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Year : 2021  |  Volume : 32  |  Issue : 4  |  Page : 489-494
The time taken for retrieval of separated instrument and the change in root canal volume after two different techniques using CBCT: An in-vitro study

Department of Conservative Dentistry and Endodontics, Thai Moogambigai Dental College and Hospital, Dr M.G.R. Educational and Research Institute, Chennai, Tamil Nadu, India

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Date of Submission03-May-2021
Date of Decision07-Aug-2021
Date of Acceptance27-Aug-2021
Date of Web Publication18-May-2022


Introduction: Retrieval of separated instrument from a root canal may lead to increased dentin loss and prolonged treatment time. Aim: The aim of this study was to evaluate the retrieval time and increase in root canal volume after instrument retrieval with two retrieval techniques under dental operating microscope. Materials and Methods: Forty extracted human mandibular molars with 30°–40° mesiobuccal root canal curvature were selected based on cone-beam computed tomography (CBCT) and divided into two groups (n = 20 each). Group 1: Terauchi group (Terauchi ultrasonic tips) and Group 2: Satelec group (Satelec ET25 ultrasonic tip) based on the retrieval technique. Groups 1 and 2 were further divided into two subgroups (n = 10 each) based on the size of the separated instrument; Groups 1a and 2a with ProTaper Gold (PTG) F1 and Groups 1b and 2b with PTG F2. The time taken for retrieval was calculated and increase in root canal volume was evaluated using CBCT. Results were statistically analysed with paired t-test and post hoc analysis by Tukey's HSD test. Results: All separated instruments were successfully retrieved. The mean time for instrument retrieval was lower in Terauchi group than in the Satelec group (P > 0.05) but not significantly. The mean increase in total root canal volume post-retrieval as well as the mean volume corresponding to the coronal part of the separated instrument was significantly lower in Terauchi group than in Satelec group (P < 0.05). Conclusion: Terauchi ultrasonic instruments resulted in reduced instrument retrieval time with lower loss of root dentin.

Keywords: Canal curvature, CBCT, dental operating microscope, mandibular molars, mesiobuccal canal, separated instrument retrieval

How to cite this article:
Kumar BS, Krishnamoorthy S, Shanmugam S, PradeepKumar AR. The time taken for retrieval of separated instrument and the change in root canal volume after two different techniques using CBCT: An in-vitro study. Indian J Dent Res 2021;32:489-94

How to cite this URL:
Kumar BS, Krishnamoorthy S, Shanmugam S, PradeepKumar AR. The time taken for retrieval of separated instrument and the change in root canal volume after two different techniques using CBCT: An in-vitro study. Indian J Dent Res [serial online] 2021 [cited 2023 Mar 21];32:489-94. Available from:

   Introduction Top

The overall incidence of separated rotary nickel–titanium (NiTi) files after clinical use by endodontists ranges from 0.8% to 21%.[1],[2] Instrument separation in the root canal can hinder canal cleaning beyond the obstruction and affect the outcome.[3]

NiTi instruments demonstrate superior resistance to fracture compared with stainless steel instruments.[4] However, rotary NiTi instruments can fracture more frequently than stainless steel hand instruments during root canal preparation.[5] Instrument separation has been reported to occur more frequently in molars,[6] especially in mandibular molars[7] and most commonly in the apical third of a root canal.[8]

Intentionally leaving a separated instrument in the root canal may be considered only when nonsurgical retrieval of the separated instrument has been attempted without success.[9] The retrieval of fractured NiTi instrument is influenced by factors such as the tooth anatomy, root canal curvature, location of the fragment and visibility of instrument under the dental operating microscope.[7],[10] The use of a microscope along with an ultrasonic device has been reported to be an effective and safe method for the removal of separated instruments.[11],[12]

After instrument retrieval procedures, excessive root dentin loss is evidenced.[3] Cone-beam computed tomography (CBCT) provides accurate and quantifiable 3D images[11] and is a reliable tool for linear measurement of root dentin thickness and root canal volume.[13] Thus, evaluation of root canal volume and root dentin thickness after instrument retrieval procedures will be useful to evaluate the risk of root perforation and root fracture.[14]

Terauchi File Removal System (Dental Engineering Laboratories, Santa Barbara, CA) [Figure 1]a has been reported to reduce the amount of root dentin removed and the time required to retrieve separated instruments from root canals of extracted mandibular incisors[15] and also in clinical cases.[16]
Figure 1: Terauchi and Satelec ultrasonic tips used for instrument retrieval. (a) Terauchi File Retrieval Kit. (b) ET 25 Satelec tip. (c) TFRK-6 (spoon tip). (d) TFRK-S (straight tip)

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In the present in-vitro study, two instrument retrieval tips, Satelec ET25 ultrasonic tip [Figure 1]b and the Terauchi ultrasonic tips, were compared for time taken and increase in root canal volume (dentin loss) after instrument retrieval from the apical thirds of mesiobuccal (MB) canals of molar root canals under dental operating microscope using CBCT. The null hypothesis tested was that there is no difference between two instrument retrieval systems in terms of dentin loss and time taken for removal of separated instruments.

   Materials and Methods Top

This study was approved by the Institutional Review Board of Dr MGR Educational and Research Institute, Chennai (Dr MGR/TMDCH/ERB/2019-2020/0602001). Extracted permanent mandibular first molars were collected from the Department of Oral and Maxillofacial surgery, Thai Moogambigai Dental College and Hospital and three private dental clinics. Mandibular molars with closed apices and no history of previous root canal treatment were included. Teeth with open apex, root fracture, resorption and calcified canals were excluded. All samples collected were initially scanned by CBCT[17] to select teeth with two separate mesial canals and apical foramina and with MB canal curvature between 30° and 40° for this study.[11]

Sample-size calculation

A pilot study was done with 20 samples (5 samples each in Groups 1a, 1b, 2a and 2b) to arrive at the required sample size. A power analysis was done using G Power software version 3.0 for Windows (University of Düsseldorf, Düsseldorf, Germany) with 0.90 power, 95% confidence level and margin of alpha error at 0.05[18] and the estimated sample size was 10 per subgroup.

In Group 1 Terauchi ultrasonic tips (Dental Engineering Laboratories, Santa Barbara CA, US)[19] and in Group 2 Satelec ET25 ultrasonic tip (Satelec Corp, Merignac Cedex, France)[20] were used to retrieve the separated instrument. Groups 1 and 2 were further subdivided into two subgroups based on the size of the separated instrument in the root canal: Groups 1a and 2a: ProTaper Gold (PTG) F2 (#25.08) and Groups 1b and 2b: PTG F3 (30.09).

Pre-operative root canal volume measurement

The teeth were placed in a custom made specimen holder and preoperatively scanned [Figure 2]a and [Figure 2]b using the CS 9300CBCT scanner (CS 9300, Carestream Dental LLC, Atlanta, Georgia, USA) operating at 140 kVp and 2–15 mA (5 × 5 cm) and voxel size 180 μm. The MB root canal volume from canal orifice to the apical constriction[11] was measured using OsiriX MD DICOM viewer (version 12.0 64 bit, Pixmeo, Geneva, Switzerland) installed in MacBook Air (OS version 10.13.6; Apple Inc., California, USA).
Figure 2: (a and b) Pre-operative CBCT of MB canal before instrument separation and retrieval. (c and d) Post-operative CBCT of MB canal after instrument separation and retrieval

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Experimental protocol

The principal investigator (BSK) performed all the experimental procedures. The access cavity was prepared using an endo-access bur 2 (Dentsply Maillefer, Ballaigues, Switzerland) and the canals were located. A 10 K-file (Dentsply/Tulsa Dental, Tulsa, OK, USA) was inserted into the canal until its tip was visible at the apical foramen.[21] The working length was established at 1 mm short of the apical foramen. Next, each canal was prepared in a crown-down sequence by using PTG S1, S2, and F1 (Dentsply/Tulsa Dental, Tulsa, OK, USA) in Groups 1a and 2a and up to PTG F2 in Groups 1b and 2b. Between each instrument copious irrigation with 5 mL 5.25% sodium hypochlorite (NaOCl) using 30 gauge needle was done. Once the instrumentation was completed, 5 mL 17% EDTA was irrigated for 3 min, followed by final irrigation with 3 mL NaOCl, while patency was maintained using a size 10-K file.

Instrument separation

Initially, a notch was made at 4 mm from the tip of the instrument to be separated using a low-speed diamond disk[11] to a depth of one half of the instrument thickness.[11] The notched instrument was then inserted into the prepared MB canal at 250 rpm until the instrument separated.[15] A radiograph was taken to confirm the position of the separated instrument beyond the curvature in the apical third of the canal [Figure 3].[11]
Figure 3: Radiograph to confirm the position of the separated instrument inside the root canal

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Instrument retrieval procedure

The procedure of retrieval was performed under magnification using dental operating microscope (Sanma Lumin Pro, Sanma Medineers Vision Pvt Ltd., Chennai, India) with a time limit of 90 min.[15] Prior to any root dentin removal techniques, cotton pellets were placed into other exposed root canal orifices to prevent re-entry of the fragment into other canal systems.[22]

Straight line access to the separated instrument was achieved with Gates Glidden (GG) nos. 1 and 2. Staging platform was prepared by using modified GG no. 3 (Dentsply Maillefer, Ballaigues, Switzerland).[20] The bud of GG drill no. 3 was modified by cutting it perpendicular to its long axis at its maximum cross-sectional diameter.[12] This modified GG bur at 300 rpm was carried into the pre-enlarged canal and directed apically until it contacted the dentin surrounding the coronal end of the separated instrument to create a staging platform.[23] Next, the coronal end of the separated instrument was visualized with the aid of dental operating microscope (16×).

Group 1 (Terauchi group)

Initially, the TFRK-6 (spoon tip) [Figure 1]c with 18 mm length, 0.3 mm width and 0.1 mm tip thickness with the concave portion facing the separated file[24] was activated on the inner curvature of the root canal wall in dry condition with air coolant from the three-way syringe[19] to create a space 1-mm deep and one quarter turn of a circle around the coronal end of the separated instrument.[24]

Next, the TFRK-S (straight tip) [Figure 1]d with 30 mm length and 0.2 mm tip diameter was used (dry) to expand the created space in the inner curvature of the root canal wall to complete the semicircular space.[24] Then, the canal was filled with EDTA solution[19] and TFRK-S was then activated in push–pull motion until the separated file was removed.

Group 2 (Satelec group)

Satelec ET25 with tip diameter of 0.3 mm, length 20 mm and 3% taper was used (dry) in the inner curvature of root canal wall to free the coronal end of the separated instrument.[20] Next, the canal was filled with EDTA[19] and the ET25 tip was activated[19] around the instrument in counter-clockwise direction[25] until the instrument was removed.[23] Ultrasonic instruments in both groups were used intermittently at a low power setting of 2-3.

Post-operative CBCT measurements

The tooth was rescanned with CBCT post-operatively [Figure 2]c and [Figure 2]d to evaluate the total volume of dentin removed for each MB root canal by subtracting the recorded postoperative root canal volume measurement from preoperative root canal volume using OsiriX MD DICOM software.[11] Also the increase in root canal volume corresponding to the entire separated instrument (0–4 mm), the coronal part (0–2 mm) and the apical part (2–4 mm) was measured.

Statistical analysis

Statistical analysis of data was done using Statistical Package for Social Sciences (SPSS), IBM Corporation, SPSS Inc., Chicago, IL, USA version 21 software package. Descriptive statistics including mean and standard deviation were calculated for various parameters. Normality of the data was assessed using Shapiro–Wilk test. Paired t test was used to compare the mean differences between pre and post-operative group comparisons. One-way ANOVA was used for more than two groups. Post hoc analysis and multiple in-between group comparison were made using Tukey's HSD test. The level of significance was kept at P < 0.05.

   Results Top

All 40 separated instruments were successfully removed corresponding to an overall success rate of 100%. Post hoc test by Tukey's HSD showed that the mean time [Table 1] taken for removal of separated instrument was lower in Terauchi group than in the Satelec group which was not significant (P > 0.05).
Table 1: Intergroup comparison of time taken for retrieval of separated instruments

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Mean increase in total root canal volume after removal of separated instrument was significantly lower in Terauchi group (P < 0.05) when compared to Satelec group [Table 2].
Table 2: Intergroup comparison of mean increase in total root canal volume post-instrument retrieval

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Mean increase in root canal volume [Table 3] corresponding to the entire separated instrument (0–4 mm) and also corresponding to the apical part (2–4 mm) after retrieval was similar in Terauchi and Satelec groups (P > 0.05).
Table 3: Intergroup comparison of mean increase in segmental root canal volume post-instrument retrieval

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However, mean increase in root canal volume corresponding to the coronal part of separated instrument (0–2 mm) after retrieval was significantly less (P < 0.05) in Terauchi group than in Satelec group.

   Discussion Top

Fracture of NiTi rotary instruments occurs mainly due to flexural fatigue or torsional failure.[1] A clinical study evaluating ProTaper rotary instruments had reported that most cases of instrument separation occurred in MB canals of mandibular molars and mainly in curved canals.[26] As diameter and taper of the files increased, the time to fracture decreased.[27] Therefore, in this study, PTG F2 and F3 instruments were intentionally separated in mandibular molar MB canals with a curvature of 30°–40° and retrieval was attempted.

Dentin removal should be minimized in order to improve the long-term prognosis of the tooth.[28] In this study, instruments were intentionally separated in the apical third of root canals and the increase in root canal volume[11] which corresponds to the amount of dentin removed was evaluated after retrieval. A system designed by Terauchi et al.[16] has been reported to reduce dentin removal as well as the time required to retrieve the separated instrument from the root canal.

In a previous in-vitro study, both ProUltra tips and the TFRK were successful in removing the majority of fractured files from the coronal and middle thirds of root canals with moderate curvature, with success rates of 90% and 95%, respectively.[29] In the present study, retrieval of fractured files from the apical third of curved root canals was done.

The TFRK is based on two types of ultrasonic instruments: a straight tip (TFRK-S, 0.2 mm diameter and 1% taper) and a spoon tip (TFRK-6, 0.1 mm thickness, 0.3 mm width).[16] Also, it contains a micro-trephine bur, a loop device and other instruments. In this study, two ultrasonic instruments present in TFRK (TFRK-S and the TFRK-6) were used in group 1a and 1b while another ultrasonic instrument, the Satelec ET25 tip, was used in group 2a and 2b. Micro-trephine bur, loop device and other components of TFRK were not used to reduce confounding.

A previous study has reported that there was an increase in mean canal volume and decrease in root dentin thickness after removal of separated instrument which was significantly more with EMS ultrasonic tips (112.52%) as compared with ProUltra ultrasonic tips (55.35%).[11]

In the present study, Group 1a (Terauchi 25/0.08) and Group 1b (Terauchi 30/0.09) had a significantly lesser increase in overall mean root canal volume as compared with Group 2a (Satelec 25/0.08) and Group 2b (Satelec 30/0.09), respectively, after removal of separated instruments. This may be due to the smaller dimensions of the Terauchi ultrasonic tips as compared to the Satelec ultrasonic tip. The mean time taken for removal of separated instrument was lower in Terauchi group than in the Satelec group which was not significant. Thus, the null hypothesis was partially rejected.

A previous report proposed that root canal volume increase in the lower portion of the root canal (corresponding to the coronal 1.5 mm of the separated instrument) after retrieval was significantly greater than that in the upper portion (from the CEJ to the coronal aspect of the separated instrument).[6]

In the present study, all separated instruments had a length of 4 mm. Increase in root canal volume after retrieval was evaluated corresponding to the entire (0–4 mm) separated instrument and there was no significant difference between groups. The increase in mean root canal volume in the coronal part (0–2 mm) was significantly less in the Terauchi group when compared to the Satelec group. However, there was no significant difference in the mean root canal volume increase corresponding to the apical part (2–4 mm) between groups. Thus, it was concluded that there was more dentin removed corresponding to the coronal part of the separated instrument rather than in the apical part, mainly in the Satelec group.

The time required to remove the separated instrument using ultrasonics can vary based on the location of the tooth, the root canal anatomy and position of the separated instrument in the root canal.[30] A decreased success rate was reported when retrieval time exceeded 45–60 min.[31] The more apical that the fragment was located, with corresponding increased use of ultrasonic vibration to loosen it, a greater potential for root perforation resulted.[31] The time taken to remove the separated instrument in our study varied from 11 to 42 min, which was similar to a previous report by Nagai et al.[30] (3–40 min).

Retrieval of separated instrument can lead to excessive removal of dentin and decreased root strength which could make the tooth vulnerable to fracture.[11] Steps to reduce the risk of separating instruments within root canals include preparing the apical third of curved canals with stainless steel hand files before the use of rotary instruments[32] and regularly examining all files after each use under stereomicroscope[25] However, NiTi rotary instruments can fracture without visible defects or deformation.[25]

Limitations of this study include the in-vitro study design. Also, the use of micro-CT to evaluate dentin removal may have led to more accurate measurements. Further research with separated instruments in canals with greater curvature (>30°) is recommended.

   Conclusion Top

Within the limitations of this study, retrieval of separated instruments using Terauchi ultrasonic tips resulted in reduced mean root dentin removal with lesser mean time as compared to Satelec ultrasonic ET 25 tip. Removal of separated instruments with ultrasonic tips of smaller dimensions can result in reduced root dentin removal with lesser time.

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Conflicts of interest

There are no conflicts of interest.

   References Top

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Correspondence Address:
Dr. Angambakkam Rajasekharan PradeepKumar
Department of Conservative Dentistry and Endodontics, Thai Moogambigai Dental College and Hospital, Dr. MGR Educational and Research Institute (Deemed to be University), Chennai, Tamil Nadu - 600095
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijdr.ijdr_403_21

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  [Table 1], [Table 2], [Table 3]


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