- Email: [email protected]
A Microcomputed Tomographic Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars in a Chinese Population
Clinical Research
A Microcomputed Tomographic Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars in a Chinese Population Lisha Gu, MS,* Xi Wei, MDS, PhD,* Junqi Ling, PhD, and Xiangya Huang, MS Abstract Untreated isthmuses can be a cause of endodontic treatment failure. We investigated the anatomic features of the isthmus in the mesial root of mandibular first molars using microcomputed tomography scans. Thirty-six extracted mandibular first molars were collected from the Chinese population and divided into three age groups as follows: 20 to 39 years (group A), 40 to 59 years (group B),and $60 years (group C). Each tooth was scanned and reconstructed, and then the prevalence and type of isthmus were recorded. The percentage of sections showing isthmuses for groups A, B, and C were 50%, 41%, and 24%, respectively. The chi-square test indicated a significant correlation of the distribution of isthmuses with age (p < 0.001). The ratio of partial isthmus to complete isthmus for group C (17.1:1) was significantly higher than group A (5.9:1) and group B (7.0:1) (p < 0.001). By understanding the configuration and location of isthmus, a more efficient endodontic microsurgery can be guaranteed. (J Endod 2009;35: 353–356)
Key Words Canal anatomy, isthmus, mandibular first molar, microcomputed tomography, three-dimensional morphology
From the Department of Operative Dentistry and Endodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou, Guangdong, China. * Lisha Gu and Xi Wei contributed equally to this article. Supported by Key Clinical Program of the Ministry of Health in 2007 and Guangzhou Sanitary Bureau (grant number: 2005ZDa-002), China. Address requests for reprints to Dr Junqi Ling, Department of Operative Dentistry and Endodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou 510055, China. E-mail address: [email protected]. 0099-2399/$0 - see front matter Copyright ª 2009 American Association of Endodontists. doi:10.1016/j.joen.2008.11.029
JOE — Volume 35, Number 3, March 2009
A
n isthmus is defined as a narrow, ribbon-shaped communication between two root canals that contains pulp tissue (1). Isthmuses within the root canal system, specifically of the maxillary and mandibular molars, may contain necrotic debris, tissue remnants, or organic substrates that support the growth of microorganisms leading to the failure of endodontic treatment. The buccolingual (buccopalatal) orientation of small isthmuses does not permit its detection radiographically before endodontic procedures. Therefore, a thorough knowledge of this anatomic feature in the apical third of posterior teeth plays an important role in successful endodontic therapy. Any root containing two canals has a high prevalence of isthmuses (1, 2). The prevalence of isthmuses in the mesial root of mandibular first molars has been reported in previous studies in which observations were performed using different methods and at varying distances from the apex (2–7) (Table 1). The most commonly used method for experimental observation of the canal isthmus is visual inspection of sectioned or cleared teeth. However, magnification may not be sufficient to detect the presence of isthmuses, even if stereomicroscopes or surgical microscopes are used (2, 5). Moreover, these methods are destructive to the specimens. The sectioning procedures may result in the deposition of the smear layer that can obliterate the thin isthmuses and preclude the final analyses. As an alternative method, microcomputed tomography (mCT) scanning has been increasingly used in recent years for in-depth studies of root canal anatomy (7–12). Among them, only one study investigated the prevalence of isthmuses by mCT scanning nondestructively in the mesial root of mandibular first molars (7). It is known that the root canal anatomy shows complicated morphologic changes because of the deposition of secondary dentine with age. The same is applied to the isthmus within the root canal system. However, anatomic changes with age and three-dimensional (3-D) mCT observation of the isthmus were not reported in that study. Previous studies have shown that the prevalence of an isthmus is higher at the apical 3- to 6-mm level in the mesial roots of mandibular first molars (2, 5–7). It has been reported that root canal anatomy varies according to race (9, 13–17), and few studies have been performed on canal isthmus of mandibular first molars in a Chinese population. In this in vitro study, we investigated the prevalence and morphology of isthmuses in the apical 6 mm of the mesial root of mandibular first molars in a Chinese population as well as the anatomic changes with age using mCT scanning.
Materials and Methods Mature mandibular first molars with similar root length were collected from a native Chinese population along with the record of the patients’ age at the time of extraction. All the teeth were extracted for periodontal reasons. Teeth were excluded if there were extensive caries, fractures, or restorations. Extracted teeth were stored in 10% neutral buffered formalin solution. Attached soft tissue and calculus were removed using an ultrasonic scaler and by soaking the teeth in 2% sodium hypochlorite. Based on the patients’ age at extraction, the specimens were classified into three age groups: group A was from young adults (20-39 years), group B from middle-aged adults (40-59 years), and group C from older adults ($60 years). Each group consisted of 12 teeth. The mean age of each group was 33.8, 51.4, and 69.1, respectively. Thirty-six teeth were scanned by using a mCT system (GE Explore Locus SP; GE Healthcare, Waukesha, WI) from the apex to crown with voxel sizes of 15 15
mCT Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars
353
Clinical Research TABLE 1. Prevalence of Isthmuses in the Mesial Roots of Mandibular First Molars Reported in Previous Studies
Authors
Method used
Skidmore (3)
Plastic casts
Vertucci (4)
Cleared teeth
Number of roots
Level from apex
100
Coronal 1/3 Middle 1/3 Apical 1/3 Coronal 1/3 Middle 1/3 Apical 1/3 1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 34 mm 1 mm 2 mm 3 mm 4 mm 5 mm
100
Hsu (5)
Sectioned teeth
Not reported
Teixeira (2)
Sectioned teeth
50
von Arx (6) Mannocci (7)
Endoscope (in vivo) mCT
56 20
Isthmuses (%) 6.1 18.1 60.6 7.6 47.3 8.2 6 30 64 80 80 74 6.7 11.9 20.6 30.3 33.3 32.4 83 17.3 33 50.3 36.8 17.3
15 mm, and the cross-section was perpendicular to the long axis of the root. For each tooth, 400 sections were examined for the apical 6 mm of the mesial roots, giving a total of 4,800 sections for analysis in each group. 3-D images were reconstructed from the sliced images using 3-D reconstruction software (MicroView 2.0+ABA, GE Healthcare). Three examiners observed each section simultaneously and determined the presence and type of an isthmus. An isthmus was classified as complete or partial. A complete isthmus (CI) had a continuous opening between the two main root canals, whereas a partial isthmus (PI) was defined as a narrow projection of one root canal opening toward the second in the same root section but not merging (1). An agreement was reached among the examiners in case of any controversies in the evaluation. The ratio of PI to CI was calculated for each tooth.
Statistical Analyses The chi-square test was adopted to analyze the differences in isthmus prevalence and type distribution among three age groups. The distribution of isthmus in the apical 6 mm was also subjected to the chi-square statistical test. The statistical significance level was established at 0.05. The SPSS13.0 software package (SPSS for Windows; SPSS Inc, Chicago, IL) was used for the statistical analyses.
Results The main morphologic characteristic of isthmuses in mandibular first molars is the presence of a fin, web, or ribbon connecting the individual root canals (Fig. 1). One of the intriguing findings of the present study was a ribbon-shaped CI running in the coronal-apical direction, 354
Gu et al.
as viewed in the transverse section, that seemed to be a third or an accessory canal between the two main root canals in the elongated, peanut-shell shape root surface (Fig. 2). Of the 36 mesial roots, 32 had isthmus somewhere along its length in the apical 6 mm. Only seven of the 32 roots had an isthmus that was continuous from its coronal part until the apical end, whereas the other 25 roots showed a pattern of sections with and without isthmus. The prevalence of an isthmus was higher at the apical 4- to 6-mm level. The chi-square test indicated a significant difference in the distribution of isthmuses with sections (p < 0.001). The data for the prevalence of each type of isthmus were summarized in Table 2. The percentage of sections showing isthmus in descending order were 50% in group A, 41% in group B, and 24% in group C. The chi-square test indicated a significant difference in the distribution of isthmus with age (p < 0.001). The proportion of PI to CI for group C was 17.1:1, which was significantly higher than group A (5.9:1) and group B (7.0:1) (p < 0.001).
Discussion The canal isthmus has its clinical significance in two fields of endodontics: nonsurgical endodontic treatment and periapical surgery. Proper cleaning, shaping, and obturation of the apical third has been regarded as an important determinant to good prognosis after nonsurgical orthograde root canal treatment. Despite the current progress in endodontic treatment, cleaning and shaping the isthmus with nonsurgical methods is still problematic (18). More recent studies showed that although the addition of 1 minute of ultrasonically activated irrigation after hand/rotary instrumentation significantly increased the isthmus cleanliness values, complete debridement of the isthmuses were not always achieved (19, 20). Some in vivo studies showed that none of the isthmuses could be filled by the orthograde root canal treatment (6, 21). In periradicular surgery, untreated isthmus can be a nidus for recurrent infections and lead to persistent inflammation (18). Therefore, the localization and management of the isthmus is an important factor that may improve the long-term prognosis of nonsurgical endodontic treatment or periapical surgery in posterior teeth (18, 22). Mannocci et al (7) reported that the prevalence of isthmuses ranged from 17.25% to 50.25% in the apical 5 mm of the mesial root of mandibular first molars, with the highest prevalence at the 3mm level. Our results also revealed a high prevalence of isthmuses (up to 81%) in the apical 6 mm of the mesial root. Moreover, the prevalence of isthmus increases as the observed level goes up, with the highest prevalence at 4 to 6 mm from the apex. These findings are consistent with other two studies using sectioned teeth (2, 5). Currently, a 3-mm root-end resection is suggested in periapical surgery to facilitate the removal of apical root canal complexities that could not be cleaned, shaped, and obturated from an orthograde approach (23). However, when performing periapical surgery in the mesial root of mandibular molars, simply arbitrarily resecting 3 mm of the root apex and preparing a retrofilling could result in a less than ideal treatment. Insufficient root resection would not expose the isthmus tissue for retropreparation and retrofilling. Thus, failure to locate more than one canal and lack of isthmus management would suggest that further resection is necessary (24). After apical root resection, root-end preparation should extend at least 3 mm into the canals to ensure an effective apical seal. Observations in the present study showed that the 3-mm deep retropreparation would be frequently performed in areas (apical 4-6 mm) occupied by isthmuses. In view of this, it should be normal clinical practice to include the isthmus into retrograde preparation and retrofilling to ensure complete debridement and sealing of the root canal system.
JOE — Volume 35, Number 3, March 2009
Clinical Research
Figure 1. 3-D classification of canal isthmus configuration. (A) A fin extending from one canal into the isthmus area is defined as a fin-shaped isthmus, whereas (B) a complex and interconnected structure formed by irregular extension and fusion of canals in the isthmus area is defined as a web-shaped isthmus. (C) A definite ribbon-shaped connection between the two main canals is classified as a ribbon-shaped isthmus.
In the present study, there were much more sections at each level of the root containing PI than CI, which was consistent with the study investigating the isthmus in the mesiobuccal root of maxillary first molars (1). The management of a CI is relatively easy with the advent of microsurgical techniques using dental operating microscope and microsurgical instruments, whereas the PI requires a careful examination with thin ultrasonic tips troughing along it (25). However, in clinical practice, the use of loupe or even surgical
microscopes may not provide sufficient magnification to detect the presence of isthmuses and distinguish PI from CI (2, 5). Kim et al (18) reported that even at high magnifications the presence of isthmuses can be overlooked, and some seemingly separate canal orifices are found to be connected microscopically when examined under scanning electron microscope. Therefore, whenever periapical surgery is performed and canal orifices are located in the root-end surface of mandibular molars, a 3-mm deep class I retropreparation
Figure 2. The apical 6-mm cross-sections of the complete isthmus shown in Figure 1C. The ribbon-shaped complete isthmus viewed at (A) 1-mm and (B) 2-mm levels from the apex is considered a PI, whereas at (C) 3-mm, (D) 4-mm, (E) 5-mm, and (F) 6-mm levels, the isthmus seems to be a third or an accessory canal between the mesiobuccal and mesiolingual canals.
JOE — Volume 35, Number 3, March 2009
mCT Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars
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Clinical Research TABLE 2. Number of Sections Showing Isthmuses at Each Level in the Apical 6 mm of the Mesial Root of Mandibular First Molar Level from apex (mm)
CI
PI
Cumulative No.
Cumulative %
CI
PI
Cumulative No.
Cumulative %
CI
PI
Cumulative No.
Cumulative %
6 5 4 3 2 1 Total
90 0 14 74 110 60 348
548 652 432 236 80 112 2060
638 652 446 310 190 172 2408
79 81 56 39 24 22 50
26 62 0 30 54 72 244
488 500 350 104 120 148 1710
514 562 350 134 174 220 1954
64 70 44 17 22 28 41
0 0 40 24 0 0 64
228 262 204 166 78 154 1092
228 262 244 190 78 154 1156
28 33 31 24 10 19 24
Group A (20 to 39 years)
Group B (40 to 59 years)
Group C ($60 years)
CI, complete isthmus; PI, partial isthmus.
and retrofilling connecting the mesiobuccal and mesiolingual canals is recommended. Oi et al (8) have described both 3-D mCT observations of the root canal orifices of maxillary first premolars and their morphologic changes with age. It was shown that root canal orifices were constricted mesiodistally with age. A changing distribution pattern of isthmus with age was also revealed in the present study. There was a decreasing prevalence of isthmus per root from group A to group C. Conversely, the ratio of PI to CI increased from group A (20-39 years) to group C ($60 years). It was speculated that the amount of the secondary dentine was greater in the mesiodistal direction than that in the buccolingual direction, and thus the isthmuses were constricted mesiodistally with age. In our study, the occurrence of an isthmus in group C was lower than the other two groups. However, the vast majority of isthmuses in group C were constricted, barely traceable communication lines between two canals and might be easily missed. This can make surgical endodontic treatment of mandibular molars more difficult in older patients. The location of canal orifices on the resected root surface is essential for the successful diagnosis and treatment of canal isthmus. Although the diameter of the canal orifice decreased with age, the enhanced location of the canal orifice with the use of an operating microscope and/or endoscope has been corroborated by several studies (6, 26–28). Therefore, in older adults, whenever the mesial root-end resection of the mandibular first molar is performed, canal orifices must be located under the microscope and/or endoscope; meanwhile, the presence of an isthmus must be assumed and treated as mentioned previously. It is advisable to make a retro preparation oval in an oval-shaped root to maximize the opportunity for including an isthmus that may not be clinically observable. Methylene blue staining might also be applied to the resected root-end surface to facilitate canal orifice and isthmus identification. In conclusion, the isthmus prevalence in the mesial root of the mandibular first molars was high, particularly at the apical 4 to 6 mm in the 20- to 39-year-old age group (up to 81%). The prevalence of an isthmus significantly decreased with age, whereas the ratio of PI to CI increased with age. The inability to treat the isthmus in mesial roots can be a cause of endodontic treatment failure in mandibular molars (2, 5, 29). Being fully aware of the configuration and location of isthmus will ensure a more efficient surgical endodontic treatment.
References 1. Weller RN, Niemczyk SP, Kim S. Incidence and position of the canal isthmus. Part 1. Mesiobuccal root of the maxillary first molar. J Endod 1995;21:380–3. 2. Teixeira FB, Sano CL, Gomes BPFA, et al. A preliminary 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. 3. Skidmore AE, Bjornal AM. Root morphology of the human mandibular first molar. Oral Surg 1971;32:778–84.
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4. Vertucci F. Root canal anatomy of the human permanent teeth. Oral Surg 1984;58: 589–99. 5. Hsu YY, Kim S. The resected root surface. The issue of canal isthmuses. Dent Clin North Am 1997;41:529–40. 6. von Arx T. Frequency and type of canal isthmuses in first molars detected by endoscopic inspection during periradicular surgery. Int Endod J 2005;38:160–8. 7. 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. 8. Oi T, Saka H, Ide Y. Three-dimensional observation of pulp cavities in the maxillary first premolar tooth using micro-CT. Int Endod J 2004;37:46–51. 9. Fan B, Cheung GS, Fan M, et al. C-shaped canal system in mandibular second molars: part I-anatomical features. J Endod 2004;30:899–903. 10. Fan B, Gao Y, Fan W, et al. Identification of a C-shaped canal system in mandibular second molars-Part II: the effect of bone image superimposition and intraradicular contrast medium on radiograph interpretation. J Endod 2008;34:160–5. 11. Min Y, Fan B, Cheung GS, et al. C-shaped canal system in mandibular second molars. Part III: the morphology of the pulp chamber floor. J Endod 2006;32:1155–9. 12. Gao Y, Fan B, Cheung GS, et al. C-shaped canal system in mandibular second molars part IV: 3-D morphological analysis and transverse measurement. J Endod 2006;32:1062–5. 13. Robinson S, Czerny C, Gahleitner A, et al. Dental CT evaluation of mandibular first premolar root configurations and canal variations. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;93:328–32. 14. Reuben J, Velmurugan N, Kandaswamy D. The evaluation of root canal morphology of the mandibular first molar in an Indian population using spiral computed tomography scan: an in vitro study. J Endod 2008;34:212–5. 15. Gulabivala K, Aung TH, Alavi A, et al. Root and canal morphology of Burmese mandibular molars. Int Endod J 2001;34:359–70. 16. Gulabivala K, Opasanon A, Ng YL, et al. Root and canal morphology of Thai mandibular molars. Int Endod J 2002;35:56–62. 17. Ahmed HA, Abu-bakr NH, Yahia NA, et al. Root and canal morphology of permanent mandibular molars in a Sudanese population. Int Endod J 2007;40:766–71. 18. Kim S, Pecora G, Rubinstein RA. The resected root surface and isthmus. In: Color Atlas of Microsurgery in Endodontics. Philadelphia: W.B. Saunders Company; 2001:98–104. 19. Gutarts R, Nusstein J, Reader A, et al. In vivo debridement efficacy of ultrasonic irrigation following hand-rotary instrumentation in human mandibular molars. J Endod 2005;31:166–70. 20. Burleson A, Nusstein J, Reader A, et al. The in vivo evaluation of hand/rotary/ultrasound instrumentation in necrotic, human mandibular molars. J Endod 2007;33: 782–7. 21. Jung IY, Seo MA, Fouad AF, et al. Apical anatomy in mesial and mesiobuccal roots of permanent first molars. J Endod 2005;31:364–8. 22. Rubinstein RA, Kim S. Long-term follow-up of cases considered healed one year after apical microsurgery. J Endod 2002;28:378–83. 23. Kim S, Pecora G, Rubinstein R. Osteotomy and apical root resection. In: Color Atlas of Microsurgery in Endodontics. Philadelphia: WB. Saunders Company; 2001:85–94. 24. Degerness R, Bowles W. Anatomic determination of the mesiobuccal root resection level in maxillary molars. J Endod 2008;34:1182–6. 25. Kim S, Kratchman S. Modern endodontic surgery concepts and practice: a review. J Endod 2006;32:601–23. 26. Go¨rduysus MO, Go¨rduysus M, Friedman S. Operating microscope improves negotiation of second mesiobuccal canals in maxillary molars. J Endod 2001;27:683–s6. 27. Baldassari-Cruz LA, Lilly JP, Rivera EM. The influence of dental operating microscope in locating the mesiolingual canal orifice. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;93:190–4. 28. Schwarze T, Baethge C, Stecher T, et al. Identification of second canals in the mesiobuccal root of maxillary first and second molars using magnifying loupes or an operating microscope. Aust Endod J 2002;28:57–60. 29. Nair PN. On the causes of persistent apical periodontitis: a review. Int Endod J 2006; 39:249–81.
JOE — Volume 35, Number 3, March 2009
A Microcomputed Tomographic Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars in a Chinese Population Lisha Gu, MS,* Xi Wei, MDS, PhD,* Junqi Ling, PhD, and Xiangya Huang, MS Abstract Untreated isthmuses can be a cause of endodontic treatment failure. We investigated the anatomic features of the isthmus in the mesial root of mandibular first molars using microcomputed tomography scans. Thirty-six extracted mandibular first molars were collected from the Chinese population and divided into three age groups as follows: 20 to 39 years (group A), 40 to 59 years (group B),and $60 years (group C). Each tooth was scanned and reconstructed, and then the prevalence and type of isthmus were recorded. The percentage of sections showing isthmuses for groups A, B, and C were 50%, 41%, and 24%, respectively. The chi-square test indicated a significant correlation of the distribution of isthmuses with age (p < 0.001). The ratio of partial isthmus to complete isthmus for group C (17.1:1) was significantly higher than group A (5.9:1) and group B (7.0:1) (p < 0.001). By understanding the configuration and location of isthmus, a more efficient endodontic microsurgery can be guaranteed. (J Endod 2009;35: 353–356)
Key Words Canal anatomy, isthmus, mandibular first molar, microcomputed tomography, three-dimensional morphology
From the Department of Operative Dentistry and Endodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou, Guangdong, China. * Lisha Gu and Xi Wei contributed equally to this article. Supported by Key Clinical Program of the Ministry of Health in 2007 and Guangzhou Sanitary Bureau (grant number: 2005ZDa-002), China. Address requests for reprints to Dr Junqi Ling, Department of Operative Dentistry and Endodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou 510055, China. E-mail address: [email protected]. 0099-2399/$0 - see front matter Copyright ª 2009 American Association of Endodontists. doi:10.1016/j.joen.2008.11.029
JOE — Volume 35, Number 3, March 2009
A
n isthmus is defined as a narrow, ribbon-shaped communication between two root canals that contains pulp tissue (1). Isthmuses within the root canal system, specifically of the maxillary and mandibular molars, may contain necrotic debris, tissue remnants, or organic substrates that support the growth of microorganisms leading to the failure of endodontic treatment. The buccolingual (buccopalatal) orientation of small isthmuses does not permit its detection radiographically before endodontic procedures. Therefore, a thorough knowledge of this anatomic feature in the apical third of posterior teeth plays an important role in successful endodontic therapy. Any root containing two canals has a high prevalence of isthmuses (1, 2). The prevalence of isthmuses in the mesial root of mandibular first molars has been reported in previous studies in which observations were performed using different methods and at varying distances from the apex (2–7) (Table 1). The most commonly used method for experimental observation of the canal isthmus is visual inspection of sectioned or cleared teeth. However, magnification may not be sufficient to detect the presence of isthmuses, even if stereomicroscopes or surgical microscopes are used (2, 5). Moreover, these methods are destructive to the specimens. The sectioning procedures may result in the deposition of the smear layer that can obliterate the thin isthmuses and preclude the final analyses. As an alternative method, microcomputed tomography (mCT) scanning has been increasingly used in recent years for in-depth studies of root canal anatomy (7–12). Among them, only one study investigated the prevalence of isthmuses by mCT scanning nondestructively in the mesial root of mandibular first molars (7). It is known that the root canal anatomy shows complicated morphologic changes because of the deposition of secondary dentine with age. The same is applied to the isthmus within the root canal system. However, anatomic changes with age and three-dimensional (3-D) mCT observation of the isthmus were not reported in that study. Previous studies have shown that the prevalence of an isthmus is higher at the apical 3- to 6-mm level in the mesial roots of mandibular first molars (2, 5–7). It has been reported that root canal anatomy varies according to race (9, 13–17), and few studies have been performed on canal isthmus of mandibular first molars in a Chinese population. In this in vitro study, we investigated the prevalence and morphology of isthmuses in the apical 6 mm of the mesial root of mandibular first molars in a Chinese population as well as the anatomic changes with age using mCT scanning.
Materials and Methods Mature mandibular first molars with similar root length were collected from a native Chinese population along with the record of the patients’ age at the time of extraction. All the teeth were extracted for periodontal reasons. Teeth were excluded if there were extensive caries, fractures, or restorations. Extracted teeth were stored in 10% neutral buffered formalin solution. Attached soft tissue and calculus were removed using an ultrasonic scaler and by soaking the teeth in 2% sodium hypochlorite. Based on the patients’ age at extraction, the specimens were classified into three age groups: group A was from young adults (20-39 years), group B from middle-aged adults (40-59 years), and group C from older adults ($60 years). Each group consisted of 12 teeth. The mean age of each group was 33.8, 51.4, and 69.1, respectively. Thirty-six teeth were scanned by using a mCT system (GE Explore Locus SP; GE Healthcare, Waukesha, WI) from the apex to crown with voxel sizes of 15 15
mCT Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars
353
Clinical Research TABLE 1. Prevalence of Isthmuses in the Mesial Roots of Mandibular First Molars Reported in Previous Studies
Authors
Method used
Skidmore (3)
Plastic casts
Vertucci (4)
Cleared teeth
Number of roots
Level from apex
100
Coronal 1/3 Middle 1/3 Apical 1/3 Coronal 1/3 Middle 1/3 Apical 1/3 1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 34 mm 1 mm 2 mm 3 mm 4 mm 5 mm
100
Hsu (5)
Sectioned teeth
Not reported
Teixeira (2)
Sectioned teeth
50
von Arx (6) Mannocci (7)
Endoscope (in vivo) mCT
56 20
Isthmuses (%) 6.1 18.1 60.6 7.6 47.3 8.2 6 30 64 80 80 74 6.7 11.9 20.6 30.3 33.3 32.4 83 17.3 33 50.3 36.8 17.3
15 mm, and the cross-section was perpendicular to the long axis of the root. For each tooth, 400 sections were examined for the apical 6 mm of the mesial roots, giving a total of 4,800 sections for analysis in each group. 3-D images were reconstructed from the sliced images using 3-D reconstruction software (MicroView 2.0+ABA, GE Healthcare). Three examiners observed each section simultaneously and determined the presence and type of an isthmus. An isthmus was classified as complete or partial. A complete isthmus (CI) had a continuous opening between the two main root canals, whereas a partial isthmus (PI) was defined as a narrow projection of one root canal opening toward the second in the same root section but not merging (1). An agreement was reached among the examiners in case of any controversies in the evaluation. The ratio of PI to CI was calculated for each tooth.
Statistical Analyses The chi-square test was adopted to analyze the differences in isthmus prevalence and type distribution among three age groups. The distribution of isthmus in the apical 6 mm was also subjected to the chi-square statistical test. The statistical significance level was established at 0.05. The SPSS13.0 software package (SPSS for Windows; SPSS Inc, Chicago, IL) was used for the statistical analyses.
Results The main morphologic characteristic of isthmuses in mandibular first molars is the presence of a fin, web, or ribbon connecting the individual root canals (Fig. 1). One of the intriguing findings of the present study was a ribbon-shaped CI running in the coronal-apical direction, 354
Gu et al.
as viewed in the transverse section, that seemed to be a third or an accessory canal between the two main root canals in the elongated, peanut-shell shape root surface (Fig. 2). Of the 36 mesial roots, 32 had isthmus somewhere along its length in the apical 6 mm. Only seven of the 32 roots had an isthmus that was continuous from its coronal part until the apical end, whereas the other 25 roots showed a pattern of sections with and without isthmus. The prevalence of an isthmus was higher at the apical 4- to 6-mm level. The chi-square test indicated a significant difference in the distribution of isthmuses with sections (p < 0.001). The data for the prevalence of each type of isthmus were summarized in Table 2. The percentage of sections showing isthmus in descending order were 50% in group A, 41% in group B, and 24% in group C. The chi-square test indicated a significant difference in the distribution of isthmus with age (p < 0.001). The proportion of PI to CI for group C was 17.1:1, which was significantly higher than group A (5.9:1) and group B (7.0:1) (p < 0.001).
Discussion The canal isthmus has its clinical significance in two fields of endodontics: nonsurgical endodontic treatment and periapical surgery. Proper cleaning, shaping, and obturation of the apical third has been regarded as an important determinant to good prognosis after nonsurgical orthograde root canal treatment. Despite the current progress in endodontic treatment, cleaning and shaping the isthmus with nonsurgical methods is still problematic (18). More recent studies showed that although the addition of 1 minute of ultrasonically activated irrigation after hand/rotary instrumentation significantly increased the isthmus cleanliness values, complete debridement of the isthmuses were not always achieved (19, 20). Some in vivo studies showed that none of the isthmuses could be filled by the orthograde root canal treatment (6, 21). In periradicular surgery, untreated isthmus can be a nidus for recurrent infections and lead to persistent inflammation (18). Therefore, the localization and management of the isthmus is an important factor that may improve the long-term prognosis of nonsurgical endodontic treatment or periapical surgery in posterior teeth (18, 22). Mannocci et al (7) reported that the prevalence of isthmuses ranged from 17.25% to 50.25% in the apical 5 mm of the mesial root of mandibular first molars, with the highest prevalence at the 3mm level. Our results also revealed a high prevalence of isthmuses (up to 81%) in the apical 6 mm of the mesial root. Moreover, the prevalence of isthmus increases as the observed level goes up, with the highest prevalence at 4 to 6 mm from the apex. These findings are consistent with other two studies using sectioned teeth (2, 5). Currently, a 3-mm root-end resection is suggested in periapical surgery to facilitate the removal of apical root canal complexities that could not be cleaned, shaped, and obturated from an orthograde approach (23). However, when performing periapical surgery in the mesial root of mandibular molars, simply arbitrarily resecting 3 mm of the root apex and preparing a retrofilling could result in a less than ideal treatment. Insufficient root resection would not expose the isthmus tissue for retropreparation and retrofilling. Thus, failure to locate more than one canal and lack of isthmus management would suggest that further resection is necessary (24). After apical root resection, root-end preparation should extend at least 3 mm into the canals to ensure an effective apical seal. Observations in the present study showed that the 3-mm deep retropreparation would be frequently performed in areas (apical 4-6 mm) occupied by isthmuses. In view of this, it should be normal clinical practice to include the isthmus into retrograde preparation and retrofilling to ensure complete debridement and sealing of the root canal system.
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Clinical Research
Figure 1. 3-D classification of canal isthmus configuration. (A) A fin extending from one canal into the isthmus area is defined as a fin-shaped isthmus, whereas (B) a complex and interconnected structure formed by irregular extension and fusion of canals in the isthmus area is defined as a web-shaped isthmus. (C) A definite ribbon-shaped connection between the two main canals is classified as a ribbon-shaped isthmus.
In the present study, there were much more sections at each level of the root containing PI than CI, which was consistent with the study investigating the isthmus in the mesiobuccal root of maxillary first molars (1). The management of a CI is relatively easy with the advent of microsurgical techniques using dental operating microscope and microsurgical instruments, whereas the PI requires a careful examination with thin ultrasonic tips troughing along it (25). However, in clinical practice, the use of loupe or even surgical
microscopes may not provide sufficient magnification to detect the presence of isthmuses and distinguish PI from CI (2, 5). Kim et al (18) reported that even at high magnifications the presence of isthmuses can be overlooked, and some seemingly separate canal orifices are found to be connected microscopically when examined under scanning electron microscope. Therefore, whenever periapical surgery is performed and canal orifices are located in the root-end surface of mandibular molars, a 3-mm deep class I retropreparation
Figure 2. The apical 6-mm cross-sections of the complete isthmus shown in Figure 1C. The ribbon-shaped complete isthmus viewed at (A) 1-mm and (B) 2-mm levels from the apex is considered a PI, whereas at (C) 3-mm, (D) 4-mm, (E) 5-mm, and (F) 6-mm levels, the isthmus seems to be a third or an accessory canal between the mesiobuccal and mesiolingual canals.
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mCT Study of Canal Isthmuses in the Mesial Root of Mandibular First Molars
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Clinical Research TABLE 2. Number of Sections Showing Isthmuses at Each Level in the Apical 6 mm of the Mesial Root of Mandibular First Molar Level from apex (mm)
CI
PI
Cumulative No.
Cumulative %
CI
PI
Cumulative No.
Cumulative %
CI
PI
Cumulative No.
Cumulative %
6 5 4 3 2 1 Total
90 0 14 74 110 60 348
548 652 432 236 80 112 2060
638 652 446 310 190 172 2408
79 81 56 39 24 22 50
26 62 0 30 54 72 244
488 500 350 104 120 148 1710
514 562 350 134 174 220 1954
64 70 44 17 22 28 41
0 0 40 24 0 0 64
228 262 204 166 78 154 1092
228 262 244 190 78 154 1156
28 33 31 24 10 19 24
Group A (20 to 39 years)
Group B (40 to 59 years)
Group C ($60 years)
CI, complete isthmus; PI, partial isthmus.
and retrofilling connecting the mesiobuccal and mesiolingual canals is recommended. Oi et al (8) have described both 3-D mCT observations of the root canal orifices of maxillary first premolars and their morphologic changes with age. It was shown that root canal orifices were constricted mesiodistally with age. A changing distribution pattern of isthmus with age was also revealed in the present study. There was a decreasing prevalence of isthmus per root from group A to group C. Conversely, the ratio of PI to CI increased from group A (20-39 years) to group C ($60 years). It was speculated that the amount of the secondary dentine was greater in the mesiodistal direction than that in the buccolingual direction, and thus the isthmuses were constricted mesiodistally with age. In our study, the occurrence of an isthmus in group C was lower than the other two groups. However, the vast majority of isthmuses in group C were constricted, barely traceable communication lines between two canals and might be easily missed. This can make surgical endodontic treatment of mandibular molars more difficult in older patients. The location of canal orifices on the resected root surface is essential for the successful diagnosis and treatment of canal isthmus. Although the diameter of the canal orifice decreased with age, the enhanced location of the canal orifice with the use of an operating microscope and/or endoscope has been corroborated by several studies (6, 26–28). Therefore, in older adults, whenever the mesial root-end resection of the mandibular first molar is performed, canal orifices must be located under the microscope and/or endoscope; meanwhile, the presence of an isthmus must be assumed and treated as mentioned previously. It is advisable to make a retro preparation oval in an oval-shaped root to maximize the opportunity for including an isthmus that may not be clinically observable. Methylene blue staining might also be applied to the resected root-end surface to facilitate canal orifice and isthmus identification. In conclusion, the isthmus prevalence in the mesial root of the mandibular first molars was high, particularly at the apical 4 to 6 mm in the 20- to 39-year-old age group (up to 81%). The prevalence of an isthmus significantly decreased with age, whereas the ratio of PI to CI increased with age. The inability to treat the isthmus in mesial roots can be a cause of endodontic treatment failure in mandibular molars (2, 5, 29). Being fully aware of the configuration and location of isthmus will ensure a more efficient surgical endodontic treatment.
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JOE — Volume 35, Number 3, March 2009