Surgical and Radiologic Anatomy

, Volume 37, Issue 3, pp 267–272 | Cite as

An anatomical investigation of the mandibular second molar using micro-computed tomography

  • Sara A. Barsness
  • Walter R. Bowles
  • Alex Fok
  • Scott B. McClanahan
  • Samantha P. Harris
Original Article

Abstract

Purpose

An understanding of root anatomy is an important foundation for providing successful endodontic treatment. The aim of this study was to use micro-computed tomography (micro-CT) to investigate the root anatomy of the mandibular second molar.

Methods

Eighteen mandibular second molars were scanned using micro-CT. Images were reconstructed, and measurements and observations were recorded regarding pulpal floor anatomy, canal configuration, root wall thickness along the root, presence of calcifications in the pulp chamber and in canals, and apical anatomy.

Results/conclusions

The most frequently found mesial root canal configuration was Vertucci Type 7 (1-2-1-2), which was seen in 33.3 % of samples. Distal canals were most frequently Vertucci Type 1 (one canal), with 61.1 % of samples showing this configuration. 11.1 % of samples had two canals, 44.4 % of samples had three canals, 33.3 % of samples had four canals, and 11.1 % of samples had five canals at some point along the length of the roots. Average root wall thickness between the mesiobuccal canal and the furcation was 1.23 mm. Mesiolingual canal root wall thickness was on average 1.29 mm, and the distal root furcation wall thickness averaged 1.41 mm. 77.8 % of samples had calcifications present in both the pulp chamber and within the canals.

Keywords

Mandibular second molar Micro-computed tomography Canal configurations Root wall thickness Calcifications 

Purpose

The main goal of endodontic therapy is to prevent or treat apical periodontitis. This goal is accomplished through disinfection of the root canal system, followed by a three-dimensional obturation of the canals and associated structures [14, 15]. The complexity of root canal anatomy, however, can create challenges for the clinician seeking to accomplish these objectives [12]. A thorough understanding of the canal anatomy of any tooth type is, therefore, an essential first step in effective endodontic therapy. In this study, the mandibular second molar was chosen as the focus due to previous findings that in the population seeking root canal therapy, molars in account for a large proportion of the teeth treated by root canal therapy, the mandibular second molar being the third most frequently endodontically treated tooth [9].

Although there is already literature dedicated to the anatomy of the mandibular second molar, most of the literature is based on older research techniques that may have introduced error into the reported results [3, 8, 18]. Micro-computed tomography (micro-CT) offers an exceptionally detailed three-dimensional representation of a tooth and canals that can be obtained without any modification to the tooth and with minimal error [7, 12].

The purpose of this study was to describe the root canal morphology of the mandibular second molars using micro-CT and either confirm or refine the findings of previous anatomical studies on this tooth type. Specifically, this study evaluated: pulp floor anatomy, root wall thickness, canal configuration, presence of calcifications in the pulp chamber and in the canals, and apical anatomy.

Materials and methods

This project was considered exempt from review by the Institution’s Review Board. Eighteen extracted mandibular second molars were obtained from local dental clinics. The age and gender of the patients from whom these samples were obtained were unknown. The reason for the extractions and whether the teeth were associated with pathology was also unknown. Teeth with fused roots or suspected C-shape anatomy were excluded. Each tooth was mounted on a cylindrical specimen holder. A series of tomographic images (at approximately 935 × 1001 × 1437 voxels, effective resolution 11.41 × 12.21 × 17.53 μm) were obtained for each specimen using micro-CT (X-Tech XT H 225). CT Pro software (Nikon Metrology, Leuven, Belgium) was used to reconstruct a three-dimensional image of each tooth. VG Studio MAX 2.1 software (Volume Graphics GmbH, Heidelberg, Germany) was then used to view the reconstructed images and make measurements regarding pulp floor anatomy, root wall thickness, and apical anatomy. Images of tooth structure were rendered translucent relative to the canals for determination of canal type in accordance with the classification system presented by de Pablo [5]. The presence or absence of calcifications in the pulp chamber and within canals was also recorded.

A representative image of the mandibular second molar is shown in Fig. 1a along with that of calcifications (Fig. 1b). In the pulpal floor area, the following measurements were taken from 0.5 mm below the coronal-most point of the pulpal floor: (1) distance between outermost aspect of mesial canal(s) (Fig. 1c), (2) distance between outermost aspect of distal canal(s) (Fig. 1d), (3) distance from the mesial aspect of mesial canals to the distal aspect of distal canal(s) (Fig. 1e), (4), measurement from the pulpal floor to the furcation (the distance from the most coronal aspect of the pulpal floor to the most coronal aspect of the furcation was used) (Fig. 1f), and (5) buccolingual and mesiodistal measurements of each canal orifice.
Fig. 1

a Representative three-dimensional image of mandibular second molar canal anatomy. b Presence of calcifications within the pulp chamber. c Distance between outermost aspect of mesial canal(s). d Distance between outermost aspect of distal canal(s). e Distance from the mesial aspect of mesial canals to the distal aspect of distal canal(s). f Distance from pulp floor to furcation. g Thickness of root dentin toward the furcation from the mesiobuccal canal. h Thickness of root dentin toward the furcation from the distal canal

Using the imaging software, both roots were sectioned in 1.0 mm increments from furcation to apex to measure the thinnest area of root dentin toward the furcation at each 1.0 mm point (Fig. 1g, h).

In the apical area of the tooth, the following were recorded: (1) number of major foramina in each root corresponding to the main canal(s), (2) number of accessory foramina, (3) distance from the most coronal aspect of the major foramen to the anatomic apex (Fig. 2a), (4) smallest width of the minor constriction (Fig. 2b), and (5) distance from the minor constriction to the major foramen (Fig. 2c). All measurements and observations were made by one observer.
Fig. 2

a Distance from the most coronal aspect of the major foramen to the anatomic apex. b Smallest width of the minor constriction. c Distance from the minor constriction to the major foramen. d Summary of apical anatomy results

Results

Pulp floor anatomy

The mesial orifice area of the pulp floor was made up of one continuous structure connecting mesiobuccal and mesiolingual canals in sixteen of the samples, with only two samples including separate mesiobuccal and mesiolingual orifices. Average buccolingual dimension of the mesial connected orifices was 1.94 mm. Mesiodistally, the width of this orifice averaged 0.56 mm. A singular distal orifice was noted in 17/18 teeth (94.4 %), with two separate orifices seen in only one sample. The average buccolingual dimension of the distal orifices was 1.47 mm. Average mesiodistal dimension of the distal orifices was 1.01 mm with standard deviation of 0.31. Distance between the mesial orifices and the distal orifices averaged 3.26 mm. The distance from the pulp floor to the furcation was on average 3.95 ± 0.86 mm.

Root wall thickness

The thinnest root wall thickness toward the furcation was 1.23 mm for the mesiobuccal canal, 1.29 mm in the mesiolingual canal, and 1.41 mm in the distal canal. Along the length of the roots, the thinnest area of dentin toward the furcation ranged from 0.86 to 2.21 mm.

Canal configurations

The most frequent mesial canal configuration found was Type 7 (1-2-1-2), with 33.3 % of samples showing this configuration. Two samples (11.1 %) had only one canal in the mesial root and three samples had three canals at some point within the mesial root (16.7 %). There was a wide variety of configurations seen in the mesial root with 10 different categories represented in 18 teeth.

The distal root samples showed fewer canal configuration types, with four types represented in this group. Type 1 (one canal) was the most commonly found system in eleven (61.1 %) of the samples and the remaining samples demonstrated two canals at some point along their length.

Overall, 11.1 % (2/18) of the samples had two canals, 44.4 % (8/18) of the samples had three canals, 33.3 % (6/18) of the samples had four canals, and 11.1 % (2/18) of the samples had five canals (Table 1).
Table 1

Mesial and distal canal configurations, number of canals per root, and number of canals per sample

Sample #

Canal configuration

Number of canals

Total number of canals

Mesial

Distal

Mesial

Distal

1

V1

V1

1

1

2

2

V6

V2

2

2

4

3

V6

V1

2

1

3

4

V7

V3

2

2

4

5

V5

V7

2

1

3

6

V7

V3

2

2

4

7

V7

V3

2

2

4

8

V7

V1

2

1

3

9

G9

V3

3

2

5

10

V7

V7

2

2

4

11

G10

V1

2

1

3

12

V6

V1

2

1

3

13

V3

V1

2

1

3

14

V7

V1

2

1

3

15

V4

V1

2

1

3

16

S17

V1

3

1

4

17

G12

V3

3

2

5

18

V1

V1

1

1

2

Calcifications

In the pulp chamber, 15 of the 18 samples (83.3 %) exhibited calcifications. Within the canal spaces, 14 of the 18 samples (77.8 %) were calcified. Three samples (16.7 %) were free from calcifications in both the pulp chamber and the canals, while 14 teeth (77.8 %) had calcifications present in both the pulp chamber and within the canals. Only one sample (5.6 %) had calcified material in the pulp chamber while the canals remained calcification-free.

Apical anatomy

In the 18 teeth studied, there were 52 major foramina that were found to deviate an average of 0.89 mm from the anatomic apex. The minor constriction was on average 0.25 mm in width at the narrowest point. When measuring the distance from the major constriction to the minor constriction, the average was 0.54 mm. In the apical region of the mesial root, 8 (44.4 %) of the roots had only 1 major foramen, 3 (16.7 %) of the roots had 1 lateral, secondary, or accessory canal, 4 (22.2 %) of the roots had 2 lateral, secondary, or accessory canals, and 3 (16.667 %) of the roots had 3 lateral, secondary, or accessory canals. The distal root had fewer lateral, secondary, or accessory canals overall, with 11 (61.1 %) roots having 0, and 7 (38.9 %) roots having 1 (Fig. 2d).

Discussion

Pulpal floor anatomy

The pulp floor has been described in terms of the symmetry of orifices along the mesiodistal midline of the tooth [10]. In this study, the average distances between the orifices were reported. The limitation of this data is the variability in the distances from tooth to tooth.

The distance from the pulp floor to the furcation has been reported as 2.96 ± 0.78 mm [6]. In this study, this distance was on average 3.95 ± 0.86 mm. Both studies included heavily restored teeth, but in this study, no access preparation was made. Perhaps there were undisturbed calcifications present on the pulp floor in this study that account for a greater distance on average from pulp floor to furcation, or differences could be due to low sample size. Clinicians can use the findings of this study and previous studies to guide them on how deep they can trough on the pulpal floor in this tooth when searching for calcified canals.

Root wall thickness

The area in the first mandibular molar that is thought to be the most vulnerable to strip perforation is located 1.5 mm below the furcation, and has been reported to be 1.2–1.3 mm thick [1]. In this study, the average root wall thickness over 10 mm of root length was 1.23 mm for the mesiobuccal canal (range: 0.91–1.47 mm), 1.29 mm in the mesiolingual canal (range: 0.86–1.71 mm), and 1.41 mm in the distal canal (range: 1.18–1.83 mm). There was a general trend of thicker root structure more coronally, and thinner structure apically. For example, the mesiobuccal canal started with an average root thickness of 1.47 at 2.0 mm from the furcation, decreasing in thickness to 0.91 at 10.0 mm down the root. The mesiobuccal canal had the thinnest root wall compared to the mesiolingual and distal canals, and the very dramatic clinical point is that the root wall is generally thin enough that iatrogenic errors that deviate the canal or procedures that over-enlarge the canal have the potential to perforate the root at any level. Dentist must be aware of the thinness of root dentin in these areas to avoid strip perforations. It may be advisable to file away from the furcation in areas of thin root dentin.

Canal configurations

A wide variety of root canal configurations were seen in the mesial root, with 10 different categories represented in 18 teeth. The distal canal expressed less variety in form, with four canal configuration types seen in the samples investigated. The multiple forms seen in this study are likely due to frequent communications between the canals such as isthmuses that may be difficult to appreciate with other methods of investigation. In this study, there were only 18 total samples. If the number of samples were increased, the number of categories represented may be even more diverse. Many of the more complex canal types involved midroot connections (isthmuses) that would be difficult to detect radiographically and manage clinically.

In this study, a higher percentage of two-canal and five-canal teeth were seen compared to classic studies of mandibular second molars. In contrast to the retrospective in vivo study by Hartwell and Bellizzi [8], which found two canals 4.1 % of the time, three canals 89.4 % of the time, four canals 5.5 % of the time, this study had 11.1 % of samples with two canals, 44.4 % with three canals, 33.3 % with four canals, and 11.1 % with five canals. It is plausible that the higher representation of four and five-canal samples is due to the high resolution available using the micro-CT scans. However, the finding of a higher percentage of two-canal mandibular second molars would not be expected in a micro-CT study. It is possible that due to smaller number of samples, this study had a higher percentage of mandibular second molars with two canals by chance. The strong representation of mandibular molars with two canals does suggest that these teeth exhibit a variety of forms, ranging from rather simple to quite complex. When treating this tooth type, clinicians must be aware of this variable anatomy and use other clues such as the radiographic appearance of the root configuration or the symmetry of the pulpal floor [9], to help determine the number of canals present.

Calcifications

The majority of the teeth (77.8 %) in this study had calcifications present in the pulp chamber and within the canals. Many of the samples had large restorations and evidence of wear, which may have contributed to the high degree of calcification seen. Another possible factor may be the age of the patients from which the teeth were extracted, which was not recorded for this study. Periodontal issues may also have had an impact on the calcification rate; however, periodontal status was unknown. The calcifications were extensive enough that they could block progress down canals. In this study, there was not an attempt to rate the degree of calcification in the pulp chamber or canals. This may be an area to address in future studies. The knowledge that a high percentage of mandibular second molars may have calcifications in the pulp chamber and canals may help dentists identify and prepare for these potential obstacles to treatment preoperatively.

Apical anatomy

Apical anatomy findings were consistent with those reported in previous studies. The distance from the major foramen to the anatomic apex on average was 0.89 mm in this study, as compared to 0.59 and 0.61 mm as reported in previous studies [2, 11]. The proximity of the major foramen to the minor constriction averaged 0.54 mm in this study, with 0.72 mm for this distance found previously [17]. As in any non-surgical root canal case, clinicians should consider using an electronic apex locator to determine working length, but the knowledge of average apical dimensions can give the dentist some additional guidance in determining working length [16]. For example, in this tooth, the fact that the major foramen deviates from the anatomic apex 0.89 mm on average gives the clinician guidance that the canal may not appear to exit at the apex radiographically.

An average minor constriction width was found to be 0.25 mm in this study, which compares rather closely to 0.189 mm found in previous studies [17]. Knowing the average width of the constriction would help the clinician to decide the size of the apical preparation within this tooth. The fact that the constriction is anatomically 0.25 mm indicates that an apical preparation size of anything smaller than an ISO size 30 file would likely not clean all of the walls in the apical few millimeters of the canal.

Lateral, secondary, and accessory canals were present in 55.6 % of mesial roots in the apical region and 38.9 % of distal roots in the apical region. Previous studies reported 17 % of roots with lateral, secondary, and accessory canals present in the apical third [4]. The higher frequency of these structures found in this study is likely related to the detail available in images from the micro-CT. The significance of these accessory canals in the apical area is that these are areas that current endodontic instruments may not be able to mechanically clean, so the dentist must focus on disinfecting these spaces by either chemical means or even surgical intervention in some cases.

Conclusion

This micro-CT morphologic evaluation of the mandibular second molar investigated pulpal floor anatomy, root wall thickness, canal configurations, calcifications, and apical anatomy. Pulpal floor anatomy was variable in this tooth. Root wall thickness toward the furcation averaged 1.23 mm from the mesiobuccal canal, 1.29 mm from the mesiolingual canal, 1.41 mm from the distobuccal canal, and 2.03 mm from the distolingual canal. The most frequent mesial canal configuration was Vertucci type 7 (1-2-1-2). The distal root samples exhibited fewer canal configuration types, with Vertucci type 1 (one canal) found most frequently. Calcifications were found in the chambers and canals of 77.8 % of samples. Measurements of apical root anatomy were similar to those found in previous studies. The goals is to expand this study in the future to include more samples and further confirm the findings of this current investigation. However, it is hoped that the results of this study will aid dentists in the endodontic treatment of the mandibular second molar.

Notes

Acknowledgments

The authors deny any conflicts of interest and any financial affiliations related to this study or its sponsors.

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Copyright information

© Springer-Verlag France 2014

Authors and Affiliations

  • Sara A. Barsness
    • 1
  • Walter R. Bowles
    • 2
  • Alex Fok
    • 3
  • Scott B. McClanahan
    • 2
  • Samantha P. Harris
    • 2
  1. 1.Private PracticeSt. PaulUSA
  2. 2.Division of EndodonticsUniversity of Minnesota School of DentistryMinneapolisUSA
  3. 3.Minnesota Dental Research Center for Biomaterials and Biomechanics, University of Minnesota School of DentistryMinneapolisUSA

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