Effects of Side Flattening on Torsional and Cyclic Fracture Resistance of Nickel-Titanium File

INTRODUCTION

The purpose of this study was to evaluate the effect of side flattening of cutting flutes on the cyclic resistance and torsional resistance of nickel-titanium files.

METHODS

Both novel flattened Platinum V.EU (PL) and standard nonflattened CC Premium V.EU (CC) rotaries were tested. For cyclic fatigue tests, all the files were rotated in an artificial root canal with a curvature of 45° and a radius of 6.06 mm at 300 rpm (n = 15 in each group). The number of cycles to failure (NCF) was calculated. For torsional tests, the files were rotated at 2 rpm clockwise until fracture occurred. The maximum torque value at fracture was measured and the toughness and distortion angle were computed. Subsequently, 5 fragments were randomly selected in each experiment, the cross-section and longitudinal direction of the fragments were photographed using a scanning electron microscope. An unpaired t-test was performed at a significance level of 95%.

RESULTS

There was a statistically significant difference in NCF between CC and PL (P < .05). CC showed higher NCF than PL. There was no statistically significant difference between CC and PL with regards to the parameters related to torsional resistance (distortion angle, ultimate strength, and toughness) (P > .05).

CONCLUSION

Within the limitations of this study, side flattening of the file did not improve cyclic resistance or torsional resistance of the files. As side flattening may reduce a file's cyclic resistance, such files should be used with caution in clinical practice.

Effect of MT Technology of Heat Treatment on Reciproc: Comparison of Reciproc, Reciproc Blue, and Reciproc MT

INTRODUCTION

This study aimed to evaluate the effects from the memory-triple (MT) heat treatment on the fatigue resistance of the Reciproc by comparison with the file systems of same geometry.

METHODS

Reciproc files subjected to MT heat treatment technology were designated as Group RMT and were compared with the original Reciproc (Group REC) and Reciproc Blue (Group REB). Each NiTi file from 3 groups (n = 15) was operated reciprocally with a repetitive up-and-down movement in the curved canal with 4 mm of pecking distance inside of the simulated canal at body temperature. When each file fractured, the time until fracture was recorded. The length of the fractured fragment was measured. Fractured fragments were observed under scanning electron microscope (SEM) to evaluate the topographic features of the surface. Differential scanning calorimetry (DSC) analysis was performed to estimate phase transformation temperatures. One-way analysis of variance and Duncan post hoc comparison were applied to compare among the groups at a significance level of 95%.

RESULTS

RMT showed significantly higher fracture resistance (P < .05), whereas there was no difference in fatigue resistance between REC and REB. SEM examination showed the files from the 3 groups had similar topographic features. RMT showed a peak of austenite peak (Ap) at a temperature (52°C) higher than body temperature, whereas REC and REB showed Ap at 37 and 32°C, respectively.

CONCLUSIONS

Under the condition of this study, the new heat treatment technique of MT technology could enhance the fatigue fracture resistance of the reciprocating files made of M-wire and Blue-wire.

The effect of ibuprofen sustained release oral premedication on intraoperative and postoperative pain: A randomised clinical trial

The aim of this study was to assess the effect of ibuprofen sustained release (SR) oral premedication on the efficacy of buccal infiltration (BI) with intraoperative and postoperative pain after single-visit root canal treatment. Sixty patients diagnosed with symptomatic irreversible pulpitis and apical periodontitis in mandibular molar were divided into two groups. Group SR received ibuprofen SR 800 mg and group PL received placebo capsule 1 h before 3.6 mL articaine BI injection. Pain was recorded using a modified visual analogue scale and postoperatively at intervals 6, 24 and 48 h. Group SR showed a significantly higher anaesthetic success rate (73.3%) compared to group PL (46.7%) (p < 0.05). Intraoperative and postoperative pain was significantly higher in group PL compared to group SR (p < 0.05). Premedication of ibuprofen SR improved the efficacy of primary BI in mandibular molars with symptomatic irreversible pulpitis and decreased postoperative pain at 6 and 48 h.

Effect of Heat Treatment on Mechanical Properties of Nickel-Titanium Instruments

INTRODUCTION

The aim of this study was to evaluate the torsional resistance, cyclic fatigue resistance, and bending stiffness of nickel-titanium (NiTi) file systems with different heat treatments and cross-sectional designs.

METHODS

WaveOne Primary treated with memory-triple (MT) heat treatment (WOMT) was compared with WaveOne Primary (WO) and WaveOne Gold Primary (WOG). Torsional resistance test was performed using a customized device, and the distortion angle, ultimate strength, and toughness were evaluated. For cyclic fatigue resistance test, the instruments were reciprocated with continuous 4 mm up-and-down movement until fracture in a customized device, and the time to fracture was compared. Fracture surfaces of each group were examined under the scanning electron microscope. Bending stiffness was measured using a custom-made device. The results were analyzed using one-way analysis of variance and the Tukey's post hoc comparison at a significance level of 95%.

RESULTS

WOMT showed higher ultimate strength and toughness than the other systems (P < .05). WOMT also showed highest cyclic fatigue resistance among the tested groups (P < .05). WO had the highest bending stiffness than others, whereas WOMT had a larger residual angle than others (P < .05).

CONCLUSIONS

This new MT heat treatment technique makes NiTi file more flexible and improves its mechanical properties. In addition, the effect of heat treatment on flexibility was found to be more significant than that of the cross-sectional area.

Chemomechanical Properties and Biocompatibility of Various Premixed Putty-type Bioactive Ceramic Cements

INTRODUCTION

This study aimed to evaluate the chemomechanical properties and biocompatibility of recently introduced premixed putty-type bioactive ceramic cements (PPBCs).

METHODS

Including ProRoot MTA (PMTA) as a control, BC RRM fast-set putty (BCPT), Well-Root PT (WRPT), One-Fil PT (OFPT), and Endocem MTA premixed (ECPM) were compared to evaluate setting time, radiopacity, pH change, and microhardness. Biocompatibility on human dental pulp cells was compared using CCK-8 assay. Mineralization potential was evaluated using alkaline phosphatase activity, Alizarin Red S (ARS) staining, and quantitative real-time polymerase chain reaction with odontogenic gene marker. For data analysis, 1-way analysis of variance and Tukey's post hoc test were used at the significance level of 95%.

RESULTS

Among the PPBCs, BCPT presented the longest (552 ± 27) setting time (minutes) and others showed significantly shorter time than PMTA (334 ± 22) (P < .05). WRPT (6.20 ± 0.54) and OFPT (5.82 ± 0.50) showed significantly higher radiopacity values (mmAl) and others showed similar value compared with PMTA (P > .05). All PPBCs showed high alkaline pH from fresh materials and tended to increase according to time periods from 30 minutes to 12 hours. ECPM showed the highest value of microhardness (81.62 ± 5.90), WRPT showed similar, and others showed lower than PMTA (P < .05). All PPBCs showed biocompatibility in CCK-8 assay. All PPBCs showed similar or better value compared with PMTA in ALP and ARS staining, and ALP and DSPP marker expression (P < .05).

CONCLUSIONS

The PPBCs showed clinically acceptable chemomechanical properties and favorable mineralization potential.

Long-term follow-up of single-visit pulp revascularisation for the dens evaginatus and dens invaginatus: Cases report

This article documents cases of single-visit pulp revascularisation for dens evaginatus and dens invaginatus, without using intracranial medicaments or antibiotics, aiming to provide a potentially applicable protocol for pulp revascularisation procedure in a single-visit. Two patients with chief complaints of pain and swelling visited a dental hospital. Radiographs revealed that the causative teeth had an open apex and periapical radiolucency, and the teeth were diagnosed as pulp necrosis and acute apical abscess or symptomatic apical periodontitis. For both cases, single-visit revascularisation was completed without intracanal medicaments or antibiotics. The patients were periodically recalled to evaluate periapical healing after treatment. The apical lesion healed, and the root dentin thickening was observed. The single-visit pulp revascularisation procedure without using specific intracanal medicaments can produce clinically favourable results for these dental anomalies.

Physicochemical properties and biocompatibility of various bioceramic root canal sealers: in vitro study

INTRODUCTION

The aim of this study was to compare the physicochemical properties and biocompatibility of various calcium silicate-based bioceramic sealers (CSBSs).

METHODS

Four recently developed CSBSs, including AH Plus Bioceramic Sealer (AHB), EndoSequence BC Sealer (ESB), TotalFill BC Sealer (TTB), and Bio-C Sealer (BIC), were compared with the epoxy resin-based sealer AH Plus (AHP). Their physical properties, including flow, setting time, radiopacity, dimensional stability, and pH, were evaluated according to International Organization for Standardization (ISO) 6876. Their cytotoxicity in human periodontal ligament fibroblast (hPDLF) was assessed through the MTT assay and compared. Additionally, cell attachment to the sealer surface was analysed using green fluorescent protein and confocal laser scanning microscopy (CLSM) to evaluate cell viability. Data were analysed using one-way analysis of variance to determine the difference between groups for categorical variables, followed by Tukey's post-hoc test at the significance level of 95%.

RESULTS

The flow, setting time, and radiopacity of all tested CSBSs satisfied the ISO 6876/2012 standards. Further, these CSBS showed shrinkage after immersion in distilled water for 30 days and complied with the ISO 6876/2001 requirements. The pH of AHB, ESB, TTB, and BIC was greater than 11 whereas AHP had a pH of 6.69 after 4 weeks. CSBS showed excellent biocompatibility compared with that of AHP (P < .05). CLSM showed that alive hPDLFs were attached well to all the tested CSBS but not to AHP.

CONCLUSIONS

CSBS have similar physical characteristics within the ISO standards and higher biocompatibility than epoxy resin-based sealers.

Torque Generation of the Endodontic Instruments: A Narrative Review

As the use of nickel-titanium (NiTi) file systems for root canal therapy has become popular; hence, knowledge and understanding of the characteristics of NiTi files is essential for dentists. Unintended sudden fracture can occur during root canal shaping, and it is important to understand the conditions that may cause instrument fractures. Torque is defined as the force required to rotate the NiTi file and can be considered of as a parameter for the stress generated. The endodontic engine maintains a constant rotational speed by adjusting torque regardless of the root canal conditions. The process of root canal shaping by rotary instruments is a series of actions that requires torque and generates stress to both the teeth and the NiTi instruments. The generated stress may induce the strain accumulation on NiTi instrument and the canal wall and lead to the development of microcrack in the instrument and dentinal wall. Therefore, understanding of torque and stress generated is important to prevent the fractures to the instrument and the teeth. This stress has been measured using various experimental approaches, including microcrack observation by using a microscope or computed tomography, attaching strain gauges to the teeth, and finite element analysis. This review focuses on the stress generated to the teeth and the instrument during instrumentation under various experimental conditions. The factors related to torque generation are also discussed.

Characterisation of deformed or separated nickel-titanium retreatment instruments after clinical use - A multicentre experience: Defect profiles of clinically-used retreatment instruments

OBJECTIVES

The present study examined the defect characteristics of clinically-used, discarded nickel titanium rotary retreatment instruments and analysed the impact of clinical use on their metallurgical properties.

METHODS

92 XP-endo Shaper (XPS; FKG Dentaire) and 20 XP-endo Finisher (XPFR; FKG Dentaire) instruments with structural deformation or separation were collected after retreatment from four endodontic clinics over a 20-month period. The types of defects and their relative locations were recorded. The lateral and fractured surfaces of the separated instruments were examined with scanning electron microscopy. Differential scanning calorimetry was used to investigate the thermal behaviour of new, deformed and fractured instruments.

RESULTS

77 (84%) XPS and 4 (20%) XPFR had an area with structural change while 15 (16%) XPS and 16 (80%) XPFR were fractured. All unfractured, deformed XPFR showed unwinding close to the coronal end of the flute. Fractures in XPS and XPFR were often close to the coronal end of the flutes or the expanding segments of the insruments. Most of the XPS fractures were torsional failure (67%) while XPFR failed predominantly by cyclic fatigue (81%). The austenite-finishing temperature of XPFR (40°C) was higher than that of XPS (35°C). Both XPS and XPFR exhibited 2-stage phase transformations.

CONCLUSIONS

Torsional failure was more prevalent in XPS instruments and fatigue failure was more prevalent in XPFR instruments. Among the investigated instruments, XPFR were more likely to separate without warning whereas XPS frequently exhibited plastic deformation. The latter may be used as a pre-separation forewarning sign during clinical retreatment.

CLINICAL SIGNIFICANCE

The failure mode of XP-endo Shaper and XP-endo Finisher used clinically for retreatment appeared to be different. Plastic deformation, the forewarning sign of instrument seaparation, occurs when XP-endo instruments are used for retreatment.

Heat Treatment and Surface Treatment of Nickel–Titanium Endodontic Instruments

Knowledge and thorough understanding of the characteristics of endodontic nickel–titanium (NiTi) files is paramount for dentists performing root canal treatments to patients. Understanding the behavior of the NiTi files guides the clinicians in choosing the correct instruments for different clinical and anatomical situations. This review focuses on the metallurgical properties of endodontic NiTi files, with a special emphasis on recent developments and improvements in metallurgy and the effects of heat treatment and surface treatment. In this study, the impact that such developments have on the properties of endodontic NiTi files is discussed.

Torsional Resistance of Heat-Treated Nickel-Titanium Instruments under Different Temperature Conditions

This study compared the torsional resistance of heat-treated nickel-titanium (NiTi) instruments under different temperature conditions. Four thermomechanically treated single-use NiTi rotary instruments were selected for this study: OneShape (OS), OneCurve (OC), WaveOne Gold (WOG) and HyFlex EDM (HFE). Each instrument was further subdivided by temperature into 2 subgroups. Maximum torque and the distortion angle until fracture occurred were evaluated. Differential scanning calorimetry analysis was performed to measure the phase transformation temperature. Statistical analysis was performed using a two-way ANOVA and t-test (p < 0.05). Fractured fragments were observed using scanning electron microscopy (SEM). The two-way ANOVA showed no significant differences for different temperature conditions. At both room (RT) and body temperature (BT), OS was predominantly austenite while HFE was martensite. OC and WOG were predominantly martensite at RT and mixed phase at BT. At BT, more than half of WOG was martensite, while half of OC was austenite. SEM examination showed no topographical differences between instruments in different temperature groups. In relation to a limitation of this study, there was no difference in torsional resistance of NiTi rotary instruments between the BT and RT conditions. This implies that clinicians do not need to consider a decrease of torsional resistance of heat-treated NiTi instruments at BT.

Comparison of the effects from coronal pre-flaring and glide-path preparation on torque generation during root canal shaping procedure

This study aimed to evaluate the effect of glide-path preparation and coronal pre-flaring on torque generation of rotary nickel-titanium files. Sixty mesiobuccal root canals of 3D printed resin teeth (Dentalike) were randomly divided into 4 groups (n = 15) according to the preparation method; Group 1: OneCurve only; Group 2: OneG and OneCurve; Group 3: OneFlare and OneCurve; Group 4: OneFlare, OneG, and OneCurve. During the final canal preparation procedure, the torque generated was recorded. The sum of generated torque and maximum torque were calculated. Statistical analysis was performed with one-way ANOVA and Duncan's post hoc comparison. The summed torque was reduced by glide-path preparation and coronal pre-flaring (P < 0.05), whereas the maximum torque was not reduced by the glide-path preparation (P > 0.05). It will be advantageous to create a glide-path with coronal pre-flaring to reduce the stress and reactive forces to the NiTi file and root dentin.

Effects of Root Canal Curvature and Mechanical Properties of Nickel-Titanium Files on Torque Generation

INTRODUCTION

This study aimed to compare the torque generated by 4 different files in root canals with 4 different curvature angles.

METHODS

Four brands of nickel-titanium (NiTi) endodontic files were selected: WaveOne Primary (Dentsply Sirona, Ballaigues, Switzerland), WaveOne Gold Primary (Dentsply Sirona), ProTaper Universal F2 (Dentsply Sirona), and ProTaper Next X2 (Dentsply Sirona). A tempered steel block containing artificial canals with 4 different canal curvatures (15°, 25°, 35°, and 45°) was constructed. Each file was used according to the manufacturer's instructions in the dynamic model, with an added 15 axial up-and-down movements of 4 mm at the end of the canal. The generated torque was recorded, and the total and maximum torque values were measured. Two-way analysis of variance and the Duncan post hoc comparison test were performed at a significance level of 95%.

RESULTS

A significant correlation between the curvature angle and the type of file system was observed (P < .05). As the degree of canal curvature increased, the generated total and maximum torque increased. At 15° and 25°, the NiTi files with reciprocating motion generated a higher total and maximum torque than files with continuous rotation. ProTaper Universal of conventional NiTi alloy showed the steepest increase in the generated total and maximum torque with the increasing curvature angle. The ProTaper Next file had the lowest torque values at the higher canal angles of 35° and 45° (P < .05).

CONCLUSIONS

Despite the study limitations, it can be concluded that root canal curvature, design, and heat treatment of NiTi files and file kinematics affect the generated torque during instrumentation.

Physicochemical properties and cytocompatibility of newly developed calcium silicate-based sealers

The purpose of this study was to compare the physical properties and cytocompatibility of contemporary calcium silicate-based sealers. Four calcium silicate-based sealers (BrightEndo MTA sealer, CeraSeal, EndoSeal TCS and One-Fil) were compared to an epoxy resin-based sealer (AH Plus). Flow, setting time, radiopacity and dimensional change were evaluated according to ISO 6876 standards. Cytotoxicity on human periodontal ligament fibroblast (hPDLF) cells was compared for biological properties using MTT assay. The surface of the sealer was analysed using scanning electron microscopy to evaluate cell attachment. Flow and radiopacity of all sealers met ISO standards, while setting time and dimensional stability did not meet the ISO standards. Calcium silicate-based sealers showed favourable cytocompatibility, and hPDLF cells were well attached to the calcium silicate-based sealers. Calcium silicate-based sealers have clinically acceptable flow and radiopacity, and cytocompatibility. However, these sealers had longer setting time and higher dimensional change than those required by ISO 6876.

Numeric Evaluation of Innovate Spring Machined Nickel-Titanium Rotary Instruments: A 3-dimensional Finite Element Study

INTRODUCTION

The purpose of this study was to investigate if machined springs in nickel-titanium (NiTi) rotary instruments can improve their mechanical properties. The bending and torsion properties were assessed using finite element (FE) model analysis.

METHODS

A basic 3-dimensional file model without a spring was created with apical size #25, 25-mm full length, and 16-mm cutting flutes. Three other models were created with a spring machined into their shaft portion with different numbers of spring coils: standard (STspr), 10% more (INspr), and 10% less (DEspr). To compare the mechanical responses among the 4 FE models, file bending and torsion were simulated using FE analysis.

RESULTS

Spring machined NiTi rotary instruments showed higher torsional resistance and less bending stiffness than the same instrument without. The spring machined models required more torque to bend or rotate the DEspr model than was required for the STspr and INspr models; however, the STSpr and INSpr models were similar.

CONCLUSIONS

Within the limitations of this study, the FE analysis indicated that machining a spring into the shaft of NiTi rotary instruments improved torsional resistance and bending flexibility. Therefore, spring machining has the potential to increase the durability of the NiTi rotary instruments.

Buckling Resistance of Various Nickel-Titanium Glide Path Preparation Instruments in Dynamic or Static Mode

INTRODUCTION

The aim of this study was to compare the buckling resistance of nickel-titanium (NiTi) instruments for glide path preparation depending on the test mode (static vs dynamic).

METHODS

The conventional PathFile (PF; Dentsply Sirona, Ballaigues, Switzerland) and heat-treated ProGlider (PG, Dentsply Sirona) and WaveOne Gold Glider (WG, Dentsply Sirona) were evaluated. The instrument tips were placed in a small dimple prepared in an aluminum cube in a customized device. The file was then pushed in the axial direction at a 1.0-mm/s crosshead speed with rotation (dynamic mode) or without rotation (static mode). The dynamic mode of WG used its dedicated reciprocating rotation, whereas the others were rotated continuously at 300 rpm. The axial load and lateral buckling displacement were simultaneously measured. Data were analyzed statistically using 2-way analysis of variance (P = .05).

RESULTS

The buckling resistance in the dynamic mode was higher than in the static mode for PG and WG (P < .05), whereas PF was not influenced by test modes (P > .05). In the dynamic mode, the PG required the highest buckling load followed by PF and WG (P < .05). In the static mode, the WG showed the lowest load (P < .05). The dynamic mode showed significantly more upper level lateral buckling displacement than in the static mode (P < .05).

CONCLUSIONS

When the glide path preparation instruments moved in the dynamic mode as in clinical situations, the buckling resistance of the heat-treated NiTi glide path instruments was higher than in the static condition. The heat-treated instruments may have better buckling resistance than the conventional NiTi instrument in clinical situations.

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.

Effect of Shaft Length on the Torsional Resistance of Rotary Nickel-titanium Instruments

INTRODUCTION

This study was conducted to evaluate the effect of file length on the torsional resistance of nickel-titanium rotary files.

METHODS

Forty-five new files with a #25 tip size and variable taper for ProTaper Next (PTN; Dentsply Sirona, York PA), ProTaper Gold (PTG, Dentsply Sirona), and HyFlex CM (HCM; Coltene-Whaledent, Allstetten, Switzerland) were divided into 3 subgroups (n = 15) according to the file length (21, 25, and 31 mm). Torsional tests were performed using a custom-made device (AEndoS; DMJ System, Busan, Korea). The maximum torsional load and distortion angle were measured until file fracture occurred. The toughness was calculated using these data. All data were statistically analyzed using 2-way analysis of variance at a confidence level of 95%. One-way analysis of variance and Duncan post hoc comparison were also conducted to assess the difference among the independent variables, shaft length, and file system.

RESULTS

A statistically significant interaction was observed between the file system and instrument length with respect to the maximum torsional load (P < .05). The 31-mm shaft length of HCM showed higher maximum torsional load than that of the 25- and 21-mm shaft length in the HCM groups (P < .05). Among the instrument systems, PTG showed the significantly highest maximum torsional load followed by PTN and HCM (P < .05). The 31-mm shaft length of PTG showed significantly higher toughness than that of the 21- and 25-mm shaft lengths (P < .05), whereas PTN and HCM did not show any difference in relation to the shaft length.

CONCLUSIONS

Within the limitations of this study, the instrument with a longer shaft may have higher maximum torsional load or toughness than that with a shorter shaft.

Mechanical Properties of Various Glide Path Preparation Nickel-titanium Rotary Instruments

INTRODUCTION

The aim of this study was to compare the cyclic fatigue, torsional resistance, and bending stiffness of single-file glide path preparation nickel-titanium instruments.

METHODS

ProGlider (#16/progressive taper; Dentsply Sirona, Ballaigues, Switzerland), One G (#14/.03; Micro-Mega, Besancon, France), and EdgeGlidePath (#16/progressive taper; Edge Endo, Albuquerque, NM) were tested. For the bending stiffness test, the instrument was bent to a 45° angle along the main axis while 3 mm of the tip was secured. Cyclic fatigue resistance was measured during pecking movement, whereas the file was rotated within a reproducible simulated canal with a 3-mm radius and a 90° angle of curvature. The ultimate torsional strength and toughness were evaluated using a custom device. While tightly holding the end of the file at 3 mm, a rotational load of 2 rpm speed was applied until fracture. The results were analyzed using 1-way analysis of variance and Tukey post hoc comparison. A microscopic inspection was performed on the surface of a separate instrument using a scanning electron microscope.

RESULTS

EdgeGlidePath showed superior cyclic fatigue resistance compared with the other systems. One G showed higher maximum torsional strength than the others. One G showed the largest distortion angle and the highest toughness followed by EdgeGlidePath and ProGlider (P < .05). One G also showed larger bending stiffness than the others, whereas EdgeGlidePath showed a larger residual angle than the others (P < .05).

CONCLUSIONS

Conventional wire showed higher toughness and torsional resistance than M-Wire and heat-treated nickel-titanium instruments. However, heat-treated wire showed higher cyclic resistance than conventional wire and M-Wire.

Mechanical Properties of Orifice Preflaring Nickel-titanium Rotary Instrument Heat Treated Using T-Wire Technology

INTRODUCTION

This study examined whether the use of T-Wire heat treatment enhanced the resistance to torsional force, cyclic fatigue, and bending stiffness of orifice preflaring nickel-titanium instruments.

METHODS

Ninety heat-treated OneFlare (OFT; Micro-Mega, Besançon, France) and conventional OneFlare (OFC, Micro-Mega) nickel-titanium files of size #25/.09 with identical geometries were selected (n = 15 per group for each test). Torsional resistance was evaluated through the measurement of ultimate torsional strength, distortion angle, and toughness during rotational loading. Cyclic fatigue resistance was evaluated through measurement of the number of cycles to failure for each instrument. Stiffness was evaluated by observation of the bending moment on attaining a 45° bend. Independent t tests were performed for statistical analysis. After torsional and cyclic fatigue tests, all fractured fragments were examined under a field emission scanning electron microscope to observe characteristics of the fractured surfaces.

RESULTS

OFT showed better cyclic fatigue resistance and lower bending stiffness than OFC (P < .05); however, OFT and OFC did not differ significantly in terms of torsional resistance. The fractured cross-sectional surfaces had characteristics typical of cyclic fatigue and torsional fractures.

CONCLUSIONS

The OFT instruments made using T-Wire heat treatment had better cyclic fatigue resistance and lower bending stiffness with no decline in torsional strength.

Safe pseudoelastic limit range under torsional loading with Reciproc Blue

AIM

To determine the distortional angle and torsional load at the pseudoelastic limit of Reciproc Blue instruments and to verify the safety of using pre-set dedicated motors designed for use with the original Reciproc instruments.

METHODOLOGY

Two torsional conditions of Reciproc R25 and Reciproc Blue R25 were tested using a custom device. The first condition fixed the file tips at 3 mm and repetitively rotated them, with gradually increasing angles, from 10° to 270°, and the resulting torque was recorded (n = 15). The second test involved a single continuous rotation until fracture (n = 15). The pseudoelastic limits of the instruments were determined from their torque-rotation curves. For statistical analysis, two-way analysis of variance and t-tests were used, at a 95% significance level. Tested specimens were examined using field-emission scanning electron microscopy (FE-SEM).

RESULTS

The angle at the pseudoelastic limit was significantly greater for Reciproc Blue than Reciproc, regardless of test mode (P < 0.05). When torsional loading was repeated, using gradually increasing rotational angles, the torsional resistance was significantly lower than for a single rotation (P < 0.05). However, under all test conditions, the pseudoelastic limit was below the pre-set 170° of the dedicated reciprocating motor. FE-SEM evaluation of the lateral aspects of the instruments revealed numerous longitudinal microcracks running along their long axis. For Reciproc Blue groups, the file shaft machining grooves were distorted after repetitive and continuous torsional tests.

CONCLUSIONS

The 170° pre-set angle of the dedicated endodontic motors for the Reciproc system was safe for Reciproc Blue in single or time-restricted use.

Microscopic Features of Fractured Fragment of Nickel-Titanium Endodontic Instruments by Two Different Modes of Torsional Loading

This study compared the microscopic features of the fractured endodontic nickel-titanium (NiTi) rotary instruments by two different torsional loadings: repetitive torsional loading (RTL) and single torsional loading (STL) based on the International Organization for Standardization (ISO). ProTaper Next, HyFlex EDM, and V-Taper 2 were compared in this study. In the STL method, the torsional load was applied after fixing the 3 mm tip of the file, by continuous clockwise rotation (2 rpm) until fracture. In the RTL method, a preset rotational loading (0.5 N·cm) was applied and the clockwise loading to the preset torque and counterclockwise unloading to original position were repeated at 50 rpm until the file fractured. Fractured fragments by two methods were compared under a scanning electron microscope (SEM) to examine the topographic features of the fractured surfaces and longitudinal aspects. SEM examinations showed significantly different features according to the loading methods. Specimens from the RTL method showed ruptured aspects on cross sections, with multiple areas of initiated cracks while the STL method showed the typical features of torsional failure, such as circular abrasion marks and fatigue dimples. This study suggested a new repetitive torsional loading method which is much more clinically relevant and may result in a different fracture feature from STL method.

Evaluation of dynamic and static torsional resistances of nickel-titanium rotary instruments

Background/purpose

This study evaluated the torsional resistances of nickel-titanium rotary instruments by two methods: i) dynamic resistance: repetitive torsional loading (RTL) and ii) static resistance: single torsional loading (STL) based on the International Organization for Standardization.

Materials and methods

In RTL method, a pre-set rotational loading (0.5 N cm) was applied after fixing the 3-mm tip of the file, and this clockwise loading to the pre-set torque and counter-clockwise unloading to original position was repeated at 50 rpm until the file fractured. The number of repetition cycles until fracture was counted. In STL method, the torsional strength was determined by continuous clockwise rotation (2 rpm) until fracture. Results from both methods were compared after testing the torsional resistances of four instrument systems (n = 15): Hyflex CM, HyFlex EDM, V-Taper2, and V-Taper2H. A scanning electron microscope (SEM) was used to examine the topographic features of the fractured surfaces and longitudinal aspects (n = 5) from both methods.

Results

The RTL and STL methods had similar results: V-Taper2 had the highest resistance and the Hyflex CM had the lowest (P < 0.05). Spearman correlation test showed the results from two methods were strongly correlated (coefficient = 1). Under the SEM, specimens from the RTL showed ruptured aspects on cross-sections with multiple areas of crack propagation, while the STL showed the typical features of torsional failure such as circular abrasion marks and fatigue dimples.

Conclusions

This study suggests the clinically relevant torsional test (RTL) method yield similar results with the STL method, but they have different topographic findings.

Effect of surface treatment on the mechanical properties of nickel-titanium files with a similar cross-section

OBJECTIVES

The aim of this study was to compare the mechanical properties of various nickel-titanium (NiTi) files with similar tapers and cross-sectional areas depending on whether they were surface-treated.

MATERIALS AND METHODS

Three NiTi file systems with a similar convex triangular cross-section and the same ISO #25 tip size were selected for this study: G6 (G6), ProTaper Universal (PTU), and Dia-PT (DPT). To test torsional resistance, 5 mm of the straightened file's tip was fixed between polycarbonate blocks (n = 15/group) and continuous clockwise rotation until fracture was conducted using a customized device. To evaluate cyclic fatigue resistance, files were rotated in an artificial curved canal until fracture in a dynamic mode (n = 15/group). The torsional data were analyzed using 1-way analysis of variance and the Tukey post-hoc comparison test, while the cyclic fatigue data were analyzed using the Mann-Whitney U test at a significance level of 95%.

RESULTS

PTU showed significantly greater toughness, followed by DPT and G6 (p < 0.05). G6 showed the lowest resistance in ultimate torsional strength, while it showed a higher fracture angle than the other files (p < 0.05). In the cyclic fatigue test, DPT showed a significantly higher number of cycles to failure than PTU or G6 (p < 0.05).

CONCLUSIONS

Within the limitations of this study, it can be concluded that the torsional resistance of NiTi files was affected by the cross-sectional area, while the cyclic fatigue resistance of NiTi files was influenced by the surface treatment.

Anatomical relationship of maxillary posterior teeth with the sinus floor and buccal cortex

This study aimed to evaluate the possibility of root fenestration or oroantral communication by evaluating the distance from root apex to the sinus floor and buccal cortex in maxillary posterior teeth using cone-beam computed tomography (CBCT) images. The study included 2182 roots of the maxillary posterior teeth from 219 patients after reviewing CBCT images of 462 patients according to the location of roots by two endodontists. The distances from each root apex to the maxillary sinus floor and buccal and palatal cortices were evaluated according to sex and age, and the mean values were compared by one-way analysis of variance and Mann-Whitney U-test. The distance between root apex and maxillary sinus floor was the greatest in maxillary first premolars and shortest in the mesio-buccal roots of maxillary second molars. The distances from root apex to the buccal and palatal cortical bones were significantly greater in male patients than those in female patients (P < 0·05). The palatal roots of maxillary first molars exhibited the highest incidence as well as the greatest mean length (1·96 mm) of protrusion into the maxillary sinus. The distance from root apex to the sinus floor was found to increase with age, except in case of maxillary second premolars. Understanding the relationship of maxillary posterior teeth with the sinus floor and buccal cortex could provide clinicians valuable information to help reduce iatrogenic damage.

Vibrations Generated by Several Nickel-titanium Endodontic File Systems during Canal Shaping in an Ex Vivo Model

INTRODUCTION

This study aimed to compare the vibration generated by several nickel-titanium (NiTi) file systems and transmitted to teeth under 2 different motions (continuous rotation motion and reciprocating motion).

METHODS

Sixty J-shaped resin blocks (Endo Training Bloc-J; Dentsply Maillefer, Ballaigues, Switzerland) were trimmed to a root-shaped form and divided into 2 groups according to the types of electric motors: WaveOne motor (WOM, Dentsply Maillefer) and X-Smart Plus motor (XSM, Dentsply Maillefer). Each group was further subdivided into 3 subgroups (n = 10 each) according to the designated file systems: ProTaper Next (PTN, Dentsply Maillefer), ProTaper Universal (PTU, Dentsply Maillefer), and WaveOne (WOP, Dentsply Maillefer) systems. Vibration was measured during the pecking motion using an accelerometer attached to a predetermined consistent position. The average vibration values were subjected to 2-way analysis of variance as well as the t test and Duncan test for post hoc comparison at the 95% confidence interval.

RESULTS

Both motor types and instrument types produced significantly different ranges of average vibrations. Regardless of the instrument types, the WOM group generated greater vibration than the XSM group (P < .05). Although PTN and PTU did not show significant differences, the WOP group showed significantly greater vibration than the other groups regardless of motor types (P < .05).

CONCLUSIONS

Under the limitations of this study design, the reciprocating NiTi file system may generate greater vibration than the continuous rotation NiTi file systems. The motor type also has a significant effect to amplify the vibrations.

The geometric effect of an off-centered cross-section on nickel-titanium rotary instruments: A finite element analysis study

Background/purpose

Geometric design dictates the mechanical performance of nickel–titanium rotary instruments. Using finite element (FE) analysis, this study evaluated the effects of an off-centered cross-sectional design on the stiffness and stress distribution of nickel–titanium rotary instruments.

Materials and methods

We constructed three-dimensional FE models, using ProTaper-NEXT type design (PTN) as well as three other virtual instruments with varied cross-sectional aspect ratios but all with the same cross-sectional area. The cross-sectional aspect ratio of the PTN was 0.75, while others were assigned to have ratios of 1.0 (square), 1.5 (rectangle), and 2.215 (centered-rectangle). The PTN center of the cross-section was ‘k’, while others were designed to have 0.9992k, 0.7k, and 0 for the square, rectangle, and centered-rectangle models, respectively. To compare the stiffness of the four FE models, we numerically analyzed their mechanical response under bending and torque.

Results

Under the bending condition, the square model was found to be the stiffest, followed by the PTN, rectangle, and then the centered-rectangle model. Under the torsion, the square model had the smallest distortion angle, while the rectangular model had the highest distortion angle.

Conclusion

Under the limitation of this study, the PTN type off-centered cross-sectional design appeared the most optimal configuration among the tested designs for high bending stiffness with cutting efficiency while rotational stiffness remained similar with the other designs.

The Effects of Torsional Preloading on the Torsional Resistance of Nickel-titanium Instruments

INTRODUCTION

This study evaluated the effect of torsional preloading on the torsional resistance of nickel-titanium (NiTi) endodontic instruments.

METHODS

WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper Universal F2 (Dentsply Maillefer) files were used. The ultimate torsional strength until fracture was determined for each instrument. In the phase 1 experiment, the ProTaper and WaveOne files were loaded to have a maximum load from 2.0 up to 2.7 or 2.8 Ncm, respectively. In the phase 2 experiment, the number of repetitions of preloading for each file was increased from 50 to 200, whereas the preloading torque was fixed at 2.4 Ncm. Using torsionally preloaded specimens from phase 1 and 2, the torsional resistances were calculated to determine the ultimate strength, distortion angle, and toughness. The results were analyzed using 1-way analysis of variance and Duncan post hoc comparison. The fracture surfaces and longitudinal aspect of 5 specimens per group were examined under a scanning electron microscope.

RESULTS

All preloaded groups showed significantly higher ultimate strength than the unpreloaded groups (P < .05). There was no significant difference among all groups for distortion angle and toughness. Although WaveOne had no significant difference between the repetition groups for ultimate strength, fracture angle, and toughness, ProTaper had a higher distortion angle and toughness in the 50-repetition group compared with the other repetition groups (P < .05). Scanning electron microscopic examinations of the fractured surface showed typical features of torsional fracture.

CONCLUSIONS

Torsional preloading within the ultimate values could enhance the torsional strength of NiTi instruments. The total energy until fracture was maintained constantly, regardless of the alloy type.

Screw-in forces during instrumentation by various file systems

OBJECTIVES

The purpose of this study was to compare the maximum screw-in forces generated during the movement of various Nickel-Titanium (NiTi) file systems.

MATERIALS AND METHODS

Forty simulated canals in resin blocks were randomly divided into 4 groups for the following instruments: Mtwo size 25/0.07 (MTW, VDW GmbH), Reciproc R25 (RPR, VDW GmbH), ProTaper Universal F2 (PTU, Dentsply Maillefer), and ProTaper Next X2 (PTN, Dentsply Maillefer, n = 10). All the artificial canals were prepared to obtain a standardized lumen by using ProTaper Universal F1. Screw-in forces were measured using a custom-made experimental device (AEndoS-k, DMJ system) during instrumentation with each NiTi file system using the designated movement. The rotation speed was set at 350 rpm with an automatic 4 mm pecking motion at a speed of 1 mm/sec. The pecking depth was increased by 1 mm for each pecking motion until the file reach the working length. Forces were recorded during file movement, and the maximum force was extracted from the data. Maximum screw-in forces were analyzed by one-way ANOVA and Tukey's post hoc comparison at a significance level of 95%.

RESULTS

Reciproc and ProTaper Universal files generated the highest maximum screw-in forces among all the instruments while M-two and ProTaper Next showed the lowest (p < 0.05).

CONCLUSIONS

Geometrical differences rather than shaping motion and alloys may affect the screw-in force during canal instrumentation. To reduce screw-in forces, the use of NiTi files with smaller cross-sectional area for higher flexibility is recommended.

Preference of undergraduate students after first experience on nickel-titanium endodontic instruments

Objectives

This study aimed to compare two nickel-titanium systems (rotary vs. reciprocating) for their acceptance by undergraduate students who experienced nickel-titanium (NiTi) instruments for the first time.

Materials and Methods

Eighty-one sophomore dental students were first taught on manual root canal preparation with stainless-steel files. After that, they were instructed on the use of ProTaper Universal system (PTU, Dentsply Maillefer), then the WaveOne (WO, Dentsply Maillefer). They practiced with each system on 2 extracted molars, before using those files to shape the buccal or mesial canals of additional first molars. A questionnaire was completed after using each file system, seeking students' perception about 'Ease of use', 'Flexibility', 'Cutting-efficiency', 'Screwing-effect', 'Feeling-safety', and 'Instrumentation-time' of the NiTi files, relative to stainless-steel instrumentation, on a 5-point Likert-type scale. They were also requested to indicate their preference between the two systems. Data was compared between groups using t-test, and with Chi-square test for correlation of each perception value with the preferred choice (p = 0.05).

Results

Among the 81 students, 55 indicated their preferred file system as WO and 22 as PTU. All scores were greater than 4 (better) for both systems, compared with stainless-steel files, except for 'Screwing-effect' for PTU. The scores for WO in the categories of 'Flexibility', 'Screwing-effect', and 'Feeling-safety' were significantly higher scores than those of PTU. A significant association between the 'Screwing-effect' and students' preference for WO was observed.

Conclusions

Novice operators preferred nickel-titanium instruments to stainless-steel, and majority of them opted for reciprocating file instead of continuous rotating system.

Debris extrusion by glide-path establishing endodontic instruments with different geometries

Background/purpose

Glide-path preparation is an important step during initial endodontic procedure to reduce shaping-instrument fracture. The aim of this study was to evaluate the amount of apically extruded debris produced by glide-path preparation instruments with different geometric designs.

Materials and methods

Forty teeth extracted for periodontal reasons were randomly divided into four groups (n = 10). The working length was standardized at 17 mm from the apical foramen by a flattening reference point. The glide-path was created using repetitive up-and-down movement three times with one of following four selected instruments: One G, ProGlider, a size 15 ScoutRace, and a size 15 stainless-steel K-file. To collect the apically extruded debris, the customized apparatus was used, and the collected debris was stored in an incubator. The weight of the debris was measured using an analytical balance with an accuracy of 0.00001 g. The data were analyzed by one-way analysis of variance and Duncan's multiple comparison test at a significance level of 95%.

Results

The ProGlider group produced significantly less debris extrusion relative to the other groups (P < 0.05). The One G and ScoutRace groups showed no significant difference, but debris production was lower than that observed for the stainless-steel group (P < 0.05).

Conclusion

Creating the glide-path using nickel-titanium rotary files produced lower amounts of debris extrusion than using manual stainless-steel files. The progressive taper design of ProGlider, the center-off cross-section of One G, and the alternative-pitch design of ScoutRace may have increased the efficiencies of debris removal with minimal extrusion during glide-path preparation. Glide-path preparation using NiTi rotary files have better clinical efficiency than the manual stainless-steel file.

Effects of Pitch Length and Heat Treatment on the Mechanical Properties of the Glide Path Preparation Instruments

INTRODUCTION

This study aimed to compare the effects of pitch length and heat treatment on the mechanical properties of glide path establishing instruments.

METHODS

Prototypes of glide path preparation files (#14/.03 taper) were made to evaluate the effects of different pitch lengths and heat treatments. The files were divided into 4 groups according to the pitch length (pG and OneG) and heat treatment (pGH and OneGH). For the torsional resistance test, ultimate strength and fracture angle were measured, and the file tip was fixed at 3 different levels of 2, 4, and 6 mm from the tip (n = 10 for each level). The toughness was calculated by multiplying the ultimate strength and the fracture angle. The cyclic fatigue resistance was compared by measuring the number of cycles to fracture in a curved metal canal (n = 10). The screw-in forces were measured during instrumentation motions with a sequential increase in the pecking distance of 1 mm until the file reached the end of the simulated resin canal (n = 10).

RESULTS

The heat-treated groups showed lower toughness and higher cyclic fatigue resistance than the non-heat-treated groups. The short pitch groups showed significantly higher torsional strengths than the long pitch groups. The heat-treated groups had significantly lower screw-in forces than the non-heat-treated groups.

CONCLUSIONS

Under the limitations of this study, the torsional strength of the experimental file was reduced by heat treatment and increased by the short pitch length. Thus, a non-heat-treated file with a shorter pitch length would be favorable as a rotary glide path instrument.