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Comparative evaluation of the effect of hand file, different nickel-titanium retreatment files, and self-adjusting file system on the incidence of dentinal microcrack formation during the removal of root canal filling material: An in vitro stereomicroscopic study

 Department of Conservative and Endodontics, Dr. D.Y. Patil Dental College and Hospital, Dr. D.Y. Patil Vidyapeeth (Deemed to be University), Pune, Maharashtra, India

Date of Submission06-May-2021
Date of Decision16-Jun-2021
Date of Acceptance16-Jun-2021

Correspondence Address:
Karan Bhargava,
Department of Conservative and Endodontics, Dr. D.Y. Patil Dental College and Hospital, Dr. D.Y. Patil Vidyapeeth (Deemed to be University), Sant Tukaram Nagar Pimpri, Pune - 411 018, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mjdrdypu.mjdrdypu_328_21


Aim: The aim of the study is to evaluate the effect of hand files, rotary nickel-titanium files, and self-adjusting file (SAF) on the incidence of crack formation during retreatment. Materials and Methods: Eighty extracted human mandibular molar teeth with intact mesial roots were selected. They were divided into 8 groups of 10 teeth each; Group 1: Control, Group 2: Obturated control, Group 3: Hand files, Group 4: ProTaper Universal Retreatment, Group 5: Twisted File Adaptive, Group 6: Reciproc Blue, Group 7: XP-endo Finisher R, Group 8: SAF system. All teeth had the coronal portions and distal roots removed by a diamond-coated disc with water cooling, leaving roots approximately 13 mm in length. Except for Group 1, all canals were similarly instrumented and obturated at an apical size of 40.02. Retreatment was performed in each group according to manufacturer's recommendations except for Groups 1 and 2. The roots were sectioned horizontally at 3, 6, and 9 mm from the apex with a low-speed saw. Stereomicroscopic evaluation of the sections was done to assess the presence or absence of defects. Results: The groups that were not retreated (Groups 1 and 2) showed no defects. Hand files showed the least number of defects followed by the SAF. Reciproc Blue showed the most dentinal defects. Conclusion: All NiTi instruments tend to induce various degrees of dentinal damage during retreatment procedures. The SAF and hand instrumentation presented satisfactory results with minimal dentinal microcracks.

Keywords: Dentinal microcracks, root canal filling material, self-adjusting files

How to cite this URL:
Newase P, Bhargava K, Paunikar M, Bhawalkar A, Kumar T, Sarode G. Comparative evaluation of the effect of hand file, different nickel-titanium retreatment files, and self-adjusting file system on the incidence of dentinal microcrack formation during the removal of root canal filling material: An in vitro stereomicroscopic study. Med J DY Patil Vidyapeeth [Epub ahead of print] [cited 2023 Mar 20]. Available from: https://www.mjdrdypv.org/preprintarticle.asp?id=339396

  Introduction Top

Mechanical instrumentation is an important phase of root canal preparation as it creates the space for irrigants and antibacterial medicaments to more effectively eradicate bacteria and eliminate bacterial by-products. Nonsurgical endodontic retreatment is often indicated as the first choice to eliminate or reduce microbial infection when initial root canal treatment fails. The retreatment aims to completely remove the filling material from the canal system followed by repeat cleaning, shaping, and filling of the root canal.[1] The strength of a root-filled tooth is directly related to the amount of remaining sound tooth structure. Removal of dentin during canal preparation may compromise the fracture strength of the root and found to have a significant effect on the incidence of incomplete apical cracks and dentinal defects.[2] These defects may act as stress concentration areas. As a result of repeated stress generation due to further endodontic and restorative procedures, incomplete dentinal defects may get propagated and ultimately develop into a vertical root fracture.[3]

Stainless steel root canal instruments clean the canal superficially and can create canal aberrations such as ledges, zips, and elbows. To eliminate these shortcomings of stainless steel instruments, nickel-titanium (Ni-Ti) instruments have been developed.[4] Recently, a few studies have assessed the effect of endodontic procedures, including canal preparation and different obturation techniques on apical crack initiation.[2],[5],[6] However, there has been no study evaluating the effects of hand files and rotary NiTi retreatment systems on the incidence of apical crack initiation and propagation during retreatment. Therefore, the purpose of this study was to evaluation of the effect of Hand file, 4 Ni-Ti retreatment files (ProTaper Universal Retreatment, Twisted File Adaptive (TFA), Reciproc Blue and XP-endo Finisher R [XP-FR]), and Self-adjusting File (SAF) system on the incidence of dentinal microcrack formation during the removal of root canal filling material.

  Materials and Methods Top

Tooth selection

A total of eighty extracted human mandibular molar teeth with intact mesial roots were selected. Radiographs were taken to verify anatomical variation. The root surfaces of each tooth were observed under a stereomicroscope (B × 60; Olympus, Tokyo, Japan) at ×20 magnification for evidence of fracture lines, open apices, or anatomic irregularities and were discarded if any of these characteristics were found. Following extraction, teeth were cleaned and then stored in purified filtered water at room temperature. To ensure standardization,[7] the coronal portions and distal roots of all teeth were removed by a diamond-coated disc with water cooling, leaving roots approximately 13 mm in length.

Tooth preparation

Cavity access was achieved by a diamond bur (Brasseler USA, Savannah, GA) and water cooling on all teeth except the control group. A size 10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was introduced into each canal until it appeared at the apical foramen. The working length was determined by subtracting 1 mm from this measurement. The canals were prepared to a master apical size 40 with K-files (Dentsply Maillefer) by using the balanced force technique as described by Roane et al.[8] Step back was performed by using K-file sizes 45–80. The canals were irrigated with 2 mL 5% sodium hypochlorite (NaOCl) between each file size by using a syringe and a 29-G needle (NaviTip; Ultradent, South Jordan, UT). After completion of the preparation, the canals were irrigated with 2 mL 17% EDTA for 1 min and subsequently rinsed with 2 mL distilled water. All roots were covered with gauze and kept moist throughout the preparation to avoid drying.[9]

Root canal obturation

Before beginning the obturation process, the canals were dried with paper points (Dentsply Maillefer). E and Q plus thermoplasticized gutta-percha obturation system (Meta Dental Corp. Cheongju, Korea) was used for obturation of canals according to the manufacturer's instructions.[10] The root canal openings of all specimens were sealed with temporary filling material (Cavit; 3M ESPE, Seefeld, Germany). Radiographs were then taken from the buccolingual and mesiodistal directions to ensure quality of the obturation. The roots were reexamined for defects under a stereomicroscope; no visible external defects were detected. The specimens were then stored at 100% humidity for 2 weeks to allow for the complete setting of the sealers.

10 of the 70 obturated teeth were set aside, and no retreatment procedure was performed. Root canal cleaning, shaping, filling, and retreatment procedures were performed by a single operator to avoid inter-operator variability.

Retreatment procedure

The surfaces of sixty roots were coated with a silicone impression material to simulate periodontal ligament space. These teeth were then embedded in a metal mold filled with self-curing acrylic resin. The sixty roots were then randomly divided into six groups of ten. Root fillings were removed using the following techniques.

Hand instrument group (n = 10 teeth)

Gates Glidden Drills (Dentsply Maillefer) size 3 and subsequently size 2 at 2000 rpm were used to remove coronal filling material. The canals were reinstrumented with Hedstrom files (Dentsply Maillefer; sizes 30, 25, 20, and 15) in a circumferential, quarter-turn, push-pull, filing motion to remove filling material until the working length was achieved. Once the working length had been reached (with a size 15 file), sizes 20, 25, 30, 35, and 40 were used at the working length.[9]

ProTaper Universal Retreatment System group (n = 10 teeth)

ProTaper UR files D1, D2, and D3 were sequentially used in a crown-down manner. The D1 (30/09) was used for the removal of the coronal third and D2 (25/08) for the middle third of the root canal filling. Finally, the D3 (20/7) was used up to the WL. Apical preparation was then performed with size F4 (40/05). The instruments were used with an XSmart electric motor (Dentsply Maillefer) at a speed of 300 rpm and with a torque of 2 N cm, according to the manufacturer's instructions.[11],[12]

Twisted file adaptive group (n = 10 teeth)

TFA files were used with the aid of an Elements Motor in the TFA program (SybronEndo) at 500 rpm and the torque at 2 Ncm in the crown-down technique.[13] In this group, the TFA SM3 (35/04) and SM2 (25/06) were used until gutta-percha was removed from the coronal one-third and middle one-third of the canal, respectively. Then, TFA SM1 (20/04) with light pressure was used into more deeply positioned obturating material which was augured out of apical one-third of the canal. The final apical enlargement was performed with a TFA SM3 (35/04) file (SybronEndo) and then using Endostar file 40/0.02.

Reciproc blue file group (n = 10 teeth)

The instrument was introduced into the canal, activated by a VDW.CONNECT Drive® and applied in a reciprocating motion. It was then moved toward the apex using an in-and-out pecking motion with an amplitude of approximately 3 mm, according to the manufacturer's instructions.[14],[15] Initially, RECIPROC® blue instrument R25 (25/08) was used for retreatment until estimated working length (WL) was reached. Then, final apical enlargement was done with RECIPROC® blue R40 (40/06).

XP-endo finisher R group (n = 10 teeth)

XP-FR was used after the shaping procedure was done with 30 H file. The XP-FR instrument was inserted in a contra-angle handpiece (VDW Silver), cooled, and removed from the plastic tube in rotation mode. It was inserted in the root canal without rotation and then activated (1000 rpm and 1 Ncm) using slow and gentle 7–8 mm lengthwise movements up to the WL for 30 s. The instrument was pressed against the canal walls during this procedure.[16],[17],[18]

Self-adjusting file group (n = 10 teeth)

SAF 2 mm instrument was operated in the EndoStation motor (ReDent/Acteon-Satelec, Merignac, France), with 5000 gentle vertical vibrations per minute and continuous irrigation flow of 4 mL/min according to the manufacturer's directions. The SAF was activated for 3 min under simultaneous irrigation with 2.5% NaOCl delivered by the EndoStation peristaltic pump (ReDent Nova). Next, SAF was activated with a manual irrigation of 2 mL 17% EDTA for 30 s, followed by another use with 2 mL 2.5% NaOCl for 30 s.[18],[19]

In all groups, 0.1 mL eucalyptol was introduced into each canal to soften the gutta-percha. In all other groups except SAF, root canals were irrigated with 1 ml 5% NaOCl at each instrument change. The root canals were then irrigated with 1 ml 17% EDTA to eliminate suspended debris and subsequently rinsed with 2 ml distilled water.

The retreatment procedure was considered complete when no gutta-percha or sealer was detected on the instrument surfaces or inside the root canal or dentinal walls.

Sectioning of the samples

The silicone impression material was removed and all roots were cut horizontally at 3, 6, and 9 mm from the apex with a low-speed saw (Leica SP1600; Leica Microsystems, Wetzlar, Germany) under water coolant.

Microscopic examination

Digital images of each section were captured at ×20 magnification using a digital camera attached to a stereomicroscope (Magnus MSZ-Bi).

A total of 30 slices were examined in each group. The slices were reviewed for defects by 2 operators who were blinded to the technique used to retreat the canal. If the 2 operators each had a different score for a slice, a consensus between them was reached. To avoid confusing definitions of root defects, 2 distinct categories were established: “no defect” and “defect.”

No defect was defined as root dentin devoid of any lines or cracks where both the external root surface and the internal root canal wall had no defects. Defect was defined as all lines observed on the section that extended either from the outer root surface into the dentin or from the root canal lumen to the dentin. This also included teeth with a fracture, which was defined as a line extending from the root canal space to the outer surface of the root.

All 30 specimens of tooth section in different groups were evaluated, and frequency of incidence of cracks in different areas was reported as numbers and percentages.

Ethical approval

The study protocol was approved by the Institute Ethics committee of Vasantdada Patil Dental College and Hospital, Sangli letter number 956/2016-2017, dated 05-10-2017.

Statistical analysis

All the data were entered into Microsoft Excel 2010. Descriptive statistics were expressed as frequency distribution and percentage for 8 groups as defected and not defected. It was done for section wise (coronal, middle, and apical). For all the above test, P value is considered statistically significant when it was <0.05. The software used was SPSS (Statistical Package for the Social Sciences) version 17.

  Results Top

A total of 80 extracted mandible molar teeth with intact mesial roots were divided into 8 groups of 10 each, respectively, Control, Obturated control, Hand files, ProTaper Universal Retreatment, TFA, Reciproc Blue, XP-FR, SAF system [Graph 1] and [Table 1].

Table 1: Comparison of eight groups with respect to status of number of roots with defect by Kruskal–Wallis test

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The cracks were as follows in decreasing order Reciproc blue file (12), ProTaper universal retreatment file (9), TFA (7), XP-FR file (2), and SAF and hand file (1).

For Reciproc Blue file, majority cracks (8) were at coronal section followed by middle section (3) and apical section (1). For ProTaper universal retreatment file group (5) cracks were at coronal section, (3) cracks at the middle section and (1) at apical section. TFA file (4) was at coronal section, middle section (3), and apical section (1). XP-FR file group had (1) at coronal section. Hand file and SAF groups had Crack (1) each in coronal section.

Both the control groups of unobturated and the obturated showed no cracks. Thus, proving that experiment was well controlled.

The incidence of cracks was highest in Reciproc blue (40%) followed by ProTaper retreatment file (30%), TFA (23.3%), XP-FR (6.7%), SAF (3.3%), hand files (3.3%), and control (0%).

[Table 2] shows that Significant difference (P < .05) were found in relation with total number of cracks in:
Table 2: Pairwise comparison of all eight groups with status of number of roots with defects

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  • Reciproc blue versus control, obturated control, H file, XP-FR, SAF
  • Control versus ProTaper universal retreatment system
  • Obturated control versus ProTaper universal retreatment system.

There is no significant difference between other groups in relation with total number of cracks (P > .05).

[Table 3] and [Graph 2] show a statistically significant P value for coronal sections and middle sections.
Table 3: Comparison of eight groups with respect to status of number of roots at coronal middle and apical with defect by Kruskal–Wallis test

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As Kruskal–Wallis test shows P = 0.644 for apical group, there will be no pairwise comparison at apical section as shown in [Table 4].
Table 4: Pairwise comparison of all eight groups at coronal middle and apical with status of number of roots with defects

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It was found that apical and middle sections showed no significant statistical differences among the file systems owing to a smaller number of cracks observed. However, the Coronal section showed highest number of cracks between Reciproc blue file compared to control.

  Discussion Top

Root canal preparation may result in weakening of the remaining tooth structure and lead to the formation of crack lines and microcracks. In response to the application of stress, such as that caused by occlusal forces and restorative or endodontic procedures, these crack lines and microcracks can propagate and produce oblique root fractures, leading to endodontic treatment failure.[20] Retreatment procedures require additional mechanical preparation of the root canal, greater damage to the root canal wall may be caused after this procedure.[14]

Ni-Ti retreatment systems with different file designs, metallurgical alloys, and rotational motions enable them to cut not only gutta-percha but also superficial layers of dentin.[21] However, instrumentation of the root canal using hand files may not have a damaging effect on the canal wall. This has been attributed to the less aggressive movements of the hand files in the canal compared with engine-operated files and less taper (0.02) as compared to rotary Ni-Ti instruments.[9] Bier et al. suggested that fractures did not occur immediately after canal preparation. However, only 4% to 16% of craze lines may be developed into fractures during retreatment or after long-term functional stresses such as chewing.[22]

Purified filtered water medium was recommended as the storage medium for investigations of human dentin because it causes the least amount of change in dentin over time.[9] In the present study, multiroot teeth, especially the mesial root of lower first molars, were used, which are at a higher risk of strip perforation and are more prone to be influenced by forces during instrumentation as a result of their smaller dimensions and thin dentinal walls. If large tapered files cannot induce cracks in mandibular molar, it is unlikely that rotary files induce cracks in other teeth.[23]

Acrylic blocks and a silicone impression material used to simulate bone and periodontal ligament, respectively. Silicon layer allows limited freedom of movement while avoiding external reinforcement.[24] In the present study, 5% NaOCl was used for irrigation. As all teeth were irrigated following the same protocol and roots prepared with hand files and SAF did not show any dentinal defects (1/30) as shown in [Figure 1], we may consider that the irrigation using NaOCl in this study did not contribute to the appearance of dentinal defects, seems justified. As all teeth that were obturated following the same protocol did not show any dentinal defects in Group 2, we may consider that the thermoplasticized gutta-percha obturation in this study did not contribute to the appearance of dentinal defects. In the present study, eucalyptol was used as a solvent. Eucalyptol has been reported to be safe and efficient.[25]
Figure 1: No defects

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In our study, we found out the maximum cracks 12/30 (40%) with the Reciproc Blue in the coronal and middle thirds as shown in [Figure 2] and [Figure 3]. The reason behind this could probably be that the Reciproc blue system is a single file system. Bürklein et al. stated that when using only one instrument for complete preparation, more stress will be generated during mechanical instrumentation compared with canal instrumentation by using full-sequence systems.[26] Bier et al. stated that the taper of the files could be a contributing factor in the generation of dentinal defects because of increased stress on the canal walls caused by the tapered files.[22]
Figure 2: Fracture

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Figure 3: Incomplete defect

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ProTaper retreatment file has triangular cross-section.[11],[12] This causes less space for collection of dentin chips thus generating stress concentration on root canal walls and dentin crack formation.[27] Kim et al. found that file design affected apical stress and strain concentrations during instrumentation, which was linked to an increase in dentinal defects and canal deviation.[28] Jain et al. concluded that ProTaper rotary files produced microcracks due to a continuous rotational force and constant torque applied by the NiTi rotary file on the root canal walls.[29]

The TFA system has a triangular cross-section and uses continuous rotation or reciprocation, depending on the amount of pressure placed on the file.[13],[30] The adaptive motion combined with the high flexibility and the multifile system seems to decrease torsional forces resulting in less dentine defects.[31] XP Endo finisher R file showed least number of cracks 2/30 (6.7%) of all rotary files used in this study. The reason behind this could be scraping action of file on the exposed dentin walls to remove bacteria and biofilm, its zero taper and the ability of the file to work in mixed M and A phases, adaptation to canal morphology, and preservation of the dentine.[16],[17],[32]

In the SAF group, we found only one crack in the coronal section out of the total 30 sections examined under the microscope. System works with a back and forth motion that removes dentin from the canal walls in a scraping action due to a Ni-Ti abrasive coating. In addition, it neither has a cutting edge nor does it have flutes.[18],[19] The SAF comes in intimate contact with the inner canal wall as a result of compressible and expansive structure of lattice.[33] The less stress generation and no microcracks formed in the radicular dentin could be attributed to the higher fracture resistance of the teeth instrumented by the SAF in the study reported by Pawar et al.[34]

In this study, the outer surface of all roots was inspected with a stereomicroscope and also with periapical radiographs for the presence of preinstrumentation and postobturation dentinal microcracks. However, ruling out the presence of dentinal cracks before the start of the experiment is impossible because some of the cracks could be internal and may not be visible on the external surface of the root. Standardization of the root dentin thickness among all groups is also not possible. Although we have used only mandibular molars in all groups, there would still be differences in dentin thickness. Thickness variation would give rise to significant changes in strength and hence its response to stresses during instrumentation. Files used in this study were used at varying recommended speed and torque which cannot be standardized for all files. The use of different speed and torque settings for each file system could be a limitation of the present study.

Future studies could employ the use of micro CT for the assessment of cracks and also the effect of masticatory forces on root canal retreated teeth.

  Conclusion Top

Even though this in vitro study did not reflect the clinical settings, we can conclude that all NiTi instruments tend to induce various degrees of dentinal damage during retreatment procedures. The findings of present study revealed that more crack generation occurs at coronal third as compared to middle or apical third. This may be due to increased taper of files in the coronal third. On the other hand, SAF and hand instrumentation presented satisfactory results with the fewest dentinal microcracks.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3], [Table 4]


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    -  Newase P
    -  Bhargava K
    -  Paunikar M
    -  Bhawalkar A
    -  Kumar T
    -  Sarode G
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