Dynamic Torque and Vertical Force Analysis during Nickel-titanium Rotary Root Canal Preparation with Different Modes of Reciprocal Rotation

Effect of repetitive up-and-down movements on torque/force generation, surface defects and shaping ability of nickel-titanium rotary instruments: an ex vivo study

BACKGROUND

The screw-in effect is a tendency of a nickel-titanium (NiTi) rotary endodontic file to be pulled into the canal, which can result in a sudden increase in stress leading to instrument fracture, and over-instrumentation beyond the apex. To reduce screw-in force, repeated up-and-down movements are recommended to distribute flexural stress during instrumentation, especially in curved and constricted canals. However, there is no consensus on the optimal number of repetitions. Therefore, this study aimed to examine how repeated up-and-down movements at the working length affect torque/force generation, surface defects, and canal shaping ability of JIZAI and TruNatomy instruments.

METHODS

An original automated root canal instrumentation device was used to prepare canals and to record torque/force changes. The mesial roots of human mandibular molars with approximately 30˚ of canal curvature were selected through geometric matching using micro-computed tomography. The samples were divided into three groups according to the number of up-and-down movements at the working length (1, 3, and 6 times; n = 24 each) and subdivided according to the instruments: JIZAI (#13/0.04 taper, #25/0.04 taper, and #35/0.04 taper) or TruNatomy (#17/0.02 taper, #26/0.04 taper, and #36/0.03 tape) (n = 12 each). The design, surface defects, phase transformation temperatures, nickel-titanium ratios, torque, force, shaping ability, and surface deformation were evaluated. Data were analyzed with the Kruskal-Wallis and Dunn's tests (α = 0.05).

RESULTS

The instruments had different designs and phase transformation temperatures. The 3 and 6 up-and-down movements resulted in a smaller upward force compared to 1 movement (p < 0.05). TruNatomy generated significantly less maximum torque, force, and surface wear than JIZAI (p < 0.05). However, TruNatomy exhibited a larger canal deviation (p < 0.05). No statistical differences in shaping ability were detected between different up-and-down movements.

CONCLUSIONS

Under laboratory conditions with JIZAI and TruNatomy, a single up-and-down movement at the working length increased the screw-in force of subsequent instruments in severely curved canals in the single-length instrumentation technique. A single up-and-down movement generated more surface defects on the file when using JIZAI. TruNatomy resulted in less stress generation during instrumentation, while JIZAI better maintained the curvature of root canals.

Effect of preset torque setting on torque/force generation, shaping ability and surface changes of nickel titanium rotary instrument in different root canal curvature locations: An ex vivo study

The aim of this study was to evaluate how preset torque settings influence the torque, vertical force, and root canal-centering ability of ProGlider and ProTaper NEXT nickel-titanium rotary instruments in canals with different curvature locations. Based on micro-computed tomography, mesial roots of human mandibular molars (25°-40° curvature) were allocated to the apical curvature (apical 1-5 mm) or the middle curvature (apical 5-9 mm) groups, and mandibular incisors (curvature <5°) to the straight canal group. Each group was subjected to automated instrumentation and torque/force measurement with the preset torque of 1, 2.5, or 5 N•cm. Canal-centering ratios were determined with micro-computed tomography. Instrument fracture occurred only in the 2.5 and 5 N•cm groups in curved canals. The preset torque setting and curvature location did not influence canal shaping ability.

Dynamic torque and screw-in force of four different glide path instruments assessed in simulated single- and double-curved canals: An in vitro study

Background/purpose

The glide path instruments are the introductory instruments into the canals; thus, they should be durable enough to withstand torsional stress/screw-in force. This study aimed to investigate the torque and screw-in force of TruNatomy Glider (TN), ProGLIDER (PG), Hyflex EDM (EDM) and Dent Craft RE (RE) glide path instruments in single- and double-curved canals.

Materials and methods

Each instrument brand was divided into two groups (n = 7 each) according to the canal configuration. Torque and screw-in force were recorded during automated instrumentation of simulated resin canals with XSmart IQ cordless motor after the canal patency was checked with a #10 K-file. The values were statistically analyzed using the Kolmogorov-Smirnov test followed by the Kruskal Wallis test and the Mann-Whitney U test with Bonferroni correction (α = 0.05).

Results

TN produced significantly higher torque than RE in single-curved canals and PG in double-curved canals (P < 0.05). EDM yielded significantly higher screw-in force than TN and RE in single-curved canals (P < 0.05), but there was no significant difference in double-curved canals (P > 0.05). A significant effect of different canal configurations was only detected for screw-in force in EDM (P < 0.05).

Conclusion

TN in single-curved canals and RE in double-curved canals yielded higher torque values, while EDM exhibited greater screw-in force in both canal configurations. No effect of different canal configurations was detected for torque, but a significant impact was detected for screw-in force in EDM.

Health prediction of reciprocating endodontic instrument based on the machine learning and exponential degradation models

This study proposes an intelligent health prediction and fault prognosis of the endodontic file during the root canal treatment. Root canal treatment is the procedure of disinfecting the infected pulp through the canal with the help of an endodontic instrument. Force signals are acquired with the help of a dynamometer during the canal preparation, and statistical features are extracted. The extracted features are selected through the window-wise feature extraction process. Characteristic features for endodontic file prognostics include time-domain features of the signals are evaluated. The extracted feature has inappropriate information, that is, noise between the signals; hence the smoothing of the feature is required at this stage to observe a trend in the signals. Based on the smoothing feature and post-processing of the feature, defined the health index to calculate the health condition of the endodontic instruments. A machine learning algorithm and exponential degradation model are used to predict the health of the endodontic instrument during the root canal treatment. This model is used to forecast the degradation of the endodontic file so that actions can be taken before actual failures happen. The proposed methodology can analyze the failures and micro-crack initiation of the endodontic instruments. Endodontics practitioners can use the machine learning models as well as an exponential model for estimating the health condition of the endodontic instrument. This study may help the clinician to progress the efficiency of the root canal treatment and the competence of the endodontic instruments.

Mechanical Properties and Root Canal Shaping Ability of a Nickel-Titanium Rotary System for Minimally Invasive Endodontic Treatment: A Comparative In Vitro Study

Selection of an appropriate nickel−titanium (NiTi) rotary system is important for minimally invasive endodontic treatment, which aims to preserve as much root canal dentin as possible. This study aimed to evaluate selected mechanical properties and the root canal shaping ability of TruNatomy (TRN), a NiTi rotary system designed for minimally invasive endodontic shaping, in comparison with existing instruments: HyFlex EDM (HEDM), ProTaper Next (PTN), and WaveOne Gold (WOG). Load values measured with a cantilever bending test were ranked as TRN < HEDM < WOG < PTN (p < 0.05). A dynamic cyclic fatigue test revealed that the number of cycles to fracture was ranked as HEDM > WOG > TRN > PTN (p < 0.05). Torque and vertical force generated during instrumentation of J-shaped artificial resin canals were measured using an automated instrumentation device connected to a torque and vertical force measuring system; TRN exhibited smaller torque and vertical force values in most comparisons with the other instruments. The canal centering ratio for TRN was smaller than or comparable to that for the other instruments except for WOG at the apex level. Under the present experimental conditions, TRN showed higher flexibility and lower torque and vertical force values than the other instruments.

Effect of Rotational Modes on Torque/Force Generation and Canal Centering Ability during Rotary Root Canal Instrumentation with Differently Heat-Treated Nickel-Titanium Instruments

This study aimed to evaluate how various rotational modes influence the torque/force production and shaping ability of ProTaper Universal (PTU; non-heat-treated) and ProTaper Gold (PTG; heat-treated) nickel−titanium instruments. J-shaped resin canals were instrumented with PTU or PTG using an automated instrumentation device operated with reciprocating rotation [150° clockwise and 30° counterclockwise (R150/30) or 240° clockwise and 120° counterclockwise (R240/120)], optimum torque reverse motion (OTR), or continuous rotation (CR) (n = 10 each). Maximum force and torque were recorded, and canal centering ratios were calculated. Statistical analysis was performed with two-way ANOVA and a Bonferroni test (α = 0.05). The results were considered with reference to previous studies on the microstructure of the instruments. The upward force generated by R240/120 and OTR was smaller than that generated by R150/30 and CR in PTG (p < 0.05). The clockwise torque produced by OTR was lower than that produced by R150/30 in PTU and R240/120 and CR in PTG (p < 0.05). R240/120 and OTR induced less canal deviation compared to CR in PTU at 0 mm from the apex (p < 0.05). In conclusion, R240/120 and OTR reduced the screw-in force in PTG and improved the canal centering ability in PTU, which may be associated with the heat treatment-induced microstructural difference of the two instruments.

Effect of Different Downward Loads on Canal Centering Ability, Vertical Force, and Torque Generation during Nickel-Titanium Rotary Instrumentation

This study aimed to examine how downward loads influence the torque/force and shaping outcome of ProTaper NEXT (PTN) rotary instrumentation. PTN X1, X2, and X3 were used to prepare J-shaped resin canals employing a load-controlled automated instrumentation and torque/force measuring device. Depending on the torque values, the handpiece was programmed to move as follows: up and down; downward at a preset downward load of 1 N, 2 N or 3 N (Group 1N, 2N, and 3N, respectively; each n = 10); or upward. The torque/force values and instrumentation time were recorded, and the canal centering ratio was calculated. The results were analyzed using a two-way or one-way analysis of variance and the Tukey test (α = 0.05). At the apex level, Group 3N exhibited the least canal deviation among the three groups (p < 0.05). The downward force was Group 3N > Group 2N > Group 1N (p < 0.05). The upward force, representing the screw-in force, was Group 3N > Group 1N (p < 0.05). The total instrumentation time was Group 1N > Group 3N (p < 0.05). In conclusion, increasing the downward load during PTN rotary instrumentation improved the canal centering ability, reduced the instrumentation time, and increased the upward force.

Improvement and analysis of mechanistic modeling of root canal preparation by a computer-based method

BACKGROUND AND OBJECTIVE

Root canal preparation is a cutting process between nickel-titanium (Ni-Ti) file and root canal, which aims to remove the bacteria and to keep teeth from infection. A mechanistic model in root canal preparation is proposed to investigate the mechanical mechanism of Ni-Ti file, which is essential to prevent physical and thermal damage on root canal.

METHODS

First, the mathematic modeling is introduced based on oblique cutting theory, which the loading condition of Ni-Ti file is derived from each cutting element by expressing a function of geometric parameters. For the modeling improvement, a cutting simulation algorithm (CSA) based on Boolean operation is proposed to achieve the complicated cutting situation between root canal and Ni-Ti file instantaneously. After establishment of model, the predictive precision is verified by conducting in vitro experiments. Eighteen artificial root canals were prepared in 6 mm straight and 2 mm C-shaped curved specification with 0.3 mm diameter, which was a single canal for each position, all the canals do not have connections with each other. During experiments, root canals were prepared using Wave One Gold (GWO) instruments with reciprocating rotational motion. Different influential factors (curvatures of root canal and movements of Ni-Ti file) and cutting parameters (feed rate and spindle speed) were analyzed by conducting a series of simulations under the mechanistic model.

RESULTS

Experiment results show that the predictive error of thrust force based on the proposal model is around 15%. The thrust force will increase dramatically after Ni-Ti file gets into craved canal. It can be indicated that the curvatures of root canal, movements of Ni-Ti file have a strong influence on root canal preparation. 20° increasement of curved degrees can lead to the 0.73 N increase of thrust force, while pecking movement can decrease 19.88% of thrust force compared with continues one. Furthermore, investigation on pecking distance represent that 2-1 mm movement can effectively reduce the thrust force of 15.82% compared to 4-2 mm movement.

CONCLUSION

Based on simulation results, 2-1 mm pecking movement is recommended for dentists compared with 4-2 mm pecking movement or continues movement. In addition, this paper provides a novel insight of interactive mechanism between Ni-Ti file and root canal, so as to contribute to both the theoretical and practical research by elucidating mechanisms and providing quantitative predictions that can be validated. Compared with conventional analytical model, both calculated precision and efficiency are improved in the proposed model.

Effect of kinematics on the torque/force generation, surface characteristics, and shaping ability of a nickel titanium rotary glide path instrument: an ex-vivo study

AIM

To evaluate the effect of various rotational motions on the torque/force generation, surface wear, and shaping ability of the ProGlider glide path instrument (Dentsply Sirona).

METHODOLOGY

Mesiobuccal and mesiolingual canals of mandibular molars were selected based on the canal volume, length, angle of curvature (25°-40°), and radius of curvature (4-8 mm) after micro-computed tomographic scanning. The samples were randomly assigned to four groups (n = 13, each) according to movement kinematics [continuous rotation (CR; 300 rpm), optimum torque reverse motion (OTR; 180° forward and 90° reverse when torque > 0.4 N·cm), time-dependent reciprocal motion (TmR; 180° forward and 90° reverse), and optimum glide path motion (OGP; a combination of 90° forward, 90° reverse, 90° forward, and 120° reverse)]. Instrumentation was performed with an automated root canal instrument and torque/force analysing device. Maximum torque/force values, canal volume changes, and canal-centring ratios at 1, 3, 5, and 7 mm were evaluated. Surface defects (pits, grooves, microcracks, blunt cutting edges, and disruption of cutting edges) and spiral distortion on the ProGlider instrument were scored at the tip and 5 mm short of the tip before and after five consecutive uses with scanning electron microscopy. The Kruskal-Wallis test followed by Dunn's post-test with Bonferroni correction and Wilcoxon signed-rank test were used to analyse the data (α = 0.05).

RESULTS

OGP generated significantly less clockwise torque and greater upward force than other groups (P < 0.05). OGP resulted in significantly fewer surface defects than CR (P < 0.05). In OGP and CR, the tip exhibited more surface defects than 5 mm short of the tip (P < 0.05). CR resulted in greater volume changes than OGP and TmR (P < 0.05) and greater centring ratios (i.e., more deviation) than OGP at 1 mm and OTR at 3 mm (P < 0.05).

CONCLUSIONS

Under laboratory conditions using the ProGlider instrument, OGP generated significantly less clockwise torque and greater upward force than the other rotatory motions. OGP generated fewer superficial defects than CR, and the three modes of reciprocal rotation better maintained the apical curvature of root canals than CR with the ProGlider instrument.

Impact of Radial Lands on the Reduction of Torque/Force Generation of a Heat-Treated Nickel-Titanium Rotary Instrument

This study investigated the impact of a one-sided radial-landed cross-sectional design of a heat-treated nickel-titanium rotary instrument (JIZAI, MANI, Japan; JZ) on torque/force generation and canal-shaping ability, using an experimental non-landed instrument (non-landed JZ) for comparison. Both instruments had tip sizes of 25 and 0.04 or 0.06 taper and were similar in metallurgy and geometry, except for the presence/absence of a radial land. Twenty J-shaped simulated resin canals were instrumented in a two-instrument single-length sequence using an automated root canal instrumentation device with a torque/force analyzing unit. Pre- and post-instrumentation images of the resin canals were analyzed for canal-centering ability at 0–3 mm from the apex. The mean centering ratio was not significantly different between JZ and non-landed JZ (p > 0.05). In the 2nd instrumentation, JZ showed a significantly smaller torque compared with the non-landed JZ (p < 0.05). Regardless of instrumentation sequence, JZ showed a significantly smaller maximum upward force, representing screw-in force (p < 0.05), and a larger maximum downward force (p < 0.05) than the non-landed JZ. JZ generated smaller screw-in forces and had similar canal-centering ability compared with the non-landed JZ.

Present Status and Future Directions - Canal Shaping

This narrative review will focus on the evolution, present and future of engine‐driven root canal preparation. Root canal preparation changed drastically when Walia in 1988 introduced the use of nickel‐titanium (NiTi) alloys in Endodontics. In 2013, five generations of NiTi endodontic instruments had been established based on their metallurgical, mechanical properties and design features. Since then, manufacturers have been introducing further major changes in instrument design and characteristics that have not been translated in new recognized generations of instruments. In general, those changes have demonstrated enhanced instrument properties, but it is not clear yet if all those improvements are directly translated to an improvement in clinical success. This narrative review attempts to address the present status of engine‐driven instruments in terms of both evidence from laboratory‐based studies and clinical data, to identify potential further generations of instruments, and last to anticipate future directions for research and development.

Kinematics of 'Adaptive Motion' under constant torque values

INTRODUCTION

To demonstrate the unknown kinematics of complex adaptive motion with respect to disclosed values, the aim of the present study was (i)to analyze the adaptive motion and (ii)to compare the effects of constant torque values on kinematics.

METHODS

A new endodontic motor(Aseptico AEU-28SYB Elements) and 8:1 contra-angle handpiece were used for analysis. A pilot test was conducted for synchronizing the external experimental stress on the shaft of the target object and the inbuilt torque sensor of the motor source. A load setup was developed by attaching universal precision weights to the shaft of the of the target object. The groups were adaptive motion with no load(TFA-0), adaptive motion with 100 gcm(TFA-100), and 200 gcm(TFA-200) constant load. The peak-to-peak based phase identification method was used to analyze the kinematics. Student's t and Mann Whitney U tests were used where applicable(α=0.05).

RESULTS

A noteworthy observation was the inconsistent kinematics of the TFA-100 compared to the kinematics of TFA-0 and TFA-200. TFA-100 presented mixed-phased kinematics, which consisted of both quaternary(n=33) phases similar to that of TFA-200, and dual(n=17) phases similar to that of TFA-0. The sequence of these quaternary and dual phases of TFA-100 was arbitrarily changing. The kinematic parameters of dual-phase intermittent motion of TFA-0 and TFA-100 and quaternary-phase reciprocating motion of TFA-100 and TFA-200 revealed significant differences(p<0.05).

CONCLUSIONS

The main conclusions of the present study are the kinematics of adaptive motion was demonstrated and the 100 gcm load is not sufficient for predictable and constant operation in reciprocating mode.

An Update on Nickel-Titanium Rotary Instruments in Endodontics: Mechanical Characteristics, Testing and Future Perspective-An Overview

Since the introduction of Nickel-Titanium alloy as the material of choice for the manufacturing of endodontic rotary instruments, the success rate of the root canal therapies has been significantly increased. This success mainly arises from the properties of the Nickel-Titanium alloy: the biocompatibility, the superelasticity and the shape memory effect. Those characteristics have led to a reduction in time of endodontic treatments, a simplification of instrumentation procedures and an increase of predictability and effectiveness of endodontic treatments. Nevertheless, the intracanal separation of Nickel-Titanium rotary instruments is still a major concern of endodontists, with a consequent possible reduction in the outcome rate. As thoroughly demonstrated, the two main causes of intracanal separation of endodontic instruments are the cyclic fatigue and the torsional loads. As results, in order to reduce the percentage of intracanal separation research and manufacturers have been focused on the parameters that directly or indirectly influence mechanical properties of endodontic rotary instruments. This review describes the current state of the art regarding the Nickel-Titanium alloy in endodontics, the mechanical behavior of endodontic rotary instruments and the relative stresses acting on them during intracanal instrumentation, highlighting the limitation of the current literature.

Force and vibration analysis in biomechanical preparation of root canals using reciprocating endodontic file system: In vitro study

The shaping and cleaning of the root canal are very important in root canal treatment. The excessive force and vibration during biomechanical preparation of the root canal may result in failure of the endodontic file. In this study, force and vibration analysis was carried out during root canal preparation. The samples of human extracted (premolar) teeth were provided by the College of Dental Science and Hospital. Endodontic instruments for reciprocating motion, such as the WaveOne Gold file system, had been used for root canal preparation. Force and vibration signals were recorded by dynamometer and accelerometer, respectively. The acquired signals were denoised using the db4 (SWT denoising 1-D) wavelet. Four levels of decomposition were carried out for each signal. The signal denoising technique was used to remove unwanted noise from the acquired signal. FESEM analysis was used to visualize the levels of severity of endodontic files during the cleaning and shaping of the root canal. In most of the cases, the failure occurred due to the improper use of the root canal instrumentation. The optimum amount of force was used to avoid the file failure and provided the proper instrumentation. The curve fitting regression model was used to find the interdependency between force and vibration.

Influence of rotational speed on torque/force generation and shaping ability during root canal instrumentation of extracted teeth with continuous rotation and optimum torque reverse motion

AIM

To evaluate how different rotational speeds affect the torque/force generation and shaping ability of rotary root canal instrumentation using JIZAI (MANI, Utsunomiya, Japan) nickel-titanium instruments in continuous rotation and optimum torque reverse (OTR) motion.

METHODOLOGY

Mesial root canals of extracted mandibular molars were instrumented up to size 25, 0.04 taper using JIZAI instruments, and anatomically matched canals were selected based on geometric features of the canal [canal volume (mm³ ), surface area (mm² ), length, 15°-20° curvature and radius of curvature (4-8 mm)] after micro-computed tomographic scanning. An automated root canal instrumentation and torque/force analysing device was programmed to permit a simulated pecking motion (2 s downward and 1 s upward at 50 mm min^-1 ). The selected canals were prepared with size 25, 0.06 taper JIZAI instruments using continuous rotation or OTR motion and further subdivided according to the rotational speed (300 or 500 rpm, n = 10 each). Real-time clockwise/counterclockwise torque and downward/upward force were recorded using a custom-made torque/force analysing device. Then, the registered pre- and post-operative micro-computed tomographic datasets were examined to evaluate the canal volume changes and centring ratios at 1, 3, 5 and 7 mm from the apical foramen. Data were analysed using two-way analysis of variance or the Kruskal-Wallis test and Mann-Whitney U test with Bonferroni correction (α = 5%).

RESULTS

Maximum upward force and clockwise torque were significantly smaller in 500 rpm groups than in 300 rpm groups (P < .05); however, no significant difference was found between continuous rotation and OTR motion (P > .05). OTR motion developed higher maximum counterclockwise torque than continuous rotation (P < .05). Maximum downward force, canal volume changes and centring ratios were not significantly different among all groups (P > .05). There was no file fracture in any of the groups.

CONCLUSIONS

Under laboratory conditions using JIZAI instruments, a rotational speed of 500 rpm generated significantly lower maximum screw-in forces and torque values than rotational speed of 300 rpm. Continuous rotation and OTR motion performed similarly in shaping the canals.

Analysis of Torque and Force Induced by Rotary Nickel-Titanium Instruments during Root Canal Preparation: A Systematic Review

The aim of this review was to provide a detailed literature analysis of torque and force generation during nickel-titanium rotary root canal instrumentation. We followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. An electronic search was performed using in PubMed and in journals for articles published in English from 1987 to June 2020 on studies that investigated dynamic torque and force in vivo or in vitro. We assessed article titles and abstracts to remove duplicates, and the titles and abstracts of the remaining articles were screened for eligibility. Full texts were read to verify eligibility by considering predetermined inclusion and exclusion criteria. Fifty-two out of 4096 studies met the inclusion criteria, from which we identified 26 factors that influence torque or force generation. Factors associated with higher torque or force generation and supported by multiple studies with mostly consistent results included convex triangle cross-sectional design, regressive taper, short pitch length, large instrument size, small canal size, single-length preparation technique, long preparation time, deep insertion depth, low rate of insertion, continuous rotation (torque), reciprocating motion (force), lower rotational speed and conventional alloy. However, several factors are interrelated, which obscured the independent effect of each factor, and there was insufficient scientific evidence supporting the influence of some factors.

Comparison of canal transportation and centering ability of manual K-files and reciprocating files in glide path preparation: a micro-computed tomography study of constricted canals

Background

Optimum Glide Path (OGP) is a new reciprocating motion aiming to perform efficient glide path preparation in constricted canals. The aim of this study was to investigate and compare manual and OGP movement in terms of canal transportation and centering ability in glide path preparation of constricted canals.

Methods

Thirty constricted mesial root canals of mandibular molars, with initial apical size no larger than ISO#8, were selected and negotiated with #6–#8 K-files under the microscope. Canals were randomly divided into two experimental groups: Group 1 (MAN, n = 15): Glide path was established by using #10-#15 stainless steel K-files manually; Group 2 (OGP, n = 15): #10-#15 Mechanical Glide Path super-files were used with OGP motion (OGP 90°, 300 rpm). Each instrument was used to prepare only 2 canals (as in one mesial root). Canals were scanned before and after glide path preparation with micro-computed tomography (micro-CT) to evaluate root canal transportation and centering ratio at 1, 3 and 5 mm levels from the root apex. File distortions and separations were recorded. Paired t-test was used to statistically evaluate the data (P < .05).

Results

Group 2 showed a significantly lower transportation value than group 1 at 1-mm and 3-mm levels (P < .05), however the difference at 5-mm level was not significant. There was no significant difference regarding the centering ratio between the groups. Six #10 K-files were severely distorted in group 1, while no file separation or distortion was found in group 2.

Conclusions

OGP motion performed significantly less canal transportation (apical 3 mm) and file distortion during glide path establishment in constricted canals comparing to manual motion, while the centering ability between the two was similar.

Clinical relevance

OGP reciprocating motion provides a safer and efficient clinical approach compared to traditional manual motion in glide path establishment with small files in constricted canals.

Comparative evaluation of mechanical properties and shaping performance of heat-treated nickel titanium rotary instruments used in the single-length technique

This study aimed to evaluate the mechanical properties of contemporary heat-treated nickel-titanium (NiTi) rotary instruments used in the single-length technique [ProTaper Next (PTN), HyFlex EDM (EDM), and JIZAI (JZ)]. Bending loads, cyclic fatigue resistance, torque/force values and canal-centering ratios were evaluated for the three instruments and a non-heat-treated experimental NiTi instrument with the same geometry as JZ (nJZ). EDM and JZ exhibited significantly lower bending load and more cycles to failure compared with nJZ and PTN (p<0.05). PTN and JZ exhibited significantly better centering ability than nJZ and EDM (p<0.05). JZ and nJZ generated significantly smaller upward force and maximum torque than PTN and EDM (p<0.05). Under the present experimental condition, JZ exhibited flexibility and cyclic fatigue resistance comparable to EDM, better maintained the canal curvature than the other instruments, and generated smaller torque and screw-in force than PTN and EDM.

Development of Innovative Contra-angle Handpiece Device with Piston Movement for Root Canal Preparation

INTRODUCTION

The purpose of this study was to assess the optimal amplitude and weight of the newly developed contra-angle handpiece. The handpiece uses piston movement without using an endodontic motor and enables a safe, quick, and reliable canal preparation.

METHODS

A prototype handpiece was designed. Instrumentation was performed on root canal resin blocks by 20 operators in 3 groups: the prototype handpiece with an H file (a stainless steel #25 manual H file, the piston group), a manually standardized technique with a K file (stainless steel #15-25 K files, the manual group), and a nickel-titanium (NiTi) reciprocating file with an endodontic motor (Reciproc Blue R25 [VDW, Munich, Germany], the NiTi group). Transportation of the canal center line and the time required for preparation were measured and statistically analyzed.

RESULTS

The optimal condition was an amplitude of 1.35 mm and a weight of 61.0 g. Transportation of the canal center was observed in all groups. A statistically significant difference was found at 2.0-3.0 mm from the apical foramen between the piston or NiTi group and the manual group, but no significant difference was found between the piston and NiTi groups. The least transportation was found in the NiTi and piston groups. The handpiece with a #25 H file demonstrated a good centering ability, similar to the NiTi file, which enabled speedy preparation. The time required for preparation between the piston or NiTi group and the manual group was statistically different. No significant difference was observed between the piston and NiTi groups (P < .05).

CONCLUSIONS

We concluded that the newly designed handpiece achieved efficient canal preparation and negotiation. The handpiece could avoid endodontic accidents, including ledge formation, instrument separation, and perforation.

Dynamic torque analysis of rotary and reciprocating instruments during root canal instrumentation in simulated canals by an endodontist or a general dentist

Background:

Excessive torque is associated with engine-driven file fracture.

Aims:

The aim of this study to evaluate the real-time torque of rotary and reciprocating instruments, working time, and the occurrence of procedural errors during root canal preparation of simulated canals by an endodontist and a general dentist.

Methods:

Thirty-six commercially available simulated “J-shaped” root canals in resin blocks were used. Instrumentation was performed using WaveOne, WaveOne Gold, ProTaper Next, Reciproc, Reciproc Blue, and Mtwo. The real-time torque analysis and the number of times the maximum torque applied to the instrument were evaluated. Images were obtained to assess the occurrence of procedural errors, and working time was recorded.

Statistical Analysis:

The one-way analysis of variance with a Bonferroni post hoc test, Mann Whitney test and the t-test was used for statistical analysis (P < 0.05).

Results:

Reciprocating instruments showed lower values in the number of times that reached maximum torque and percentage time in the area of critical torque, with significant differences compared to rotary instruments (P < 0.05). Operators influenced torque values only with rotary motion instruments. There was no significant difference in mean working time between the operators or instruments. No fracture of instruments or canal transportation occurred.

Conclusions:

Rotary instruments were associated with higher peaks in real-time torque variation during the preparation of simulated root canals.

Influence of shaft length on torsional behavior of endodontic nickel-titanium instruments

Torsional stresses are one of the most frequent causes of intracanal separation of nickel–titanium endodontic instruments, which represents a great concern of endodontists. For this reason, torsional resistance of rotary instruments has been deeply investigated by determining all parameters that can influenced it, that can be summarized in: (1) Tooth-related factors, (2) Strategy-related factors and (3) Instrument-related factors. This study was conducted to examine the influence of shaft length on torsional resistance of a nickel–titanium rotary instrument and if it should be considered as an Instrument-related factor. With this aim, 120 Twisted Files Adaptive M-L (TFA M-L) NiTi instruments (SybronEndo, Orange, CA, USA) were divided into 6 experimental groups (n = 20), according to instruments length and size: Group 1, 20 TFA M-L1 25/08 23 mm; Group 2, 20 TFA M-L1 25/08 27 mm; Group 3, 20 TFA M-L2 35/06 23 mm; Group 4, 20 TFA M-L2 35/06 27 mm; Group 5, 20 TFA M-L3 50/04 23 mm; and Group 6, 20 TFA M-L3 50/04 27 mm. All instruments were submitted to a static torsional test, blocking each instrument at 3 mm from the tip and rotating it until its fracture. Torque to Fracture (TtF) and fragments length were registered and all data were statistically analyzed. Results showed that Groups 2, 4 and 6 had a higher TtF, respectively, than Groups 1, 3 and 5, which differ from the former just for shaft length. Group 6 showed the highest torsional resistance (1.31 ± 0.08 Ncm), whilst Group 1 the lowest (0.40 ± 0.08 Ncm). According to that, it can be stated that the longer the instrument, the higher the torsional resistance is, proving that shaft length should be considered as an important factor about torsional resistance.

Effect of Flexural Stress on Torsional Resistance of NiTi Instruments

INTRODUCTION

Previously published studies have deeply investigated the characteristics of flexural and torsional resistance of nickel-titanium rotary instruments, but none of them investigated the relationship between the 2 stresses. The objective of the present study was to evaluate the influence of flexural stresses over torsional resistance.

METHODS

Sixty S-One 20.04 files (Fanta Dental, Shanghai, China) were used in the present study (N = 60) and divided into 3 test groups of 20 files. A customized device made of a mobile structure with a connection that holds the handpiece and the artificial canal was used for the experiment to make the measurements repeatable. The artificial canals were created with a 90° curvature, a 60° curvature, and lastly a straight canal. Each file was rotated at 300 rpm with a maximum torque value of 5.5 Ncm with the apical 2 mm firmly secured in a vise. The torque at fracture and the time to fracture were recorded by the software integrated in the handpiece and evaluated through statistical analysis.

RESULTS

Statistical analysis found significant differences in the values of torque to fracture (TtF) between these 3 groups (P < .05). The 90° curved canal group showed the highest TtF value, and the 60° curved canal group showed a higher TtF value than the straight canal group.

CONCLUSIONS

The results of the present study demonstrated a positive influence of flexural stresses over torque at fracture of rotary files. When nickel-titanium instruments were used in a 90° or 60° curvature, the values of torque at fracture increased compared with the same instruments that rotated in the straight canal.

Comparison of Torque, Screw-in Force, and Shaping Ability of Glide Path Instruments in Continuous Rotation and Optimum Glide Path Motion

INTRODUCTION

This study aimed to analyze torque/force generation and transportation in double-curved canals instrumented with 3 types of glide path files using optimum glide path (OGP) motion in comparison with continuous rotation.

METHODS

Sixty simulated double-curved canals were prepared with #10/0.05 or #15/0.03 HyFlex EDM Glidepath files (Coltene/Whaledent, Altstätten, Switzerland) or a #13/0.04 prototype MANI Glidepath file (Tochigi, Japan) using OGP motion or continuous rotation (n = 10 each). Canals were sequentially prepared to 20 mm and 22 mm (full working length) using automated root canal instrumentation and a torque/force analyzing device. Transportation was calculated at 1-9 mm from the apex. Data were compared using 2-way analysis of variance followed by a post hoc simple main effect test with Bonferroni correction and a Kruskal-Wallis test (α = 5%).

RESULTS

All #10/0.05 instruments fractured. In the 22-mm preparation, the OGP motion resulted in lower clockwise torque and screw-in force than did continuous rotation (P < .05). In the 20-mm preparation, #15/0.03 instruments recorded a lower screw-in force for OGP motion than for continuous rotation (P < .05). Comparing the 2 preparation phases, OGP motion generated no significant differences; however, continuous rotation developed higher clockwise torque and screw-in force in the 22-mm preparation than in the 20-mm preparation (P < .05). There was no significant difference among the tested groups for transportation values.

CONCLUSIONS

Compared with continuous rotation, OGP motion generated less screw-in force, lower clockwise torque, and similar transportation. The #15/0.03 HyFlex EDM instrument and the #13/0.04 prototype MANI instrument performed similarly well.

Torque Range, a New Parameter to Evaluate New and Used Instrument Safety

The aim of the present study is to evaluate operative torque, torque at failure and the difference between these two values—the “torque range”—of two different NiTi files. We also sought to evaluate and compare these parameters for new and used files. Forty S-One 20.06 and forty M-Two 20.06 were tested, divided in four equal groups (n = 10) for each brand. Ten instruments from each brand performed three root canal treatments each on standardized extracted single-rooted teeth. Afterwards, each group was subjected to the following two tests: operative torque and torsional resistance. Mean values for all the tested groups were calculated. The results for new instruments showed that S-One developed significantly higher operative torque, but higher torsional resistance. The results for used instruments showed that the S-One 20.06 developed less operative torque and higher mean torsional resistance value compared to the M-Two. Moreover, the percentage reduction of both values was significantly higher for M-Two. The results of the present study showed a safer torsional behavior of the S-One. These results could be related to the heat treatment and the manufacturing process.

Ex-Vivo Comparison of Torsional Stress on Nickel-Titanium Instruments Activated by Continuous Rotation or Adaptive Motion

This study aimed to evaluate the effect of adaptive motion applied to conventional nickel-titanium (NiTi) rotary instruments on torsional stress generation during shaping procedure. One hundred and twenty mesio-buccal canals of molars were randomly assigned to two groups according to the kinetics; adaptive motion (AD) and continuous rotation (CR). Each group was divided into four subgroups (n = 15) according to the NiTi instrument systems: HyFlex EDM, One Curve, Twisted File Adaptive, and ProTaper Next. A glide path was established with PathFile #1, for each file group being used with either of the kinetic movements. During the instrumentation with the designated motion and file system, the generated torque was measured via the control unit and acquisition module. Based on the acquired data, the maximum and total torque were calculated. The data were statistically analyzed using Kruskal–Wallis and Mann–Whitney tests at a significance level of 95%. The maximum and total torque generated by all instruments were significantly reduced by the adaptive motion (p < 0.05). In the CR group, HyFlex EDM generated the highest maximum and total stress. In the AD group, HyFlex EDM showed the highest maximum torsional stress, and One Curve showed the highest total torsional stress (p < 0.05). The TF Adaptive instrument with adaptive movement produced the lowest maximum and total torsional stress (p < 0.05). Under the conditions of this study, the use of adaptive motion would be useful to reduce the torsional stress of instrument and root dentin. The reduction of torsional stress through adaptive motion may enhance the durability of instruments and reduce the potential risk of dentinal cracks.

Assessment of mechanical properties of WaveOne Gold Primary reciprocating instruments

This study aimed to evaluate cyclic fatigue resistance and bending properties and torque/force generation of WaveOne Gold (WOG) Primary in comparison with WaveOne (WO) Primary and Reciproc (RE) R25. A cyclic fatigue test revealed that the WOG Primary took significantly longer time to fracture compared with the WO Primary (p<0.05). The WOG Primary had the smallest load values at a deflection of 0.5 and 2 mm (p<0.05), as measured with a cantilever bending test. Torque/force measurement demonstrated that maximum upward force and maximum counterclockwise torque values in the WOG Primary were significantly lower than those in the RE R25 (p<0.05). Under the present experimental condition, the WOG Primary showed a higher cyclic fatigue resistance compared with the WO Primary, a higher flexibility compared with the WO Primary and RE R25, and generated a significantly lower maximum torque compared with the RE R25.