Evaluation and Comparison of Microcracks in Radicular Dentine Following the Use of Different Root Canal Instruments –In Vitro Study

INTRODUCTION

Endodontic diseases are associated with multiple bacterial infections, where root canal therapy serves as a significant canal modality.^[1] The goal of endodontics is to create a flawless 3D seal of the root canal system. To achieve this, many kinds of endodontic instruments have been manufactured.^[2]

The biomechanical preparation is a crucial phase for eliminating bacteria and debris from the root canal, which is essential for achieving effective root canal treatment.^[3] Stresses are generated from within the root canals, which are higher in the apical region and more along the walls, so the distribution of stress in the apical area leads to the development of cracks and fractures. Bacteria can proliferate in the crack line and later establish biofilms on the root surface.^[1]

Hand instrumentation, which was once a cornerstone of endodontic practice, has seen a decline in popularity. Despite the setback, it is a vital component of canal preparation. Hand ProTaper Universal (Dentsply, Maillefer, Switzerland) files are well-regarded in endodontic procedures due to their enhanced taper, active cutting action, and ability to remove a greater amount of dentin from the coronal sections compared to other systems. A distinctive characteristic of ProTaper files, in contrast to other filing systems, is that each tool features varying percentage tapers along the length of its cutting blades. When utilized, this gradually tapered design mimics the Schilderian Envelope of Motion technique and significantly enhances flexibility, cutting efficiency, and safety. Another attribute of the ProTaper instruments is their convex, triangular cross-section, which improves the cutting performance while reducing the rotational friction between the file’s blade and the dentin. ProTaper files also possess a variable helical angle and pitch along their cutting blades, minimizing the risk of an instrument inadvertently screwing into the canal. HERO Shaper files, characterized by their distinct tapers, are intended for use with the crown-down technique to systematically eliminate limitations and widen the canal. The differing helical pitchesand varying lengths of the cutting segments of the files offer a remarkable balance of efficiency and flexibility. The cleaning process becomes significantly more effective due to the gradual widening of the canal as the instrument is inserted, which enables the irrigation needle to reach the apical third. Rotary instruments, by theirinnate behaviour in the canal, result in more friction, which may increase microcrack formation in comparison to hand instruments.^[4]

The introduction of nitinol in endodontics provided superior flexibility and resistance to torsional fracture. They lead to a lower possibility of instrument fracture and leave a thickerlayer of dentine thanstainless steel, and hence area safe alternative for the treatment of canals.^[5]

ProTaper Next (Dentsply Tulsa Dental Specialities, Johnson City, TN, USA) is a recently launched NiTi rotary system made from M-wire NiTi alloy (Sportswire, Langley, OK). The ProTaper Next system features a variable taper and an off-centered rectangular cross-section design, which operates with a rotational motion thatminimizestorsional stress on the instrument.^[6]

Recently, reciprocating filesystems have become popular in endodontic practice.Theycan prepare the canal with only one instrument, thereby requiring lesstime than a full sequence rotary system.^[7] The WaveOne gold reciprocating files were manufactured with Gold wire super metal alloy and used with a dedicated reciprocating motion motor.^[8]

The RECIPROC (VDW, Munich, Germany) isused with automated devices.^[9] The reciprocating motion reduces stress on the instrument, and it has been noted that the reciprocating method for root canal preparation represents an advancement of the balanced force technique.

The aim and objective of this study wereto evaluate and compare the development of microcracks in radicular dentin following the use of different root canal instruments.

• To determine the cracks caused by hand, rotary, and reciprocating systems.

• To determine the absence or presence of cracks in the control cohort.

• To determine the prevalenceof cracks at 2mm, 4mm, and 6mm.

• To determine the different types of cracks induced by different engine drives.

MATERIAL AND METHODS

The present study was carried out at the Department of Conservative Dentistry and Endodontics, Maharana Pratap College of Dentistry and Research Centre, Gwalior.

One hundred andtwenty freshly extracted teeth were selected for the study. These teeth were extracted for orthodontic or periodontal reasons. The teeth were carefully rinsed under running water to eliminate any blood and saliva. They were subsequently cleaned of calculus and other soft tissue remnants using an ultrasonic scale. The samples were then preserved in normal saline to avoid dehydration until they were needed.

Specimen preparation

The teeth were then adjusted to 16mm from the apex to standardizethe specimens. Subsequently, the teeth were re-evaluated using a stereomicroscope for the detection of cracks and fractures. The teeth were then divided into fourgroups as shown in Table 1. To prepare the access cavity, the teeth were placed in acrylic blocks. Each tooth was wrapped in aluminum foil, which was peeled off later, and the periodontal ligament was simulated using hydrophilic vinyl polysiloxane impression material that replaced the space created by the foil, and the root was immediately repositioned until the material set [Figure 1].

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Figure 1:

Light body used for the simulation of periodontal Ligament.

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Table 1: Distribution of specimens into various groups.

Group	Subgroups	No. of sample	Files used for cleaning and shaping	Company
Group A Reciprocating [Figure 5]	WaveOne Gold	15	Size#40, Taper:0.08	Reciprocating motion until working length
	RECIPROC	15	Size#40, Taper:0.06	Reciprocating motion until working length
Group B Rotary [Figure 6]	HERO Shaper	15	Size#25. Taper:0.06	These files were used till X2. Each file was used with a brushing motion.
	ProTaper Next	15	Size#25. Taper:0.06	The first penetration is performed with a conventional hand instrument and then .06 taper files are brought to 2/3 of the working length. 04 taper files are brought to working length.
Group C Hand [Figure 7]	ProTaper Universal	15	Size#20, Taper:0.06	First three shaping files were used in brushing motion before light resistance & last were used until working length.
	Hand HERO Shaper	15	Size#25, Taper:0.06	Short in and out movement using light pressure using crown down technique, is used till .06 25 %.
Group D Control [Figure 8]	2% K files	30	Size#25, Taper:0.02	Used as a control group 2% k files were used till file size 25 with copious irrigation.

Microscopic evaluation

After preparation, all samples were injected with methylene blue and then aspirated back and washed with water. All roots were sectioned at 2mm, 4mm, and 6mm from the apex with a low speed saw under cooling water to prevent artefactsfrom dehydration. The teeth were kept moist in distilled water throughoutall the experimental procedures [Figure 2]. The slices were then examined through a stereomicroscope [Figures 3 & 4].

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Figure 2:

Tooth sectioning at root apex.

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Figure 3:

Section with no cracks examinedThrough a stereomicroscope.

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Figure 4:

Section with a complete crack Examined through a stereomicroscope.

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RESULT

At 2 mm: In the Wave one group (n = 15, 100.00%), dentinal microcracks were not observed in 12 (80.00%) of the samples; only three (20.00%) teeth samples showed dentinal microcracks. In the RECIPROC group [Figure 5] (n = 15, 100.00%), dentinal microcracks were detected in nine (60.00%) teeth samples, whereas six(40.00%) didnot havedentinal microcracks. In the ProT taper next group (n = 15, 100.00%), seven (46.67%) teeth samples had dentinal microcracks, and eight (53.33%) didnot havedentinal microcracks. In the HERO shaper group [Figure 6] (n = 15, 100.00%), dentinal microcracks were detected in 13 (86.67%) samples and not detected in the remaining two(13.33%) samples. In the Hand HERO Shaper group [Figure 7] (n = 15, 100.00%), 10 (66.67%) had dentinal microcracks, and five(33.33%) didnot haveany dentinal microcracks. In the Hand ProTaper group (n = 15, 100.00%), the maximum teeth samples,i.e., 12 (80.00%), were detected with dentinal microcracks; only three(20.00%) samples showed no dentinal microcracks. In the control group, 2% K-file [Figure 8], none of the samples had dentinal microcracks (n =30, 100.00%). In total, out of 120 teeth samples (100.00%), dentinal microcracks were present in 54 (45.00%) and not detected in 66 (55.00%). Chi-square test showed significant difference for dentinal microcracks between the groups (χ² = 50.505, df = 6, P <0.001, very highlysignificant). [Table 2]

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Figure 5:

Reciprocating files.

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Figure 6:

Rotary files.

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Figure 7:

Rotary hand files.

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Figure 8:

K-files

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At 4 mm: In the WaveOne Gold group (n = 15, 100.00%), dentinal microcracks were not observed in 13 (86.67%); however,02(13.33%) teeth samples showed dentinal microcracks. In the RECIPROC group (n = 15, 100.00%), dentinal microcracks were detected in 05(33.33%) teeth samples, whereas 10 (66.67%) of the samples didnot have dentinal microcracks. In the ProTaper next group (n = 15, 100.00%), 04 (26.67%) samples had dentinal microcracks, and 11 (73.33%) did not have dentinal microcracks. In the HERO Shaper group (n = 15, 100.00%), dentinal microcracks were detected in 13 (86.67%) samples and not detected in the remaining 02(13.33%) samples. In the Hand HERP Shaper group (n = 15, 100.00%), 09 (60.00%) samples had dentinal microcracks, and 06 (40.00%) didnot haveany dentinal microcracks. In the Hand ProT taper group (n = 15, 100.00%), 10 (66.67%) samples had dentinal microcracks, and 05 (33.33%) didnot haveany dentinal microcracks. In the control group, 2% K files, none of the samples had dentinal microcracks (n =30, 100.00%). In total, out of 120 teeth samples (100.00%), dentinal microcracks were present in 43 (35.83%) and not detected in 77 (64.17%). Chi-square test showed significant difference for dentinal microcracks between the groups (χ² = 47.514, df = 6, P <0.001, very highly significant). [Table 2]

At 6 mm: In the WaveOne Gold group (n = 15, 100.00%), none of the samples had dentinal microcracks. In the RECIPROC group (n = 15, 100.00%), dentinal microcracks were detected in 02 (13.33%) teeth samples, whereas 13 (86.67%) did not have dentinal microcracks. In the ProTaper next group (n = 15, 100.00%), 01 (6.67%) tooth sample had dentinal microcracks, and 14 (93.33%) didn ot have dentinal microcracks. In the Heroshaper group (n = 15, 100.00%), 10 (66.67%) had dentinal microcracks, and 05 (33.33%) did not have any dentinal microcracks. In the Hand HERO Shaper group (n = 15, 100.00%), 07 (46.67%) samples had dentinal microcracks, and 08 (53.33%) did not have any dentinal microcracks. In the Hand ProTaper group (n = 15, 100.00%), 10 (66.67%) samples had dentinal microcracks, and 05 (33.33%) did not have any dentinal microcracks. In the control group, 2% K files, none of the samples had dentinal microcracks (n =30, 100.00%). In total, out of 120 teeth samples (100.00%), overall dentinal microcracks were present in 30 (25.00%) and not detected in 90 (75.00%). Chi-square test showed significant difference for dentinal microcracks between the groups (Yates’ χ² = 41.022, df = 6, P <0.001, very highlysignificant). [Table 2]

DISCUSSION

Endodontic treatment focuses on addressing both vital and non-vital pulp to enable patients to keep their natural teeth for both functional and esthetic purposes. While successful endodontic treatment relies on various factors, one of the critical steps in root canal procedures is the preparation and shaping of the canals.^[10]

The biomechanical preparation process can lead to temporary points of stress concentration within the dentin. Such concentrations of stress might result in dentinal defects, such as microcracks or craze lines.^[11] Which may further develop into VRF.^[12-15] Adorno et al indicated that the start of cracking was closely associated with preparation,whilethe method of filling root canals was linked to the spread of these fractures.^[16] Bier et al indicated that fractures did not happen right after canal preparation. The occurrence of craze lines ranged from 4% to 16%, which could potentially evolve into fractures during retreatment or after prolonged functional stresses, such as chewing.^[17] These damages must be prevented as much as possible.^[18]

The difference between the various root canal preparation instruments in terms of dentinal cracks can be associated with preparation techniques and the cross-section of files.^[18] Historically, root canal preparation was performed with stainless steel endodontic files that were handled manually. In this study, no defectswere seen in the control group (2% K-hand files). The reason for this may be that (1) there were feweraggressive movements of the hand files in the canal, (2) continuousrotational motion was avoided, and (3) there was less taper in 2% of the files used for enlarging and shaping. This finding agrees with Bier et al (2009), who stated that they did not observe any defect in the hand file group.^[17]

The ProTaper Universal instruments were designed by Dr. Cliff Ruddle,Dr. John West, and Dr. Pierre Machtou and originally consisted of just six instruments: three shaping files and three finishing files.^[19] Sim et al (2001)stated that there is decreased microhardness of radicular dentine after exposure to NaOCl in concentrations of 5.25%,but not when NaOCl was used in lower concentrations. In the present study 3% NaOCl was used to minimize alterations of the mechanical properties of dentine during the experimental procedures.^[20] Aydin U, Aksoy F, Karatasliogu E (2014) evaluated the incidence of crack formation while using Reciproc (VDW), WaveOne (Dentsply Maillefer), and Twisted File Adaptive (Kerr Dental) with and without ethylenediaminetetraacetic acid (EDTA). The authors concluded that no microcracks were observed in the control group, whereas all other NiTi instruments resulted in dentinal defects. However, there was no significant difference in crack formation between the experimental groups. Also, EDTA gel did not reduce the incidence of crack occurrence. In the present study, EDTA was used in accordance with the above study.^[21]

In the present study, comparison of dentinal cracks at 2mm,4 mm, and 6mm from the root apex produced by rotary (ProTaperNext and HERO Shaper) was significantly lower when compared with hand HERO Shaper and hand ProTaper Universal. This maybe due to the force generated on the dentinal walls because of the difference in speed of the rotary files and hand files. It hasalso been stated that since PTU has more taper than HS and PTNfiles, it is likely to cause more dentinal cracks.

In this study, it was found that the reciprocating system significantly caused fewermicrocracks as compared to the hand HERO Shaper and the hand ProTaper. Thereciprocal motion seems to allow the continuous release of file when it is engaged in the inner surface of the root canal during cleaning and shaping.Theflexural and torsional stress are reduced by repeating reciprocating clockwise and counterclockwise rotations because the counterclockwise motion disengages the instrument blade and reduces stress.

It was observed that the rotary file system produced more cracks than the reciprocating system because the rotary filesystem uses multiple files for root canal shaping, whereas RECIPROC and WaveOneGold usea single-file system, so there would be fewer chances of cross-contamination in the reciprocating group. The reciprocating system works in a reverse balanced force cutting motion where the unequal CW /CCW angles enable a file to more readily advance towards the desired working length without using excessive and potentially dangerous inward pressure, causing fewer cracks (Sahu et al 2016).^[22]

CONCLUSION

The present study revealed that all rotary as well as hand systems, except for 2% stainless steel K hand files, caused microcracks at each level, significantly more at the apical level.In the limitations of the study, stereomicroscopic evaluation proved to be useful for the evaluation and comparison of microcracks using the different endodontic systems.