The internal anatomy of danger zone of mandibular molars: A cone-beam computed tomography study.

AIM: The aim of this study was the assessment of the anatomical thickness of danger zone in the cervical third of mesial canals of mandibular molars.
MATERIALS AND METHODS: Fifty mandibular molars were selected and scanned with cone-beam computed tomography. Data were compared using a length tool provided by scanner software. The measured areas were the mesial and distal walls of the cervical third of the mesial roots, which correspond to the safety and danger zones, respectively. In addition, dentin thickness at the furcation was evaluated.
RESULTS: Dentin thicknesses of the safety zone were higher than in the danger zone in all teeth examined. The thinnest dentin of the safety zone was found at a point located 4 mm below the canal orifice, with a mean value of 1.03 mm; conversely, in the danger zone, the thinnest point was located 3 mm below the orifice, with a mean value of 0.81 mm. As for the distance from the pulp chamber floor to the furcation, the average value was 2.23 mm.
CONCLUSION: These results show that mean thicknesses at the danger zone of mandibular molar mesial roots were <1.0 mm. These data reinforce the importance of understanding anatomy and the need for conservative preparations when assessing and instrumenting these root canals.

INTRODUCTION

Understanding the internal configuration of the tooth, as well as its variations, is necessary for successful endodontic treatment, as it enables efficient root canal preparation and proper filling of the canal. The complexity of the internal anatomy, however, brings difficulties that can lead to the failure of endodontic therapy from complications such as perforations in the molar's furcation region.[1]

The furcation region of molars is recognized as one of the most vulnerable areas in cervical preparation,[2] especially in the dentin walls located in so-called risk areas. These risk areas are located in the distal walls of the mesial canals and have thin dentin walls interposed between the root canal and the furcation region.[3]

The occurrence of perforations and root fractures during endodontic instrumentation occurs more frequently in these areas,[4] thus triggering underlying inflammatory processes and the subsequent collapse of support structures.[5] Understanding the internal anatomy can help prevent these accidents.

Advances in imaging technology, such as cone-beam computed tomography (CBCT), enable a three-dimensional analysis of internal dental morphology.[6] providing accurate measurements of dental structures – such as the configuration of the root canal system, the thickness of dentin, and the concavity of the root surface in risk areas.[7]

This study aimed to measure the average thickness of the mesial and distal walls – corresponding to secure and risky zones, respectively, – of the cervical third in the mesial roots of mandibular molars. Furthermore, the distance between the floor of the pulp chamber and the furcation was measured using images obtained through CBCT.

MATERIALS AND METHODS

Sample selection

Fifty mandibular molars were selected from a pool of 412 mandibular molars of Estacio de Sá University, from both gender and age between 20 and 40 years old. Molars with intact pulp chambers, complete root tips, root morphologies, curvatures and approximate lengths, pulp chambers and root canals without resorptions or calcifications, two canals in the mesial root, and complete separation of the mesial roots from the distal roots were included in the study. The teeth were placed in 2.5% sodium hypochlorite solution for 1 h (Briluz Bleach, Prodisa Producer and Distributor Ltda, Rio de Janeiro, Brazil) and were evaluated by direct visual inspection to check for possible periodontal ligament remains attached to the root, which, if present, were removed using periodontal curettes, ultrasonic tips, and bicarbonate jet. Following this cleaning procedure, the teeth were stored in 10% formalin.

Tomographic cone-beam examination

After mounting the teeth in an epoxy resin platform, they were scanned using the ACCUITOMO (J Morita Mfg. Corp, Japan), with the cut calibrated at 1.0 mm. The data obtained were transferred to the image analysis program ODVIEWER (One Data Viewer Plus, Kyoto, Japan) for evaluation.

Measurement of dentin thickness of risk and safety areas in mesial roots

Using the ODVIEWER (One Data Viewer) software, the starting point was set from the furcal, which was considered point 0 mm; the thicknesses of the dentinal structures of the distal and mesial walls of the mesial canal were evaluated at every millimeter in the apical direction until the fourth millimeter [Figure 1], which corresponds to the beginning of the curve (Cunningham and Senia, 1992).[8] To measure the shortest distance between the inner walls of the canals and the distal (risk areas) and mesial (security areas) outer walls of the mesial root of mandibular molars, the examiner traced a line starting from the inner wall of the canal and moving perpendicularly to an imaginary line that was tangential to the outer walls of the root [Figure 2].

Figure 1

Levels selected for measuring the cervical third of mesial root of mandibular molars

Figure 2

Measurement of dentin thickness of the risk and safety area of mesial root of mandibular molars

Measuring the distance of the pulp chamber floor to the furcation

The same ODVIEWER (One Data Viewer) software was used. A Z-line was placed on top of the floor, and the corresponding measurement was recorded. Then, the same line was placed at the exact point of the furcal where the mesial and distal roots split, and this measurement was also recorded, and the value of the floor-furcation distance was obtained by subtracting these two measurements.

Statistical analysis

Statistical analyses were conducted using an analysis of variance and complemented by the Student–Newman–Keuls multiple comparison test when necessary. The chosen significance level was 5% (0.05) in all tests.

RESULTS

Measurements of dentin thickness in the distal wall of the lower molars (risk zone) were evaluated from the furcal at every millimeter in the apical direction until the fourth millimeter. The same measurements were taken on the mesial wall of the mesial buccal canal. The Table 1 shows the mean and the standard deviation of cervical third danger and security areas thickness of cervical third at different levels. The mean of furcal dentin thickness of the 50 molars was 2.23 mm (standard deviation: 890), according to Table 2.

Table 1

Mean±standard deviation of the dentin thickness of risk and safety areas of the mesial buccal root of lower molars in the five levels assessed

Table 2

Mean±standard deviation of the furcal dentin thickness

DISCUSSION

The success of endodontic treatment requires extensive knowledge of the internal anatomy of the dental element to be treated. This knowledge guides the coronary access, the location of canals, and their instrumentation.[9]

A lack of knowledge of the internal anatomy can lead to instrumentation failures, which may weaken the structure and cause dentinal perforations or even root fractures.[10] These complications cause communication between the root canal system and its support tissue, promoting the spread of bacteria and inflammatory reactions that can impede the success of endodontic therapy.[11]

Many techniques can be used for the measurement of anatomical references, such as microscopic analysis, silicone printing, clarification, scanning electron microscopy, histological section, radiographic comparison, and computed microtomography. The techniques' various limitations, such as only a two-dimensional image, high cost, high time consumption, or the required destruction of the specimens,[10] have made the CT scan as the method of choice for numerous authors.[7101213]

The most subjected to accidents areas are the internal root canal walls of the root canal, located in the distal wall of the mesial canal, considered the risk zone. This is because of the limited amount of dentin associated with the concavity and root curvature of these roots.

Tabrizizadeh et al.[4] reported an average thickness of 1.2 mm at the distal wall of the mesial canal of the mandibular first molars, which lies 4 mm below the canal inlet port. The authors noted that the distal wall has the lowest average thickness at this same spot as compared to the mesial (1.96 mm), buccal (2.17 mm), and lingual (2.2 mm) walls.

In the present study, the average dentin thickness of the distal wall of the mesiobuccal root of mandibular molars was higher in the point located at the canal entrance, at the same height of the furcation region, with 1.14 mm, and smaller at 3-mm distance from the canal inlet to 0.81 mm. In addition, there was an increase in this average at the 4-mm point, with a mean value of 0.86 mm, a value lower than those reported in previous studies of this same region.[13]

Asgary et al.[13] also reported a decrease in dentin thickness of the risk zone in the first 4 mm of root extension from the furcation in the first molars. The authors reported a reduction in average thickness from 1.17 to 0.90 mm, confirming that the greatest reduction in dentin occurs at the level of the cervical third of the root. Saberi et al.,[7] however, measured the average thickness of these teeth based on three levels of risk zones: below the furcal, at the end of the third cervical, and between these areas, finding the following thicknesses, respectively, 1.12, 1.01, and 1.06 mm.

In all studies cited, there is a significant reduction in dentin thickness of the distal wall in the first several millimeters below the entrance of the mesial canal. The present study found the lowest thickness at a point located 3 mm below the entrance of the furcation, with a value <1.0 mm. This fact serves as a warning and justifies the need for extra care in cervical canal preparation to prevent excessive wear. The preexpansion procedures of the canal mouths may favor less invasive preparation and more emphasis on the root canal.

Root perforations are the second biggest cause of accidents, accounting for 10% of endodontic treatment failures.[14] Akbar[15] highlighted professional training as a preventative measure to improve the prognosis of treatment and to promote the use of instruments that promote lower iatrogenic errors, such as rotary instruments made with more flexible alloys such as nickel titanium. In addition, endodontic instrumentation in the counter-bending direction may be noted as an important safety procedure. The findings of this study corroborate with those of other authors, as they found the highest average dentin thicknesses on the mesial walls of inferior mesial root molars,[4] a fact that justifies the name “security zone” for these anatomical regions.

Furthermore, knowledge of the distance between the floor of the pulp chamber and the furcal also reduces the risk of perforation.[16] Thus, in our study, these values were also measured, coming to an average of 2.23 mm. This mean value was very close to that reported by Azim et al.,[12] who found an average thickness in the furcal of 2.24 mm, according to data obtained from the tomographic evaluation of 104 inferior molars in an American population.

CONCLUSION

This study showed that the smallest thicknesses of the dentinal walls of the risk zone are located 3 mm below the furcation. These locations deserve more attention during cervical preflaring. This highlights the importance of adequate canal opening preparations leading to a safer, more predictable, and centered preparation in the main canal.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.