- Email: [email protected]
Correlation of Root Dentin Thickness and Length of Roots in Mesial Roots of Mandibular Molars
Basic Research—Technology
Correlation of Root Dentin Thickness and Length of Roots in Mesial Roots of Mandibular Molars Shweta Dwivedi, MDS,* Chandra Dhar Dwivedi, MDS,† and Neelam Mittal, MDS* Abstract Introduction: The purpose of this study was to analyze the relation of tooth length and distal wall thickness of mesial roots in mandibular molars at different locations (ie, 2 mm below the furcation and at the junction between the middle and apical third). Methods: Fortyfive mandibular first molars were taken, and the length of each tooth was measured. Then, specimens were divided into three groups according to their length: group I–long (24.2 mm 1.8), group II–medium (21 mm 1.5) and group III–short (16.8 mm 1.8). mesial root of each marked at two levels - at 2 mm below the furcation as well as at junction of apical and middle third of roots. The minimum thickness of the distal root dentine associated with the buccal and lingual canals of the mesial roots was measured, The distance between the buccal and lingual canals and the depth of concavity in the distal surface of the mesial roots were also measured. Results: Statistical analysis was performed by using analysis of variance and the Student-Newman-Keuls test. The minimum thickness of the distal wall of the mesiobuccal canal was significantly different (P < .001) between groups 1 (long) and 3 (short). Conclusions: Distal wall thickness of the mesiobuccal root and distal concavity of the mesial root of mandibular first molars were found to be thinner in longer teeth compared with shorter teeth. (J Endod 2014;-:1–4)
Key Words Mandibular molars, mesial roots, stereomicroscope
From the Departments of *Conservative Dentistry and Endodontics and †Oral and Maxillofacial Surgery, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi, India. Address requests for reprints to Dr Shweta Dwivedi, Conservative Dentistry and Endodontics, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2014.02.011
JOE — Volume -, Number -, - 2014
T
he mandibular first molar seems to be the most frequent tooth in need of endodontic treatment. It usually has 2 roots but occasionally 3, with 2 canals in the mesial root and 1 or 2 canals in the distal root. The distal root is readily accessible to endodontic instrumentation, but the mesial roots are usually curved, with the greatest curvature in the mesial-buccal canal (1). The main goal of endodontics is cleaning, shaping, and disinfecting the root canal system to restore the function and esthetic of the involved tooth (2) and to avoid transportation, ledge formation, and perforation (3). The mesial roots of mandibular first molars, approximately 2 mm below the furcation, have a greater concavity, and the thickness of dentin is limited (2). They are particularly subject to strip perforation (4). Thus, this is described as a danger zone (3). Cervical preflaring is recommended in order to eliminate cervical interferences during preparation of the apical third to the proper size, which could create an increased risk for iatrogenic procedural errors (4–9), especially in curved mesial root canals of mandibular molars (10). Abou-Rass et al (11) first described the anticurvature filing method to maintain the integrity of canal walls at their thin portion and reduce the possibility of root perforation or stripping. Although the thickness of the dentin in the danger zone has been studied widely, there is little information in the literature concerning the relation of the thickness of radicular dentin and tooth length. Therefore, the aim of this study was to analyze the correlation between dentin thickness of the mesial roots at the distal aspect and different lengths of the mandibular first molars.
Materials and Methods Ninety-eight mandibular first molars were collected from the Department of Oral and Maxillofacial Surgery, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh, India. Teeth with open apices, resorption, or calcification; endodontically treated teeth; and teeth with root caries were excluded from the study. Age, sex, and the systemic condition of the patients were unknown. Sixty-four teeth that showed fully formed roots and intact external morphology were selected. The degree of the curvature was standardized as described by Abou-Rass et al (11) and Schneider (12). Radiographs were taken in the mesial-distal and buccal-lingual directions to confirm that the mesial roots had 2 separate canals. On the basis of radiographic appearance, 45 teeth were selected to undergo further study. Selected teeth were placed in 3% sodium hypochlorite for 15 minutes for disinfection and then stored in normal saline with 0.2% thymol to inhibit microbial growth. The tooth length of each specimen was recorded from the mesiolingual cusp tip to the apex and categorized according to the length of teeth as follows: Group 1: Long teeth (ie, 23–26 mm, mean = 24.2 1.8 mm) Group 2: Medium teeth (ie, 20–23 mm, mean = 21 1.5 mm) Group 3: Short teeth (15–19 mm, mean = 16.8 1.8 mm) Mesial roots were marked at 2 mm below the furcation (level X) and at the junction between the apical and middle third of the roots (level Y) (Fig. 1). Then, they were sliced off horizontally at level X and level Y of the roots using a diamond disc with a thickness of 0.2 mm under water spray. In this study, both the coronal and apical surfaces were used for study (ie, the coronal surface as level X and the apical surface as level Y). The specimens were observed under a stereomicroscope (Stemi SV6; Zeiss Microsystems Ltd, Gottingen, Germany).
Strip Perforation and Length of Roots of Mandibular Molars
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Basic Research—Technology
Figure 1. A diagram showing level X and level Y and measurement parameters (ie, A, A1, B, and C).
Images of the sectioned surface were taken with the help of the integrated camera system under 36 magnification, and all measurements were collected. The minimum distal wall thickness of the buccal (A) and lingual (A1) canals of the mesial roots were measured and represented the least dentin thickness from the inner border of the canals to the external root surface. Parameter (B) is the distance between the mesiobuccal and mesiolingual canals. The depth of concavity (C) in the distal surface of the mesial roots was measured as the distance between a line joining together 2 points in the convex root surface and the deepest point in the concave root surface (Fig. 1). Statistical analysis was performed by using SPSS software (SPSS Inc, Chicago, IL). The analysis of variance test was used to measure variance within groups and among groups. For pair-wise multiple group comparison between groups, the Student t test (StudentNewman-Keuls test) was used. The level of significance was set at P < .001.
Results The mean at level X of parameter A in groups 2 and 3 was higher compared with group 1 (Table 1); this difference was statistically signif-
icant. The mean of parameter A at level X in group 3 was also significantly higher than in group 2. The mean of parameter A at level Y in group 1 was lower than in groups 2 and 3 (Table 1), but there was no significant difference between groups 1 and 2. Other differences were significant (Table 2). The mean of parameter A1 at level X in group 3 was higher compared with groups 1 and 2 (Table 1). This difference was statistically significant. For the mean of parameter A1 at level X, there was no significant difference between groups 1 and 2 (Table 2). The mean of parameter A1 at level Y in group 1 was lower than in groups 2 and 3, but all differences were insignificant. The mean at level X of parameter B in group 3 was higher compared with groups 1 and 2 (Table 1), but all differences were statistically insignificant (Table 2). The mean of parameter B at level Y in groups 1 and 2 were higher than in group 3 (Table 1); the differences were significant, but there was no significant difference between groups 1 and 2 (Table 2). The mean at level X of parameter C in groups 1 and 2 was higher compared with group 3 (Table 1); this difference was found statistically significant. The mean at level X of parameter C in group 2 was higher compared with group 1, but this difference was
TABLE 1. Mean Standard Deviation (in mm) of the Measurement Criteria at 2 Levels (X and Y) according to Groups and Its Intergroup Comparison A (in mm) Group 1 2 3
Level X
Level Y
A1 (in mm) Level X
Level Y
B (in mm) Level X
Level Y
C (in mm) Level X
Level Y
1.0721 0.105 0.8225 0.130 1.1406 0.201 0.8100 0.111 2.7121 0.495 2.1275 0.215 0.8683 0.045 0.8390 0.067 1.3340 0.078 0.9000 0.133 1.0361 0.106 0.8785 0.132 2.7615 0.385 1.8105 0.513 0.9522 0.116 0.4539 0.112 1.8780 0.169 1.0631 0.074 1.6920 0.296 0.9137 0.169 3.1787 0.664 1.0144 0.335 0.4498 0.193 0.2116 0.210
Intergroup comparison: 1-way analysis of variance F P
F = 164.949 P < .001
F = 16.907 P < .001
F = 39.995 P < .001
F = 2.134 P = .131
F = 3.538 P = .038
F = 35.007 P < .001
F = 61.429 P < .001
F = 73.209 P < .001
Bold signifies P < .001.
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Dwivedi et al.
JOE — Volume -, Number -, - 2014
Basic Research—Technology TABLE 2. Post Hoc Test (SNK test) for Pair-wise Multiple-group Comparison A Post hoc test by SNK test 1 vs 2 1 vs 3 2 vs 3
A1
B
C
X
Y
X
Y
X
Y
X
Y
Q = 5.785 P < .001 Q = 17.802 P < .001 Q = 12.016 P < .001
Q = 1.835 P = .202 Q = 5.696 P < .001 Q = 3.861 P < .001
Q = 1.325 P = .467 Q = 6.997 P < .001 Q = 8.323 P < .001
Q = 1.340 P = .457 Q = 2.031 P = .137 Q = 0.690 P = .867
Q = 0.255 P = .992 Q = 2.421 P = .058 Q = 2.165 P = .103
Q = 2.312 P = .074 Q = 8.120 P < .001 Q = 5.807 P < .001
Q = 1.728 P = .246 Q = 8.617 P < .001 Q = 10.346 P < .001
Q = 7.264 P < .001 Q = 11.996 P < .001 Q = 4.632 P < .001
Bold signifies P < .001.
insignificant. The mean of parameter C at level Y in group 1 was higher than in groups 2 and group 3; all differences were significant (Table 2).
Discussion The mandibular first molar is the most frequently endodontically treated tooth and is extremely anatomically challenging (13). According to Tabrizizadeh et al (14), the distal surface of the mesial root was reported thinnest (1.2 mm) and the lingual wall thickest (2.2 mm) at the furcation point. Micro–computed tomographic evaluation of the mandibular first molar quantitatively investigated the morphology and indicated that the furcal aspect of the entire mesial root should be considered a ‘‘danger zone.’’ Mesial canals were found to be much more variable than distal canals in morphology (13). Cervical preflaring recommended decreasing the tension of manual and rotary instruments during apical instrumentation (6, 7). The variation of root dentin thickness in different areas supports the notion (15) that preflaring of the cervical third could result in iatrogenic strip perforation. Therefore, care should be taken to avoid perforation at danger zones during shaping and post space preparation procedures (16). In this study, the length and danger zone of mesial roots in mandibular molars were analyzed and correlated. According to a previous study, the danger zone is located 4–6 mm below the canal chamber orifice (17). Knowledge of the remaining dentin thickness in the root, especially in the distal aspect of the mesial roots, would minimize, if not eliminate, the occurrence of strip perforations (18, 19). Our study showed that the distal wall thickness of mesial roots was significantly less in longer teeth than in shorter teeth, both at the area of furcation and at the junction between the apical and middle third of the roots. In mesiobuccal roots, the mean of dentin thickness in longer teeth was 1.0721 mm at the distal aspect and in shorter roots it was 1.8780 mm at level X, whereas it was 0.8225 and 1.0631 mm, respectively, at level Y. According to Berutti and Fedon (20), even differences of tenths or hundreds of a millimeter can be critical in avoiding strip perforation. Therefore, this study suggests that long molars may have a higher risk of strip perforation in mesiobuccal canals if flared to a larger size (20). The distal (furcal) surface concavity was deeper in teeth with long roots compared with those with short roots. Observations of root concavity is supported by the result of a previous study by Bower (19), who found a mean value of 0.7 0.19 mm ranging from 0.3–1.3 mm. However, no information on the length of the teeth was given in Bower’s study, which makes it difficult to compare the results. Additionally, the distance between the mesiobuccal and mesiolingual canals was measured in this study. Short molars presented the longest distance between the canals (3.1787 0.66421 mm), which was higher than the other groups (P < .000), but these results are not in agreement with previous studies. However, the range of mean disJOE — Volume -, Number -, - 2014
tances (2.7121–3.1787 mm) is similar to that found in previous studies (2–4 mm) (21). To prevent strip perforations, coronal flaring should be limited and directed at the outer and thicker aspects of the canal walls. Lim and Stock (22) have determined an arbitrary value of 0.3 mm as the minimum canal wall thickness that should remain after instrumentation to prevent perforation or vertical root fracture. Stereomicroscopic imaging was used to study the surface topography of the tooth surface, and it provides 3-dimensional information. This is basically a macroscope that allows users to slowly enlarge the object. Another feature of stereomicroscopes is a dual illuminator system that provides more than enough light to view specimens 3-dimensionally. A calibrated scale was integrated into the stereomicroscope, which makes it possible to measure dimensions of objects (ie, root dentin thickness at the microscopic level).
Conclusion The present study concludes that the root wall on distal surfaces of mesial roots in longer mandibular molars is thinner than in shorter teeth. Similarly, the mesial roots were found to be more concave in the distal aspect in the longer teeth. The distal wall of mesial roots in the mandibular first molars are more prone to strip perforation, particularly in longer teeth.
Acknowledgments The authors deny any conflicts of interest related to this study.
References 1. Burns RC, Buchnan LS. Tooth morphology and access openings. In: Cohen S, Burns RC, eds. Pathways of Pulp, 6th ed. St Louis: CV Mosby; 1994:160. 2. Skidmore AE, Bjorndal AM. Root canal morphology of the human mandibular first molar. Oral Surg Oral Med Oral Pathol 1971;32:778–84. 3. Sp angberg LSW. Instruments, materials and devices. In: Cohen S, Burns RC, eds. Pathways of the Pulp, 7th ed. St Louis: CV Mosby; 1998:463–75. 4. Sorenson JA, Martinhoff JT. Intracanal reinforcement and coronal coverage: a study of endodontically treated teeth. J Prosthet Dent 1984;51:780–4. 5. Sedgley CM, Messer HH. Are endodontically treated teeth more brittle? J Endod 1992;18:332–5. 6. Pilo R, Corcino G, Tamse A. Residual dentin thickness in mandibular premolars prepared by hand and rotary instruments. J Endod 1998;24:401–5. 7. Pilo R, Tamse A. Residual dentin thickness in mandibular premolars prepared with Gates-Glidden and ParaPost drills. J Prosthet Dent 2000;83:617–23. 8. Guttmann JL. The dentin-root complex: anatomic and biologic considerations in restoring endodontically treated teeth. J Prosthet Dent 1992;67:458–67. 9. Tamse A, Katz A, Pilo R. Furcation groove of buccal root of maxillary first premolars—a morphometric study. J Endod 2000;26:359–63. 10. Duarte MA, Bernardes RA, Ordinola-Zapata R, et al. Effects of Gates-Glidden, LA Axes and orifice shaper burs on the cervical dentin thickness and root canal area of mandibular molars. Braz Dent J 2011;22:28–31. 11. Abou-Rass M, Frank AL, Glick DH. The anticurvature filling method to prepare the curved root canal. J Am Dent Assoc 1980;101:792–4.
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Basic Research—Technology 12. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg Oral Med Oral Pathol 1971;32:271–5. 13. Harris SP, Bowles WR, Fok A, McClanahan SB. An anatomic investigation of the mandibular first molar using micro-computed tomography. J Endod 2013;39: 1374–8. 14. Tabrizizadeh M, Reuben J, Khalesi M, et al. Evaluation of radicular dentin thickness of danger zone in mandibular first molars. J Dent (Tehran) 2010;7:196–9. 15. Kessler JR, Peters DD, Lorton L. Comparison of the relative risk of molar root perforations using various endodontic instrumentation techniques. J Endod 1983;9: 439–47. 16. Gu YC, Zhang YP, Liao ZG, Fei XD. A micro-computed tomographic analysis of wall thickness of C-shaped canals in mandibular first premolars. J Endod 2013;39: 973–6.
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17. Estrela C, Bueno MR, Sousa-Neto MD, Pecora JD. Method for determination of root curvature radius using cone-beam computed tomography images. Braz Dental J 2008;19:114–8. 18. Filho PFG, Letra A, Menezes R, Carmo AM. Danger zone in mandibular molars before instrumentation: an in vitro study. J Appl Oral Sci 2003;11:324–6. 19. Bower RC. Furcation morphology relative to periodontal treatment. J Periodontol 1979;50:366–74. 20. Berutti E, Fedon G. Thickness of cementum/dentin in mesial roots of mandibular first molars. Journal of Endodontics 1992;18:545–8. 21. Isom TL, Marshall JG, Baumgartner JC. Evaluation of root thickness in curved canals after flaring. J Endod 1995;21:368–71. 22. Lim SS, Stock CJ. The risk of perforation in the curved canal: anticurvature filing compared with the step back technique. Int Endod J 1987;20:33–9.
JOE — Volume -, Number -, - 2014
Correlation of Root Dentin Thickness and Length of Roots in Mesial Roots of Mandibular Molars Shweta Dwivedi, MDS,* Chandra Dhar Dwivedi, MDS,† and Neelam Mittal, MDS* Abstract Introduction: The purpose of this study was to analyze the relation of tooth length and distal wall thickness of mesial roots in mandibular molars at different locations (ie, 2 mm below the furcation and at the junction between the middle and apical third). Methods: Fortyfive mandibular first molars were taken, and the length of each tooth was measured. Then, specimens were divided into three groups according to their length: group I–long (24.2 mm 1.8), group II–medium (21 mm 1.5) and group III–short (16.8 mm 1.8). mesial root of each marked at two levels - at 2 mm below the furcation as well as at junction of apical and middle third of roots. The minimum thickness of the distal root dentine associated with the buccal and lingual canals of the mesial roots was measured, The distance between the buccal and lingual canals and the depth of concavity in the distal surface of the mesial roots were also measured. Results: Statistical analysis was performed by using analysis of variance and the Student-Newman-Keuls test. The minimum thickness of the distal wall of the mesiobuccal canal was significantly different (P < .001) between groups 1 (long) and 3 (short). Conclusions: Distal wall thickness of the mesiobuccal root and distal concavity of the mesial root of mandibular first molars were found to be thinner in longer teeth compared with shorter teeth. (J Endod 2014;-:1–4)
Key Words Mandibular molars, mesial roots, stereomicroscope
From the Departments of *Conservative Dentistry and Endodontics and †Oral and Maxillofacial Surgery, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi, India. Address requests for reprints to Dr Shweta Dwivedi, Conservative Dentistry and Endodontics, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2014.02.011
JOE — Volume -, Number -, - 2014
T
he mandibular first molar seems to be the most frequent tooth in need of endodontic treatment. It usually has 2 roots but occasionally 3, with 2 canals in the mesial root and 1 or 2 canals in the distal root. The distal root is readily accessible to endodontic instrumentation, but the mesial roots are usually curved, with the greatest curvature in the mesial-buccal canal (1). The main goal of endodontics is cleaning, shaping, and disinfecting the root canal system to restore the function and esthetic of the involved tooth (2) and to avoid transportation, ledge formation, and perforation (3). The mesial roots of mandibular first molars, approximately 2 mm below the furcation, have a greater concavity, and the thickness of dentin is limited (2). They are particularly subject to strip perforation (4). Thus, this is described as a danger zone (3). Cervical preflaring is recommended in order to eliminate cervical interferences during preparation of the apical third to the proper size, which could create an increased risk for iatrogenic procedural errors (4–9), especially in curved mesial root canals of mandibular molars (10). Abou-Rass et al (11) first described the anticurvature filing method to maintain the integrity of canal walls at their thin portion and reduce the possibility of root perforation or stripping. Although the thickness of the dentin in the danger zone has been studied widely, there is little information in the literature concerning the relation of the thickness of radicular dentin and tooth length. Therefore, the aim of this study was to analyze the correlation between dentin thickness of the mesial roots at the distal aspect and different lengths of the mandibular first molars.
Materials and Methods Ninety-eight mandibular first molars were collected from the Department of Oral and Maxillofacial Surgery, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh, India. Teeth with open apices, resorption, or calcification; endodontically treated teeth; and teeth with root caries were excluded from the study. Age, sex, and the systemic condition of the patients were unknown. Sixty-four teeth that showed fully formed roots and intact external morphology were selected. The degree of the curvature was standardized as described by Abou-Rass et al (11) and Schneider (12). Radiographs were taken in the mesial-distal and buccal-lingual directions to confirm that the mesial roots had 2 separate canals. On the basis of radiographic appearance, 45 teeth were selected to undergo further study. Selected teeth were placed in 3% sodium hypochlorite for 15 minutes for disinfection and then stored in normal saline with 0.2% thymol to inhibit microbial growth. The tooth length of each specimen was recorded from the mesiolingual cusp tip to the apex and categorized according to the length of teeth as follows: Group 1: Long teeth (ie, 23–26 mm, mean = 24.2 1.8 mm) Group 2: Medium teeth (ie, 20–23 mm, mean = 21 1.5 mm) Group 3: Short teeth (15–19 mm, mean = 16.8 1.8 mm) Mesial roots were marked at 2 mm below the furcation (level X) and at the junction between the apical and middle third of the roots (level Y) (Fig. 1). Then, they were sliced off horizontally at level X and level Y of the roots using a diamond disc with a thickness of 0.2 mm under water spray. In this study, both the coronal and apical surfaces were used for study (ie, the coronal surface as level X and the apical surface as level Y). The specimens were observed under a stereomicroscope (Stemi SV6; Zeiss Microsystems Ltd, Gottingen, Germany).
Strip Perforation and Length of Roots of Mandibular Molars
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Basic Research—Technology
Figure 1. A diagram showing level X and level Y and measurement parameters (ie, A, A1, B, and C).
Images of the sectioned surface were taken with the help of the integrated camera system under 36 magnification, and all measurements were collected. The minimum distal wall thickness of the buccal (A) and lingual (A1) canals of the mesial roots were measured and represented the least dentin thickness from the inner border of the canals to the external root surface. Parameter (B) is the distance between the mesiobuccal and mesiolingual canals. The depth of concavity (C) in the distal surface of the mesial roots was measured as the distance between a line joining together 2 points in the convex root surface and the deepest point in the concave root surface (Fig. 1). Statistical analysis was performed by using SPSS software (SPSS Inc, Chicago, IL). The analysis of variance test was used to measure variance within groups and among groups. For pair-wise multiple group comparison between groups, the Student t test (StudentNewman-Keuls test) was used. The level of significance was set at P < .001.
Results The mean at level X of parameter A in groups 2 and 3 was higher compared with group 1 (Table 1); this difference was statistically signif-
icant. The mean of parameter A at level X in group 3 was also significantly higher than in group 2. The mean of parameter A at level Y in group 1 was lower than in groups 2 and 3 (Table 1), but there was no significant difference between groups 1 and 2. Other differences were significant (Table 2). The mean of parameter A1 at level X in group 3 was higher compared with groups 1 and 2 (Table 1). This difference was statistically significant. For the mean of parameter A1 at level X, there was no significant difference between groups 1 and 2 (Table 2). The mean of parameter A1 at level Y in group 1 was lower than in groups 2 and 3, but all differences were insignificant. The mean at level X of parameter B in group 3 was higher compared with groups 1 and 2 (Table 1), but all differences were statistically insignificant (Table 2). The mean of parameter B at level Y in groups 1 and 2 were higher than in group 3 (Table 1); the differences were significant, but there was no significant difference between groups 1 and 2 (Table 2). The mean at level X of parameter C in groups 1 and 2 was higher compared with group 3 (Table 1); this difference was found statistically significant. The mean at level X of parameter C in group 2 was higher compared with group 1, but this difference was
TABLE 1. Mean Standard Deviation (in mm) of the Measurement Criteria at 2 Levels (X and Y) according to Groups and Its Intergroup Comparison A (in mm) Group 1 2 3
Level X
Level Y
A1 (in mm) Level X
Level Y
B (in mm) Level X
Level Y
C (in mm) Level X
Level Y
1.0721 0.105 0.8225 0.130 1.1406 0.201 0.8100 0.111 2.7121 0.495 2.1275 0.215 0.8683 0.045 0.8390 0.067 1.3340 0.078 0.9000 0.133 1.0361 0.106 0.8785 0.132 2.7615 0.385 1.8105 0.513 0.9522 0.116 0.4539 0.112 1.8780 0.169 1.0631 0.074 1.6920 0.296 0.9137 0.169 3.1787 0.664 1.0144 0.335 0.4498 0.193 0.2116 0.210
Intergroup comparison: 1-way analysis of variance F P
F = 164.949 P < .001
F = 16.907 P < .001
F = 39.995 P < .001
F = 2.134 P = .131
F = 3.538 P = .038
F = 35.007 P < .001
F = 61.429 P < .001
F = 73.209 P < .001
Bold signifies P < .001.
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Dwivedi et al.
JOE — Volume -, Number -, - 2014
Basic Research—Technology TABLE 2. Post Hoc Test (SNK test) for Pair-wise Multiple-group Comparison A Post hoc test by SNK test 1 vs 2 1 vs 3 2 vs 3
A1
B
C
X
Y
X
Y
X
Y
X
Y
Q = 5.785 P < .001 Q = 17.802 P < .001 Q = 12.016 P < .001
Q = 1.835 P = .202 Q = 5.696 P < .001 Q = 3.861 P < .001
Q = 1.325 P = .467 Q = 6.997 P < .001 Q = 8.323 P < .001
Q = 1.340 P = .457 Q = 2.031 P = .137 Q = 0.690 P = .867
Q = 0.255 P = .992 Q = 2.421 P = .058 Q = 2.165 P = .103
Q = 2.312 P = .074 Q = 8.120 P < .001 Q = 5.807 P < .001
Q = 1.728 P = .246 Q = 8.617 P < .001 Q = 10.346 P < .001
Q = 7.264 P < .001 Q = 11.996 P < .001 Q = 4.632 P < .001
Bold signifies P < .001.
insignificant. The mean of parameter C at level Y in group 1 was higher than in groups 2 and group 3; all differences were significant (Table 2).
Discussion The mandibular first molar is the most frequently endodontically treated tooth and is extremely anatomically challenging (13). According to Tabrizizadeh et al (14), the distal surface of the mesial root was reported thinnest (1.2 mm) and the lingual wall thickest (2.2 mm) at the furcation point. Micro–computed tomographic evaluation of the mandibular first molar quantitatively investigated the morphology and indicated that the furcal aspect of the entire mesial root should be considered a ‘‘danger zone.’’ Mesial canals were found to be much more variable than distal canals in morphology (13). Cervical preflaring recommended decreasing the tension of manual and rotary instruments during apical instrumentation (6, 7). The variation of root dentin thickness in different areas supports the notion (15) that preflaring of the cervical third could result in iatrogenic strip perforation. Therefore, care should be taken to avoid perforation at danger zones during shaping and post space preparation procedures (16). In this study, the length and danger zone of mesial roots in mandibular molars were analyzed and correlated. According to a previous study, the danger zone is located 4–6 mm below the canal chamber orifice (17). Knowledge of the remaining dentin thickness in the root, especially in the distal aspect of the mesial roots, would minimize, if not eliminate, the occurrence of strip perforations (18, 19). Our study showed that the distal wall thickness of mesial roots was significantly less in longer teeth than in shorter teeth, both at the area of furcation and at the junction between the apical and middle third of the roots. In mesiobuccal roots, the mean of dentin thickness in longer teeth was 1.0721 mm at the distal aspect and in shorter roots it was 1.8780 mm at level X, whereas it was 0.8225 and 1.0631 mm, respectively, at level Y. According to Berutti and Fedon (20), even differences of tenths or hundreds of a millimeter can be critical in avoiding strip perforation. Therefore, this study suggests that long molars may have a higher risk of strip perforation in mesiobuccal canals if flared to a larger size (20). The distal (furcal) surface concavity was deeper in teeth with long roots compared with those with short roots. Observations of root concavity is supported by the result of a previous study by Bower (19), who found a mean value of 0.7 0.19 mm ranging from 0.3–1.3 mm. However, no information on the length of the teeth was given in Bower’s study, which makes it difficult to compare the results. Additionally, the distance between the mesiobuccal and mesiolingual canals was measured in this study. Short molars presented the longest distance between the canals (3.1787 0.66421 mm), which was higher than the other groups (P < .000), but these results are not in agreement with previous studies. However, the range of mean disJOE — Volume -, Number -, - 2014
tances (2.7121–3.1787 mm) is similar to that found in previous studies (2–4 mm) (21). To prevent strip perforations, coronal flaring should be limited and directed at the outer and thicker aspects of the canal walls. Lim and Stock (22) have determined an arbitrary value of 0.3 mm as the minimum canal wall thickness that should remain after instrumentation to prevent perforation or vertical root fracture. Stereomicroscopic imaging was used to study the surface topography of the tooth surface, and it provides 3-dimensional information. This is basically a macroscope that allows users to slowly enlarge the object. Another feature of stereomicroscopes is a dual illuminator system that provides more than enough light to view specimens 3-dimensionally. A calibrated scale was integrated into the stereomicroscope, which makes it possible to measure dimensions of objects (ie, root dentin thickness at the microscopic level).
Conclusion The present study concludes that the root wall on distal surfaces of mesial roots in longer mandibular molars is thinner than in shorter teeth. Similarly, the mesial roots were found to be more concave in the distal aspect in the longer teeth. The distal wall of mesial roots in the mandibular first molars are more prone to strip perforation, particularly in longer teeth.
Acknowledgments The authors deny any conflicts of interest related to this study.
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