Effect of root canal preparation, obturation, and retreatment on the induction of dentinal microcracks: A microcomputed tomography study.

INTRODUCTION: This study evaluated the effect of root canal preparation, obturation, and different techniques of retreatment on the percentage increase in the number of dentinal microcracks using microcomputed tomography (CT) analysis.
MATERIALS AND METHODS: Forty-five mandibular premolars after micro-CT scanning were prepared using Protaper universal files up to F4 and obturated. After the second micro-CT scanning again, the samples were randomly divided into three groups (n = 15) for retreatment procedure. In Group 1, retreatment was done with Protaper universal D1, D2, and D3 retreatment files followed by Protaper universal file till F4. In Group 2, retreatment was performed with R-Endo reciprocating motion, Re, R1, R2, R3, and Rs followed by Hero Shaper size #40. In Group 3, retreatment was performed with H-files and xylene till size #40. After gutta percha removal, the samples were again scanned for micro-CT for the evaluation of percentage increase in dentinal microcracks. Data were analyzed using "Kruskal-Wallis test ANOVA" and "student t-test" at 5% level of significance.
RESULTS: Few unprepared samples showed preexisting microcracks. There were statistically significant percentage increase dentinal cracks after root canal preparation and obturation as well as after retreatment procedure with statistically significant percentage increase in the number of dentinal microcracks between all three groups.
CONCLUSION: Intraradicular procedures such as shaping and cleaning and obturation as well as the use of retreatment file lead to the induction and propagation of dentinal microcracks. In the present study, the maximum microcracks were produced with Protaper universal retreatment file system followed by R-Endo system. H-files produced minimum cracks during retreatment procedures.

INTRODUCTION

Safe and efficient removal of all root filling materials from the canal system is essential for optimal nonsurgical.[1] Obturating material removable techniques include stainless steel files, nickel–titanium rotary instruments, heat-bearing instruments, ultrasonics, and solvents.

Various endodontic procedures such as root canal preparation, obturation, and retreatment procedures may produce dentinal defects on the root canal walls.[234] Moreover, as retreatment procedure requires more dentin tissue to be removed from root canal walls compared with initial endodontic treatment, the possibility of dentinal defects may increase after these procedures.[45]

Dentinal defects such as craze lines and microcracks may further develop into vertical root fractures, which might lead to tooth loss,[678] and thus, it is important to identify and compare the procedures causing dentinal defects.

Various commonly used methods for the diagnosis of microcracks include operative microscopic examination, sectioning, radiography, fiber optic illumination, staining, optical coherence tomography, surgical microscopes, endoscopes, and microcomputed tomography (micro-CT).

Micro-CT, a miniaturized form of CT, is a noninvasive imaging techniques which uses X-rays to create higher resolution cross-section images through a specimen, and data can be saved for future comparative or qualitative assessment.[9]

Thus, the present study evaluated the effect of root canal preparation, obturation, and different techniques of retreatment on the percentage increase in the number of dentinal microcracks using micro-CT analysis.

MATERIALS AND METHODS

Forty-five freshly extracted single-rooted single canal mandibular premolars were collected and stored in chloramine-T solution for 24 h followed by normal saline until used in the study. Teeth with caries, restoration, external cracks, defects, calcified canal, internal, or external resorption were excluded from the study.

The samples were decoronated and were standardized to 17-mm root length using water-cooled diamond disc. Micro-CT scans (phoenix v/tome/x m, Germany) were captured before and after instrumentation at 75 kV and 400 μA. The long axis of the roots was adjusted to be perpendicular to the beam to provide scans in the same sagittal positions. Each root was rotated 360° with a total scan time of approximately 2000s. The distance from the X-ray source to the sample was 124 mm and to the detector was 812 mm. Generally, 1000 CT slices (pixel pitch 200 micron, voxel resolution 30.5 microns) were obtained per root.

After working length determination, samples were prepared with rotary Protaper universal files up to the size F4 to the working length. During instrumentation, the canals were irrigated with 5 ml of 3% sodium hypochlorite solution (Amble healthcare, Delhi), 10 ml of 17% ethylenediaminetetraacetate for 1 min, and finally, using 5 ml distilled water. Thereafter, the roots were dried with corresponding absorbent points.

Dried root canals were obturated with corresponding gutta percha and AH Plus sealer (Dentsply, De Trey, Germany) using single cone obturation technique. The coronal 3-mm gutta percha was removed and sealed with Cavitemp (Ammdent, Saronno-Italia). Roots were stored for a period of 1 week at 37°C and 100% humidity to allow the sealer to set.

Second micro-CT imaging was done to determine dentinal cracks after root canal preparation and obturation.

For retreatment, the samples were randomly divided into three equal groups (n = 15) on the basis of method for retreatment procedure.

Group 1 (Protaper universal retreatment files)

Protaper universal D1, D2, and D3 retreatment files were used in the crown-down technique according to the manufacturer recommended settings in a brushing action with lateral pressing movements to remove the coronal, middle, and apical thirds of the root canal filling materials, respectively. Thereafter, Protaper universal files were used from F1 to F4 till the working length in a gentle brushing action.

Group 2 (R-Endo files)

Initially, reciprocating motion stainless steel hand file (size 25, 0.04 taper) was used with ¼ turn pressure directed toward the apex to create a pathway at a speed of 350 rpm, thus allowing the centering and alignment of the next instrument. The re instrument (size 25, 0.12 taper) was used to remove the coronal 2–3 mm of the filling. R1 (size 25, 0.08 taper) was placed and repeated penetration with limited pushing action in apical direction (push and retain) from coronal third to beginning of middle third is prepared, and R2 (size 25, 0.06 taper) file was placed and repeated penetration with limited pushing action in apical direction (push and retain) from middle third to beginning of apical third is prepared. R3 (size 25, 0.04 taper) and Rs. (size 30, 0.04 taper) instruments were then used at the working length with a circumferential filing movement from the apical third to the coronal third. The final apical preparation was performed with a Hero Shaper (Micro-Mega) instrument (size 40, 0.04 taper) at the working length at a speed of 450 rpm and a torque of 1.2 Ncm.

Group 3 (H-files and xylene)

A drop of xylene was deposited in the space created after the removal of temporary. Thereafter, gutta-percha was removed using H-files in a crown-down manner in a circumferential, quarter-turn, push-pull, filing motion until the working length was achieved. The H-files were used in the sequence of #20, #25, #30, #35, and #40. Thereafter, the third micro-CT imaging was done.

Microcrack evaluation

All the three micro-CT image horizontal sections of the same root levels were evaluated on 17-mm apical segments of roots at 0.25-mm intervals to detect microcracks. In total, 3060 slices were evaluated. If there were no microcracks or craze lines both on the external surface of the root and on the internal root canal wall, the slice was accepted as having “no crack” and was numbered zero. If there were any craze lines, microcracks, or fractures in the root dentine, these were categorized as “cracks” and the total number of cracks were noted [Figure 1].

Figure 1

Mean percentage of dentinal cracks preoperatively, after root canal preparation and obturation and after use of retreatment files

As the number of preexisting microcracks differs from sample to sample, and this can influence the number of microcracks observed in subsequent imaging. Thus, the percentage increase in the number of microcracks was evaluated and analyzed to standardize the procedure and minimize the error.

The statistical analysis was performed using SPSS software version 20.0 (IBM Corporation, Armonk, NY, USA). The values were represented in number (%), and mean ± standard deviation comparisons of between and within the groups were performed using student t-test.

RESULTS

The results showed that the mean percentage of cracks observed in uninstrumented canals, increased during root canal preparation, and obturation. In addition, the use of retreatment files further increased the mean percentage of cracks [Figure 2], and there was statistically significant increase in the percentage number of microcracks between preoperative and after obturation (P ≤ 0.001), after obturation, and after use of retreatment files (P ≤ 0.005) and preoperative and after use of retreatment files (P ≤ 0.001) [Table 1].

Figure 2

Sample showing (a) preoperative microcracks (b) propagation of microcracks after root canal preparation and obturation (c) induction of microcracks after retreatment

Table 1

The mean percentage of micro-cracks observed preoperatively and percentage increase after biomechanical preparation and obturation as well as after retreatment

Intergroup comparison among retreatment group showed significantly more microcracks formation with Protaper retreatment files as compared to R-Endo and H-files (P = 0.00). R-Endo files produced significantly more microcracks than H-files (P = 0.00) [Table 2].

Table 2

Mean percentage number of dentinal cracks increase and standard deviation after use of different retreatment files on observation under micro-computed tomography imaging

DISCUSSION

In cases of posttreatment endodontic diseases, nonsurgical root canal retreatment is the first choice to reestablish the healthy periapical tissues which require complete removal of the filling material from the root canals to allow effective cleaning, shaping, and refilling.[10]

As retreatment requires further mechanical manipulations in the canal, this can cause further damage to the root canal wall.[4] Yoldas et al.[11] indicated that root canal preparation with nickel–titanium rotary systems and all additional subsequent procedures in endodontics, such as obturation and retreatment with rotary systems, can create craze lines or fractures.

In the present study, few preoperative or uninstrumented samples showed the presence of microcracks which is in agreement with Ceyhanli et al.[12]

In the present study, in total, there was the percentage increase in the number of dentinal cracks after root canal preparation and obturation when compared with preoperative images and this increase was statistically significant (P < 0.05). It is in accordance with the various studies by Daokar et al.,[13] Karataş et al.,[14] Harandi et al.,[15] and Li et al.[16] who concluded that there is an increase in the initiation of dentinal cracks after cleaning and shaping. In addition, Bier et al.[17] and Blum et al.[18] concluded in their studies that microcracks increase after obturation.

In the present study, Protaper universal files were used for cleaning and shaping. Ashwinkumar et al.[19] and Al-Zaka et al.[20] concluded that Protaper rotary files produced the maximum number of microcracks. This may be due to a continuous rotational force and constant torque applied by the NiTi rotary file on the root canal walls and the design features of instruments such as tip design and cross-sectional geometry, taper, and flexibility.

In the present study, there was the percentage increase in the number of dentinal cracks after removal of gutta-percha irrespective of the technique used for removal of gutta-percha and this increase was found statistically significant (P < 0.05) which is in accordance with the study by Topçuoğlu et al.[21] who concluded that there was significant increase in the number of dentinal cracks after retreatment procedure.

In the present study, gutta-percha removal done with Protaper retreatment file system and finished with Protaper universal group had the highest percentage increase in the number of initiation and propagation of dentinal cracks after retreatment procedure as compared to R-Endo and H-files. H-files group showed the least percentage increase in the number of dentinal cracks (P < 0.05).

In our study, rotary instrumentation (Protaper and R-Endo) produced more dentinal cracks as compared to hand instrumentation (H-files). Despite the clinical advantages of rotary NiTi instrumentation, it results in significantly more rotations of the instrument inside the canal,[22] which may cause more friction between the files and the canal walls leading to dentinal defects. These NiTi instruments increase the risk of dentinal damage to root in the form of complete cracks, incomplete cracks, craze lines, or fractures. In addition, the total volume of dentin removed from the root canals is significantly greater with NiTi rotary systems in comparison with the hand files.[23] Furthermore, rotary NiTi instrumentation could potentially cause dentinal defects in the walls of the canal which may act as areas of stress concentration and crack initiation. Defect formation may be related to the tip design of rotary instruments, taper, cross-sectional geometry, and constant or variable pitch.[11]

In the present study, among rotary systems, there was more percentage increase in the number of dentinal cracks in Protaper universal retreatment file group as compared to R-Endo, and the difference was statistically significant (P < 0.05). Protaper universal retreatment file system has a convex triangular cross section whereas R-Endo files have a triangular cross-section and noncutting tip.[21] In addition, these NiTi retreatment files have a different taper type which may be the reason for the creation of dentinal cracks. Furthermore, Protaper retreatment file system is a three file system whereas R-Endo is five file system, so as a number of files are less in Protaper retreatment group, it is more aggressive and thus possibly increasing the chances of creation of dentinal cracks.

H-files resulted in minimal percentage increase in the number of dentinal cracks when compared to the other two groups. There was the statistically significant difference (P < 0.05) between Protaper universal retreatment file and H-file and R-Endo files and H-files. This may be attributed to the less aggressive movements of the hand files in the canal compared with engine-operated files,[21] and also, amount of contact of the instrument on the canal is less compared to rotary files. Because the roots prepared with rotary Protaper universal retreatment file (9% taper) and R-Endo (12% taper) have greater taper as compared to H-files (2% taper), it should be realized that these instruments removed more dentin thus leading to increased stress on the canal walls so this can be the reason for more dentinal cracks in rotary instrumentation.

The limitations of this study are the smaller sample size, and samples with canal curvature >20° were not included.

CONCLUSION

Within the limitation of this study, it can be concluded that microcracks were also present in relation to uninstrumented roots. Root canal preparation with Protaper rotary files and obturation significantly increased the percentage number of microcracks, all the techniques used in the study for removal of obturation material significantly increased the percentage number of microcracks than uninstrumented as well as root canal prepared and obturated root canals. Protaper retreatment files resulted in maximum percentage increase in the number of microcracks formation followed by R-Endo and H-file.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.