Evaluation of Radicular Dentin Microcracks Formation after Instrumentation with NiTi Hand and Rotary File System: A Stereomicroscopic Study

INTRODUCTION

One of the significant steps in endodontic therapy is root canal preparation. This involves elimination of necrotic and vital tissues and infected dentin from the root canals. The objective of such a root canal preparation is to aid use of disinfecting medicaments and irrigants, subsequently eradicating infection, and avoiding repeated infections accomplished through providing a fluid tight endodontic filling and restoration of tooth crown. The mechanical and biological objectives of endodontic therapy depend on thorough irrigating and shaping of the root canals.¹

Over the past several years, preparation of root canals was being performed efficaciously with the use of hand instruments. Advanced automated systems involving various mechanics have led to the use of nickel–titanium instruments resulting in an alternate enhanced technique, thus enhancing the ability to shape and to reduce the incidence of fracture of instrument during endodontic therapy.²

Since the last 10 years, full-fledged popularity of nickel–titanium (NiTi) rotary instruments use has been growing and different registered systems have been familiarized commercially. The most important benefits of NiTi files are reduced working time and enhanced flexibility.³

Sometimes the root dentin may get damaged unavoidably during instrumentation resulting in endodontic treatment failure due to the formation of tiny but complex fractures and dentinal cracks.⁴ Dentinal microcracks pose difficulty to instrumentation and restorative procedures which further decreases tooth resistance and leads to vertical root fracture (VRF). The most complicated feature of endodontic therapy failure is VRF, which ultimately leads to extraction of tooth. Frequent and recurrent occlusal forces lead to propagation of microcrack and subsequent root fracture.⁵ Therefore, the present study was conducted to assess the formation of microcracks in root dentin post-instrumentation with NiTi hand and rotary file system.

MATERIALS AND METHODS

The present study was conducted in the department of conservative dentistry and endodontics, Indira Gandhi Government Dental College, Jammu, India. Totally, 80 freshly extracted mandibular premolar teeth with single roots were chosen for this study. The teeth were extracted for various orthodontic reasons. Calculus, soft tissue, and debris were cleaned off the extracted teeth with the use of an ultrasonic scaler and were kept in saline until further use. A stereomicroscope was used to observe all the roots under a magnification of ×10 to identify any fracture or crack that was previously present, and such specimens were excluded from the study. Presence of single canal was identified using radiographs.

A diamond coated bur was used to flatten each specimen’s coronal portion under running water for cooling. The access cavity was opened; canal patency was established with #10 K-file (Micro-Mega, Besancon, France) in each root. A working length short of 0.5 mm of the file seen at the apical foramen was established. About 3 mL of sodium hypochlorite (NaOCl) solution of 2.5% concentration by a 30-G needle was used for irrigation.

RESULTS

Table 1 shows the overall radicular dentin microcracks in all the groups, in which self-adjusting file showed least number of defects with a percentage of 75% followed by XP-Shaper and NiTi hand K-files with a values of 65 and 60%, respectively. Use of hand K-files resulted in greater number of incomplete cracks (30%) and use of XP-Shaper demonstrated greater number of craze lines (15%).

When each system of file was compared for the results achieved at sections of apical third (3 mm), middle third (6 mm), and cervical third (9 mm), statistically significant difference was found between the experimental groups in the formation of dentinal defects of root at apical third (3 mm) (p < 0.031) and middle third (6 mm) (p < 0.001), whereas the sections at cervical third (9 mm) did not show any statistically significant difference (p %3E; 0.312), as shown in Tables 2 to 4.

The inference of the present study indicates that the self-adjusting file systems have potential favorable results in their ability to clean the canals thoroughly and induce least root dentin microcrack occurrences in comparison with NiTi hand K-files and XP-Shaper files system.

DISCUSSION

The utmost significant step in endodontic therapy is preparation of root canals. The idea of “Cleaning and shaping of the root canals” was familiarized by Schilder around 30 years ago and the objective of this is canal space preparation to enable disinfection by irrigating solutions and use of other medicaments and further facilitate root canal space obturation three-dimensionally. Therefore, root canal preparation is the most important step of endodontic therapy that eradicates infection.⁷

During the process of root canal preparation, momentary stress absorption gets developed along the contact between the walls of the canal and instrument in root dentin, which may result in dentinal defects, thus initiating VRF. The formation of microcracks may be linked to features of instrument, such as taper, design of tip, pitch design, cross-sectional geometry, and flute form.⁸

In the present study, a microscope was used to observe all roots under a magnification of ×10 to identify the occurrence of any fracture or crack which previously existed. This is in accordance with the trial by Langaliya et al.⁹ who examined the presence of preexisting fracture or cracks in all teeth. Yet, identifying the cracks prior to the initiation of the experiment is problematic as few of these cracks may be present internally and may not be seen on the exterior. However, the control group did not show any cracks, suggesting that the microcracks seen were due to procedures of preparation with NiTi files.

In the present study, improved results in terms of cleaning ability and least occurrence of microcracks in root dentin were demonstrated by self-adjusting file system than XP-Shaper and NiTi hand K-files system. This result in contradictory to the in vitro trials by Bier et al.¹⁰ and Ashwinkumar et al.¹¹ found minimal incidence of microcracks in teeth that were instrumented using hand file than in those using mechanical instrumentation. In the past decade, several novel NiTi rotary and reciprocating arrangements have been established and presented in the market. A possible relationship could exist between frequency of microcrack formation and NiTi instruments’ design, because it has been shown the design of file may impact concentration of strain and stress in apical region during instrumentation of root canals.¹²

A trial by Mohammed et al.¹³ showed least dentinal defects incidence in radicular dentin after use of XP-Shaper for instrumentation and it was found that XP-Shaper is prefabricated with the Ni–Ti MaxWire. Due to metallurgical alloy, the instrument has super elasticity, high flexibility, and increased fatigue resistance. The formation of microcracks in dentin may be prevented due to the enhanced flexibility of these files.¹⁴

The use of self-adjusting files to instrument the tooth specimens showed least cracks in the present study. This is due to non-application of rotational movement and application of constant delicate pressure on the walls of radicular dentin by these files and this permits an even dentin removal along the entire root canal perimeter. The application of vibrating movement in an in and out direction in place of complete rotation results in reduced concentration of stress in the root canal. Additionally, the non-stop irrigation with the use of self-adjusting file led to reduced friction and enabled the easy dentin removal. These findings are similar to those obtained by Yoldas et al.,⁸ Hof et al.,¹⁵ Peters and Paque¹⁶ who demonstrated lack of dentinal cracks in root canals instrumented with self-adjusting file system.

The technique used to store the samples is important too as a desiccated tooth will have diminished resistance to the applied forces in each path throughout the process of instrumentation and filling or the examination through scanning or cutting the samples. It has been testified that in a tooth root which is not dehydrated, radicular dentin microdefect formation or increase in remaining microstrain does not take place.¹⁷ Yet, another factor which may impact the outcome of such trials is the tooth’s age. In about 30% of older teeth, a decrease in crack propagation and an increase in resistance to fracture have been reported because of the reduced area of lumen and mineral deposition.¹⁸ The results of Bajaj et al.¹⁹ have reported a reduction of crack growth and an increase in fracture resistance of about 30% for older teeth due to the reduction of the lumen area and deposition of mineralized structure.

A diverse suggested torque and speed was used for the files system in the present study which cannot be regularized for all other files. The limitation of this study could be the use of varying torque and speed situation for each file system. Similarly, it was not easy to regularize the force used in downward motion during instrumentation. Moreover, teeth chosen for this study were those with no anatomic complications and with straight root canals which actually failed to duplicate real clinical scenario.

CONCLUSION

The present study concluded that the, self-adjusting file system gives promising better results in cleaning ability with minimal incidence of radicular dentin microcracks than XP-Shaper and NiTi hand K-files system.

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