Comparative evaluation of shaping ability of sequential rotary, single rotary, and single reciprocating file systems in simulated curved canals using cone-beam computed tomography: An in vitro study

Abstract

INTRODUCTION Shaping and cleaning comprise the most important phase in root canal treatment. Schilder's principles for canal preparation mainly insisted on preserving the apical foramen with the narrowest cross-sectional diameter and not altering the original canal curvature.[1] However, iatrogenic errors such as ledges, zips, perforations, and root canal transportation (CT) can occur during preparation, especially in curved canals.[2] CT is the removal of canal wall structure on the outside curve in the apical half of the canal due to the tendency of files to restore themselves to their original linear shape during canal preparation.[3] Centering ability (CA) is the ability of an instrument to remain centered in the canal, thus preserving the canal anatomy.[4] Persistently, newer instruments with proprietary thermomechanical processes have been developed to avoid or reduce the drawback such as the unexpected fracture of the traditional NiTi files.[5] To overcome these disadvantages, various thermomechanical procedures and the improvement of composition of the alloy are introduced, which improve the flexibility of NiTi files and, in turn, would minimize the intracanal irregularities. With the introduction of novel technologies such as gold treatment, ProTaper Gold (Dentsply Maillefer, Ballaigues, Switzerland) continuous rotation sequential file system was introduced. PTG files have a convex triangular cross-section, progressive taper and consist of three shaping files – Sx, S1, and S2 – and three finishing files – F1, F2, and F3.[6] The newest innovations are reciprocating systems and single-file systems. A recent innovation is a reciprocating motion where the stress on the instrument is relieved by reciprocating movement which could reduce the risk of cyclic fatigue caused by tension and compression. Further the single file systems have gained wide popularity with their advantages like shorter working time, reduction in number of instruments required to obtain desired canal shaping and reduced instrument fatigue compared to sequential rotary system. WaveOne GOLD (Dentsply Maillefer, Ballaigues, Switzerland) is a reciprocal single-file system, made from GOLD alloy technology and available as 20/07, 25/07, 35/06, and 45/05. They have a fixed taper from D1-D3, yet a progressively decreasing percentage tapered design from D4-D16 and semi-active tip and a unique, unequal bidirectional reciprocation movement.[7] The most recently introduced file, One Curve (Micro Mega, Besancon, France), was manufactured from C Wire. It is a continuous rotary single-file system with a continuous taper and variable pitch, available as 25/04, 25/06, 35/04, and 45/04.[8] Among the various methods used for evaluation of shaping ability of endodontic instruments, CBCT has been widely and also in this study as it is noninvasive and reproducible three-dimensional evaluation of the morphology of the tooth.[910] The purpose of this in vitro study was to evaluate and compare the CT and CA between ProTaper Gold, One Curve, and WaveOne Gold in the preparation of curved simulated root canals using CBCT imaging. The null hypothesis tested was that the different file systems used in the curved canals would not differ in the degree of CT and CA. MATERIALS AND METHODS Sample selection Thirty curved simulated root canals in clear resin blocks (Endo Training Blocks ISO 15, Dentsply Maillefer), with 60° angle of curvature, 10 mm radius of curvature, 2% taper, and 17 mm length, were randomly divided into three groups according to the instruments used, namely GROUP I: ProTaper Gold, GROUP II: One Curve, and GROUP III: WaveOne Gold. Sample preparation For all groups, a stainless steel 10 and 15 K-file (Dentsply Maillefer) scouted the canal up to the working length (WL) until it is visible at the apical exit and WL was established 0.5 mm short of this length, creating an initial and standardized glide path. The simulated resin blocks were positioned in a custom-made dental stone holder with five resin blocks per mold and scanned before and after instrumentation. Root canal preparation A single operator with experience in rotary and reciprocating motions executed all instrumentation procedures and instruments were discarded after single use. Apical patency was confirmed between each step using a size 10 K-file just beyond the WL. The glycerine was used as a lubricant and 1.0 mL sterile water was used as an irrigant using a 30-G side-vented needle (Max-i-Probe, Dentsply Rinn, Elgin, IL, USA) placed to a depth just short of binding. The flutes of the used instruments were cleaned using gauze soaked in 0.9% Normal saline after three in and out movements. All the groups were instrumented using Gear reduction handpiece or Endodontic motor (X-smart TM plus, Dentsply Tulsa Dental, Tulsa, OK). Group I The canals were instrumented to the WL with PTG instrument with a length of 25 mm, at 300 rpm with a torque of 150–520 gcm as recommended by the manufacturer as follows: SX file (1/2 of the WL), followed by S1 file, S2 file, F1, and finally F2 files. Group II Canals were prepared with one curve file system with size #25 and taper of 0.06 at continuous rotation speed 300 rpm and torque 2.5 N.cm with direct downward pecking movement until the WL. Group III Canals were prepared with the Primary reciprocating WaveOne Gold file with size #25 and taper of 0.07, in the WaveOne GOLD mode by the endo motor in brushing action at 350 rpm with a gentle inward “stroking” motion minimum apical pressure. After final irrigation with 1.0 mL sterile water, the samples were positioned for post instrumentation scan. Cone-beam computed tomography scan specifications All the groups were scanned before and after instrumentation using cone-beam computed tomography (NewTom go 3D CBCT Machine). The CBCT scans were done using the protocol supplied by the CBCT scanner, at 84Kvp, 5.0 mA, 90 mm Voxel, Exposure time 20 s, 1-mm-thick axial sections, 32 cm display field of view, and beam incidence at the central portion on the device used to fix the specimens aligned perpendicularly to the beam. Cone-beam computed tomography measurements CBCT images were subjected for the measurement using NNT viewer software and were calculated at three cross-section levels, namely Level I (apical) – 3 mm, Level II (middle) – 5 mm, and Level III (coronal) – 7 mm representing distance from the apical end using the equation as defined by Gambill et al.[9] For CT – (a1 - a2) - (b1– b2) For CA – (a1 - a2) / (b1– b2) Where, a1 will be the shortest distance from the mesial edge of the root to the mesial edge of the un instrumented canal, b1 will be the shortest distance from distal edge of the root to the distal edge of the uninstrumented canal, a2 will be the shortest distance from the mesial edge of the root to the mesial edge of the instrumented canal, and b2 will be the shortest distance from the distal edge of the instrumented canal to distal edge of the root. For CT – A result other than 0 indicates that transportation has occurred in the canal, a negative value indicates transportation toward the distal portion, a positive value denotes transportation toward the mesial portion, and null denotes the absence of transportation. For canal CA – If these numbers were not equal, then the lower figure was considered to be the numerator of the ratio. A result of 1 indicated the optimal CA and closer the result to zero, the worse the ability of the instrument to keep itself in canal central axis. The values attained were subjected to statistical analysis. Statistical analysis The mean values of the CT and CA recorded for all the groups were analyzed using IBM SPSS Statistics for Windows Version 19 (IBM Corp., Armonk, NY, USA). The values between groups were analyzed using one way analysis of variance (ANOVA), and multiple intergroup comparisons were analyzed using the Tukey honestly significant difference (HSD) (post hoc) test. The confidence interval was set at 95%, and the P value was set for 0.05. RESULTS [Tables 1a and 1b] show the mean, standard deviation values, and one-way ANOVA and multiple intergroup comparisons (Tukey Post hoc test) of the CT and CA of groups at three levels. Among the groups, at Level I, Group III (WOG) showed a significantly lower mean transportation value compared to Group II and Group I (P = 0.04 and P = 0.01, respectively). However, there was no significant difference in CT values between Group I (PTG) and Group II (OC) (P = 0.867). Among the groups at Level II, Group III (WOG) showed the least mean transportation value (P = 0.01 and P = 0.001, respectively). However, there was no significant difference in CT values between Group III (WOG) and Group II (OC) (P = 0.867). At Level III, there was no significant difference in CT among any of the groups (P > 0.05).Table: No title available.Among the groups, at Level I, WOG Group showed a significantly higher mean CA compared to PTG Group and OC Group (P = 0.028). However, there was no significant difference in CA values between PTG Group and OC Group (P = 0.991). At Level II, WOG Group showed significantly higher mean CA compared to other groups (P = 0.001) and there was no significant difference between PTG Group and OC Group (P = 0.217). At Level III, there was no significant difference in CA among any of the groups (P > 0.763). DISCUSSION On comparing between the three groups, the least CT and the highest CA resulted in WaveOne Gold–Single-file reciprocation system followed by One Curve – Single-file continuous rotation and followed by ProTaper Gold – Sequential file continuous rotation. Thus, the null hypothesis is rejected. Moreover, for all the groups, higher mean values of CT and lowest mean values of CA were in the apical level followed by the middle and coronal level. On evaluating the cross-sectional images, the direction of transportation showed a greater tendency toward the outer curvature of the root canal with a higher tendency in curved parts of the canal in the apical and middle thirds for all three groups. Although several methods are used for assessment of shaping ability of files such as reassembly technique, radiographic comparisons, scanning electron microscope, serial sectioning technique, photographic assessment, computer manipulation, and silicone impressions of canals, the main limitation was they are two-dimensional, not accurate, invasive, and leading to loss of specimen.[9] In the present study, the CBCT tool was used to evaluate as it is noninvasive, feasibility to evaluate the anatomic structure of the root canal before and after instrumentation, and the quality of the three-dimensional images acquired by this method is superior to other techniques.[1112] The use of resin blocks and extracted natural teeth are the most common methods available for the evaluation of CT. Ideally, it should be studied using human teeth, but the standardization of teeth in terms of apical patency and the angle of curvature is difficult, whereas in simulated canals in resin blocks, the root canal diameter, length, taper, and angle of curvature are easily standardized and serve as an ideal experimental model for the analysis of the endodontic preparation technique. However, the microhardness of dentine could not be standardized.[13] According to Buchanan,[14] the amount of CT increases with apical preparation greater than size 25. Hence, the apical preparation was standardized to size 25 in the present study. In this study, the ProTaper Gold (Sequential continuous rotation) showed higher mean values of CT and lower CA. The results obtained were similar to studies by Singla et al.,[15] who reported ProTaper Gold to show higher mean values of CT and lower CA. Furthermore, similar results were reported by Jasim and Sh. Al-Gharrawi[16] and Singh et al.,[17] who reported higher mean transportation by PTG compared to the TwoShape. This may be attributed to sharp cutting flutes, the larger number of files required, clustering blades stress on the canal wall straightening curvatures, decreased flexibility with tip stiffness associated with the progressive taper, and relative larger tapers of ProTaper system.[1819] Moreover, studies by Ruddle[20] and De-Deus et al.[21] reported better shaping ability and benefits of a single-file system over full sequential file rotary systems as shorter working time, reduction in the number of instruments required to obtain the desired shape reduces cross-contamination and instrument fatigue. The one curve (single-file continuous rotation) has a marginally higher centering ratio (values were closer to 1 compared to that of PTG) and lower CT compared to ProTaper Gold which may be due to benefits of the single-file system, unique asymmetrical cutting profile, its snake-like movement into the canal, safety tip of size 25, continuous taper of 0.06, and variable pitch which reduces instrument screwing effects.[8] The results were similar to studies supporting better shaping ability of single-file continuous rotation over sequential file systems.[2223] WaveOne Gold (Single-file reciprocation system) has shown the highest mean value CA and least CT compared to other groups. These results are similar to Singla et al.,[15] and Dhingra et al.[24] but contrary to Mamede-Neto[25] who reported ProTaper Gold files to have no significant differences among the different instruments evaluated. Hence, it is valid to state that Single-file systems (WaveOne Gold and One Curve) have better shaping ability compared to the sequential file system (ProTaper Gold). On the comparison between WaveOne Gold and One Curve, WaveOne Gold has shown higher centering ratio and lesser CT with a significant difference in apical level. Jellil[26] and Vallabhaneni et al.[27] reported single-file reciprocation (WOG) to have file respected original canal anatomy than single-file continuous rotation files. However, the results were not similar to Kolhe et al.,[28] who have reported OC to have less CT and better CA compared to WOG. The performance of WaveOne Gold may probably be attributed to its reciprocating motion. The instruments when used in reciprocating motion enable more centered instrumentation and maintains original canal contour better than that when used in continuous rotating motion, especially in the apical third as aggressive continuous rotating motion tends to remove the dentine toward the outer wall of the canal.[2729] The WOG engages and cuts in a 150° counterclockwise direction and then disengages with 30° in a clockwise direction to prevent taper lock. The net file movement is a 120° cutting cycle. They have a fixed taper from D1-D3, yet a progressively decreasing percentage tapered design from D4-D16. Furthermore, it has semi-active and modified guiding tip to reduce the mass of the center and aid its penetration into any canal with a reproducible glide path. They have a parallelogram cross-section that offers two 85° cutting edges in contact with the canal wall, a patented off-centered design with only one cutting edge in contact with the canal wall, and a constant helical angle of 24° that ensures little or no screwing effect. Moreover, they are manufactured from advanced proprietary thermal process called gold process that yields super-elastic NiTi file having 2 stage A-R-M transformation.[7] Nevertheless, Wu et al.[30] have quantified 0.3 mm as the critical CT value and beyond this value, leakage was found to occur more frequently. However, none of the files have exceeded this value. The limitations in the present study could be the differences in the taper, the metallurgy of the files compared, and the microhardness of the resin model. Hence, to validate the outcomes of the present study, complex clinical studies evaluating different aspects of the files need to be done. CONCLUSION Within the limitations, it can be concluded that All the systems showed the highest CT at apical level and least CT at coronal level WOG exhibited least transportation and most centered preparation PTG (Sequential file system) exhibited highest CT and least centering compared to WOG or OC (Single-file systems) Among the Single-file systems, WOG and OC, WOG (reciprocation motion) produced the least transportation and more CA compared to OC (continuous rotation motion). Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.