Analysis of mandibular second molars with fused roots and shallow radicular grooves by using micro-computed tomography.

AIM: This study aimed to describe the morphological and morphometric aspects of fused mandibular second molars with radicular shallow grooves using micro-computed tomography (CT).
MATERIALS AND METHODS: Eighty-eight mandibular second molars with fused roots were scanned in a micro-CT scanner at a voxel size of 19.6 μm. After reconstruction, only molars without C-shaped roots and presenting shallow radicular grooves were selected. 30 molars were chosen for further analysis. Canal cross-sections were classified according to Fan's modified classification (C1, C2, C3, and C4) and morphometric parameters at the apical region, examination of accessory foramina and tridimensional configuration were evaluated.
RESULTS: Three-dimensional reconstructions indicated a higher prevalence of merging type (n = 22). According to Fan's modified classification, the C4 configuration was predominant in the 3 apical mm. Roundness median values revealed a more round-shaped canals at 3 mm (0.72) than at 2 (0.63) and 1 (0.61) mm from the apex. High values of major and minor diameters were observed in the canals of these evaluated sections. In addition, few accessory apical foramina were observed at 1 and 2 mm from the apex. The average distance between last accessory foramina and the anatomic apex was 1.17 mm. A less complex internal anatomy is found when a mandibular second molar presents fused roots with shallow radicular grooves. The merging type canal was frequently observed. Moreover, the C4 configuration was predominant at a point 3 mm from the apex and presented rounded canals, large apical diameters, and few accessory foramina. The cervical and middle thirds presented C3 and C1 canal configurations most frequently.
CONCLUSION: A minor morphological complexity is found when fused mandibular second molars present shallow radicular grooves.

INTRODUCTION

The molar group possesses one of the most complex anatomies among all teeth[1] and understanding its internal anatomy and possible variations is of utmost importance for the clinician so it can determine a predictable root canal treatment approach. Normally, the mandibular molar has two separate roots, one mesial with two or three canals and one distal with one or two canals. A common variation in their external anatomy of these teeth is the presence of fused roots with major incidence in the mandibular second molar in comparison to the mandibular first molar[23] with a more unusual root canal system.[4]

When root fusion is present the aspect of the external surface of these teeth may be conical- or square-shaped,[56] and a shallow or deep radicular groove can be present individually or in both the buccal and lingual aspects of the root.[7] Such grooves may continue to the apical area or less than half of the root length.[78]

When a deep radicular groove is encountered (as given by a high groove-to-thickness ratio), a C-shaped canal system can majorly be found.[7] However, when a shallow groove or even no groove is present, the C-shaped root canal system may or may not exist[8] and single wide canals can be expected.[9]

In a previous study[7] of a total of 54 teeth, six teeth presented shallow radicular grooves, and four of them did not contain a C-shaped canal system. Two teeth presented single root canals. These wide single root canals have been reported[910] with a incidence 14 teeth out of 33 teeth[4] and 4 of 416 teeth (1%).[11] Whereas considerable information regarding the anatomy of mandibular second molars with fused roots can be found in the literature,[121314151617] anatomical studies revising the fused mandibular second molars with shallow radicular grooves are lacking. Thus, in this study using micro-computed tomography (CT) technology the morphological and morphometric aspects of these types of teeth were assessed.

MATERIALS AND METHODS

After the Ethics Committee Protocol Approval (Protocol #703.054), 88 extracted mandibular second molars with fused roots from a Brazilian population were selected. All teeth were scanned in a micro-CT system (SkyScan 1174 v2; Bruker-microCT, Kontich, Belgium) with 19.6 μm of voxel size, 50 kV, 800 mA, and 360° of rotation. The system includes a charge-coupled device camera (1304 × 1024 pixels). Radiographic images of each tooth were reconstructed with dedicated software (NRecon v. 1.6.3; Bruker-microCT) providing axial cross-sections of the inner structure of the samples.

Following the reconstruction, the cross-sections of all samples were analyzed on the DataViewer v1.5 (Bruker-microCT, SkyScan, Belgium) software. Only fused molars without C-shaped roots and presenting a shallow radicular groove that continued to less than half of root length as classified by Kotoku[8] were chosen [Figure 1]. The shallow groove was selected according to the groove/thickness ratio.[7] Following these criteria, a total of 30 molars were chosen for further analysis.

Figure 1

Representative images of the canal types and the five cross-sections levels evaluated in mandibular second molars with shallow radicular grooves: (A) merging, (B) symmetrical and (C) asymmetrical. E, F and G (merging, symmetrical and asymmetrical, respectively) show representative images of the remaining 58 mandibular molars with C-shaped roots and deep radicular grooves that did not meet the study criteria.

Tridimensional classification

The automatic segmentation and surface modeling with CTAn v. 1.12 software (Bruker-microCT) were used to reconstruct the 3D models. The qualitative evaluation of the specimens was conducted using the CTVol v. 2.2.1 and DataViewer v1.5.1 software (Bruker-microCT, SkyScan, Belgium). Then, the root canal configurations were classified according to Gao et al.[18] into merging (Type I), symmetrical (Type II), and asymmetrical (Type III) [Figure 1].

Classification and morphometric analysis of the root canal cross-sections

The cross-sections images were selected at 1, 2, and 3 mm from the apex, in the middle and the cervical third of the root. The middle third of the root was defined as the section equidistant between the root apex and the pulp chamber floor of the root. The cervical portion was chosen 1 mm apically from the pulp chamber floor. The number and morphology of the root canals in the evaluated levels were classified according to the Melton's modified configuration proposed by Fan et al.[7] as follows:

C1: An uninterrupted C-shaped canal with no separation or division, C2: The canal shape resembled a semicolon resulting from a discontinuation of the C-shaped canal outline, C3: Two or three separate canals were present in the cross-section, C4: Only one round or oval canal was present in the cross-section, and C5: No canal lumen was observed.

After classifying all the cross sections, the cross-sections images of the apical 3 mm were analyzed quantitatively for the area, major diameter, minor diameter, roundness, and aspect ratio (AR) using the CTAn v. 1.12 software (Bruker-microCT). Area and perimeter were calculated using the Pratt algorithm. The cross-sectional appearance, round or more ribbon shaped, was expressed as roundness. The roundness of a discreet two-dimensional (2D) object is defined as 4.A/(π.(dmax) 2), where A is the area and dmax is the major diameter. The value of roundness ranges from 0 to 1, with 1 meaning the perfect circle. The major diameter was defined as the distance between the 2 most distant pixels in that object. The minor diameter was defined as longest chord through the object that can be drawn in the direction orthogonal to that of the major diameter.[1219]

According to the standard practice for characterization of particles, the AR is a common measure of shape and is defined as “a ratio of the major to the minor diameter of a particle, which can be used when the major axis does not cross a particle outline:” AR = dmax/dmin.[2021] Examination of accessory foramina Cross-sections from 1 to 4 mm were recorded to analyse the presence of any accessory foramina of the 22 molars with merging type root canals. Also the average distance between the last accessory foramina and the anatomic apex was calculated.[15] Both analyses were performed using the DataViewer software v 1.5 (Bruker-microCT, SkyScan, Belgium). The median, minimum and maximum range values of all above-mentioned measurements were processed using the Prism 6.0 software (GraphPad Software Inc, La Jolla, CA, USA).

RESULTS

According to Gao's classification, the three-dimensional (3D) reconstruction of the canals showed a higher prevalence of the merging type (n = 22) in which mesial and distal canals merges into a single foramen at the apical third. The symmetrical type and asymmetrical type with four samples each. Illustrations on Figure 1 exemplify the encountered anatomical variations of the second mandibular molars with fused roots and shallow radicular grooves.

Root canal cross-section classification

In this part of the analysis, three teeth did not have a C-shaped cross-section in any of the analyzed sections and were considered as a single canal with circular or oval shaped; thus; they were excluded for posterior analyses. The other 27 molars exhibited a variable distribution of diverse canal shapes according to the Melton's classification having at least one cross-section level classified as C1, C2, C3, or C4.

At 1, 2, and 3 mm from the apex the occurrence of the C4 configuration was predominant at the 1 mm level with 18 teeth, at the 2 and 3 mm level, 16 and 14 molars showed this characteristic. In the middle and cervical third, few samples of this kind of configuration were observed in Table 1. At the middle and cervical thirds, the C3 configuration (two or three separated canals) was mostly observed followed by the appearance of the C1 cross-section configuration. The complete distribution of the cross-sectional configuration is presented in Table 1.

Table 1

Distribution of Melton's categories in mandibular second molars with C-shaped roots at the five evaluated levels

Quantitative analysis

The complete median and ranges of the 2D cross-section analysis of the apical third of the evaluated samples are described in Figure 2.

Figure 2

Distribution of morphometric data found at the apical, middle and cervical third of second mandibular molars with shallow grooves and merging type canals. The red lines indicate the medians.

According to Melton's modified classification, the C4 configuration was predominant in the first apical 3 mm. The roundness median values revealed a more rounded-shaped canal at 3 mm (0.72) than at 2 (0.63) and 1 (0.61) mm from the apex. The minor median diameters at 1, 2, and 3 mm were of 0.52, 0.52, and 0.55 mm, respectively. High values of major and minor diameters were observed in the canals of these evaluated sections [Figure 2].

DISCUSSION

Mandibular second molars can present three different types of external configuration: two rooted, C-shaped root and fused rooted. Previous studies have reported the root canal anatomy of two rooted and C-shaped canals in mandibular second molars.[67141516] However, detailed description of fused mandibular second molars with non-C-shaped roots is scarce.

In the 2D and 3D observations assessed in this study, an important finding was that the merging type canal was predominant in mostly all the fused molars studied. This merging type anatomy showed a predominant prevalence in comparison to the symmetrical and asymmetrical types. Previous studies[1219] showed a similar prevalence of these 3 anatomical variants in Chinese and Brazilian populations (merging, symmetrical and asymmetrical). Apparently, the extension and deep of the radicular grooves determine the anatomical configuration of the cross-sections, and the lower deepness of the groove or its absence in the apical third allowed an oval- or round-shaped canal [Figure 1].

The merging type canal that was predominant in this stud starts its internal canal configuration at the cervical third mainly as separate canals (C3) or as a C-shaped cross-sectional configuration (C1) and merges apically into one single canal (C4). At the apical third, a variety of apical diameters in the C4 configuration were observed. Since the shape of the canals at this region was majorly rounded instrumentation can be more predictable at this region. In our study, even though the wall thickness measurement was not assessed the image of the cross-sections in Figure 1, shows a thicker dentin toward the shallow groove. Possibly, higher file diameters could be used for debridement without increasing the risk of perforation. Previous reports[1822] related that the dentinal thickness in the lingual surface of the C-shaped roots was minimum due to its deep groove, showing low values of wall thickness of (0.27 and 0.74) mm in the apical, (0.45 and 0.95) mm middle, and (0.85 and 1.49) mm cervical portion, respectively.

When trying to identify a C-shaped molar, the use of surgical operative microscope can be of significant aid to identify and treat a C-shaped canal[23] since wide single canals due to the type of the cross-sections as the C1 or C4 can often be confused with perforation.[24] Related that it is not always possible to estimate the anatomical configuration of a C-shaped canal over its length based only in canal orifice shape.

Furthermore, the periapical radiography is the most common tool used for diagnosis, but it has the limitation of showing a 2D vision of a structure only. Thus, other aid as cone-beam computed tomography could allow to differentiate a fused molar with a shallow groove from a C-shaped root and to distinguish accurately the internal and external anatomy of this type of teeth.[13] Periapical radiographs in different angulations can be also useful.

Apical surgery in the second mandibular molar is frequently associated with the greatest number of surgical complications due to its limited accessibility, its alveolar bone thickness and the risk of nerve bundle damage.[25] Intentional reimplantation has some advantages over apical surgery being less time consuming and invasive where a considerable amount of bone would be required to be removed to contact the apices.[25] Its success rate has been reported to be 80%–82%.[2627] The examination of the apical foramina showed the presence of few apical foramina basically at 1 and 2 mm of the apex. In cases were tooth reimplantation of these mandibular molars can be indicated, sectioning 3 mm of the root might promote total elimination of apical foramina. Furthermore, the simplicity of the apical cross-section type (C4) facilitates retrograde preparation and filling with mineral trioxide aggregate (MTA).

CONCLUSIONS

Based on the results of this study, a less complex internal anatomy is found when fused mandibular second molars present shallow radicular grooves. It seems that a lower depth of the radicular groove or its absence permitted more rounded root canals. The merging type canal was frequently observed and the C4 canal configuration was predominant at a point 3 mm from the apex with rounded canals, large apical diameters and few accessory foramina. The cervical and middle thirds presented C3 and C1 canal configurations as dominant.

Financial support and sponsorship

This work was supported by the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) grant (2014/03009-1).

Conflicts of interest

There are no conflicts of interest.