Yu-feng Zheng

Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments.

INTRODUCTIONnThe performance and mechanical properties of nickel-titanium (NiTi) instruments are influenced by factors such as cross-section, flute design, raw material, and manufacturing processes. Many improvements have been proposed by manufacturers during the past decade to provide clinicians with safer and more efficient instruments.nnnMETHODSnThe mechanical performance of NiTi alloys is sensitive to their microstructure and associated thermomechanical treatment history. Heat treatment or thermal processing is one of the most fundamental approaches toward adjusting the transition temperature in NiTi alloy, which affects the fatigue resistance of NiTi endodontic files. The newly developed NiTi instruments made from controlled memory wire, M-Wire (Dentsply Tulsa Dental Specialties, Tulsa, OK), or R-phase wire represent the next generation of NiTi alloys with improved flexibility and fatigue resistance. The advantages of NiTi files for canal cleaning and shaping are decreased canal transportation and ledging, a reduced risk of file fracture, and faster and more efficient instrumentation. The clinician must understand the nature of different NiTi raw materials and their impact on instrument performance because many new instruments are introduced on a regular basis.nnnRESULTSnThis review summarizes the metallurgical properties of next-generation NiTi instruments, the impact of thermomechanical treatment on instrument flexibility, and the resistance to cyclic fatigue and torsion.nnnCONCLUSIONSnThe aim of this review was to provide clinicians with the knowledge necessary for evidence-based practices, maximizing the benefits from the selection and application of NiTi rotary instruments for root canal treatment.

In Vitro Cytotoxicity Evaluation of a Novel Root Repair Material

INTRODUCTIONnThis study examined the effect of a new bioactive dentin substitute material (Biodentine) on the viability of human gingival fibroblasts.nnnMETHODSnBiodentine, White ProRoot mineral trioxide aggregate (MTA), and glass ionomer cement were evaluated. Human gingival fibroblasts were incubated for 1, 3, and 7 days both in the extracts from immersion of set materials in culture medium and directly on the surface of the set materials immersed in culture medium. Fibroblasts cultured in Dulbecco modified Eagle medium were used as a control group. Cytotoxicity was evaluated by flow cytometry, and the adhesion of human gingival fibroblasts to the surface of the set materials was assessed by using scanning electron microscopy. The data of cell cytotoxicity were analyzed statistically by using a one-way analysis of variance test at a significance level of P< .05.nnnRESULTSnCells exposed to extracts from Biodentine and MTA showed the highest viabilities at all extract concentrations, whereas cells exposed to glass ionomer cement extracts displayed the lowest viabilities (P< .05). There was no significant difference in cell viabilities between Biodentine and MTA during the entire experimental period (P> .05). Human gingival fibroblasts in contact with Biodentine and MTA attached to and spread over the material surface after an overnight culture and increased in numbers after 3 and 7 days of culture.nnnCONCLUSIONSnBiodentine caused gingival fibroblast reaction similar to that by MTA. Both materials were less cytotoxic than glass ionomer cement.

Physical Properties of 5 Root Canal Sealers

INTRODUCTIONnThe aim of this study was to evaluate the pH change, viscosity and other physical properties of 2 novel root canal sealers (MTA Fillapex and Endosequence BC) in comparison with 2 epoxy resin-based sealers (AH Plus and ThermaSeal), a silicone-based sealer (GuttaFlow), and a zinc oxide-eugenol-based sealer (Pulp Canal Sealer).nnnMETHODSnISO 6876/2001 specifications were followed. The pH change of freshly mixed and set sealers was evaluated during periods of 1 day and 5 weeks, respectively. The viscosity was investigated at different injection rates (72, 10, and 5 mm/min) at room temperature by using a syringe-based system that was based on the Instron 3360 series universal testing system.nnnRESULTSnThe flow, dimensional change, solubility, and film thickness of all the tested sealers were in agreement with ISO 6876/2001 recommendations. The MTA Fillapex sealer exhibited a higher flow than the Endosequence BC sealer (P < .05). The MTA Fillapex and Endosequence BC sealers showed the highest film thicknesses among the tested samples. The Endosequence BC sealer exhibited the highest value of solubility, which was in accordance with 3% mass fraction recommended by the ISO 6876/2001, and showed an acceptable dimensional change. The MTA Fillapex and Endosequence BC sealers presented an alkaline pH at all times. The pH of fresh samples of the AH Plus and ThermaSeal sealers was alkaline at first but decreased significantly after 24 hours. The viscosity of the tested sealers increased with the decreased injection rates.nnnCONCLUSIONSnThe tested sealers were pseudoplastic according to their viscosities as determined in this study. The MTA Fillapex and Endosequence BC sealers each possessed comparable flow and dimensional stability but higher film thickness and solubility than the other sealers tested.

Metallurgical Characterization of Controlled Memory Wire Nickel-Titanium Rotary Instruments

INTRODUCTIONnTo improve the fracture resistance of nickel-titanium (NiTi) files, manufacturers have introduced new alloys and developed new manufacturing processes for the fabrication of NiTi files. This study aimed to examine the phase transformation behavior and microstructure of NiTi instruments from a novel controlled memory NiTi wire (CM wire).nnnMETHODSnInstruments of EndoSequence (ES), ProFile (PF), ProFile Vortex (Vortex), Twisted Files (TF), Typhoon (TYP), and Typhoon™ CM (TYP CM), all size 25/.04, were examined by differential scanning calorimetry (DSC) and x-ray diffraction (XRD). Microstructures of etched instruments were observed by optical microscopy and scanning electron microscopy with x-ray energy-dispersive spectrometric (EDS) analyses.nnnRESULTSnThe DSC analyses showed that each segment of the TYP CM and Vortex instruments had an austenite transformation completion or austenite-finish (A(f)) temperature exceeding 37°C, whereas the NiTi instruments made from conventional superelastic NiTi wire (ES, PF, and TYP) and TF had A(f) temperatures substantially below mouth temperature. The higher A(f) temperature of TYP CM instruments was consistent with a mixture of austenite and martensite structure, which was observed at room temperature with XRD. All NiTi instruments had room temperature martensite microstructures consisting of colonies of lenticular features with substantial twinning. EDS analysis indicated that the precipitates in all NiTi instruments were titanium-rich, with an approximate composition of Ti(2)Ni.nnnCONCLUSIONSnThe TYP CM and Vortex instruments with heat treatment contribute to increase austenite transformation temperature. The CM instrument has significant changes in the phase transformation behavior, compared with conventional superelastic NiTi instruments.

Influence of cross-sectional design and dimension on mechanical behavior of nickel-titanium instruments under torsion and bending: A numerical analysis

INTRODUCTIONnThe aim of this study was to examine the influence of the cross-sectional configuration and dimensions (size and taper) on the torsional and bending behavior of nickel-titanium rotary instruments, taking into account the nonlinear mechanical properties of material.nnnMETHODSnTen cross-sectional configurations, square, triangular, U-type, S-type (large and small), convex-triangle, and 4 proprietary ones (Mani NRT and RT2, Quantec, and Mtwo), were analyzed under torsion or bending by using a 3-dimensional finite element method. The von Mises stresses were correlated with the critical values for various phases of the nickel-titanium material.nnnRESULTSnDifferent loading conditions led to unequal patterns of stress distribution. Increasing the applied torque or bending angle resulted in a rise in the corresponding stresses in the instrument. Favorable stress distribution without dangerous stress concentration was observed if the material was undergoing superelastic transformation at that applied load. The ultimate strength of the material was not exceeded when the instrument was bent up to a 50-degree curvature. On the other hand, when a torsional moment of greater than 1.0 N*mm was applied, the maximum stresses developed in some designs would exceed the ultimate strength of the material. Little variation in the von Mises stresses was observed for instruments of different nominal sizes and tapers on bending to similar extent.nnnCONCLUSIONSnThe cross-sectional design has a greater impact than taper or size of the instrument on the stresses developed in the instrument under either torsion or bending. Certain cross-sectional configurations are prone to fracture by excess torsional stresses.

In Vitro Cytotoxicity of Calcium Silicate–containing Endodontic Sealers

INTRODUCTIONnThe cytotoxicity of 2 novel calcium silicate-containing endodontic sealers to human gingival fibroblasts was studied.nnnMETHODSnEndoSequence BC (Brasseler, Savannah, GA), MTA Fillapex (Angelus Indústria de Produtos Odontológicos S/A, Londrina, PR, Brazil) and a control sealer (AH Plus; Dentsply DeTrey GmbH, Konstanz, Germany) were evaluated. Human gingival fibroblasts were incubated for 3 days both with the extracts from fresh and set materials in culture medium and cultured on the surface of the set materials in Dulbecco-modified Eagle medium. Fibroblasts cultured in Dulbecco-modified Eagle medium were used as a control group. Cytotoxicity was evaluated by flow cytometry, and the adhesion of the fibroblasts to the surface of the set materials was assessed using scanning electron microscopy. The data of cell cytotoxicity were analyzed statistically using a 1-way analysis of variance test at a significance level of P < .05.nnnRESULTSnCells incubated with extracts from BC Sealer showed higher viabilities at all extract concentrations than cells incubated with extracts from freshly mixed AH Plus and fresh and set MTA Fillapex, esspecially for the high extract concentrations (1:2 and 1:8 dilutions). Extracts from set MTA Fillapex of 2 weeks and older were more cytotoxic than extracts from freshly mixed and 1-week-old cement. With extract concentrations of 1:32 and lower, MTA Fillapex was no longer cytotoxic. After setting, AH Plus was no longer cytotoxic, and the fibroblast cells grew on set AH Plus equally as well as on BC Sealer.nnnCONCLUSIONSnBC Sealer and MTA Fillapex, the 2 calcium silicate-containing endodontic sealers, exhibited different cytotoxicity to human gingival fibroblasts.

Mechanical Properties of Controlled Memory and Superelastic Nickel-Titanium Wires Used in the Manufacture of Rotary Endodontic Instruments

INTRODUCTIONnThe aim of this study was to investigate the structure and mechanical properties of newly developed controlled memory (CM) nickel-titanium wires used in the manufacture of rotary endodontic instruments.nnnMETHODSnThe composition and the phase transformation behavior of both types of wires were examined by x-ray energy dispersive spectroscopy and differential scanning calorimetry, respectively. Conventional superelastic (SE) nickel-titanium wire was used as a control. The mechanical properties of the wires at selected temperatures (room temperature, 37°C, and 60°C) were evaluated with tensile, cyclic tensile, and cantilever bending tests by using an Instron 3365 universal testing machine. The data of austenitic transformation finishing temperature (A(f)) were analyzed statistically by using 1-way analysis of variance test at a significance level of P < .05.nnnRESULTSnThe raw CM wires contained a nickel content of 50.7% ± 0.5% and possessed a relatively higher A(f) than SE wires (P < .05). The critical plateau stress and ultimate tensile strength of the CM wires were lower than they were for the SE wires, but the maximum strain before fracture of the CM wires (58.4% ± 7.5% to 84.7% ± 6.8%) was more than 3 times higher than it was for SE wires (16.7% ± 3.8% to 27.5% ± 5.4%). The maximum strain of the CM wires with a diameter of 1.22 mm tested at room temperature (23°C ± 2°C) was up to 84% ± 6.4%. CM wires were not SE at either room temperature or 37°C; however, they exhibited superelasticity when heated to 60°C.nnnCONCLUSIONSnThe raw CM wires exhibited different phase transformation behavior and mechanical properties when compared with SE wires, attributing to the special heat treatment history of CM wires. This study suggested greater flexibility of endodontic instruments manufactured with CM wires than similar instruments made of conventional SE wires.

A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments.

AIMnTo evaluate the bending fatigue lifetime of nickel-titanium alloy (NiTi) and stainless steel (SS) endodontic files using finite element analysis.nnnMETHODOLOGYnThe strain-life approach was adopted and two theoretical geometry profiles, the triangular (TR) and the square cross-sections, were considered. Both low-cycle fatigue (LCF) lifetime and high-cycle fatigue (HCF) lifetime were evaluated.nnnRESULTSnThe bending fatigue behaviour was affected by the material property and the cross-sectional configuration of the instrument. Both the cross-section factor and material property had a substantial impact on fatigue lifetime. The NiTi material and TR geometry profiles were associated with better fatigue resistance than that of SS and square cross-sections.nnnCONCLUSIONSnWithin the limitations of this study, finite element models were established for endodontic files to prejudge their fatigue lifetime, a tool that would be useful for dentist to prevent premature fatigue fracture of endodontic files.

HyFlex nickel–titanium rotary instruments after clinical use: metallurgical properties

AIMnTo analyse the type and location of defects in HyFlex CM instruments after clinical use in a graduate endodontic programme and to examine the impact of clinical use on their metallurgical properties.nnnMETHODOLOGYnA total of 468 HyFlex CM instruments discarded from a graduate endodontic programme were collected after use in three teeth. The incidence and type of instrument defects were analysed. The lateral surfaces of the defect instruments were examined by scanning electron microscopy. New and clinically used instruments were examined by differential scanning calorimetry (DSC) and x-ray diffraction (XRD). Vickers hardness was measured with a 200-g load near the flutes for new and clinically used axially sectioned instruments. Data were analysed using one-way anova or Tukeys multiple comparison test.nnnRESULTSnOf the 468 HyFlex instruments collected, no fractures were observed and 16 (3.4%) revealed deformation. Of all the unwound instruments, size 20, .04 taper unwound the most often (nxa0=xa05) followed by size 25, .08 taper (nxa0=xa04). The trend of DSC plots of new instruments and clinically used (with and without defects) instruments groups were very similar. The DSC analyses showed that HyFlex instruments had an austenite transformation completion or austenite-finish (Af ) temperature exceeding 37xa0°C. The Af temperatures of HyFlex instruments (with or without defects) after multiple clinical use were much lower than in new instruments (Pxa0<xa00.05). The enthalpy values for the transformation from martensitic to austenitic on deformed instruments were smaller than in the new instruments at the tip region (Pxa0<xa00.05). XRD results showed that NiTi instruments had austenite and martensite structure on both new and used HyFlex instruments at room temperature. No significant difference in microhardness was detected amongst new and used instruments (with and without defects).nnnCONCLUSIONSnThe risk of HyFlex instruments fracture in the canal is very low when instruments are discarded after three cases of clinical use. New HyFlex instruments were a mixture of martensite and austenite structure at body temperature. Multiple clinical use caused significant changes in the microstructural properties of HyFlex instruments. Smaller instruments should be considered as single-use.

Phase Transformation Behavior and Mechanical Properties of Thermomechanically Treated K3XF Nickel-Titanium Instruments

INTRODUCTIONnThe bending and torsional properties of thermomechanically treated K3XF (SybronEndo, Orange, CA) nickel-titanium instruments in relation to their phase transformation behavior were evaluated.nnnMETHODSnNiTi instruments K3 (SybronEndo) and K3XF, both in sizes 25/.04 and 40/.04, were examined by differential scanning calorimetry and X-ray diffraction. The metal composition was determined by scanning electron microscopy with X-ray energy-dispersive spectrometric analyses. The bending property of K3 and K3XF instruments was measured in a cantilever-bending test with a maximum deflection of 4.00 mm. A torsional test of the instruments was evaluated according to the American National Standards Institute/American Dental Association Specification No.xa028.nnnRESULTSnK3 and K3XF instruments had approximately the same chemical composition with a nickel content of 48-49 atomic %. The differential scanning calorimetry analyses showed that each segment of the K3XF instruments (24.89°C ± 1.98°C) had a higher austenite finish temperature than the K3 instruments (17.63°C ± 1.76°C) (P < .05). The bending load values were significantly lower for K3XF than for K3 in the superelastic ranges (P < .05). There was no statistically significant difference between K3 and K3XF in the maximum torque or maximum angular deflection before failure. The torque at fracture values of K3 and K3XF increased significantly with the diameter (P < .05).nnnCONCLUSIONSnK3XF exhibited different phase transformation behavior and flexibility when compared with K3, which may be attributed to the special heat treatment history of K3XF instruments.