Comparative Evaluation of Microcrack Formation in Different Kinematics Using Rotary and Reciprocating File Systems: An In Vitro Study

INTRODUCTION

The success of endodontic therapy can be achieved by bacterial elimination, unexpurgated debris removal, and a complete three-dimensional obturation.¹

Vertical root fractures are a common complication that results after root canal therapy often leading to extraction of the tooth.² The root fracture might occur as a result of a microcrack or craze line that is propagated with repeated stress application by occlusal forces.³ Over the past few decades, new nickel titanium rotary systems have been developed and was preferred due to their advantages such as resistance to flexural and torsional failure as well as improved cutting ability. Root canal instrumentation with continuous rotation and reciprocating motion is reported to cause dentinal microcracks, regardless of the technique or kinematics used.^3–5

In 2008, Yared proposed a canal preparation technique using F2 ProTaper in reciprocation and this has shown to maintain the original root canal anatomy with least possible distortion.⁶ The ProTaper Next rotary system, which consists of instruments with variable taper, has an off-centered rectangular cross-sectional design.⁷

The flexibility of rotary instruments as a result of heat treatment has been stated as a significant reason for microcrack formation. The Waveone gold rotary system has an offset parallelogram-shaped cross-section with a semiactive tip.^7,8

There are very few reported studies that evaluate the rate of microcrack formation with different reciprocating kinematics. Research on relationship between tooth structural integrity and instrumentation techniques and kinematics is certainly required. Therefore, the aim of the study was to evaluate microcracks formed after canal preparation using Waveone gold (WOG) in reverse reciprocating motion and the ProTaper Next (PTN) file system in a sequential manner in rotation or forward reciprocating motion.

The null hypothesis was that there would be no significant difference in the formation of dentinal microcrack between ProTaper Next in rotation or forward reciprocating motion and Waveone gold in reverse reciprocating motion.

MATERIALS AND METHODS

The present study was carried out in the Department of Conservative Dentistry and Endodontics and Nano Sciences, Kochi, India. Sixty human maxillary premolars indicated for extraction due to orthodontic reasons with moderately curved buccal roots (curvature <10°) with mature apices according to the Schneider method⁹ were selected for the study. The curvatures of the roots were determined using the CBCT (CS 9300) 3D imaging software. The extracted teeth were stored in distilled water. The cusps were flattened using carbide disks to maintain the tooth length to 20 mm. The roots of all teeth were inspected with transmitted light and stereomicroscopy under 12× magnification to detect any preexisting craze lines or cracks if present, if so, were excluded from the study group. A rubber-based impression was made and all the roots were embedded in acrylic blocks.¹⁰ After achieving coronal access, the apical patency of the canals was confirmed using a 10 K size file (Dentsply Maillefer, Switzerland). The samples were randomly divided into three groups as follows:

RESULTS

The three groups examined showed the presence of dentinal microcracks at apical 3 and 6 mm (Figs 1 to 3). In the current study, the presence of microcrack at 3 mm was found to be 30% in both group I and II. However, microcrack was comparatively more in group III (60%) (Table 1). About 30% of group I, 35% of group II, and 55 % of group III showed presence of microcrack at 6 mm (Table 2). The presence of microcrack was detected in all three groups; however, there is no significant difference between the three groups in 3 and 6 mm sections (p values 0.082 and 0.23).

DISCUSSION

When rotational forces are applied onto the canal walls using nickel titanium instruments, microcracks are formed in the root dentin.¹¹ Even after meticulous shaping and cleaning with various versions of rotary or hand instrumentation, microcracks occur.¹² Vertical root fracture (VRF) may result as a gradual propagation of microcracks within the tooth structure and a delayed effect of root canal therapy.¹³ The effect of root canal preparation techniques and kinematics has been found to induce the formation of microcracks, resulting in VRF.¹⁴

According to the results of the present study, microcrack formation occurred independent of the kinematics and the type of instrument used. The single-file reverse reciprocating system (group III) showed maximum dentinal microcrack when compared to the sequential file system in continuous rotation or forward reciprocation (groups I and II). The results were consistent with previous studies, which demonstrated that no method could completely prevent crack formation.^3–5 The obtained differences may be due to the preparation technique, the cross-sectional design, the taper of the instruments, and kinematics used such as continuous rotation and forward and reverse reciprocating motion.^3,15,16

The cross-section of Waveone gold is a parallelogram with two 85° cutting edge, alternating with a patented off-centered cross-section where only one cutting edge will be in contact with the root dentinal wall. The tip of WOG is semiactive, which helps in improving its penetration into any secure canal with a smooth and reproducible glide path, eliminating the need to push the file apically. During a 360° rotation, the WOG single file with a cross-section that alternatively touches the dentinal wall might increase the contact of the instrument with the dentinal wall, thus promote the formation of dentinal defect.¹⁷

Liu et al. reported that the use of Reciproc files caused less microcrack than the ProTaper universal file in a sequential file system.¹⁰ However, Priya et al. had reported that the use of single-file reciprocating motion in the root canal may result in higher levels of stress in the root dentin.¹²

The PTN file system is a M wire technology and has an off-centered mass with rectangular cross-section reducing the screwing effect. The cross-section design with a two-point contact onto the dentinal wall serves to reduce the engagement between the files and dentin and also the torque over the file.^7,15,18

Deveci et al. reported that the rate of microcrack formation was observed more in Reciproc than ProTaper Next.¹⁵ Capar et al. had reported that PTN caused less microcrack than ProTaper universal and hyflex.¹⁹ Cassimiro et al. stated that kinematics is not related to the formation of microcracks.²⁰

However, in the present study the files used in forward reciprocation were found to cause less microcrack compared to ones in reverse reciprocation, though the results were not statically significant. Therefore, taper and kinematics used for root canal preparation need to be taken into consideration while instrumenting the root canals. Images of dentinal microcracks were captured using a digital camera attached to the stereomicroscope. However, the limitation of the study is that there are high chances that these imaging procedures may develop heat, leading to dehydration of samples inducing cracks, which might affect the results of the study. Therefore, future studies using other methods like microcomputed tomography or optical coherence tomography will eliminate the sectioning procedure and hence could be less detrimental.

This is the first study to assess the presence of microcrack when forward reciprocation motion was used with the ProTaper next system. The presence of microcrack was observed to be similar when PTN was used in forward reciprocation as in rotation.

CONCLUSION

Nickle titanium rotary instruments tend to induce varied degrees of root dentinal damage during canal instrumentation. ProTaper Next files in rotation as well as in forward reciprocating motion represented satisfactory results with minimal microcrack defects when compared with Waveone gold in reverse reciprocation.

REFERENCES

1. Seltzer S, Naidorf IJ. Flare-ups in endodontics: I. etiological factors. J Endod 1985;11(11):472–478. DOI: 10.1016/S0099-2399(85)80220-X.

2. Tsesis I, Rosen E, Tamse A, et al. Diagnosis of vertical root fractures in endodontically treated teeth based on clinical and radiographic indices: a systematic review. J Endod 2010;36(9):1455–1458. DOI: 10.1016/j.joen.2010.05.003.

3. Yoldas O, Yilmaz S, Atakan G, et al. Dentinal microcrack formation during root canal preparations by different NiTi rotary instruments and the self-adjusting file. J Endod 2012;38(2):232–235. DOI: 10.1016/j.joen.2011.10.011.

4. Karataş E, Gündüz HA, Kırıcı DÖ, et al. Dentinal crack formation during root canal preparations by the twisted file adaptive, ProTaper next, ProTaper universal, and WaveOne instruments. J Endod 2015;41(2):261–264. DOI: 10.1016/j.joen.2014.10.019.

5. Kansal R, Rajput A, Talwar S, et al. Assessment of dentinal damage during canal preparation using reciprocating and rotary files. J Endod 2014;40(9):1443–1446. DOI: 10.1016/j.joen.2014.02.015.

6. Yared G. Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. Int Endod J 2008;41(4):339–344. DOI: 10.1111/j.1365-2591.2007.01351.x.

7. Tanalp J, Güngör T. Apical extrusion of debris: a literature review of an inherent occurrence during root canal treatment. Int Endod J 2014;47(3):211–221. DOI: 10.1111/iej.12137.

8. Pedullà E, Genovesi F, Rapisarda S, et al. Effects of 6 single-file systems on dentinal crack formation. J Endod 2017;43(3):456–461. DOI: 10.1016/j.joen.2016.10.038.

9. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg Oral Med Oral Pathol 1971;32(2):271–275. DOI: 10.1016/0030-4220(71)90230-1.

10. Liu R, Hou BX, Wesselink PR, et al. The incidence of root microcracks caused by 3 different single-file systems versus the ProTaper system. J Endod 2013;39(8):1054–1056. DOI: 10.1016/j.joen.2013.04.013.

11. Shemesh H, Van Soest G, Wu MK, et al. Diagnosis of vertical root fractures with optical coherence tomography. J Endod 2008;34(6):739–742. DOI: 10.1016/j.joen.2008.03.013.

12. Priya NT, Chandrasekhar V, Anita S, et al. Dentinal microcracks after root canal preparation” a comparative evaluation with hand, rotary and reciprocating instrumentation. J Clin Diagn Res 2014;8(12):ZC70–ZC72. DOI: 10.7860/JCDR/2014/11437.5349.

13. Shemesh H, Bier CAS, Wu M-K, et al. The effects of canal preparation and filling on the incidence of dentinal defects. Int Endod J 2009;42(3):208–213. DOI: 10.1111/j.1365-2591.2008.01502.x.

14. Ustun Y, Aslan T, Sagsen B, et al. The effects of different nickel-titanium instruments on dentinal microcrack formations during root canal preparation. Eur J Dent 2015;9(1):41–46. DOI: 10.4103/1305-7456.149638.

15. Deveci Taç M, Kaya S, Falakaloğlu S. Evaluation of dentinal micro-cracks caused by the ProTaper universal, ProTaper next and Reciproc rotary file systems used in root canal preparation. Int Dent Res 2018;8(3):111–116. DOI: 10.5577/intdentres.2018.vol8.no3.3.

16. Haridas K, Hariharan M, Singh P, et al. Effect of instrumentation techniques and kinematics on apical extrusion of debris: An in vitro study. J Contemp Dent Pract 2019;20(9):1067–1070. DOI: 10.5005/jp-journals-10024-2656.

17. Karataş E, Ersoy İ, Gündüz HA, et al. Influence of instruments used in root canal preparation on amount of apically extruded debris. Artif Organs 2016;40(8):774–777. DOI: 10.1111/aor.12675.

18. Ruddle CJ, Machtou P, West JD. The shaping movement: fifth-generation technology. Dent Today 2013;32(4):94. , 96–9.

19. Capar ID, Arsalan H, Akcay M, et al. Effect of ProTaper universal, PTN and hyflex instruments on crack formation in dentin. J Endod 2014;40(9):1482–1484. DOI: 10.1016/j.joen.2014.02.026.

20. Cassimiro M, Romeiro K, Gominho L, et al. Effects of reciproc, ProTaper next and WaveOne gold on root canal walls: a stereomicroscope analysis. Iran Endod J 2018;13(2):228–233.