in a population to estimate a predictive value for prognosis and root canal treatment success, but few studies have used tomographic images to detect and describe RCIs in all root thirds. This study evaluated RCI frequency in human permanent teeth by using CBCT.

Materials and Methods Image Selection The databases of patients with different diagnoses who were referred to the dental radiology service of private clinics in Goi^ania, Brazil were searched to select CBCT scans of 1400 teeth (1146 roots of maxillary teeth and 900 roots of mandibular teeth) of 618 patients (224 men; mean age, 43.4 years) from January 2012 to August 2014. Inclusion criteria for CBCT images were no root canal treatments, posts, or crowns; no calcified root canals; no internal or external root resorption; fully formed apex; no history of orthodontic treatment; no developmental disorders; and no pathologies. Third molars were excluded. Only high-resolution images were included to ensure that the analyses were accurate. This study was approved by the Research Ethics

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Basic Research—Technology Committee of the institution where it was conducted (approval #7968214.8.0000.5083).

Imaging Methods CBCT images were obtained by using a PreXion 3D scanner (PreXion Inc, San Mateo, CA) by using the following settings: thickness, 0.100 mm; dimensions, 1.170 mm  1.570 mm  1.925 mm; field of view, 56.00 mm; voxel, 0.100 mm, 33.5 seconds (1024 views); tube voltage, kVp; tube current, 4 mA; and exposure time, 33.5 seconds. Images were examined by using the scanner’s proprietary software (PreXion 3D Viewer; TeraRecon Inc, Foster City, CA) in an Intel Core 2 Duo-6300 1.86 MHz (Intel Corp, Santa Clara, CA) PC workstation with a NVIDIA GeForce 6200 turbo cache video card (NVIDIA Corporation, Santa Clara, CA) running Windows XP professional SP-2 (Microsoft Corp, Redmond, WA) and with an EIZO-Flexscan S2000 monitor at a resolution of 1600  1200 pixels (EIZO NANAO Corp, Hakusan, Japan). RCI Detection RCIs were recorded when scanning showed a narrow (ribbonshaped) communication between 2 root canals on an axial image, a mesiodistal or buccolingual projection of a root canal measuring about one-third of the main canal, or a C-shaped root canal. RCIs in maxillary and mandibular anterior teeth and premolars with 1 or 2 roots were analyzed by using axial scanning of all anatomic regions. For teeth with 3 or more roots, scanning was individualized for each root; in maxillary molars, it started in the mesiobuccal root and ended in the distobuccal and palatal roots; in mandibular molars, it started in the mesial root and continued to the distal root. Bifurcated roots were scanned at the same time. The presence or absence of RCI in each tooth was analyzed by using a map-reading strategy described in a previous study (9); examination followed longitudinally in the axial plane from the pulp orifice to the root apex (Fig. 1). Findings were recorded into 7 categories according to the site of RCI beginning and end: 1. 2. 3. 4. 5. 6. 7.

Both in the cervical third (CT-CT) Begin in the cervical third and end in the middle third (CT-MT) Begin in the cervical third and end in the apical third (CT-AT) Both in the middle third (MT-MT) Begin in the middle third and end in the apical third (MT-AT) Both in the apical third (AT-AT) No isthmus

Scans of the teeth that had RCIs were obtained in different planes (sagittal, coronal, and axial) at 0.1-mm thickness. Axial scanning of 0.1mm/0.1-mm slices moved from coronal to apical and from apical to coronal region. This map-reading technique provided valuable information and improved visualization and identification of RCI frequency and position. All images were analyzed by 2 observers, an endodontist and a radiologist, both with 10 or more years of experience, calibrated by evaluating 10% of the sample. When differences were found, a consensus was reached after the image was discussed with a third observer.

Statistical Analysis Categorical variables were described as frequencies and percentages and quantitative variables as means and standard deviations. Frequencies were reported with their confidence intervals (95%). Categorical variables were analyzed by using a c2 test with Yates correction or the Fisher exact test. Quantitative variables were compared by 2

Estrela et al.

Figure 1. Overview of study model used to determine RCI in maxillary molars viewed by navigation in coronal to apical direction on 0.1-mm/0.1-mm CBCT axial slices.

using the Student t test for independent samples. The level of significance was set at a = 0.05. Statistical analysis of data was performed by using the Statistical Package for Social Sciences 18.0 (IBM 21; IBM Co, New York, NY).

Results The difference in RCI frequency between sexes was significant only when mandibular central incisors (teeth #24 and #25) were compared. There were no significant differences between mean age and tooth groups (P > .05), except for mandibular lateral incisors (teeth #23 and #26). RCI frequency was lower among older patients. The confidence intervals (95%) of RCI frequency in human permanent teeth are shown in Figure 2 (Table 1). In maxillary teeth, RCI frequency in first molars was 60.8%, and most RCIs were in mesiobuccal roots (93.5%) (Fig. 3A). In maxillary second molars, RCIs were found in 46.5% of the cases, and 78.8% of these were in the mesiobuccal roots. In this group, 1 tooth had only 1 root, and 29 had 2 roots. Several specimens had fused roots (Table 1). The highest RCI frequency (87.9%) was found in the group of mandibular first molars (Fig. 3B), 57.8% (67) in the mesial root and 42.2% (49) in the distal root. In this group of teeth, 51% had 4 root canals. In the mandibular second molars, RCI frequency was 66.3%, and 94.1% of the RCIs were found in mesial roots (Table 1). Maxillary second premolars (Fig. 3C) had more RCIs (50.5%) than maxillary first premolars (18.8%). Inversely, mandibular first JOE — Volume -, Number -, - 2015

Basic Research—Technology

Figure 2. Confidence intervals (95%) of frequencies of isthmi in human permanent teeth.

premolars had more RCIs (18.8%) than mandibular second premolars (3%). In maxillary anterior teeth, the number of RCIs was negligible, whereas in mandibular anterior teeth, the highest frequency was found in lateral incisors (47.6%), followed by central incisors (33.3%) and canines (24%) (Table 1) (Fig. 3D). The frequency of RCIs that begin in the middle or in the apical third ranged from 50% for maxillary first molars and 42.2% for mandibular first molars and from 82.3% for mandibular central incisors to 55.1% for mandibular lateral incisors (Table 1). Six teeth had C-shaped roots: 2 in maxillary second molars, 2 in mandibular second molars, 1 in a mandibular second premolar, and 1 in a mandibular first premolar.

Discussion RCI is a common anatomic structure in human permanent teeth, except in maxillary anterior teeth. Numerous methods have been used to evaluate RCIs: periapical radiography, vertical and transversal sectioning, clearing and staining, stereomicroscopy, surgical microscopy, dissecting microscopy, plastic casting, scanning electronic microscopy,

CBCT, and MCT (4–19, 24–27). CBCT images provide a satisfactory visualization of RCIs, and their use in association with a longitudinal map-reading strategy to identify isthmi seems to yield satisfactory results. In our study, teeth were scanned from pulp orifice to root apex. Our study methods were similar to those used by Pecora et al (9), and our results confirmed their findings. In their study, RCI frequency was evaluated only in the mesiobuccal root of maxillary molars and in the mesial root of mandibular molars. RCIs in maxillary molars ranged from 86% in the ex vivo assay to 62% in the in vivo assay, whereas in mandibular molars it was 70% in the ex vivo assay and 72% in the in vivo assay. The method used in their study revealed that RCI frequency in the apical third of maxillary molars (mesiobuccal roots) was 4% (ex vivo) and 6% (in vivo), and of mandibular molars (mesial roots), it was 16% (ex vivo) and 26% (in vivo). The differences between studies may be assigned to the fact that our analysis included all root canals, whereas Pecora et al (9) evaluated only the mesiobuccal root of maxillary molars and the mesial root of mandibular molars. Several investigations used three-dimensional CBCT images to describe root canal anatomy (9, 12, 16, 19, 23–25). Studies that used map-reading strategies showed the potential of this CBCT tool (9, 28), which evaluates sequential 0.1-mm/0.1-mm axial slices of each root from the coronal to the apical (or the apical to the coronal) region. The movement in this direction provides valuable information about the exact position of images that suggest anatomic structures, as well as points of communication between root canals and the periodontal space associated with radiolucent areas (9, 28), for example. This method supersedes periapical radiography because it provides a dynamic visualization of what is static on periapical radiography images. In our study, this CBCT strategy was used to scan 0.1-mm/0.1-mm axial slices from the coronal to the apical region as well as from the apical to the coronal region. Map reading provided valuable information to detect RCIs and define their frequency and position on high-resolution images (28). RCI types have been grouped according to the cross-sectional view of the root and the distance to root apex (4–8, 25). Hsu and Kim (5) classified isthmi into 5 types: 1. Two or 3 root canals with no notable communication between them

TABLE 1. Distribution of Root Isthmus by Navigation in CBCT Axial Slices in Coronal to Apical Direction in Human Permanent Teeth CT-CT

CT-MT

CT-AT

MT-MT

MT-AT

AT-AT

Total

No isthmus

0 0 2 4 7 19 13

0 0 0 0 5 2 6

0 1 1 7 11 17 10

0 0 0 1 10 10 6

0 0 0 2 9 4 7

0 (0.0%) 2 (2.0%) 5 (5.0%) 19 (18.8%) 53 (50.5%) 62 (60.8%) 47 (46.5%)

100 (100.0%) 98 (98.0%) 95 (95.0%) 82 (81.2%) 52 (49.5%) 40 (39.2%) 54 (53.5%)

0 11 12 3 1 24 13

4 5 0 1 0 13 16

13 10 6 5 0 18 9

8 9 2 3 1 7 12

7 8 1 4 0 24 12

34 (33.3%) 49 (47.6%) 24 (24.0%) 19 (18.8%) 3 (3.0%) 116 (87.9%) 67 (66.3%)

68 (66.7%) 54 (52.4%) 76 (76.0%) 82 (81.2%) 97 (97.0%) 16 (12.1%) 34 (33.7%)

Maxillary teeth (N = 700) #8–#9 0 #7–#10 1 #6–#11 2 #5–#12 5 #4–#13 11 #3–#14 10 #2–#15 5 Mandibular teeth (N = 700) #24–#25 2 #23–#26 6 #22–#27 3 #21–#28 3 #20–#29 1 #19–#30 30 #18–#31 6

The occurrence of more than one type of isthmi by each root was considered, which explains the difference between the frequency of root canal isthmi (RCIs) analyzed and the number of roots. The isolated frequency of RCIs in the molar roots was described in the results section. AT-AT, both in the apical third; CT-AT, begin in cervical third and end in apical third; CT-CT, both in the cervical third; CT-MT, begin in cervical third and end in middle third; MT-MT, both in the middle third; MTAT, begin in middle third and end in apical third.

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Figure 3. (A) RCI in maxillary first and second molars, (B) mandibular first molar, (C) maxillary second premolar, and (D) mandibular central and lateral incisors viewed by navigation in coronal to apical direction on 0.1-mm/0.1-mm CBCT axial slices. Arrows indicate isthmus.

2. Two root canals with a clear connection between the 2 main root canals 3. Three root canals instead of 2 or incomplete C-shaped canals with 3 canals 4. Canals extending to the isthmus 5. A true connection or corridor throughout the section Our study evaluated RCI frequency on CBCT images by using a longitudinal map-reading strategy from pulp orifice to the apical third. It was not our objective to classify isthmi in each root canal third; however, various forms of RCI, as defined earlier, were detected in the axial planes of CBCT images. The mandibular first molars had the highest RCI frequency (87.9%), and RCIs were distributed from the coronal to the apical third. RCI frequency was significantly different between women and men only in mandibular central incisors (teeth #24 and #25). There were no significant differences between mean age and tooth groups, except for mandibular lateral incisors (teeth #23 and #26). RCI frequency was lower among older patients. Studies in the literature (29) evaluated the root canals of permanent mandibular first molars and found a mean RCI frequency of 55% in the mesial root and 20% in the distal root. MCT has also been used as an alternative method to evaluate RCI areas (10–13). Gu et al (10) evaluated the anatomic features of RCIs in 36 extracted human teeth (mesial root of mandibular first molars) by using MCT scans. The occurrence of RCI was high, particularly in the apical 4-mm to 6-mm area of the 20- to 39-year-old age group (up to 81%). The prevalence of an isthmus significantly decreased with age. Fan et al (11) found RCIs in the mesial roots of 126 first and second mandibular molars by using MCT. The occurrence of RCIs in the 5-mm apical region of the mesial roots was 85%. The first molar had more isthmi of single and mixed types, whereas second molars had more RCIs with connections. We found that the frequency of RCIs in maxillary first molars was 60.8%. Of these, most were detected in the mesiobuccal roots (93.5%). Maxillary second molars had 46.5% of all RCIs, and 78.8% of these were in the mesiobuccal roots. In these teeth, 1 tooth had just 1 root, and 29 had 2 roots. Fused roots were found in several cases (Table 1). Weller et al (4) found that the prevalence of complete RCIs in the mesiobuccal 4

Estrela et al.

roots of the maxillary first molars was 5%–14.8% (1–6 mm from apex), and partial RCI was 23.1%–88%. Jung et al (7) found a lower prevalence of partial isthmi (2.6%–15.8%). Mannocci et al (13) found a higher frequency of RCI at 3 mm from the apex (50.25%) than at 1 mm (17.24%). In our study, the frequency of RCI in maxillary second premolars (50.5%) was greater than in maxillary first premolars (18.8%). The opposite was found in mandibular premolars; the first premolars had more RCIs (18.8%) than the second premolars (3%). Zhu et al (12) used CBCT to study the incidence of RCIs in maxillary first premolars with a single root and 2 canals before and after root canal preparation. RCI incidence was different at each 1 mm within the apical 0- to 6-mm region in the single roots and 2 canals before and after root canal preparation and was lowest in the most apical 1-mm region and highest at 6 mm from the apex. The rate of partial RCI was significantly higher than that of complete RCI. After root canal preparation, the incidence of RCI in the apical 0- to 6-mm region decreased, but the proportion of complete isthmi increased. The difference in RCI structure before and after root canal preparation may be accurately defined by using CBCT. On the basis of a review of the root and root canal morphology in mandibular first premolars, Cleghorn et al (30) reported that about 98% of the teeth in these studies had only 1 root. The incidence of 2 roots was 1.8%. Three roots were found in only 0.2% of the teeth included in the study. A single apical foramen was found in 78.9% of the teeth, whereas 21.1% had 2 or more apical foramina. The frequencies of root canals in our study were close to those rates. The role of genetic and racial variation may explain differences in the incidence of roots and canals in human populations (30). In our study, the presence of RCI in maxillary anterior teeth was negligible, whereas in mandibular anterior teeth the highest frequency was detected in lateral incisors (47.6%), followed by central incisors (33.3%) and canines (24%) (Table 1). In the analysis of 100 randomly selected mandibular incisors, Mauger et al (31) found RCIs in 55%, 30%, and 20% of the teeth at 3 mm, 2 mm, and 1 mm, respectively, from the root apex. Canal shapes were classified into 4 distinct types: round, oval, long oval, and ribbon-shaped. Access by cervical pre-flaring, mechanical action of rotary instruments, or ultrasound is easier in cases of RCI that begin in the cervical third. In clinical situations in which RCI begins in the middle third and

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Basic Research—Technology ends in the apical third or begins and ends in the apical third, the access to these areas is challenging (9). Clinically, this finding indicates that it is difficult to use mechanical means to clean and shape canals, to break bacterial biofilm, and to reach these inaccessible areas. A rigorous strategy of irrigation and intracanal dressing has been recommended to improve bacterial control and to address all the challenges associated with this complex anatomy and with root canal preparation protocols (1–3, 15–18, 32–35). The variations of RCI frequencies found in this study and in others may be explained by several factors such as method differences, sample size, definition of isthmus, differences between RCI definitions, and tooth age. Together with patient sex and ethnicity, these characteristics are often not known in ex vivo studies, whereas age and sex may be controlled in in vivo studies. Most RCI frequencies reported in the literature were found in serial static and cross-sectional slices. Therefore, the variability of human tooth anatomy described in the literature should be taken into consideration before starting any root canal treatment. The frequency of number of roots, canals, apical foramina, isthmi, ramifications, and canal shapes may not match any perfect standard. In summary, the highest frequencies of RCI (87.9%) in human permanent teeth were found in mandibular first molars. The frequencies of RCIs according to mean age and tooth groups were not significantly different (P > .05), except in mandibular central incisors. RCI frequency was lower among older patients.

Acknowledgments This study was supported in part by grants from the National Council for Scientific and Technological Development (CNPq grants 306394/2011-1 to C.E.). The authors deny any conflicts of interest related to this study.

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11. Fan B, Pan Y, Gao Y, et al. Three-dimensional morphologic analysis of isthmuses in the mesial roots of mandibular molars. J Endod 2010;36:1866–9. 12. Zhu LN, Qian WH, Hong J. A cone-beam computed tomography study of changes in canal isthmus of maxillary first premolars before and after instrumentation. Shanghai Kou Qiang Yi Xue 2013;22:41–5. 13. Mannocci F, Peru M, Sherriff M, et al. The isthmuses of the mesial root of mandibular molars: a micro-computed tomographic study. Int Endod J 2005;38:558–63. 14. Wu MK, R’Oris A, Barkis D, Wesselink PR. Prevalence and extent of long oval canals in the apical third. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89: 739–43. 15. Silva EJ, Nejaim Y, Silva AV, et al. Evaluation of root canal configuration of mandibular molars in a Brazilian population by using cone-beam computed tomography: an in vivo study. J Endod 2013;39:849–52. 16. Endal U, Shen Y, Knut A, et al. A high-resolution computed tomographic study of changes in root canal isthmus area by instrumentation and root filling. J Endod 2011;37:223–7. 17. Adcock JM, Sidow SJ, Looney SW, et al. Histologic evaluation of canal and isthmus debridement efficacies of two different irrigant delivery techniques in a closed system. J Endod 2011;37:544–8. 18. Paque F, Laib A, Gautschi H, et al. Hard-tissue debris accumulation analysis by high resolution computed tomography scans. J Endod 2009;35:1044–7. 19. Matherne RP, Angelopoulos C, Kulild JC, Tira D. Use of cone-beam computed tomography to identify root canal systems in vitro. J Endod 2008;34:87–9. 20. Moura MS, Guedes AO, Alencar AH, et al. Influence of length of root canal obturation on apical periodontitis detected by periapical radiography and cone beam computed tomography. J Endod 2009;35:805–9. 21. Estrela C, Bueno MR, Azevedo B, et al. A new periapical index based on cone beam computed tomography. J Endod 2008;34:1325–31. 22. Estrela C, Bueno MR, Alencar AH, et al. Method to evaluate inflammatory root resorption by using cone beam computed tomography. J Endod 2009;35:1491–7. 23. Estrela C, Silva JA, Decurcio DA, et al. Monitoring nonsurgical and surgical root canal treatment of teeth with primary and secondary infections. Braz Dent J 2014;25: 494–501. 24. Baratto-Filho F, Zaitter S, Haragushiku GA, et al. Analysis of the internal anatomy of maxillary first molars by using different methods. J Endod 2009;35:337–42. 25. Lima FJ, Montagner F, Jacinto RC, et al. An in vitro assessment of type, position and incidence of isthmus in human permanent molars. J Appl Oral Sci 2014;22:274–81. 26. Plotino G, Tocci L, Grande NM, et al. Symmetry of root and root canal morphology of maxillary and mandibular molars in a white population: a cone beam computed tomography study in vivo. J Endod 2013;39:1545–8. 27. Kim SY, Kim BS, Woo J, Kim Y. Morphology of mandibular first molars analyzed by cone-beam computed tomography in a Korean population: variations in the number of roots and canals. J Endod 2013;39:1516–21. 28. Bueno MR, Estrela C, Figueiredo JA, Azevedo BC. Map-reading strategy to diagnose root perforations near metallic intracanal posts by using cone beam computed tomography. J Endod 2011;37:85–90. 29. Pablo OV, Estevez R, Sanchez MP, et al. Root anatomy and canal configuration of the permanent mandibular first molar: a systematic review. J Endod 2010;36:1919–31. 30. Cleghorn BM, Christie WH, Dong CC. The root and root canal morphology of the human mandibular first premolar: a literature review. J Endod 2007;33:509–16. 31. Mauger MJ, Schindler WG, Walker WA. An evaluation of canal morphology at different levels of root resection in mandibular incisors. J Endod 1998;24:607–9. 32. Nair PN, Henry S, Cano V, Vera J. Microbial status of apical root canal system of human mandibular first molars with primary apical periodontitis after one-visitendodontic treatment. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005; 99:231–52. 33. Peters OA. Current challenges and concepts in the preparation of root canal systems: a review. J Endod 2004;30:559–67. 34. Duggan JM, Sedgley CM. Biofilm formation of oral and endodontic Enterococcus faecalis. J Endod 2007;33:815–8. 35. Kishen A, Sum CP, Mathew S, Lim CT. Influence of irrigation regimens on the adherence of Enterococcus faecalis to root canal dentin. J Endod 2008;34:850–4.

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Frequency of Root Canal Isthmi in Human Permanent Teeth Determined by Cone-beam Computed Tomography.

This study evaluated the frequency of root canal isthmi (RCIs) in human permanent teeth by using cone-beam computed tomography...
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