| Abstract|| |
Aim: The aim of the present study is to evaluate the residual root thickness in mesiolingual (ML) canals of mandibular molars after pre-flaring using different sequences of Gates-Glidden (GG) drills.
Materials and Methods: Computed tomography (CT) scans of the ML canals of 60 mandibular first molars were provided from areas within 1, 2, 3 and 4 mm of the furcation area and minimum initial root thickness (MIRT) of concave areas was measured at each level. The samples were divided into two groups (G1 and G2) which were prepared by the step-back and crown-down techniques, respectively. In both groups, anti-curvature filing was used. On secondary CT images of the samples in the same position, the minimum residual root thickness (MRRT) and the amount of removed dentin were analyzed by t-test and ANOVA.
Results: There was no significant difference for MIRT between the two groups. The MRRT was greater in the G2 group in all the sections, with no significant difference between them. The average of MRRT was not <0.5 mm in any of the groups or sections.
Conclusion: Based on the results of this study when the appropriate root thickness is present initially, GG drills used in any sequence may be suitable, safe and cost effective for pre-flaring of mesial canals of mandibular first molars with preserving the root thickness of furcation areas.
Keywords: Crown-down; dentin removal; Gates-Glidden; residual root thickness; step-back
|How to cite this article:|
Akhlaghi NM, Naghdi A, Bajgiran LM, Behrooz E. Computed tomography evaluation of residual root thickness after pre-flaring using gates Glidden drills: The sequence effect. J Conserv Dent 2014;17:142-5
|How to cite this URL:|
Akhlaghi NM, Naghdi A, Bajgiran LM, Behrooz E. Computed tomography evaluation of residual root thickness after pre-flaring using gates Glidden drills: The sequence effect. J Conserv Dent [serial online] 2014 [cited 2019 Oct 15];17:142-5. Available from: http://www.jcd.org.in/text.asp?2014/17/2/142/128052
| Introduction|| |
Pre-flaring is recommended for elimination of cervical interferences. One problem due to use of the instruments for pre-flaring is whether they could create an increased risk for perforations, especially in mesial canals of mandibular molars.  One of the iatrogenic factors in different methods of root canal preparation is the residual root thickness, which is an indicator of the future fracture resistance of the root.  The mesial canals of mandibular first molars are not located in the center of the root and the areas between the canals and also between the canals and the furcation area have thin walls and are therefore called danger zones.  Danger zones have less dentin in the ramification areas in comparison with the peripheral safe root areas.  Therefore, over-preparing the cervical and middle thirds of the root canal might result in thinning of the dentinal walls and sometimes in strip perforations in the furcation area. , In addition, thin dentinal walls increase permeability and fracture rate of teeth. 
Correction the perforations is very difficult and the key is to prevent them from happening in the first place.  The low price, high cutting potential and easy use of Gates-Glidden (GG) drills have made them widely-used instruments in coronal preparation of root canals.  GG reamers to No. 2 in the coronal third of mesiobuccal canal did not significantly decrease the residual dentin thickness. , Although the use of these drills is a relatively old method in root canal preparation,  little has been done to evaluate their effect in the danger zones of mandibular molars. Despite their long-time existence, the sequence in which they are used is not yet clearly defined.  Some studies indicate that the crown-down technique is a safer method due to the fact that bigger files have lower penetration depth and prevent over-removal of the dentin. ,,
Mesial canal of mandibular molars is very close to the distal surface of the root and the distal wall thickness beneath the furcation area is about 0.7 mm.  It has been reported the thickness of distal wall of mesial roots in the mandibular molars was merely 1.2-1.3 mm in 1.5 mm apical to the furcation.  Researches have shown that 3-4 mm apical to the canal orifice is the most sensitive area in the mesial roots of mandibular first molars. ,
The serial sequence technique is the most-widely accepted technique  and was used in some studies;  however, some others recommend or used the crown-down technique. , Computed tomography (CT) is a diagnostic method used in dentistry, especially in endodontics;  due to its non-invasive nature, it has become very common in the evaluation of root canal preparations, internal and apical pathologies. 
The aim of this study was to evaluate the residual root thickness after pre-flaring using different sequences of GG drills.
| Materials and Methods|| |
For this ex-vivo study a total of 107 mandibular first molars from 35 to 55-year-old patients were collected, disinfected by immersion in 5.25% NaOCl solution for 1 h. After removing the calculus by a periodontal scaler, the teeth were stored in normal saline. All the teeth with external or internal root resorption, open apices, visible cracks, fractures, caries and previous root canal treatments were excluded. Mesial roots with the similar root lengths (from furcation to the apex) were used in the study. Access cavities were prepared and presence of two separate mesial canals and patency was confirmed by simultaneous placement of two #10 K-files (Maillefer, Ballaigus, Switzerland). Furthermore resistance to #15 K-file has been checked. Using Schneider method (1971), mesiolingual (ML) canal curvature was determined by parallel radiographs in bucco-lingual and mesio-distal directions and the teeth ranged 20°-35° curvature were included. Finally, 60 mandibular first molars were coded.
The teeth were placed halfway into the acrylic resin molds with their buccal surface facing up. To facilitate orientation of the canal in the CT scan sections (Somatom Sensation 16 CT Scanner; Siemens, Berlin, Germany), a copper filament was inserted into the resin parallel to the long axis of the tooth close to the ML line angle. The samples were stabilized on some fiber boards in a manner in which all furcations were aligned. The sections were obtained from 1, 2, 3 and 4 mm of the furcation area and the minimum initial root thickness (MIRT) of the furcation area in ML canal (distance from the outer border of the canal to the outer border of the root) was evaluated by the (Siemens Medical Solutions, Erlangen, Germany). The samples were randomly divided into 2 groups and the mean of the MIRT was assessed and hence that there would be no significant differences between them.
Pre-flaring of ML canals were passively performed by GG drills (Dentsply/Maillefer, Ballaigus, Switzerland) by a conventional motor at low speed of 2500 rpm. In group 1 (G1), GG drills No 2, 3 and 4 were respectively utilized using a step-back technique. In group 2 (G2), the crown-down technique was used (drill No 4, followed by No 3 and No 2). Penetration depth of GG drills in each of the techniques was as follows: #4 to the canal orifice, #3 to the coronal third and #2 maximum to the coronal half of the canal. In order to avoid perforations and to achieve a straight line access, the GG drills were declined and pressed toward ML wall as a safety zone (anti-curvature method).
After using each of the GG drills, the canals were irrigated with 2 mL of normal saline and 2 mL of 2.5% NaOCl and canal patency was checked. An Endodontist prepared all the canals and each of the GG drills was discarded after 5 times.
Then the samples were placed in the same initial molds and CT scans were obtained, then the minimum residual root thickness (MRRT) was assessed.
To compare the root thicknesses before and after preparation in each of the groups and in each section, paired t-test was used. T-test was used to compare the residual root thicknesses in the two groups in each of the sections; repeated measure ANOVA was used to compare the initial thickness, the residual thickness and the amount of dentin removed in each group. The data were analyzed with SPSS software version 14.0 (SPSS, Inc., Chicago, IL). The significance level was set at 5%. The relative percentage of dentin removal was calculated by dividing the amount of dentin removed to the initial root thickness multiplied by 100. 
| Results|| |
Evaluation of the MIRT
The MIRT in the furcation area of the ML canals in groups G1 and G2 were determined for all the sections in each group (intra-group) and for the same sections in the two groups (inter-group) [Figure 1].
|Figure 1: The minimum initial root thickness at furcation areas in different sections of the mesiolingual canals of mandibular first molars in the G1 and G2 groups in millimeter|
Click here to view
In intra-group comparison, there was a statistically significant difference (P < 0.001). In inter-group comparison of each section, no significant difference was detected (P > 0.05).
Evaluation of the amount of dentin removal
No significant difference was observed in the intra-group comparison (P > 0.05). The lowest percentage of dentin removal relative to the initial thickness in the 1-mm sections in groups G1 and G2 were 37.5 ± 17.2% and 32.2 ± 20.1%, respectively. The greatest percentage of dentin removal relative to the initial thickness in the 4-mm sections in groups G1 and G2 were 48.8 ± 21% and 44.4 ± 22%, respectively.
In the inter-group comparison there was no significant difference (P > 0.05).
Evaluation of the MRRT
This parameter was assessed in the intra-group and inter-group comparisons [Figure 2]. None of them showed significant differences (P > 0.05). In comparison of before and after preparations, significant differences were detected (P < 0.05).
|Figure 2: The minimum residual root thickness of furcation area in various sections of the mesiolingual canals of mandibular first molars in the G1 and G2 groups in millimeter|
Click here to view
In terms of preparation mishaps, deformity of instruments or breakage of instruments were not seen. Only one of the G2 samples had a strip perforation within 3-4 mm from the furcation with the 0.5 and 0.4 mm initial root thicknesses respectively.
| Discussion|| |
The aim of this study was to compare the effect of the sequence of using GG drills on the MRRT in the furcation area of ML canals of mandibular first molars by CT scan method. CT scan method is capable of providing a 3-dimentional description of changes in the root canal, especially in the coronal and middle-thirds, which are the major areas altered by endodontic instruments.  The CT scan method provides a functional and non-invasive method in evaluating the intra-canal morphology before and after preparation. ,,
The samples of this study had root curvatures ranged 20°-35°; such a limited range can be seen in some studies. ,
As reported in the previous studies, , the area within 3-4 mm below the root canal orifice is the most sensitive area during preparation of the mesial roots of molars. Therefore in this study similar to some previous ones, , evaluation was conducted on the danger zones in the furcation area. In order to avoid perforations and to achieve a straight line access, the GG drills were declined and pressed towards the ML wall as a safety zone.
In the present study, the MIRT of the ML canals in the danger zone was more than 1 mm in all the levels except in the sections 3-4 mm from furcations in G1 and the amount gradually decreased from a distance of 1 mm to 4 mm from the furcation. Berutti and Fedon  reported the root thickness of the mesial canals of mandibular first molars to be the least (1.2-1.3 mm) at a distance of 1.5 mm from the furcation. In a study conducted by Coutinho-Filho et al.  the average initial root thickness in mesial canals of mandibular first molars at a distance of 3 mm from furcation area was 0.8 ± 0.15 mm.
In a study by Garala et al.  stated that the initial root thickness is the most important factor in determining the residual root thickness after root canal preparation.
In the present study, in comparing the relative percentage of the amount of dentin removal to the initial thickness between the two groups, no significant differences were detected (P > 0.05). Furthermore, there were no significant differences for the MRRT in similar sections between the groups (P > 0.05). Coutinho-Filho et al.  in their study have reported the residual root thickness was significantly greater in the crown down technique compare with the step-back technique.
The results of the current study revealed that using GG drills in anti-curvature directions can maintain an appropriate thickness in the root. Lim and Stock  showed that the MRRT after preparation should be no <0.3 mm to resist forces during root canal obturation. According to their results, the ML canals of mandibular first molars do have sufficient resistance to root canal obturation after usage the GG drills. Wu et al.  reported that using GG drills in mandibular molars weakens the furcation area regardless of the size of the instrument used or the penetration depth. They concluded that using the anti-curvature method does not lower the risk.
Carvalho-Sousa et al.  reported that the use of GG drills is as safe as ProTaper rotary files with respect to perforation on the distal wall of the mesial canals of mandibular molars.
In the present study, the average of the MRRT in all the sections and groups was <1 mm. Raiden et al.  showed that a minimum 1 mm of tooth structure has to be preserved around the post for the tooth to resist vertical fractures. Akhlaghi et al.  indicated that the thickness of the mesial canals of mandibular first molars after preparation in the distal and distolingual surfaces was <1 mm. Therefore, it appears the mesial canals of mandibular first molars are not sufficiently resistant to vertical fractures when posts are used.
| Conclusion|| |
Based on results of this study that there was no significant difference between the step-back and crown down techniques in relation to the MRRT, when optimum initial root thickness is present, GG drills used in any sequence may be suitable, safe and cost-effective for pre-flaring of mesial canals of mandibular first molars with preserving the root thickness of furcation areas.
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Nahid M Akhlaghi
No 4, 10th Neyestan, Pasdaran Avenue, Tehran 19468
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]