Journal of Dentistry Defence Section

: 2021  |  Volume : 15  |  Issue : 2  |  Page : 102--105

Development of apical root microcracks following different root canal preparation systems

Saleem Akhtar 
 Field Hospital, Basoli, Jammu and Kashmir, India

Correspondence Address:
Saleem Akhtar
1209 DU, c/o 56 APO Pin: 931710 194101, Jammu and Kashmir


Context: The most critical step in root canal therapy is cleaning and shaping of the root canal. A vast array of instruments both handheld and engine driven are available for root canal preparation. There are very few studies to compare the effect of different types of instruments on the development of apical root cracks. Hence, the aim of the study was to compare the development of apical root cracks following different root canal preparation techniques and instruments. Aims: The aim of this study is to compare the development of apical root cracks following different root canal preparation techniques and instruments. Settings and Design: This is an in vitro study. Subjects and Methods: Forty extracted mandibular premolar teeth were selected and mounted on resin blocks with simulated periodontal ligaments, after which the apex was exposed. All teeth were randomly divided into four groups; each group was earmarked for a specific technique and instrument used for root canal preparation. Digital images of external apical root surfaces of every tooth were taken before and after apical enlargement through optical microscope (×40). These images were compared, and the presence of cracks was evaluated. Statistical Analysis Used: Chi-square exact test was used to compare the four groups. Results: Overall P value was 0.167 which is not statistically significant and indicates that the proportion of teeth developing cracks was not statistically significant among the four groups. Conclusions: More cracks were developed with rotary system. However, the results were not statistically significant among the four groups.

How to cite this article:
Akhtar S. Development of apical root microcracks following different root canal preparation systems.J Dent Def Sect. 2021;15:102-105

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Akhtar S. Development of apical root microcracks following different root canal preparation systems. J Dent Def Sect. [serial online] 2021 [cited 2021 Nov 30 ];15:102-105
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Root canal treatment is a multiple step procedure to treat infected tooth and restore the functional status.

Straight line access in root canal treatment leads to loss of structural integrity, thus compromising the strength of tooth under functional loading.[1]

Wide range of instruments and techniques are available for root canal treatment. Evolution has occurred in the field of endodontics. Today, we have multiple systems available from handheld stainless steel files to nickel-titanium (NiTi) rotary files.

ProTaper Universal Files are NiTi rotary files with progressively taper and advanced flute designs providing flexibility and efficiency to achieve consistently successful cleaning and shaping results.[2] These files have been studied extensively, with respect to their tendency to induce dentinal damage in the form of root microcracks, and were reported to create more dentinal damage because of larger taper and convex triangular cross-section, leading to increased dentin removal as compared to other files.[3]

Twisted files are unique files with R phase technology, twisted design, and triangular cross-section, resulting in effective dentin cutting and allowing us to use the same file first as a reamer and then later as a Hedstrom file to effectively brush laterally.[4] The twisted file pitch configuration varies along the file rather than being constant. This reduces the “pull-in” effect and creates smooth tracking during root canal preparation. No data are available regarding their effect on dentine during their use for the cleaning and shaping of canals.

There are no data in the literature on the comparative evaluation of hand files and different rotary NiTi files, ProTaper, and twisted files on the causation of root microcracks. Hence, the purpose of the present study was to compare the development of apical root microcracks following different root canal preparation techniques and instruments.

 Subjects and Methods

Forty mandibular premolars with straight roots and single canals were extracted and stored in purified water. The external apical root surfaces were inspected under optical microscope to exclude teeth with apical cracks, irregularities, and open apices.

A single layer of aluminum foil was used to wrap all the root samples [Figure 1]a and embedded in autopolymerizing resin set in an acrylic block 9 mm high and 13 mm in diameter [Figure 1]b. Once set, all root samples were removed from the blocks and single layer of aluminum foil peeled off [Figure 1]c. To expose 2–3 mm of root apices, apical end of all acrylic blocks was cut down. Polyvinyl siloxane impression material was used to replace the space created by aluminum foil, by coating the material over root surfaces and socket walls and immediately repositioning the roots in sockets. Then, the crown portion of all samples was cut above the cementoenamel junction (CEJ) so that the length of all samples remained the same (i.e., 18 mm).{Figure 1}

Initial photomicrographs of the apices of all samples were taken by the optical microscope (×40) attached to a digital camera. Then, the samples were randomly distributed among the four experimental groups with 10 teeth per group.

Group A - Step back preparation with stainless file using balanced force techniqueGroup B - Crown down technique with hand ProTaperGroup C - Crown down technique with rotary ProTaperGroup D - Crown down with twisted files.

The working length of all samples was determined by inserting a size 10 K-type file (Mani, Tochigi, Japan) till the root canal terminus and subtracting 0.5 mm from this measurement. A glide path was prepared with a size 15 K-type file. All canals were then enlarged up to size 20 K-type file.

All samples in Group A were prepared with stainless steel hand files (Mani, Tochigi, Japan) using step back technique up to the size of 40. Group B samples were prepared with hand ProTaper till F4 using crown down technique. Group C samples were prepared with rotary ProTaper till F4 using crown down technique. Group D samples were prepared with twisted files using the sequence of 25/0.08, 30/0.06, and 40/0.04. All samples were irrigated with 2 ml 3% sodium hypochlorite between each instrument using a syringe and a NaviTip irrigation needle (NaviTip 31 gauge; Ultradent, South Jordan, UT) placed at 1 mm from the Working Length (WL). A total of 16 ml NaOCl was used for each root.

The presence or absence of cracks originating at the apical foramen (AF) was recorded and compared with preoperative images of the tooth apex [Figure 2]a and [Figure 2]b. One-way analysis of variance was used to compare the number of cracks between different root canal preparation techniques and instruments. All statistical analysis was performed at a 5% significance level.{Figure 2}


Maximum number of cracks developed in Group C followed by Group B and Group D. No cracks developed in Group A [Table 1]. Chi-square exact test was used to compare the proportions among the four groups. Overall P value was 0.167 which is not statistically significant and indicates that the proportion of crack development was not statistically significant among the four groups.{Table 1}

It is an inter-group comparison and calculation of 'P' value.

0/10 vs. 1/10, P = 1.000 (K-files vs. hand ProTaper and K-files vs. TF files)3/10 vs. 1/10, P = 0.583 (Rotary ProTaper vs. hand ProTaper, Rotary ProTaper vs. TF files)3/10 vs. 0/10, P = 0.211 (Rotary ProTaper vs. K-files).

It is an intra- group comparison between pre and post values and calculation of 'P' value by using Mc Nemar exact test (binomial distribution).

Group C 0/10 vs. 3/10 – P = 0.125 not significantGroup B and D 0/10 vs. 1/10 – P = 0.50 not significant.


Apical root anatomy is highly complex in nature. It comprises three important anatomical and histological landmarks; the apical constriction (AC), the cementodentinal junction (CDJ), and the AF. AC is narrowest in diameter and considered the terminal point for all root canal procedures.

CDJ is a histological landmark and varies considerably. It is the point where pulpal tissue meets the periodontal tissues.[5],[6]

AF is a funnel-shaped structure that separates the cemental canal from the exterior surface of the root. The canal widens as it approaches from minor AC to major AC. The space between both the constrictions is funnel shaped.[7]

Apical microcracks are visible breach in the continuity of root surface not detected by dental explorer. Normal root canal anatomy narrows down apically. During instrumentation, maximum stress generates over apical region. Moreover, these stresses initiate new microcracks in apical root surface or propagate the pre-existing cracks. Multiple authors reported that the root stresses generated from inside the root canal are higher in the apical region and along the canal wall than on the external surface.[8] Isom et al.[9] reported that the pattern of stress distribution in the apical area could lead to the development of cracks and fracture propagation.

The present study showed no microcrack development with stainless steel K-files using a step back technique. These files are machined in the canals manually and have comparatively less taper. Hence, the stresses generated by these files are less.[10]

Maximum amount of apical microcracks was developed with Rotary ProTaper. The ProTaper Universal Files have active rotating movement, resulting in high levels of stress concentration in root canal walls. This fact is corroborated by the result of the current study that shows more cracks with Rotary ProTaper than hand ProTaper, but the results were not statistically significant. Out of ten samples of hand ProTaper, only one sample showed a crack in the apical region. This finding is supported by the fact that the progressively greater taper of ProTaper files results in more coronal dentin removal.

Group D with TF files showed only one microcrack out of ten samples in the current study. Twisted files are manufactured using R phase technology with twisted design and triangular cross-section which provides increased flexibility. Hence, it was proposed that less pressure might be required in advancing the file apically, resulting in lesser stress concentration on dentinal walls and less fewer defects.

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Conflicts of interest

There are no conflicts of interest.


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