NiTi Orthodontic Archwires Research Articles

Comparison of Horizontal and Vertical Deactivation Force, Length and Superelasticity in Three Commercial NiTi Orthodontic Archwires

Objectives: The aim of this study was to investigate and to compare the deactivation force, deactivation length and superelasticity in horizontal and vertical directions of Nickel-titanium (NiTi) orthodontic archwires made by 3 different manufactures. Materials and methods: The archwires tested were NiTi-OR (Ormco), NiTi-GH (G&H) and NiTi-H (Highland) and were 0.016 x 0.022 inch2 in cross-section and 25 mm in length. The study analysed load-deflection curves from three-point bending tests performed for each type of NiTi wire in vertical (occluso-gingival) direction (0.022” in horizontal dimension) and horizontal (bucco-lingual) direction (0.016” in vertical dimension) at oral temperature (37±10 oC). Statistical Analysis: Descriptive analysis and Kruskal Wallis test were performed to assess differences in deactivation force, deactivation length and superelasticity among the three brands. A p<0.05 was considered as significant. Results: The deactivation forces ranked from low to high were NiTi-GH, NiTi-OR and NiTi-H, 2.09, 2.57 and 2.90 N, respectively in horizontal dimension (occluso-gingival direction) and 3.04, 3.54 and 3.62 N in vertical dimension (bucco-lingual direction), respectively. For the deactivation length, ranking from long to short were NiTi-GH, NiTi-OR and NiTi-H, 1.08, 1.02 and 0.63 mm in horizontal dimension and 1.63, 1.46 and 1.13 mm in vertical dimension, respectively. In regards to superelasticity, NiTi-OR showed the highest superelasticity, 15.37 in horizontal and 9.68 in vertical dimension, followed by NiTi-GH, 9.51 for horizontal and 6.40 for vertical dimension and NiTi-H, 4.12 for horizontal dimension and 2.96 for vertical dimension. Conclusion: Deactivation force was higher in vertical than horizontal dimension and deactivation length was longer in vertical than horizontal dimension. However, the superelasticity was higher in horizontal than vertical dimension, except NiTi-H. The high priced NiTi wire (NiTi-OR) had the most superelasticity following by medium (NiTi-GH) and low priced wires (NiTi H).

Effect of Temperature on Deactivation Force of Three Commercial NiTi Orthodontic Archwires

Objective: The purpose of this study was to investigate the transition temperature range (TTR) and the effect of temperatures at 10, 20, 30, 40, 50 and 60 °C on the deactivation force of three commercially available NiTi archwires. Materials and methods: Three different brands of NiTi archwires, NiTi OR (Ormco), NiTi GH (G&H) and NiTi H (Highland), with a cross sectional area of 0.016 x 0.022 inch2 were analysed for transformation temperature range (TTR) by using differential scanning calorimeter and load-deflection characteristics using a three-point bending test at temperatures of 10, 20, 30, 37, 40, 50 and 60 °C. Statistical Analysis: Descriptive analysis was used to calculate each variable and Kruskal Wallis test was performed to assess the difference in measurements among the three NiTi wires. P<0.05 was considered as statistical significant. Results: TTR showed austenitic temperature finish (Af) at 24.45 °C for NiTi OR, 27.55 °C for NiTi GH and 51.5 °C for NiTi H. The highest deactivation force was found in NiTi H followed by NiTi OR and NiTi GH at the temperatures below and higher than 37°C. There were siginificant differences (p<0.05) in the deactivation force of NiTi OR - NiTi H and NiTi GH - NiTi H at 10 °C and 20 oC, and NiTi GH – NiTi H and NiTi GH – NiTi OR at 30 °C and 37 °C. However, no significant difference was found among all NiTi wires at 40, 50 and 60 oC, except NiTi GH – NiTi H at 60 oC. A close relationship was found between temperature and unloading curves (deactivaton force); increase in the temperature led to an increase in the plateau of delivery deactivation force and decrease in temperature led to a decrease in the plateau of delivery deactivation force. Conclusion: The TTRs of commercial NiTi archwires are variable, expecially the austenitic finish temperature. The deactivation force increases in higher temperature and decreases in lower temperature.

Evaluation of the biocompatibility of NiTi dental wires: A comparison of laboratory experiments and clinical conditions

Effects of intraoral environment on the surface degradation of nickel-titanium (NiTi) shape memory alloy orthodontic wires was simulated through ex situ static immersion experiments in artificial saliva. The tested wires were compared to companion wires retrieved from patients in terms of chemical changes and formation of new structures on the surface. Results of the ex situ experiments revealed that the acidic erosion effective at the earlier stages of immersion led to the formation of new structures as the immersion period approached 30days. Moreover, comparison of these results with the analysis of wires utilized in clinical treatment evidenced that ex situ experiments are reliable in terms predicting C-rich structure formation on the wire surfaces. However, the formation of C pileups at the contact sites of arch wires and brackets could not be simulated with the aid of static immersion experiments, warranting the simulation of the intraoral environment in terms of both chemical and physical conditions, including mechanical loading, when evaluating the biocompatibility of NiTi orthodontic arch wires.

Effects of Fluoride on NiTi Orthodontic Archwires: An X-ray Diffraction Study

Effects of Fluoride on NiTi Orthodontic Archwires: An X-ray Diffraction Study

Influence of the microstructure on electrochemical corrosion and nickel release in NiTi orthodontic archwires

The aim of this work was to determine the influence of the present phases and the chemical composition on the corrosion behavior and the nickel ion release of the NiTi orthodontic archwires. Eight Ni–Ti archwires from six commercial brands, in the as-received condition, were studied. The chemical composition, roughness, microstructure and the proportion of the phases as well as the corrosion behavior were analyzed for each archwire. The nickel ion release was characterized in artificial saliva immersion settings ranging up to 4weeks. The results show that the presence of the martensitic phase improves corrosion resistance and significantly decreases Ni release into exterior medium in comparison with the austenitic specimens. In spite of the partial loss of superelasticity produced in the martensitic phase, it could be of great interest for biomedical applications, as it could minimize sensitization and allergies and improve biocompatibility and corrosion resistance of NiTi shape memory alloys.

New Ni-free superelastic alloy for orthodontic applications

A potential new Ni-free Ti alloy for biomedical applications was assessed in order to investigate the superelastic behavior, corrosion resistance and the biocompatibility. The alloy studied was Ti19.1Nb8.8Zr. The chemical composition was determined by X-ray microanalysis, the thermoelastic martensitic transformation was characterized by high sensitivity calorimeter. The critical stresses were determined by electromechanical testing machine and the corrosion behavior was analyzed by potentiostatic equipment in artificial saliva immersion at 37°C. The results were compared with six different NiTi orthodontic archwire brands. The biocompatibility was studied by means of cultures of MG63 cells. Ni-free Ti alloy exhibits thermoelastic martensitic transformation with Ms=45°C. The phase present at 37°C was austenite which under stress can induce martensite. The stress–strain curves show a superelastic effect with physiological critical stress (low and continuous) and a minimal lost of the recovery around 150 mechanical cycles. The corrosion resistance improves the values obtained by different NiTi alloys avoiding the problem of the Ni adverse reactions caused by Ni ion release. Cell culture results showed that adhered cell number in new substrate was comparable to that obtained in a commercially pure Ti grade II or beta-titanium alloy evaluated in the same conditions. Consequently, the new alloy presents an excellent in-vitro response.

NiTi superelastic orthodontic wires with variable stress obtained by ageing treatments

A new strategy to obtain NiTi orthodontic archwires with variable correcting stresses in different segments is presented. Different heat treatments are applied in selected segments, which produce variations in the superelastic properties. These allow to modulate the correcting stresses to the requirements of the specific teeth. This method avoids laser welding, which is associated with corrosion and brittleness problems. The aim of this study is to analyze the effect of the heat treatment by zones on transformation temperatures, and stresses in NiTi orthodontic archwires with identical chemical composition. A reduction of the Ti content is observed, in the matrix of the posterior segment, due to the formation of Ti2Ni precipitates. This produces a decrease of the transformation temperatures and consequently an increase of correction stresses. The changes are smaller in the lateral segment due to the lower temperature reached. No changes are observed in the anterior segment in relation to the original archwire. The titanium-rich precipitates undergo observation and characterization by Transmission Electron Microscopy (TEM) and electron diffraction.

An Evaluation of the Corrosion Behavior of Orthodontic Ni-Ti Archwires as a Function of Residence Time in the Oral Cavity

The objective of the present study was to evaluate the effect of the residence time in the oral cavity on the electrochemical and microstructural properties of nickel-titanium superelastic and thermoelastic archwires aiming at establishing optimum treatment parameters regarding materials selection and maintenance of orthodontic apparatus. The corrosion behaviour of commercial archwires of different diameters was studied after periods of use from 1 to 3 months in the oral cavity of patients submitted to orthodontic treatment. Electrochemical measurements and scanning electron microscopy (SEM) were performed. The results revealed that the stiffness of superelastic archwires increased upon use, making it unnecessary to replace archwires in the early stages of treatment with the objective of adapting the diameter of the wire. Electrochemical tests suggested extremely high corrosion resistance of the nickel-titanium alloy even in severe acid environment. SEM images revealed limited pit formation concentrated in the archwire-bracket contacts.

Influence of Soft Drinks with Low pH on Different Ni-Ti Orthodontic Archwire Surface Patterns

The aim of this study was to determine the influence of soft drinks on the surface of Ni-Ti archwires and their corrosion behavior. Archwires with different patterns (smooth, scratch, dimple, and crack) were selected and characterized by scanning electron microscopy and laser confocal microscopy. Immersion tests were performed in artificial saliva (pH 6.7) with a soft drink with a pH of 2.5 for 28 days. The results showed an increase in the surface defects and/or roughness of the dimple, crack and scratch patterns with the immersion times, and a decrease in corrosion resistance. A relationship between the surface pattern and the extent of the corrosion in Ni-Ti archwires with soft drinks at low pH has been demonstrated. Pattern should be taken into account in future studies, and manufacturing processes that produce surface defects (especially cracks) should be avoided.

Variation of the superelastic properties and nickel release from original and reused NiTi orthodontic archwires

Reuse of NiTi orthodontic wires has become increasingly common in dental clinics. For sterilization and recovery of the original superelastic properties of the wires, a heat treatment is usually performed between 500 and 600 °C. The aim of this study was to analyze the effect of these thermal treatments on the mechanical behavior and the microstructure of NiTi archwires of different compositions. A reduction of the Ni content was observed in the matrix of the thermally treated archwires, due to the formation of Ti3Ni4 precipitates. The nickel-rich precipitates were observed and characterized by Transmission Electron Microscopy (TEM) and electron diffraction. They were found to alter the mechanical properties of the wires, decreasing the transformation stresses, and causing a loss of activation of the NiTi archwires. The release of nickel was higher in the original archwires than in the reused ones, due to the matrix nickel depletion caused by the precipitation of Ti3Ni4.

Influence of topographical features on the fluoride corrosion of Ni–Ti orthodontic archwires

Different manufacturing processes of Ni-Ti archwires respond differently to corrosion due to the surface conditions involved. In this study, several topographical features and their influence upon fluoride corrosion were studied. Four topographies (smooth, dimple, scratch, and crack) according to the main surface defect were characterized (n = 40). Static corrosion tests were performed in artificial saliva with fluorated prophylactic gel (12500 ppm) for 28 days. The surface was characterized by SEM and laser confocal microscopy. Standard electrochemical corrosion (open circuit potential, corrosion potential and corrosion current density) was performed. Statistical analysis was carried out using the ANOVA test (α ≤ 0.05). An increase was observed in the surface defects and/or roughness of the cracked and scratched surfaces. These defects produced an important increase in corrosion behavior. The best surfaces for the orthodontic archwires were the smooth and dimpled surfaces, respectively. The increase in defects was independent of roughness. Manufacturing processes that produce surface cracks should be avoided in orthodontic applications.

Composition and Transitional Temperature Range of Several Nickel-Titanium Alloy Wires in Orthodontic Treatment

Objective: The study aimed to clarify the compositions of 14 brands of superelastic NiTi orthodontic wires. Materials and Methods: The compositions were analysed through scanning electron microscopy and energy-dispersive X-ray microanalysis. The wires were tested in differential scanning calorimetry to find the transitional temperature range. Results: The compositions were nickel (50.085%-51.605%), titanium (46.675%-48.140%), copper (0.995%-1.155%), aluminum (0.220%-1.325%), chromium (0%-0.275%) and iron (0%-0.030%). None contained cobalt. G&H, Ormco, AMDG, IMD, Smart, TruFlex, Force 1, NIC, Sentalloy, Ortho Supply and Flexwire were almost or completely austenitic in the oral environment, whereas Highland, Grikin and Unitex were a mixture of austenitic and martensitic phase. The wires showed austenitic finish temperatures ranging from 17°C to 59°C. Conclusions: The results show that NiTi orthodontic arch wires have differences in composition and phase transformation. This information is valuable to evaluate differences in NiTi orthodontic wires for clinical use.

Relationship between the surface defects and the manufacturing process of orthodontic Ni–Ti archwires

The surface defects and roughness of orthodontic Ni–Ti archwires are related to the manufacturing and surface finishing processes employed. On grouping archwires according to their surface characteristics instead the commercial brand, comparisons can be made of the influence of different surface finishings upon the corrosion resistance of Ni–Ti archwires. This novel approach shows that orthodontic Ni–Ti archwires can be grouped into 5 surface patterns according to a dominant surface defect. Each pattern is derived from a given manufacturing process and shows a different response to corrosion in several media.

In vitro evaluation of the electrochemical behaviour of stainless steel and Ni-Ti orthodontic archwires at different temperatures

The aim of this study was to compare the electrochemical corrosion behaviour of stainless steel (SS) and nickel-titanium (Ni-Ti) orthodontic archwires in Ringer's solution at temperatures ranging from 15 to 55°C by using linear sweep voltametry and electrochemical impedance spectroscopy. Polarization curves show that the corrosion current density of SS is greater than that of Ni-Ti alloy. Since the corrosion current density is directly proportional to the corrosion rate, a great corrosion current density shows a lower resistance against corrosion. Therefore, in comparison with SS, Ni-Ti alloys have a lower corrosion rate. Results show that the temperature of the solution affects the corrosion rates of the alloys. As the temperature increases, the corrosion resistance of both of the alloys decreases, although these variations were not so large. Impedance measurements show that the electrochemical behaviour of Ni-Ti exhibits higher polarization resistance and lower capacitance, which means that passive film formed on this alloy is more homogeneous and thicker in comparison with SS. Results also show that polarization resistance of both alloys decreases slowly with increases in the temperature of the solution.

Reduction of Ni release and improvement of the friction behaviour of NiTi orthodontic archwires by oxidation treatments

This work studies NiTi orthodontic archwires that have been treated using a new oxidation treatment for obtaining Ni-free surfaces. The titanium oxide on the surface significantly improves corrosion resistance and decreases nickel ion release, while barely affecting transformation temperatures. This oxidation treatment avoids the allergic reactions or toxicity in the surrounding tissues produced by the chemical degradation of the NiTi. In the other hand, the lack of low friction coefficient for the NiTi superelastic archwires makes difficult the optimal use of these materials in Orthodontic applications. In this study, the decrease of this friction coefficient has been achieved by means of oxidation treatment. Transformation temperatures, friction coefficient and ion release have been determined.

The Mechanical and Thermal Behaviors of Heat-Treated Ni-Rich NiTi Orthodontic Archwires

The near equiatomic nickel-titanium alloy is an outstanding intermetallic compound exhibiting distinctive properties associated with characteristic thermal and stress-induced martensitic transformations. The process of producing orthodontic wires has been modified to obtain the optimal shape memory behaviors. Phase transformation temperatures and load-deflection characteristics of this binary alloy are very significant variables in the performance of this alloy and can be manipulated by different thermomechanical treatments via inducing precipitation or dislocation networks in the matrix. In this study, one brand of commercial heat-activated nickel-titanium archwire (3 M Unitek) was selected and solution treated. Then, the wires annealed at 400 °C for 10, 30, and 60 min. Thermal transformation temperatures were determined using differential scanning calorimeter. It was showed that these temperatures increased with increasing the time of heat treatment and multistage transformation occurred as the result of inhomogeneities. In order to evaluate mechanical parameters of heat-treated archwires, they were placed on an arch-form fixture simulating maxillary dentition and load-deflection curves were obtained by three-point bending test at 37 °C. The results compared to as-received archwires and the best superelasticity was observed after 30 min aging.

Static and Cyclic Load-Deflection Characteristics of NiTi Orthodontic Archwires Using Modified Bending Tests

Near-equiatomic nickel-titanium (nitinol) has the ability to return to a former shape when subjected to an appropriate thermomechanical procedure. One of the most successful applications of nitinol is orthodontic archwire. One of the suitable characteristics of these wires is superelasticity, a phenomenon that allows better-tolerated loading conditions during clinical therapy. Superelastic nitinol wires deliver clinically desired light continuous force enabling effective tooth movement with minimal damage for periodontal tissues. In this research, a special three-point bending fixture was invented and designed to determine the superelastic property in simulated clinical conditions, where the wire samples were held in the fixture similar to an oral cavity. In this experimental study, the load-deflection characteristics of superelastic NiTi commercial wires were studied through three-point bending test. The superelastic behavior was investigated by focusing on bending time, temperature, and number of cycles which affects the energy dissipating capacity. Experimental results show that the NiTi archwires are well suited for cyclic load-unload dental applications. Results show reduction in superelastic property for used archwires after long-time static bending.

Phase transformation analysis of varied nickel-titanium orthodontic wires

The shape memory effect of nickel-titanium (NiTi) archwires is largely determined by the phase transition temperature. It is associated with a reversible transformation from martensite to austenite. The aim of this study was to characterize austenite, martensite and R phase temperatures as well as transition temperature ranges of the commonly used clinical NiTi orthodontic arch wires selected from several manufacturers. Differential scanning calorimetry (DSC) method was used to study the phase transformation temperatures and the phase transition processes of 9 commonly used clinical NiTi alloys (types: 0.40 mm (0.016 inch), 0.40 mm x 0.56 mm (0.016 inch x 0.022 inch)). The austenite finish temperatures (Af) of 0.40 mm Smart, Ormco and 3M NiTi wires were lower than the room temperature, and no phase transformation was detected during oral temperature. Therefore, we predicted that these types of NiTi did not possess shape memory property. For 0.40 mm and 0.40 mm x 0.56 mm Youyan I NiTi wires, no phase transformation was detected during the scanning temperature range, suggesting that these two types of wires did not possess shape memory either. The Af of 0.40 mm x 0.56 mm Smart, L&H, Youyan II Ni-Ti wires were close to the oral temperature and presented as martensitic-austenitic structures at room temperature, suggesting the NiTi wires listed above have good shape memory effect. Although the 0.40 mm x 0.56 mm Damon CuNiTi wire showed martensitic-austenitic structures at oral temperature, its Af was much higher than the oral temperature. It means that transformation from martensite to austenite for this type of NiTi only finishes when oral temperature is above normal. The phase transformation temperatures and transformation behavior varied among different commonly used NiTi orthodontic arch wires, leading to variability in shape memory effect.

Effect of Direct Electric Resistance Heat Treatment on Mechanical Properties of NiTi Orthodontic Arch Wires

Two types of rectangular orthodontic archwires; NiTiTM and 40oCuNiTi, were heat treated by Direct Electric Resistance Heat Treatment (DERHT) using different electric currents for 4 s. Their mechanical properties were then evaluated by micro hardness and three-point bending tests. After applying 4.5-5.5 A current, the hardness of NiTiTM increased with the increased current, whereas the change in hardness of 40oCuNiTi was slight. When 6 A current was applied, the hardness of the midspan of both wires significantly decreased. From the three-point bending test, unloading forces of NiTiTM increased after treating with 5.5 A current, while those of 40oCuNiTi decreased. However, both specimens lost their superelasticity when applied with 6 A current. In conclusion, after DERHT, various changes in mechanical properties can be noted in the different types of nickel titanium archwire.

Fatigue failure of as-received and retrieved NiTi orthodontic archwires

Aim To investigate the fracture resistance of as-received and retrieved NiTi archwires. Materials and methods NiTi archwires (German Orthodontics, CA, USA) of various cross-sections were retrieved from orthodontic patients and brand-, type- and size-matched wires were included as controls. Specimens prepared from the selected wires were subjected to bending deformation at a loading frequency of 2 Hz and cyclic loading was applied either until fatigue failure or with a maximum number of loading cycles of 2 × 10 6. For each wire cross-section a minimum of five specimens were loaded; the cycle number of the fatigue fracture was recorded and the mean of the cycles was calculated from the five specimens to provide the corresponding point in the Wöhler diagram. Scanning electron microscopy was used to assess the fracture pattern of broken wires. The results of the fracture cycles of as-received and retrieved wires were analyzed with a general linear regression model with state (as-received vs. retrieved) and wire size serving as discriminating variables and Tukey's test at the 0.05 level of significance. Results Retrieved wires fractured at a significantly lower number of cycles compared to their as-received matches. The size of the wire played a role in determining fracture with larger cross-sections showing reduced fatigue failure properties and 0.30 and 0.035-mm specimens showing no fracture at the selected strain and number of cycles. The SEM investigations revealed evidence of smoother fractured surfaces in large-sized, rectangular cross-sectioned retrieved wires. Kaplan–Meier survival analysis with log rank test demonstrated the effect of aging and size in predicting survival of NiTi wires. Significance The extended life expectancy of NiTi archwires proposed in current treatment trends is associated with higher probability of fatigue fracture of wires. Large diameter and square or rectangular cross-sections possess an increased chance of failure relative to smaller diameter wires. A necessity may emerge to monitor patients to identify wire failures despite the commonly employed practice of increased time intervals between appointments in patients treated with NiTi wires.