Interrelationship between root dentin thickness and root length in mesial root of mandibular first molar: An in vitro study

Geeta Asthana; Malav J Thakrar; Aakash Solanki; Girish J Parmar

doi:10.4103/endo.endo_7_17

ORIGINAL ARTICLE

Year : 2017 | Volume : 29 | Issue : 1 | Page : 43-46

Interrelationship between root dentin thickness and root length in mesial root of mandibular first molar: An in vitro study

Geeta Asthana, Malav J Thakrar, Aakash Solanki, Girish J Parmar
Department of Conservative Dentistry and Endodontics, Government Dental College and Hospital, Ahmedabad, Gujarat, India

Date of Web Publication

25-May-2017

Correspondence Address:
Malav J Thakrar
Room No. 8-9, Department of Conservative Dentistry and Endodontics, Government Dental College and Hospital, Ahmedabad - 380 016, Gujarat
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/endo.endo_7_17

Abstract

Introduction: The aim of this study was to investigate the relation between the distal wall thickness and length of the mesial root of mandibular first molars at different positions along the length of the root.
Methods: Forty-five mandibular first molars were selected, and their lengths, from the tip of the mesiolingual cusp to the root apex, were recorded. The collected teeth were then divided into three groups according to their length: Group I – long (24.2 mm ± 1.8 mm), Group II – medium (21 mm ± 1.5 mm), and Group III – short (16.8 mm ± 1.8 mm). The mesial root of each tooth was marked at two levels – first at 2 mm below the furcation and second at the junction of the apical and middle third of the root. The thinnest portion of the distal root dentin of the buccal and lingual canals of the mesial roots was recorded. 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 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 < 0.001) 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.

Keywords: Danger zone; remaining dentin thickness; strip perforation.

How to cite this article:
Asthana G, Thakrar MJ, Solanki A, Parmar GJ. Interrelationship between root dentin thickness and root length in mesial root of mandibular first molar: An in vitro study. Endodontology 2017;29:43-6

How to cite this URL:
Asthana G, Thakrar MJ, Solanki A, Parmar GJ. Interrelationship between root dentin thickness and root length in mesial root of mandibular first molar: An in vitro study. Endodontology [serial online] 2017 [cited 2021 Feb 25];29:43-6. Available from: https://www.endodontologyonweb.org/text.asp?2017/29/1/43/207008

Introduction

The prime goal of root canal treatment is cleaning, shaping, and disinfection of the root canal space and to restore the function of the involved tooth. Mandibular first molar is most frequently involved, endodontically. Usually, mandibular first molar has two roots, but occasionally three roots are found. A curvature is usually found in the mesial root while the distal root is straighter.^[1] Care should be taken to prevent transportation, ledge formation, and perforation, especially in the mesial root while doing biomechanical preparation.^[2] A greater concavity is found 2-mm cervical to the furcation on the mesial root with limited dentin thickness.^[3] Strip perforation is one of the most common errors committed iatrogenically.^[4] Hence, this is described as a danger zone.^[2]

The procedure of prewidening cervical third to remove interferences while reaching the crucial apical third could land the tooth susceptible to mishaps,^{[4],[5],[6],[7],[8],[9]} especially in curved mesial root canals of mandibular molars.^[10] The anticurvature filing method was described first by Abou-Rass et al.^[11] 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 discussed extensively, yet there is very less information in the literature about the relation of the thickness of radicular dentin and tooth length. Therefore, the aim of this study was to analyze the interrelationship between dentin thickness of the mesial roots at the distal aspect and length of the mandibular first molars.

Methods

After exclusion criteria, 45 extracted teeth were selected. Exclusion criteria include teeth with open apices, resorption or calcification, endodontically treated teeth, and teeth with root caries. Age, sex, and the systemic condition of the patients were unknown. 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 two separate canals. Selected teeth were placed in 3% sodium hypochlorite (PREVEST DenPro Limited, Digiana, Jammu, Jammu and Kashmir, India) for 15 min for disinfection and then stored in normal saline (Zuche Pharmaceuticals Pvt. Ltd., Safdarjung Enclave, New Delhi, India) with 0.2% thymol to inhibit microbial growth.

Each specimen was measured from the mesiolingual cusp tip to the root apex and was divided into the following groups:

Group 1: Long teeth (i.e., 23–26 mm, mean = 24.2 ± 1.8 mm)
Group 2: Medium teeth (i.e., 20–23 mm, mean = 21 ± 1.5 mm)
Group 3: Short teeth (i.e., 15–19 mm, mean = 16.8 ± 1.8 mm).

Mesiolingual cusp was selected as it is the tallest cusp and is less subject to attrition being a nonfunctional cusp. On mesial roots, marking was done at 2 mm below the furcation (level M) and the junction between the apical and middle third of the roots (level N) [Figure 1]. The specimens were sectioned in a transverse plane at level M and level N of the roots with the help of a diamond disc (Sunshine Diamonds, Langenhagen, Germany) having a thickness of 0.2 mm under water spray. Both coronal and apical surfaces were used for the study (i.e., the coronal surface as level M and the apical surface as level N). The specimens were observed under a stereomicroscope (Olympus, Shinjuku, Tokyo, Japan). The sectioned surface images were taken with the help of the integrated camera system under ×16 magnification, and measurements were noted.

Figure 1: A diagram showing level M and level N and measurement parameters (i.e., X, X1, Y, and Z)

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The minimum distal wall thickness of the buccal (X) and lingual (X1) canals of the mesial roots was measured (transverse section), and these represented the least dentin thickness from the inner border of the canals to the external root surface. Parameter (Y) is the distance between the mesiobuccal and mesiolingual canals. The depth of concavity (Z) in the distal surface of the mesial roots was calculated as the distance between a line joining 2 points in the convex root surface and the deepest point in the concave portion of root surface [Figure 1].

Statistical analysis was performed using SPSS software (SPSS Inc., Chicago, IL, USA). The analysis of variance test was used to measure variance within groups and among groups. For pairwise multiple group comparisons between groups, the Student's t-test (Student–Newman–Keuls test) was used. The level of significance was set at P < 0.001.

Results

The mean of X at the level M of Group 1 was lower as compared to Groups 2 and 3 [Table 1]; this difference was statistically significant. The mean of X at level M in Group 2 was also significantly lower than that of Group 3. The mean of X at level N in Group 1 was lower than in Groups 2 and 3 [Table 2]; however, there was no significant difference between Groups 1 and 2. Other differences were also significant [Table 2]. The mean of X1 at level M in Group 3 was higher compared to Groups 1 and 2 [Table 1]. This difference was statistically significant. For the mean of X1 at level M, there was no significant difference between Groups 1 and 2 [Table 2]. The mean of X1 at level N in Group 1 was lower than in Groups 2 and 3, but all differences were insignificant. The mean at level M of Y in Group 3 was higher compared to Groups 1 and 2 [Table 1], but all differences were statistically insignificant [Table 2]. The mean of Y at level N in Groups 1 and 2 was higher than in Group 3 [Table 1]; the differences were significant. There was no significant difference between Groups 1 and 2 [Table 2]. The mean at level M of Z in Groups 1 and 2 was higher compared with Group 3 [Table 1]; this difference was statistically significant. The mean at level M of C in Group 2 was higher as compared to Group 1, but this difference was insignificant. The mean of Z at level N in Group 1 was higher than in Groups 2 and 3; all differences were significant [Table 2].

Table 1: Mean±standard deviation (mm) of the measurement criteria at two levels (M and N) according to Groups and its intergroup comparison

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Table 2: Post hoc test (Student–Newman–Keuls test) for pairwise multiple group comparison

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Discussion

Mandibular first molar is the most common tooth encountered in routine endodontic practice and poses a challenge due to its anatomical variations.^[13] The thin distal surface of the mesial root is one of the most common sites for strip perforation and hence termed as danger zone. According to Tabrizizadeh et al.,^[14] the distal surface of the mesial root reported thinnest (1.2 mm) and the lingual wall thickest (2.2 mm) at the furcation point.

Cervical preflaring is carried out to minimize the effect of torsional stress on manual and rotary instruments,^[6],[7] but the variable radicular thickness can predispose the tooth to iatrogenic strip perforation owing to cervical preflaring.^[15]

According to Harris et al., the furcal aspect of the entire mesial root should be considered as “danger zone.Ý^[13] According to a study conducted by Estrela et al., the danger zone is located 4–6 mm below the canal chamber orifice.^[16]

Novel nickel-titanium (NiTi) rotary instrumentation procedures emphasize a gradual crown-down preparation of the root canal, with the aim of improving cleaning and disinfecting the canal, better access to apical third and making the process of obturation simpler and easier. Prewidening of the coronal and middle-third aspect of the root canal system has its own share of merits, which is increased tactile control while using smaller instruments in the crucial apical third. However, injudicious removal of tooth structure probably ensues in transportation of the preparation into the danger zone, or even strip perforations of the root.^[17]

In mesiobuccal roots, the mean of dentin thickness in longer teeth was 1.22 mm at the distal aspect, and in shorter roots, it was 2.41 mm at level M, whereas it was 0.8225 and 1.0631 mm, respectively, at level N. The distal (furcal) surface concavity was deeper in teeth with longer roots compared to those with shorter roots. The measurements of root concavity in the present study were in agreement with the observations of Bower et al.,^[18] the mean value of which came to be 0.7 ± 0.19 mm ranging from 0.3 ± 1.3 mm.

The susceptibility of the distal surface of mesial roots to perforation can be minimized with this knowledge of variation in root dentin thickness. According to Berutti and Fedon,^[19] even differences of tenths or hundreds of a millimeter can prove to be critical in avoiding strip perforation.

The range of mean distances (1.13–2.85 mm) is not in agreement with the previous study (2–4 mm).^[20]

Many studies are present in the literature citing the effects of various file systems on root canal curvature and radicular thickness, but the real wisdom lies in knowing the intricacies beforehand. Hence, as to prevent any harm to the tooth, coronal flaring should be advocated only on the outer and thicker portion of the mesial root canal wall. The least canal wall thickness of 0.3 mm should be present to ensure that no perforation or root fracture occurs.^[21] The longer mesial roots of mandibular first molars are more prone to iatrogenic errors than their shorter counterparts.^[22]

Although cervical preflaring plays an important role in obtaining straight line access and determining initial apical file diameter, it should not be performed at the cost of radicular dentin. LA Axxess burs, with 20/0.06 taper, are shown to be safe and effective for preflaring mesial roots of mandibular first molars.^{[10],[23],[24]} NiTi rotary files such as Protaper Sx and HyFlex CM (0.08 taper) are shown to be efficient as well as harmless to the radicular dentin owing to their metallurgy, less aggressive cutting action, and ability to remain centered within the canal.^[25],[26]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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