Comparison of cyclic fatigue resistance of four pediatric rotary file systems at body temperature: an in vitro study.

BACKGROUND: The aim of this study was to compare the cyclic fatigue resistance (CFR) of the newly developed pediatric nickel-titanium (NiTi) rotary file systems for root canal preparation of primary teeth. METHODS: Eighty pediatric NiTi rotary file systems files were used in this study, including 20 EasyInSmile X-Baby (25/0.04), 20 Scope miniScope (25/0.04), 20 EndoArt Pedo Gold (25/0.04), and 20 EndoArt Pedo Blue (25/0.04) files. Static CFR tests; performed on a custom-made stainless steel block with an inner diameter of 1.5 mm, an angle of curvature of 60° and a curved artificial canal with a radius of curvature of 5 mm. The test system was filled with distilled water and the temperature was kept constant at 35 ± 1 °C. The files were rotated in the simulated canal until fracture occurred. The number of cycles to failure (NCF) data was recorded, and all the fracture surfaces of the files were evaluated using scanning electron microscopy (SEM). One-way ANOVA and Tukey tests were used for statistical analysis of the data, and the significance level was set at p < 0.05. RESULTS: EndoArt Pedo Blue showed the highest NCF values (2668.10 ± 755.26), while the miniScope showed the lowest NCF values (453.65 ± 72.51), with a statistically significant difference between all the tested file systems (p < 0.05). There was no statistical difference in terms of fractured fragment lengths among all tested files (p > 0.05). CONCLUSION: The EndoArt Pedo Blue file system showed the best CFR performance among the four file systems specifically designed for primary teeth.

Phase Transformation and Mechanical Behaviour of Different Heat-Treated Nickel-Titanium Rotary Instruments.

OBJECTIVES: This study evaluated the phase composition, phase transformation behaviour, and mechanical properties of five heat-treated NiTi instruments. METHODS: ProTaper NEXT (M-wire, PTN), ProTaper Gold (Gold-wire, PTG), One Curve (C-wire, OC), EdgeTaper Platinum (Fire-wire, ETP), NeoNiTi (electrical discharge machining-wire, NNA), and ProTaper Universal (conventional wire, PTU, control) with #25-tip size were tested (n = 12/group) for cyclic fatigue resistance (number of cycles to failure; NCF) and torsional resistance (angle of rotation to fracture and maximum torque at fracture [ultimate torsional strength]). The geometry and fracture surfaces of the tested instruments were examined by scanning electron microscopy. The phase transformation temperature and phase composition of the instruments were evaluated using differential scanning calorimetry and X-ray diffraction. Data were statistically analysed using one-way ANOVA and Tukey's post hoc test, with the significance level set at 5%. RESULTS: PTG showed the highest NCF (P < .05) at 37°C, while ETP exhibited the highest angle of rotation to fracture, ultimate torsional strength, and stiffness (P < .05). Scanning electron microscopy demonstrated typical clusters of fatigue striations and numerous cracks after cyclic fatigue fracture, whereas there was a concentric abrasion pattern with a dimple and microvoids at the centre after torsional fracture. In differential scanning calorimetry curves, austenite-finishing temperatures of heat-treated instruments were higher than 37°C, whereas that of PTU was lower than 37°C. PTU showed strong peaks of austenite at 25 and 37°C, whereas ETP showed a strong peak of R-phase at 25°C, but mostly austenite phase at 37°C in X-ray diffraction. CONCLUSIONS: Geometry, alloy type, and phase transformation temperatures of NiTi instruments affected their mechanical behaviour. CLINICAL RELEVANCE: PTG showed the highest NCF, suitable for markedly curved canals. ETP had the highest torsional resistance, appropriate for narrow and constricted canals.

Calculating the fatigue strength of load-bearing structures of special self-propelled rolling stock.

This research primarily focuses on the strength indicators of the bearing structures of ADM-1 special self-propelled rolling stock. The special self-propelled rolling stock used by Uzbek railroads reaching the end of their functional life is a pertinent problem as Uzbekistan's railway system is growing rapidly, but there is a lack of enough funds to buy new special self-propelled rolling stock. Hence, it is vital to fix the issues with ADM-1 special self-propelled rolling stock by overhauling them. At the outset, the researchers divided the frame of a special a self-propelled rolling stock into multiple sections. Subsequently, these individual sections were analyzed closely to spot out issues. The precise location of the fatigue defect occurrence on the longitudinal beams was determined by the analysis of the individual sections of the special self-propelled rolling stock. During the motor carriage's modernization, which is an approach to extend the service life and improve the durability of special self-propelled rolling stock, this analysis helped in pinpointing exactly the location on the frame where the stress measurements had to be calculated. Pre- and post-modernization calculations were carried out on the vehicle to determine the optimal placement of the reinforcing plates. Additionally, normative calculations were also conducted and a new design mode distinct from the repair loads was implemented. The computation results revealed that the fatigue resistance reserve coefficient and service life value prior to the bearing structure's modernization in section 1 were below the required values of n = 1.5 and 1.49, respectively. All the sections of the load-bearing structure fulfilled the fatigue resistance reserve coefficient standards after the modernization. The computational model of the motor carriage's structural strength was created in the ANSYS Workbench platform. This research intends to enhance the strength determination procedures and provides recommendations for design and restoration of modern structures of special self-propelled rolling stock.

Strong and Fatigue-Resistant Carbon Nanotube Composites Enabled by Amorphous/Crystalline Heterophase Shell.

Lightweight materials with high strength and long cyclic lifespan are greatly demanded in practical applications, yet these properties are usually mutually exclusive. Here, we present a strong, lightweight, highly deformation-tolerant, and fatigue-resistant carbon nanotube (CNT) composite enabled by an amorphous/crystalline heterophase carbon shell. In particular, we obtain nanocrystallites with CNT-induced crystalline orientation uniformly embedded within an amorphous matrix by controlled thermal annealing. The heterophase carbon shell effectively alleviates the stress concentration and inhibits crack propagation, which renders our composite superior mechanical properties and high fatigue resistance (106 compression cycles at 20% strain with high stress of 144 kPa, or 5 × 105 cycles at 50% strain with stress up to 260 kPa). This study provides a deep understanding of amorphous-crystalline phase transition and insight into utilizing phase engineering to design and develop other high-performance functional materials such as structural materials and catalysis.

Design and performance of double-layered artificial chordae.

Surgical repair with artificial chordae replacement has emerged as a standard treatment for mitral regurgitation. Expanded polytetrafluoroethylene (ePTFE) sutures are commonly employed as artificial chordae; however, they have certain limitations, such as potential long-term rupture and undesired material/tissue response. This study introduces a novel approach to artificial chordae design, termed the New Artificial Chordae (NAC), which incorporates a double-layered structure. The NAC comprises a multi-strand braided core composed of ultra-high molecular weight polyethylene (UHMWPE) fibers as the inner core, and an outer tube made of hydrophobic porous ePTFE. Compared to traditional ePTFE sutures, NAC exhibits increased flexibility, enhanced tensile strength, longer elongation and improved fatigue resistance. Moreover, NAC exhibits a more hydrophobic surface, which contributes to enhanced hemocompatibility. The study also includes in vivo investigations conducted on animal models to evaluate the biocompatibility and functional efficacy of the artificial chordae. These experiments demonstrate the enhanced durability and biocompatibility of the NAC, characterized by improved mechanical strength, minimal tissue response and reduced thrombus formation. These findings suggest the potential application of NAC as a prosthetic chordae replacement, offering promising prospects to address the limitations associated with current artificial chordae materials and providing novel ideas and approaches for the development of sustainable and biocompatible regenerative biomaterials.

Mechanical Properties of Additively Manufactured Polymeric Materials-PLA and PETG-For Biomechanical Applications.

The study presented herein concerns the mechanical properties of two common polymers for potential biomedical applications, PLA and PETG, processed through fused filament fabrication (FFF)-Material Extrusion (ME). For the uniaxial tension tests carried out, two printing orientations-XY (Horizontal, H) and YZ (Vertical, V)-were considered according to the general principles for part positioning, coordinates, and orientation typically used in additive manufacturing (AM). In addition, six specimens were tested for each printing orientation and material, providing insights into mechanical properties such as Tensile Strength, Young's Modulus, and Ultimate Strain, suggesting the materials' potential for biomedical applications. The experimental results were then compared with correspondent mechanical properties obtained from the literature for other polymers like ASA, PC, PP, ULTEM 9085, Copolyester, and Nylon. Thereafter, fatigue resistance curves (S-N curves) for PLA and PETG, printed along 45°, were determined at room temperature for a load ratio, R, of 0.2. Scanning electron microscope observations revealed fibre arrangements, compression/adhesion between layers, and fracture zones, shedding light on the failure mechanisms involved in the fatigue crack propagation of such materials and giving design reference values for future applications. In addition, fractographic analyses of the fatigue fracture surfaces were carried out, as well as X-ray Computed Tomography (XCT) and Thermogravimetric (TGA)/Differential Scanning Calorimetric (DSC) tests.

An Elastomeric Lithium-Conducting Interlayer for High-Performance LATP-Based Lithium Metal Batteries.

In response to the critical challenges of interfacial impedance and volumetric changes in Li(1+x)AlxTi(2­x)(PO4)3 (LATP)-based lithium metal batteries, an elastomeric lithium-conducting interlayer fabricates from fluorinated hydrogenated nitrile butadiene rubber (F-HNBR) matrix is introduced herein. Owing to the vulcanization, vapor-phase fluorination, and plasticization processes, the lithium-conducting interlayer exhibits a high elasticity of 423%, exceptional fatigue resistance (10 000 compression cycles), superior ionic conductivity of 6.3 × 10-4 S cm-1, and favorable lithiophilicity, rendering it an ideal buffer layer. By integrating the F-HNBR interlayer, the LATP-based lithium symmetric cells demonstrate an extended cycle life of up to 1600 h at 0.1 mA cm-2 and can also endure deep charge/discharge cycles (0.5 mAh cm-2) for the same duration. Furthermore, the corresponding lithium metal full cells achieve 500 cycles at 0.5 C with 98.3% capacity retention and enable a high-mass-loading cathode of 11.1 mg cm-2 to operate at room temperature.

Comparative study on muscle function in two different streptozotocin-induced diabetic models.

AIMS: Streptozotocin (STZ) is widely used to study diabetic complications. Owing to the nonspecific cytotoxicity of high-dose STZ, alternative models using moderate-dose or a combination of low-dose STZ and a high-fat diet have been established. This study aimed to investigate the effects of these models on muscle function. METHODS: The muscle function of two STZ models using moderate-dose STZ (100 mg/kg, twice) and a combination of low-dose STZ and high-fat diet (50 mg/kg for 5 consecutive days + 45% high-fat diet) were examined using in vivo electrical stimulation. Biochemical and gene expression analysis were conducted on the skeletal muscles of the models immediately after the stimulation. RESULTS: The contractile force did not differ significantly between the models compared to respective controls. However, the moderate-dose STZ model showed more severe fatigue and blunted exercise-induced glycogen degradation possibly thorough a downregulation of oxidative phosphorylation- and vasculature development-related genes expression. CONCLUSIONS: Moderate-dose STZ model is suitable for fatigability assessment in diabetes and careful understanding on the molecular signatures of each model is necessary to guide the selection of suitable models to study diabetic myopathy.

An in vitro evaluation of the fatigue behavior of resin composite materials as part of a translational research cycle.

PURPOSE: This study aimed to reproduce and translate clinical presentations in an in vitro set-up and evaluate laboratory outcomes of mechanical properties (flexural strength, fatigue resistance, wear resistance) and link them to the clinical outcomes of the employed materials in the Radboud Tooth Wear Project (RTWP). MATERIALS AND METHODS: Four dental resin composites were selected. 30 discs (Ø12.0 mm, 1.2 mm thick) were fabricated for each of Clearfil TM AP-X (AP), Filtek TM Supreme XTE (FS), Estenia TM C&B (ES), and Lava Ultimate (LU). Cyclic loading (200 N, 2 Hz frequency) was applied concentrically to 15 specimens per group with a spherical steatite indenter (r = 3.18 mm) in water in a contact-load-slide-liftoff motion (105 cycles). The wear scar was analysed using profilometry and the volume loss was digitally computed. Finally, all specimens were loaded (fatigued specimens with their worn surface loaded in tension) until fracture in a biaxial flexure apparatus. The differences in volume loss and flexural strength were determined using regression analysis. RESULTS: Compared to AP and FS, ES and LU showed a significantly lower volume loss (p < 0.05). Non-fatigued ES specimens had a similar flexural strength compared to nonfatigued AP, while non-fatigued FS and LU specimens had a lower flexural strength (p < 0.001; 95 %CI: -80.0 - 51.8). The fatigue test resulted in a significant decrease of the flexural strength of ES specimens, only (p < 0.001; 95 %CI: -96.1 - -54.6). CLINICAL RELEVANCE: These outcomes concur with the outcomes of clinical studies on the longevity of these composites in patients with tooth wear. Therefore, the employed laboratory test seems to have the potential to test materials in a clinically relevant way.

Comparative evaluation of marginal adaptation and fatigue resistance of endodontically treated premolars restored with direct and indirect coronal restorations: an in vitro study.

BACKGROUND: An optimum restoration for reconstructing endodontically treated teeth should provide excellent marginal adaptation, high fracture resistance as well as maximum tooth structure conservation. The purpose of this study was to evaluate the marginal adaptation and fatigue resistance of different coronal restorations in endodontically treated premolars. METHODS: Thirty sound maxillary first premolars were endodontically treated and received MOD cavities. Teeth were randomly allocated into three groups (n = 10) according to the type of coronal restoration: Group R: polyethylene fibers (ribbond), fibers-reinforced composite (everX posterior) and final layer of nano-hybrid composite. Group O: indirect lithium disilicate overlay and Group C: fiber-post, resin composite restoration, and lithium disilicate crown. Marginal gap assessment was performed before and after thermocycling (5000 cycles) using stereomicroscope. Samples were subjected to stepwise-stress loading starting at 200 N, and increased by 100 N in each step until failure occurred. Statistical analysis was done by One-way ANOVA followed Tukey`s Post Hoc test for multiple comparison. Paired t test was used to compare the marginal adaptation before and after thermocycling. Survival probability was evaluated by Life table survival analysis. Failure mode analysis was performed with Chi-square test. RESULTS: Marginal gap was significantly the lowest in group R (37.49 ± 5.05) and (42.68 ± 2.38), while being the highest in group C (59.78 ± 5.67) and (71.52 ± 5.18) in before and after thermocycling respectively (P < 0.0001). Fatigue resistance was the highest for group O (1310.8 ± 196.7), and the lowest for group R (905.4 ± 170.51) with a significant difference between groups (P < 0.0001). Crown group had the highest percentage (80%) of catastrophic failure, while, overlay group exhibited the lowest (20%). CONCLUSIONS: Direct restoration without cuspal coverage using ribbon fibers with short FRC provided better marginal adaptation than indirect overlays and crowns, but fatigue resistance wasn't significantly improved. Adhesive ceramic overlays showed the best fatigue performance and the least catastrophic failure rate compared to both direct fiber-reinforced composite and indirect ceramic full coverage restorations. CLINICAL SIGNIFICANCE: Indirect adhesive overlays are a suitable, more conservative restorative option for endodontically treated teeth than full coverage restorations, especially when tooth structure is severely compromised.

Mechanochemically Robust LiCoO2 with Ultrahigh Capacity and Prolonged Cyclability.

Pushing intercalation-type cathode materials to their theoretical capacity often suffers from fragile Li-deficient frameworks and severe lattice strain, leading to mechanical failure issues within the crystal structure and fast capacity fading. This is particularly pronounced in layered oxide cathodes because the intrinsic nature of their structures is susceptible to structural degradation with excessive Li extraction, which remains unsolved yet despite attempts involving elemental doping and surface coating strategies. Herein, a mechanochemical strengthening strategy is developed through a gradient disordering structure to address these challenges and push the LiCoO2 (LCO) layered cathode approaching the capacity limit (256 mAh g-1, up to 93% of Li utilization). This innovative approach also demonstrates exceptional cyclability and rate capability, as validated in practical Ah-level pouch full cells, surpassing the current performance benchmarks. Comprehensive characterizations with multiscale X-ray, electron diffraction, and imaging techniques unveil that the gradient disordering structure notably diminishes the anisotropic lattice strain and exhibits high fatigue resistance, even under extreme delithiation states and harsh operating voltages. Consequently, this designed LCO cathode impedes the growth and propagation of particle cracks, and mitigates irreversible phase transitions. This work sheds light on promising directions toward next-generation high-energy-density battery materials through structural chemistry design.

No sex differences in time-to-task failure and neuromuscular patterns of response during submaximal, bilateral, isometric leg extensions.

BACKGROUND: In general, it has been suggested that females are more fatigue-resistant than males, with the magnitude of difference being most pronounced during low-intensity sustained contractions. However, the mechanisms for the apparent sex difference have not yet been fully elucidated in the literature. This study aimed to examine sex-related differences in fatigability and patterns of neuromuscular responses for surface electromyographic (sEMG) and mechanomyographic (sMMG) amplitude and frequency (MPF) characteristics during a sustained submaximal bilateral, isometric leg extension muscle action. METHODS: A sample of 20 young recreationally active males and females with previous resistance training experience performed a sustained, submaximal, bilateral isometric leg extension until task failure. Time-to-task failure was compared using a nonparametric bootstrap of the 95% confidence interval for the mean difference between males and females. Additionally, patterns of response for sEMG and sMMG amplitude and MPF of the dominant limb were examined using linear mixed effect models. RESULTS: There were no differences in time-to-task failure between males and females. Additionally, neuromuscular responses revealed similar patterns of responses between males and females. Interestingly, sEMG amplitude and sMMG amplitude and MPF all revealed non-linear responses, while sEMG MPF demonstrated linear responses. CONCLUSION: These data revealed that time-to-task failure was not different between males and females during sustained submaximal bilateral, isometric leg extension. Interestingly, the parallel, non-linear, increases in sEMG and sMMG amplitude may indicate fatigue induced increases in motor unit recruitment, while the parallel decreases in sMMG MPF may be explained by the intrinsic properties of later recruited motor units, which may have inherently lower firing rates than those recruited earlier.

Effect of simulated clinical use and sterilization on the cyclic fatigue resistance of nickel titanium files.

Aim: Assess the effect of simulated clinical use and sterilization on the cyclic fatigue resistance of Race Evo and Tia Tornado Blue nickel titanium (NiTi) files. Materials and Methods: For this study, a total of sixty-four NiTi files were selected, with thirty-two files each from two different manufacturers. Files from each manufacturer were subdivided into four subgroups (n = 8) based on the test parameters. The control groups included files that were neither used nor sterilized. Files from the test groups were used to prepare the root canals of extracted mandibular premolars and then sterilized. This procedure was repeated once, twice, or thrice, depending on the test group. All files were then subjected to a cyclic fatigue test. Data was statistically analyzed using the Kruskal-Wallis and Mann-Whitney U tests. Results: No significant difference was observed in the number of cycles to failure (NCF) among the subgroups for both types of files (P = 0.869 for Tia Tornado Blue, P = 0.626 for Race Evo). Tia Tornado Blue files displayed significantly higher NCF values in the control (P = 0.021), once (P = 0.027), and thrice (P = 0.031) usage groups when compared to Race Evo files. Conclusions: Repeated clinical use and sterilization for up to three cycles did not affect the cyclic fatigue resistance of Race Evo and Tia Tornado Blue files.

Lysine-Triggered Polymeric Hydrogels with Self-Adhesion, Stretchability, and Supportive Properties.

Hydrogels, recognized for their flexibility and diverse characteristics, are extensively used in medical fields such as wearable sensors and soft robotics. However, many hydrogel sensors derived from biomaterials lack mechanical strength and fatigue resistance, emphasizing the necessity for enhanced formulations. In this work, we utilized acrylamide and polyacrylamide as the primary polymer network, incorporated chemically modified poly(ethylene glycol) (DF-PEG) as a physical crosslinker, and introduced varying amounts of methacrylated lysine (LysMA) to prepare a series of hydrogels. This formulation was labeled as poly(acrylamide)-DF-PEG-LysMA, abbreviated as pADLx, with x denoting the weight/volume percentage of LysMA. We observed that when the hydrogel contained 2.5% w/v LysMA (pADL2.5), compared to hydrogels without LysMA (pADL0), its stress increased by 642 ± 76%, strain increased by 1790 ± 95%, and toughness increased by 2037 ± 320%. Our speculation regarding the enhanced mechanical performance of the pADL2.5 hydrogel revolves around the synergistic effects arising from the co-polymerization of LysMA with acrylamide and the formation of multiple intermolecular hydrogen bonds within the network structures. Moreover, the acid, amine, and amide groups present in the LysMA molecules have proven to be instrumental contributors to the self-adhesion capability of the hydrogel. The validation of the pADL2.5 hydrogel's exceptional mechanical properties through rigorous tensile tests further underscores its suitability for use in strain sensors. The outstanding stretchability, adhesive strength, and fatigue resistance demonstrated by this hydrogel affirm its potential as a key component in the development of robust and reliable strain sensors that fulfill practical requirements.

Influence of ceramic crown design (translucent monolithic zirconia vs. bilayered) of implant-supported single crowns after mechanical cycling.

OBJECTIVES: This study aimed to assess the influence of translucent monolithic versus bilayered crowns and whether the use of a CoCr base abutments affects the fatigue and fracture resistance of screwed implant-supported single crowns with external connections under mechanical cycling. MATERIALS AND METHODS: Fifty specimens were divided into groups: (1) metal-ceramic (MC) crown, (2) veneered zirconia crown (Zr), (3) veneered zirconia crown with a CoCr base abutment (ZrB), (4) monolithic translucent zirconia crown (MZr), and (5) monolithic translucent zirconia crown with a CoCr base abutment (MZrB). Specimens underwent mechanical cycling (5 × 106 cycles; 150 N) evaluating fatigue resistance (number of failures) and those that failed were subsequently subjected to fractographic analyses (stereomicroscope and scanning electron microscope) to evaluate failure location and area, and maximum fracture load was also measured. RESULTS: The failure-related survival rate (100%) and maximum fracture resistance of the MZrB were significantly higher than those of MC and Zr (50%; p < 0.05). There were no significant differences in the failure rate and fracture resistance when a CoCr base abutment was used or not in the translucent monolithic Zr groups (p > 0.05;MZrB vs. MZr). Failure location, with MC crowns' fractures, noted at the screw area (p = 0.043), while all-ceramic crowns were mostly in the cuspid and to failure area, the Zr group had the largest mean (15.55 ± 9.17 mm2) among the groups, significant difference only when compared with MC (1.62 ± 0.81 mm2) (p = 0.025). CONCLUSIONS: Translucent monolithic zirconia crowns exhibited significantly higher fatigue and fracture resistance compared with conventional MC and bilayered crowns. CLINICAL SIGNIFICANCE: The appropriate choice of material and manufacturing technique is crucial for predicting the higher clinical performance of single crowns. Enhanced mechanical resistance in terms of fatigue and fracture resistance can be achieved by replacing MC and bilayered restorations with computer-aided design and computer-aided manufacturing monolithic zirconia.

Improving Fatigue Resistance and Autonomous Switching of pH Responsive Hydrazones by Pulses of a Chemical Fuel.

The chemically triggered reversible switching of pH-responsive hydrazones involves rotary motion-induced configurational changes, serving as a prototype for constructing an array of molecular machines. Typically, the configurational isomerization of such switches into two distinct forms (E/Z) occurs through the alteration of the pH the medium, achieved by successive additions of acid and base stimuli. However, this process results in intermittent operation due to the concomitant accumulation of salt after each cycle, limiting switching performance to only a few cycles (5-6). In this context, we introduce a novel strategy for the autonomous E/Z isomerization of hydrazones in acetonitrile using pulses of trichloroacetic acid as a chemical fuel. The use of this transient acid enabled reversible switching of hydrazones even after 50 cycles without causing significant fatigue. To test the broad viability of the fuel, a series of ortho/para-substituted hydrazones were synthesized and their switching performance was investigated. The analysis of kinetic data showed a strong dependency of switching operations including the lifetime of transient state, on the electronic properties of substituents. Finally, a distinct color change from yellow to orange due to reversible switching of the para-methoxy substituted hydrazone was employed for the creation of rewritable messages on commercially available paper.

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.

Comparative Evaluation of Different Post and Core Systems in Maxillary Molar Endodontic Treatments: An Original Study.

Background: Maxillary molar endodontic treatments require the right post and core system. This study tested cast metal, glass fiber with composite resin, carbon fiber with composite resin, and zirconia post and core materials. Materials and Methods: Sixty removed human maxillary teeth were randomly divided into four post and core material groups (n = 15 per group). The teeth acquired their post and core system after normal endodontic treatment. Fracture resistance, cyclic fatigue resistance, and microleakage analyses were statistically assessed. Results: The zirconia post and core material had the highest fracture resistance (mean: 900 N), followed by carbon fiber with composite resin, cast metal, and glass fiber with composite resin. Zirconia had the highest mean cycles to failure (120,000), followed by carbon fiber with composite resin (100,000), cast metal (110,000), and glass fiber with composite resin (90,000). Zirconia had the lowest mean microleakage score (1.8), and glass fiber with composite resin, the highest (3.0). Conclusion: The zirconia post and core material outperformed the others in fracture resistance, cyclic fatigue resistance, and sealing. The cast metal has better fracture and cycle fatigue resistance than glass fiber with composite resin. These findings help doctors choose maxillary molar endodontic post and core materials.

Effect of Radiotherapy and Taper of Root Canal Preparation on the Biomechanical Behavior of Mesial Roots of Mandibular Molars.

INTRODUCTION: This study aimed to investigate the effect of radiotherapy and taper of root canal preparation on the biomechanical behavior of mesial roots of mandibular molars. METHODS: Eighty mandibular molars with 2 canals in the mesial root were randomly allocated into 2 groups (n = 40): one group underwent irradiation (60 Gy), while the other did not. Subsequently, the mesial roots were sectioned and each group was subdivided into 5 subgroups (n = 8), according to the preparation taper: no preparation (control); 25.03; 25.04; 25.06; and 25.08. All groups were considered homogeneous regarding their dimensions, weight, and morphology. The prepared specimens were embedded in cylindrical plastic molds and subjected to a cyclic fatigue test. A failure analysis was performed according to the extension and course of the fractures. Two-way ANOVA, Tukey's post-hoc, Fisher's exact, and Kaplan-Meier tests were conducted to evaluate the obtained data (α = 5%). RESULTS: Fatigue resistance decreased as the taper of the preparation increased (P < .05). Preparation 25.03 presented significantly higher values only than 25.08 (P < .05), while 25.04, 25.06, and 25.08 were considered similar (P > .05). Irradiation significantly reduced the biomechanical performance (P < .05). Survival analysis corroborated these findings. There were no differences in the distribution of fracture types among the groups (P > .05). CONCLUSION: The biomechanical behavior of the mesial roots of the mandibular molars decreased significantly in the face of irradiation and as the taper of the preparation increased.

Advanced Solid Geopolymer Formulations for Refractory Applications.

Cement, as a construction material, has low thermal resistance, inherent fire resistance, and is incombustible up to a certain degree. However, the loss of its mechanical performance and spalling are its primary issues, and it thus cannot retain its performance in refractory applications. The present study explores the performance of geopolymer formulations that have excellent fire resistance properties for potential refractory applications. This study is unique, as it investigates advanced solid geopolymer formulations that need only water to activate and bind. Various solid geopolymer formulations with fly ash as a precursor; potassium hydroxide and potassium silicate as activators; and mullite and alumina as refractory aggregates were studied for their compressive strength at up to 1100 °C and compared with their two-part conventional liquid alkaline geopolymer counterparts. Advanced solid geopolymer formulations with mullite and alumina as refractory aggregates had mechanical strength values of 84 MPa and 64 MPa post-1100 °C exposure and were further exposed to ten thermal cycles of 1100 °C to study their fatigue resistance and post-exposure compressive strengths. The geopolymer sample with mullite as a refractory aggregate yielded 115.2 MPa compressive strength after the fourth cycle of exposure. This sample was also studied for its temperature distribution upon direct flame exposure. All the geopolymer formulations displayed a drop in compressive strength at 600 °C due to viscous sintering and then a rise in strength at 1100 °C due to phase transformation. X-ray diffraction studies revealed that the formation of crystalline phases such as leucite, sanidine, and annite were responsible for the superior strengths at 1100 °C for the alumina- and mullite-based geopolymer formulations.