Anatomical danger zone reconsidered: a micro-CT study on dentine thickness in mandibular molars

Abstract

Aim: To investigate the smallest dentine thickness in mesial canals of mandibular molars along the cervical and middle thirds of the root by means of a micro-computed tomographic (micro-CT) technology and digital image analysis.

Methodology: Fifty mesial roots of mandibular molars having two independent canals (mesiobuccal and mesiolingual), in the coronal and middle levels, were selected and scanned in a micro-CT device. After reconstruction procedures, approximately 468 slices per root covering the 7 mm below the furcation area of the mesial root were analysed to measure the smallest dentine thickness (danger zone [DZ]) in each slice from both distal and mesial regions of the mesial canals by an automatic segmentation process.

Results: The DZ values in the mesiobuccal canals varied from 0.67 to 1.93 mm, with an average of 1.13 ± 0.21 mm. For the mesiolingual canals, the DZ varied from 0.77 to 1.89 mm with an average of 1.10 ± 0.21 mm. There was no correspondence in the DZ between the mesiobuccal and mesiolingual canals at the same cross-sectional level in 71% of the specimens. Moreover, the smallest dentine thickness was towards the mesial region of the roots in 22% and 18% of the mesiolingual and mesiobuccal canals, respectively.

Conclusions: The smallest dentine thickness was on the mesial plane of the roots in about 40% of the canals. The vertical location of the DZ in relation to the furcation area was in the middle third of the root.

Introduction

Midroot perforations are normally caused by over-instrumentation on an already thin dentinal wall, which may seriously compromise the outcome of root canal treatment (Estrela et al. 2018). These perforations have been related historically to the distal area of mesial roots in mandibular molars, and thus, Abou-Rass et al. (1980) introduced the concept of the ‘danger zone’ (DZ) in the early 1980s. In fact, these authors formally reported what experienced clinicians already knew: often, mesial canals of mandibular molars do not assume a central position in the root with the distal area between the canal and root bifurcation being relatively thin, the so-called DZ, which is more vulnerable to strip perforations. On the other hand, the safety zone was described as the mesial area of the mesial root with a thicker dentine layer, which is often minimally instrumented by endodontic instruments. In short, Abou-Rass et al. (1980) pointed out the importance of this anatomical area during canal shaping and, since then, many studies have evaluated the safety of various preparation techniques in mesial canals of mandibular molars (Garcia Filho et al. 2003, Akhlaghi et al. 2010, Silva et al. 2017).

The anatomy of the DZ as well as the assessment of canal preparation techniques and instruments has been based on destructive and invasive approaches (i.e. sectioning methods) (Garcia Filho et al. 2003, Akhlaghi et al. 2010). This leads to serious experimental limitations as it allows the analysis of only a few slices per root.

In the early 2000s, the introduction of micro-computed tomographic (micro-CT) imaging opened new possibilities for endodontic research since this technology allows precise nondestructive longitudinal two-dimensional (2D) and three-dimensional (3D) assessments (Peters et al. 2001, 2003, De-Deus et al. 2015, 2016, Silva et al. 2017), because micro-CT is based on a powerful X-ray source, which allows visualization and measurement of the outer and inner structures of an opaque object without needing previous sample preparation or chemical fixation. For instance, micro-CT with mathematical modelling has provided information on the dentine thicknesses in 1-mm intervals (Harris et al. 2013). However, detailed morphological information about the DZ remains inconsistent, sparse and sometimes controversial, as the evaluation of just a few cross sections per sample is rather meaningless, especially when using micro- CT technology. Lee et al. (2015) appears to be the only micro-CT study that used high-resolution and smaller slice intervals (0.1 mm). Therefore, a comprehensive anatomical study of danger and safety zones in mesial roots of mandibular molars is timely and may help reduce either the risk of midroot perforations towards the furcation area or an unnecessary loss of sound dentine tissue, which appears to contribute to the long-term survival of teeth (Soares et al. 2008).

The current study has a descriptive nature and was designed to investigate the smallest dentine thickness in mesial canals of mandibular molars along the cervical and middle thirds of the root by means of micro-CT technology and digital image analysis.

Materials and methods

Sample selection and imaging

The present ex vivo study was approved by the Ethical Research Committee of Fluminense Federal University. Initially, one hundred moderately curved mesial roots (10–20°) of first and second mandibular molars, selected according to Schneider’s method (Schneider 1971), with 10 ± 1 mm in length, were imaged with a micro-CT scanner (SkyScan 1173; Bruker microCT, Kontich, Belgium) at 14.25 lm (pixel size), 70 kV, 114 mA, 180° rotation around the vertical axis, rotation step of 0.7°, camera exposure time of 250 milliseconds and frame average of 4, using a 1-mm-thick aluminium filter. The images were reconstructed (NRecon v. 1.7.1.6; Bruker microCT) with similar parameters for beam hardening (35–45%), ring artefact correction (3–5) and contrast limits (0–0.05). Then, fifty mesial roots with two independent canals (mesiobuccal and mesiolingual), at the coronal and middle levels, were selected. None of the specimens had a root filling, root caries, cracks, fractures, and internal or external resorption.

Image analysis

The volume of interest was selected extending from the furcation level to 3 mm from the anatomical apex of the mesial roots, corresponding to approximately 468 slices per root, for a total of 23400 slices. An image analysis routine was developed to measure the minimal dentine thickness from both distal and mesial aspects of the mesial canals of mandibular molars by using a previously validated BoneJ plug-in (Doube et al. 2010) implemented into the Fiji/ImageJ software (Fiji v.1.51n; Madison, WI, USA). First, a 3D median filter was applied to the stacks in order to reduce overall noise (Neves et al. 2015) and the dentine was binarized using a minimum threshold algorithm. Then, the BoneJ plug-in was used to identify the position and measure the smallest dentine thickness in each slice for both mesial canals.

Three-dimensional mapping of the dentine thickness was created and saved for structure thickness in CTAn v.1.15 software (Bruker microCT), and loaded into CTVox v.3.3 software (Bruker microCT) to generate colour-coded 3D models of the mesial roots of mandibular molars.

Results

Table 1 presents descriptive data for all specimens regarding the smallest dentine thickness (DZ) and its location relative to the furcation area. Table 2 shows the location of the DZ for all specimens according to the distance from the furcation area distributed in 1-mm intervals.

The DZ values in the mesiobuccal canals varied from 0.67 to 1.93 mm, with an average of 1.13 ± 0.21 mm. For the mesiolingual canals, the DZ varied from 0.77 to 1.89 mm, with an average of 1.10 ± 0.21 mm. There was no correspondence in the DZ between the mesiobuccal and mesiolingual canals at the same cross-sectional level in 71% of the specimens. Moreover, the DZ was towards the mesial region of the roots in 22% and 18% of the mesiolingual and mesiobuccal canals, respectively (Figs 1 and 2).

Figure 3 shows colour-coded representations of the dentine thickness throughout mesial roots of five representative mandibular molars. Qualitative analysis demonstrated that the noncentred position of the mesial canals and the asymmetric shape of the root resulted in variable thickness of the dentine at different levels and directions, including areas towards the mesial aspect of the root in some cases.

Discussion

The current study provided two innovative results regarding the anatomical DZ. First, the smallest dentine thickness was towards the mesial plane of the roots in around 40% of the canals (22% and 18% of the mesiolingual and mesiobuccal canals, respectively). Secondly, the vertical location of the DZ in relation to the furcation area was towards the middle third of the root.

Relevant original data revealed by the present study were about the position of the DZ in the cross-sectional plane of the root. Traditionally, the understanding from the classic concept of the DZ refers to the distal area between the main canal space and the root bifurcation, which has the thinnest dentine and is more prone to the development of strip perforations. However, the current results revealed that the DZ was displaced towards the furcation area in only 60% of the evaluated cross sections. In the other 40% of the slices, the thinnest dentine was located on the mesial region of the roots, which contrasts with the generally held view (Abou-Rass et al. 1980). Also using a micro-CT imaging analysis, Lee et al. (2015) found the thinnest root canal wall on the mesial portion of the root in between 15% and 33% of the specimens, which is consistent with the present results.

The DZ evaluated in the present study was located up to 4 mm under the furcation area in only 35% of the specimens, whilst the majority of the samples revealed that the DZ was between 4 and 7 mm below the furcation area. In other words, DZs prone to either strip perforation or unnecessary loss of dentine are more towards the middle third of the root (4.37 ± 1.68 mm under the furcation area) than previously reported (Table 1). This finding is innovative inasmuch as previous information described the anatomical position of the DZ is concentrated up to 4 mm below the furcation level (Kessler et al. 1983, Berutti & Fedon 1992, Garcia Filho et al. 2003, Sauáia et al. 2010, Tabrizizadeh et al. 2010, Akhlaghi et al. 2015).

It is worth mentioning that the mean smallest dentine thickness along the distal surfaces in mesial roots of  mandibular  molars found in this study was 0.67 mm, which is smaller than usually reported in the literature: Lim & Stock (1987) = 0.94 mm, Garcia Filho et al. (2003) = 0.79 mm, Kessler et al. (1983) = 1.08 mm, Akhlaghi et al. (2015) = 1.05 mm, Berutti & Fedon (1992) = 1.2 mm and Tabrizizadeh et al. (2010) = 1.3 mm. The information on the smallest dentine thickness values was mainly created from investigations based on destructive sectioning methods and direct optical microscopy observation of a few slices of the mesial roots. One exception is a cone beam computed tomography study with low spatial resolution (Akhlaghi et al. 2015). The other exception is a micro-CT study  which  revealed values  of 0.81 mm, but the measurements were taken only 1.5 mm below the furcation area (Harris et al. 2013). The original findings presented in this study are probably a consequence of the methodological advancement achieved with the interplay between the micro-CT imaging technology and an automatic computational routine for digital image analysis and processing. In fact, the accumulated body of evidence on the DZ was created essentially based on destructive methods and direct microscopy observation of a few root sections per tooth (Kessler et al. 1983, Berutti & Fedon 1992, Garcia Filho et al. 2003, Sauáia et al. 2010, Tabrizizadeh et al. 2010). Moreover, even studies that used micro-CT imaging technology only performed the evaluation of a few cross-sectional images (Harris et al. 2013, Ordinola-Zapata et al. 2019). In this study, a full 3D mapping of dentine thickness along the entire cervical and middle thirds was performed rendering data from hundreds of cross sections per root. Furthermore, measurements performed by an automatic computational routine are more robust and reliable. Markedly, the automated analysis allowed the rapid measurement of thousands of slices rendering the experiment less time-consuming and labour-intensive. Moreover, the age of teeth is an uncontrolled variable that may have influenced the present results at least to some degree. Stored teeth were used with unknown information about age; as long as age affects the overall canal size, it is likely to affect the overall dentinal thickness.

Conclusions

The smallest dentine thickness in extracted mandibular first and second molars was towards the mesial plane of the roots in approximately 40% of the canals, whilst the overall vertical location of the DZ was towards the middle third of the root.

Authors: G. De-Deus, E. A. Rodrigues, F. G. Belladonna, M. Simões-Carvalho, D. M. Cavalcante, D. S. Oliveira, E. M. Souza, K. A. Giorgi, M. A. Versiani, R. T. Lopes, E. J. N. L. Silva, S. Paciornik

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