TITLE: Comparison of dentinal damage induced by different nickel-titanium rotary instruments during canal preparation: An 

  ABSTRACT.AIM:
To compare dentinal damage caused by hand and rotary nickel-titanium instruments using ProTaper, K3 Endo, and Easy RaCe systems after root canal preparation.

ABSTRACT.MATERIALS AND METHODS:
One hundred and fifty freshly extracted mandibular premolars were randomly divided into five experimental groups of 30 teeth each and biomechanical preparation was done: Group 1 with unprepared teeth; Group 2 were prepared with hand files; Group 3 with ProTaper rotary instruments; Group 4 with K3 rotary; Group 5 with Easy RaCe rotary instruments. Then, roots were cut horizontally at 3, 6, and 9 mm from apex and were viewed under stereomicroscope. The presence of dentinal defects was noted.

ABSTRACT.STATISTICAL ANALYSIS:
Groups were analyzed with the Chi-square test.

ABSTRACT.RESULTS:
Significant difference was seen between groups. No defects were found in unprepared roots and those prepared with hand files. ProTaper, K3 rotary, and Easy RaCe preparations resulted in dentinal defects in 23.3%, 10%, and 16.7% of teeth, respectively. More defects were shown in coronal and middle sections, and no defect was seen in apical third.

ABSTRACT.CONCLUSION:
The present study revealed that use of rotary instruments could result in an increased chance for dentinal defects as compared to hand instrumentation.

BODY.INTRODUCTION:
Successful endodontic therapy depends upon triad of proper diagnosis, thorough biomechanical preparation and three-dimensional obturation of root canal system. Biomechanical is one of the most important factors for successful root canal treatment and determines the efficacy of all subsequent procedures.[1] It is done to completely remove organic tissue, microorganisms and debris by enlarging the canal diameter and creating a shape that allows a proper seal.[2]  Stainless steel root canal instruments clean the canal superficially and can create canal aberrations such as ledges, zips, and elbows.[3] To eliminate these shortcomings of stainless steel instruments, nickel-titanium (Ni-Ti) instruments have been developed.  Canals prepared by rotary Ni-Ti instruments show increased canal cleanliness and less straightening, apical canal transportation and perforations. These benefits are because of greater flexibility and specific design features of Ni-Ti instruments allowing the natural canal curvature to be maintained.[4] Rotary instrumentation also requires less time to prepare canals as compared to hand instrumentation.[5]  Furthermore, rotary Ni-Ti instrumentation could potentially cause dentinal defects in the walls of the canal which may act as areas of stress concentration and crack initiation. These Ni-Ti instruments increase the risk of dentinal damage to root in the form of complete cracks, incomplete cracks, craze lines or fractures. Craze lines can later propagate into vertical root fracture (VRF) if the tooth is subjected to repeated stresses from endodontic procedures.[6] And VRF is a significant clinical problem which often leads to tooth extraction.[7]  However, several factors like force of instrumentation and obturation; retreatment procedure and high concentration of hypochlorite may also be responsible for the formation of dentinal defects in different degrees.[8910] A debilitated root, as a result of flaring and instrumentation, could suffer VRFs during obturation procedures.[11]  The goal of this study is to compare the damage observed in root dentin after endodontic preparations with different Ni-Ti rotary file systems.

BODY.MATERIALS AND METHODS:
One hundred and fifty freshly extracted mandibular premolars were selected and cleaned with periodontal scaler and stored in purified filtered water. The coronal portions of all teeth were removed with diamond disk, leaving roots 16 mm in length. All root surfaces were observed with stereomicroscope (Trinocular Stereo Zoom Microscope, Nikon, NY, USA) under ×12 to exclude cracks.  
Group 1: Left unprepared and served as Group 1.Group 2: Prepared using stainless steel K-files (Dentsply Maillefer, Ballaigues, Switzerland) up to apical size 25 at the working length and step-back technique was used till file no. 60.In the remaining three groups, canal patency was established with a #10 K-file. Then, a size 15 K-file was introduced into the canal until it was visible at the apical foramen. The working length was determined by subtracting 1 mm from this measurement.Group 3: Prepared using ProTaper rotary system (Dentsply Maillefer, Ballaigues, Switzerland) sequentially at the speed of 300 rpm using a crown-down technique. Canal preparation was finished with F2 (25/.08) till working length.Group 4: K3 rotary system (SybronEndo, Orange, CA, USA) sequentially at the speed of 300 rpm using a crown-down technique. Canal preparation was done with file 25/.06 till working length.Group 5: Easy RaCe rotary system (FKG Dentaire, La Chaux-de-Fonds, Switzerland) sequentially at the speed of 300 rpm. Canal preparation was done with file 25/.06 till working length.
  In all groups, each canal was irrigated with 3% sodium hypochlorite between each instrument used in canal preparation. In groups with preparation with rotary system, Dolo Endogel (17% EDTA with 10% carbamide peroxide) was used between each sequential instrument. In all groups, EndoActivator (Dentsply Tulsa Dental, Tulsa, OK, USA) was used with no. 25 tip for 30 s to agitate the solution vigorously to clean the canals efficiently. All roots were kept moist in purified filtered water throughout the experimental procedures.

BODY.MATERIALS AND METHODS.EXAMINATION OF ROOTS:
The roots of all the teeth were sectioned horizontally at 3, 6, and 9 mm from apex. Digital images of each section were captured under stereomicroscope. Each specimen was checked by for the presence of dentinal defects. Defects were categorized as Figure 1.  Figure 1Classification of dentinal defects (a) no defect (b) vertical root fracture (c) craze line (d) partial/incomplete crack  "No defect" was defined as root dentine devoid of any lines or cracks where both the external surface of the root and the internal root canal wall had no defects [Figure 1a].  "Fracture" was defined as a line extending from the root canal space to the outer surface of the root [Figure 1b].  "Other defects" were defined as all other lines observed that did not extend from the root canal to the outer root surface. For example, craze line — line extending from the outer surface into the dentine that did not reach the canal lumen [Figure 1c], or partial crack extending from the canal wall into the dentine without reaching the outer surface of the root [Figure 1d].  Then, dentinal defects produced by these hand and different rotary systems were compared with each other. Defects at different horizontal sections in each group were also compared.  Roots were classified as "defected" if at least one of three sections showed either a craze line, partial crack or a fracture. Results were expressed as number and percentage of defected roots in each group.  Then, the data collected were put to statistical analysis to compare the appearance of defected roots between experimental groups.

BODY.MATERIALS AND METHODS.STATISTICAL ANALYSIS:
The data were analyzed using statistical software SPSS 17.0 program (SPSS Inc., Chicago). A Chi-square test was performed to determine statistically significant difference in the appearance of defected roots between the experimental groups. Chi-square test was also performed to determine the defects at different horizontal sections in each group. The level of significance was set at P < 0.05.

BODY.RESULTS:
Groups were significantly different from each other (P = 0.007). Group 1 and Group 2 showed no defected roots. Dentinal defects were found in the ProTaper, K3 SybronEndo, and Easy RaCe rotary groups. But the difference was nonsignificant among all rotary systems used in this study [Table 1 and Graph 1]. Results showed that nonsignificant differences were present for the presence of dentinal defects (at coronal and middle thirds).   Table 1 Comparison of number and percentage of teeth showing defects   Graph 1Comparison of defects in % (P = 0.007)

BODY.DISCUSSION:
In this present study, dentinal defects occurred in around 10% of samples tested, and these may develop into fractures following any additional procedure such as preparation, obturation, and retreatment or by repeated stress of occlusal forces. Bier et al. reported that craze lines occurred in 4 to 16% samples.[2] Onnink et al. were the first to report dentinal defects as a consequence of canal preparation but only found small defects entirely within dentin that did not communicate with the canal wall.[8]  Sectioning method using diamond disk used in the present study could also result in dentinal defects. However, because both the control and the hand file group did not show any defect, we may conclude that the defects seen were not due to sectioning procedure used.  Irrigation done using EndoActivator (Dentsply TulsaDental) which is a sonically driven system and comprised of a handpiece and variously sized polymer tips, can lead to dentinal damage of root. As in the current study, all teeth were irrigated following the same protocol and roots prepared with hand files did not show any dentinal defects, we may consider that EndoActivator did not contribute to the appearance of dentinal defects.[12]  As NaOCl being an alkaline material reacts with organic tissue and can change the chemical structure and the mechanical properties (elastic modulus and flexural strength) of dentine.[10] It was observed that there was marked decrease in microhardness of dentin when irrigation was done with NaOCl.[13] It may affect crack propagation. In the current study, 3% sodium hypochlorite was used for irrigation. As all teeth were irrigated following the same protocol and roots prepared with hand files did not show any dentinal defects, we may consider that the irrigation using sodium hypochlorite in this study did not contribute to the appearance of dentinal defects seems justified.  Stainless steel hand instruments clean the canals superficially and have also been shown to create canal aberrations, such as ledges, perforations, zips, and elbows.[3] But hand instrumentation does not appear to induce much damage to root canal wall. As in this study, instrumentation with hand files did not demonstrate damage to the root canal wall. This is in agreement with several other studies given by Yoldas et al.,[14] Hin et al.[15] and could be attributed to the less aggressive movements of the hand files in the canal compared with engine-operated files and less taper (0.02) as compared to rotary Ni-Ti instruments.[1415]  However, in the present study, a uniformed tapered preparation (0.06-0.08) was attempted in all rotary instrumentation groups. The observation that all groups prepared with rotary Ni-Ti files showed various degrees of damage supports the idea that greater tapered files may generate an increased stress on the dentin wall. This observation is supported by Sathorn et al. who concluded that by maintaining the canal size as small as practical, a reduction in fracture susceptibility could be expected.[16]  The amount of material removed from the root canal depends on the shape of the rotary instrument and the penetration depth in the canal. Because the roots prepared with rotary files that had a taper of at least 0.06 in this study showed defects, it should be realized that these instruments removed more dentin as compared with hand files that had a taper of 0.02. Furthermore, significantly more rotations in the canal are necessary to complete a preparation with rotary Ni-Ti files as compared with hand files. This, in itself, may contribute to the formation of dentinal defects.[5]  File design, however, is also likely to affect the shaping forces on the root dentin. Forces generated during instrumentation have been linked to an increased risk of root fracture. During preparation, a canal is shaped by the contact between instrument and dentin walls. These contacts create many momentary stress concentrations in dentin. Such stress concentrations may leave dentinal defects in which VRF can initiate. Higher stresses in the root during instrumentation can be expected to increase dentinal defects and thus increase VRF risk.[4]  ProTaper had 23.3% of dentinal defects as compared to other rotary systems which was significantly different.  ProTaper rotary files are with progressive taper design which increases stiffness and facilitates active cutting motion and removes relatively more dentin coronally compared with other systems.[17] ProTaper file F2 has a large taper of 0.08 which could explain the higher incidence of damage than other tapered rotary files. Bier et al. observed cracks in the horizontal sections of 16% of the roots instrumented with the ProTaper system and reported that ProTaper rotary files created more dentin damage than other rotary instruments.[2] Liu et al. observed cracks at the apical root surface in 25% of the roots instrumented with the ProTaper.[18]  In the present study, K3 showed 10% incidence of dentinal defects. It has an asymmetrical design with a slightly positive rake angle for optimum cutting efficiency and has peripheral blade relief which is designed to reduce friction and protects the root dentin from getting more damaged.[19]  In the present study, Easy RaCe instrumentation showed 16.7% of dentinal defects which was not significantly different than ProTaper system rather significantly different than K3 rotary system. It has noncutting safety tip with alternating sharp cutting edges which improves cutting efficacy and has variable helical angles and a variable pitch.[20]  In the current study, it was observed that more defects are seen in coronal and middle sections, and no defect was seen in the apical third.  Hence, taking preoperative radiographs is of paramount importance before cleaning and shaping procedure. As it helps in choosing the taper of instrument to be used for preparation. Also, apical gauging would be helpful in maintaining the original root canal anatomy.

BODY.CONCLUSION:
Although Ni-Ti rotary instruments have many advantages over hand instruments, Ni-Ti rotary instruments can induce various degrees of dentinal damage during root canal preparation. Reasons may be the greater taper, more rotations and aggressive cutting which can generate increased stresses on the dentin wall and subsequent formation of dentinal defects. In this study, it is seen that ProTaper rotary system causes more dentinal damage as compared to other rotary systems (K3, Easy RaCe).

Question: Compared to hand instrumentation what was the result of ProTaper, K3 SybronEndo, and Easy RaCe rotary on dentinal defects?
Evidence: Significant difference was seen between groups. No defects were found in unprepared roots and those prepared with hand files. ProTaper, K3 rotary, and Easy RaCe preparations resulted in dentinal defects in 23.3%, 10%, and 16.7% of teeth, respectively.
Answer: significantly increased