Jung Hong Ha

Effect of R-phase Heat Treatment on Torsional Resistance and Cyclic Fatigue Fracture

INTRODUCTIONnThe purpose of this study was to investigate whether the resistance to torsional stress and cyclic fatigue would be improved by the novel R-phase heat treatment by using instruments with identical geometric characteristics.nnnMETHODSnK3XF and K3 files with sizes of #30/.04 and #30/.06 were selected for the study (n = 10 per group). To evaluate the torsional resistance, the torsional load and distortion angle at yield, ultimate, and fracture points were recorded. The toughness until fracture was calculated as well. The number of cycles to failure for each instrument was determined to evaluate the cyclic fatigue resistance. Statistical analysis was performed by using the nonparametric Mann-Whitney U test (for torsional resistance) and independent t test (for cyclic fatigue). After torsional and cyclic fatigue tests, all fractured fragments were observed under a scanning electron microscope to evaluate the topographic features of the fractured surfaces.nnnRESULTSnAlthough K3XF showed superior cyclic fatigue resistance to K3 (P < .000), there was no significant difference in torsional resistance in terms of ultimate strength, fracture angle, and toughness between the 2 brands of K3XF and K3 regardless of the instruments taper. The fractured cross-sectional surfaces showed typical features of cyclic fatigue and torsional fractures. K3XF had a distinctive surface with numerous irregular micropores.nnnCONCLUSIONSnThe heat-treated K3XF instruments that used R-phase heat treatment technology showed improved cyclic fatigue resistance without decline of torsional strength.

Safety of the Factory Preset Rotation Angle of Reciprocating Instruments

INTRODUCTIONnThis study aimed to investigate the torsional resistance of 2 reciprocating nickel-titanium instruments (Reciproc [VDW, Munich, Germany] and WaveOne [Dentsply Maillefer, Ballaigues, Switzerland]) operated at the maximum rotating angle in a proprietary motor.nnnMETHODSnWith the file tip secured at various levels (3, 4, or 5 mm) of Reciproc R25 and WaveOne Primary, the distortion angles and torsional loads were monitored during counterclockwise movement at 2 rpm until fracture (n = 10 at each level) for a load-distortion graph. The rotation angles and loads at the beginning point of the plateau, the ultimate torsional strength, final fracture angle, and toughness were determined. The data were analyzed using 1-way analysis of variance and the Tukey post hoc test at α = .05. The lateral longitudinal aspect and the fracture cross-section of each specimen were examined by scanning electron microscopy after the test.nnnRESULTSnThe rotation angle at the beginning point of the plateau was significantly greater for a binding site farther away from the tip of the instrument for both systems (P < .05), and all were greater than 170° (preset in the dedicated motor from manufacturer). The ultimate strength and toughness also increased significantly at levels farther away from the instrument tip (P < .05). All specimens showed typical topographic features of torsional fracture, including the circular abrasion marks and fibrous dimples near the rotation center after the test.nnnCONCLUSIONSnIt was determined that the 2 brands of reciprocating files are safe when operated at the rotational angle in the proprietary motor.

‘Screw-in’ tendency of rotary nickel-titanium files due to design geometry

AIMnTo examine the effect of several standard geometric characteristics of rotary instruments on the screw-in forces and stresses generated on root dentine using 3D finite element analysis (FEA).nnnMETHODOLOGYnFour cross-sectional designs (triangular, slender-rectangular, rectangular and square) were evaluated. The area of the triangular cross-section and of the slender-rectangular model were the same. Another rectangular model had the same centre-core diameter as the triangular one. Each design was twisted into a file model with 5, 10 or 15 threads over its 16-mm-long working section. Three curved root canals were simulated as rigid surface models: θ = 15 degrees/R = 36 mm radius; θ = 30/R = 18; and θ = 45/R = 12. A commercial FEA package was used to simulate the file rotating in the canal to determine the screw-in force and reaction torque on the instrument.nnnRESULTSnInstruments of a square cross-section had the highest screw-in force and reaction torsional stresses followed by the rectangle, the triangle design and the slender-rectangle design, respectively. The file with closer pitch generated lower stresses, compared with that with longer pitch. The greater the root canal curvature, the higher the screw-in force and reaction torque generated.nnnCONCLUSIONnThis study demonstrated that the screw-in tendency depends on both the instrument geometry and canal curvature. Clinicians should be aware that certain instrument designs are prone to develop high screw-in forces, requiring the operator to maintain control of the handpiece or to use a brushing action to prevent instruments being pulled into the canal.

Apical Enlargement According to Different Pecking Times at Working Length Using Reciprocating Files

INTRODUCTIONnThe purpose of this study was to evaluate the apical preparation sizes resulting from repetitive pecking motions at the working length (WL) by using reciprocating files.nnnMETHODSnSixty simulated endodontic training blocks with a J-shaped root canal were instrumented using Reciproc R25 (VDW, Munich, Germany) or WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland) (n = 30 each). Each group was divided into 3 subgroups based on the repetitive pecking times at the WL: 1, 2, and 4 times. All specimens were prepared by 1 operator who was competent in instrumenting canals with both file systems. All of the procedures, including the WL measurement, were performed under an operating microscope. The replica of the prepared canal was taken with silicone impression material. After 24 hours of allowing the impression to set, each sample was evaluated under a scanning electron microscope at the apical tip, and the apical preparation size (diameter) was measured at the D0 level of the impression. The data were analyzed statistically using 2-way analysis of variance and the Tukey post hoc test at P = .05.nnnRESULTSnThe mean diameter (μm) varied between 253 and 274 and between 258 and 277 for Reciproc and WaveOne, respectively, without significant differences. However, more repetitive pecking motions at the WL resulted in a significantly larger apical preparation size than the subgroups with less pecking times (P < .05).nnnCONCLUSIONSnUnder the conditions of this study, the results indicate that a greater number of repetitive pecking times at the WL may result in an apical preparation size that is larger than the actual file size.

Elastic limits in torsion of reciprocating nickel-titanium instruments.

INTRODUCTIONnReciprocating angle and torsional load at the superelastic limit were investigated for nickel-titanium (NiTi) instruments used with reciprocating movements.nnnMETHODSnTwo reciprocating NiTi instruments (Reciproc R25 [VDW, Munich, Germany] and WaveOne Primary [Dentsply Maillefer, Ballaigues, Switzerland]) and ProTaper F2 (Dentsply Maillefer) were tested for 2 torsional conditions using a custom-designed testing device. Rotational angles were applied, and generated torque values were recorded. The first test condition fixed the files at 5 mm and repetitively rotated them with gradually increasing angles up to 250° (n = 10). The second test subjected the files to a single continuous rotation until fracture (n = 10). The superelastic limits of the instruments were determined from their torque-rotation curves. Statistical analysis was performed (Kruskal-Wallis) at a 95% significance level. The tested specimens were examined under a scanning electron microscope.nnnRESULTSnIt was found that the angle at the superelastic limit was higher for R25 and Primary files than the ProTaper file (P < .05). Repetitive torsional loading with a gradually increasing rotational angle reduced the torsional resistance compared with the single rotation motion. At the 5-mm fixation level all files had superelastic limit angles higher than the 170° set in the dedicated reciprocating motor. The scanning electron microscopic analysis showed features of torsional failure.nnnCONCLUSIONSnUnder the conditions of this study, the 170° set angle of dedicated motors for reciprocating file systems is safe at the 5-mm level. Reuse of reciprocating systems, even with the movements within the elastic limit, may cause deterioration of the instruments.

Effect from Rotational Speed on Torsional Resistance of the Nickel-titanium Instruments

Introduction: The purpose of this study was to evaluate differences in torsional resistance using up‐regulated speed of rotational spindle. Methods: Three NiTi rotary instrument systems were selected in this study: K3XF (SybronEndo, Glendora, CA), BLX (B&L Biotech, Ansan, Korea), and OneShape (MicroMega, Besançon, France). The tip size and taper for all files were #25 and 0.06. Experimental groups (n = 10/group) were assigned to 2‐, 60‐, 350‐, and 600‐rpm groups by the rotational speed of spindle. Forty new files were used for each test. The file tip of 5‐mm length was secured between brass plates. While keeping the file straight, it was rotated clockwise at a constant rotational speed until fracture occurred. The parameters of torsional resistance, torsional load (Ncm), and distortion angle (°) were measured using an AEndoS‐k (DMJ System, Busan, Korea), and the toughness until fracture was computed from these data. The 1‐way analysis of variance test was used to analyze the torsional resistance at a significance level of 95%. All fractured fragments were observed under a scanning electron microscope to evaluate the topographic features of the fractured surfaces. Results: No significant difference in torsional resistance was found among groups when they were compared for ultimate strength at the maximum torque, fracture angle, and toughness. Scanning electron microscopic examination of the fractured cross‐sectional surfaces revealed typical features of torsional fractures, concentric abrasion marks, and fibrous dimples from the torsional center. Conclusions: Under the conditions of the study, the torsional resistances of the rotary instruments were not affected by the rotational speed. HighlightsThe difference of torsional resistance was evaluated according to the up‐regulated speed of rotational spindle.Four conditions of rotational speed (2, 60, 350, and 600 rpm) were applied.The conventional nickel‐titanium alloy (BLX and OneShape) and the R‐phase heat‐treated alloy (K3XF) were tested.The different speeds of rotation did not change the torsional resistance of the rotary files.

Stress Generation during Pecking Motion of Rotary Nickel-titanium Instruments with Different Pecking Depth

INTRODUCTIONnThe aim of this study was to evaluate the effect of different pecking depth on the stress generated by the screw-in forces of a rotating endodontic file in simulated canals.nnnMETHODSnTwenty simulated resin blocks with a J-shaped curvature were used. Twenty OneG files (MicroMega, Besançon, France) were assigned for a screw-in test depending on the pecking depth in 2 groups (nxa0=xa010). The files were operated at 300xa0rpm, and the up and down speed was controlled at 1 mm/s stroke velocity and a 10-millisecond dwell time using a customized device. The distances (pecking depth) for the pecking motion were 2 mm or 4 mm for each group; 6 mm forward and 4 mm backward and 6 mm forward and 2 mm backward movements were applied, respectively, for the 2 pecking groups. During the operation, the positive and negative apical loads were recorded at a rate of 50xa0Hz using customized software attached to the device. The maximum negative apical load (screw-in force [SF]) was recorded, and the total energy during pecking motion until the file reached the working length (cumulative screw-in forces [CSFs]) was computed. The data were analyzed using an independent t test at a significance level of 95%.nnnRESULTSnNo significant difference in SF was found between the 2 groups of pecking depths. However, the longer pecking depth (4-mm group) showed a significantly larger CSF compared with the shorter pecking depth group (Pxa0<xa0.05).nnnCONCLUSIONSnThe shorter pecking depth may generate lower overall stresses for the root dentin as well as the instrument.

Root Canal Shaping Effect of Instruments with Offset Mass of Rotation in the Mandibular First Molar: A Micro–computed Tomographic Study

Introduction: The aim of this study was to evaluate the root canal shaping effect of 2 nickel‐titanium rotary instruments with offset mass of rotation compared with an instrument with a conventional centered mass of rotation. Methods: Thirty‐six extracted human mandibular first molars with 2 independent mesial canals and 1 oval distal canal were selected and divided equally into 3 groups for instrumentation with ProTaper Next (PTN; Dentsply Maillefer, Ballaigues, Switzerland), Revo‐S (RS; Micro‐Mega, Besançon, France), and ProTaper Universal (PTU, Dentsply Maillefer) systems (n = 12). For apical preparation, PTN X2 (#25/0.06), RS SU (#25/0.06), and PTU F2 (#25/0.08) were used in the mesial canals and PTN X3 (#30/0.07), RS AS30 (#30/0.06), and PTU F3 (#30/0.09) were used in the distal canals. Specimens were scanned before and after instrumentation using a SkyScan 1272 scanner (Bruker micro‐CT, Kontich, Belgium) at 10‐&mgr;m isotropic resolution. Changes in the canal area, volume, structure model index (SMI), and untouched canal area were evaluated. Canal transportation and centering ratio were measured at 1, 3, 5, and 7 mm from the apical foramen. Data were statistically analyzed using 1‐way analysis of variance with the Tukey post hoc test. Results: The PTN and RS systems showed significantly less transportation and better centering ability compared with the PTU system at 1 mm from the apical foramen (P < .05). All instrumentation increased the canal area, volume, and SMI values without significant differences among the 3 groups (P > .05). Conclusions: Regardless of the differences in the cross‐sectional design, alloy type, and taper variation, instruments with offset mass of rotation showed better root canal shaping ability compared with an instrument with a centered mass of rotation. HIGHLIGHTSInstruments with offset mass of rotation showed less canal transportation and better centering ability and a comparable amount of dentin removal compared with a system with a conventional centered mass of rotation.The resultant swaggering motion of the systems with offset mass of rotation might have a beneficial effect on dentin removal but not on untouched canal areas.

Safe pseudoelastic limit range under torsional loading with Reciproc Blue

AIMnTo 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.nnnMETHODOLOGYnTwo torsional conditions of Reciproc R25 and Reciproc Blue R25 were tested using a custom device. The first condition fixed the file tips at 3xa0mm and repetitively rotated them, with gradually increasing angles, from 10° to 270°, and the resulting torque was recorded (nxa0=xa015). The second test involved a single continuous rotation until fracture (nxa0=xa015). 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).nnnRESULTSnThe angle at the pseudoelastic limit was significantly greater for Reciproc Blue than Reciproc, regardless of test mode (Pxa0<xa00.05). When torsional loading was repeated, using gradually increasing rotational angles, the torsional resistance was significantly lower than for a single rotation (Pxa0<xa00.05). 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.nnnCONCLUSIONSnThe 170° pre-set angle of the dedicated endodontic motors for the Reciproc system was safe for Reciproc Blue in single or time-restricted use.

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 (Pxa0<xa00·05). The palatal roots of maxillary first molars exhibited the highest incidence as well as the greatest mean length (1·96xa0mm) 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.