| Abstract|| |
Objectives: This study aims to evaluate and compare the mechanical and metallurgical properties of stainless steel and titanium molybdenum alloy (TMA) archwires, with recently introduced timolium and titanium niobium arch wires.
Materials and Methods: Archwires were categorized into four groups (group I to IV) with 10 samples in each group. They were evaluated for tensile strength, yield strength, modulus of elasticity, load deflection, frictional properties and weld characteristics.
Results: The results were statistically analyzed using ANOVA test and it indicated that stainless steel has high strength, high stiffness and low friction compared to other arch wires, thereby proving that it is the best choice for both sliding as well as frictionless retraction mechanics. TMA with its high formability, low stiffness and low load deflection property is suited to apply consistent force in malaligned teeth but, high friction limits its use in retraction only with loop mechanics.
Conclusion: Timolium possesses comparatively low stiffness, better strength and behaves as an intermediate between stainless steel and TMA and hence can be tried for almost all clinical situations. Low springback and high formability of titanium-niobium archwire allows creation of finishing bends and thus it can be used as finishing archwire.
Keywords: Archwires, mechanical properties, titanium-niobium
|How to cite this article:|
Vijayalakshmi R D, Nagachandran K S, Kummi P, Jayakumar P. A comparative evaluation of metallurgical properties of stainless steel and TMA archwires with timolium and titanium niobium archwires - An in vitro study. Indian J Dent Res 2009;20:448-52
Orthodontic arch wire is the back bone for desired tooth movement. The selection of an appropriate wire size and alloy type with superior weld characteristics is necessary to provide excellent treatment results. Until the 1920's, the only orthodontic wires available were made of gold. Later, in 1929, Austenitic stainless steel which possessed greater strength, higher modulus of elasticity and good corrosion resistance was introduced. Charles Burstone  introduced Beta-Titanium (TMA), which has distinctive features of good spring back, low force delivery, weldability and good formability, in the 1980's.
|How to cite this URL:|
Vijayalakshmi R D, Nagachandran K S, Kummi P, Jayakumar P. A comparative evaluation of metallurgical properties of stainless steel and TMA archwires with timolium and titanium niobium archwires - An in vitro study. Indian J Dent Res [serial online] 2009 [cited 2019 Sep 15];20:448-52. Available from: http://www.ijdr.in/text.asp?2009/20/4/448/59450
Welding of the attachments to the archwire for proper force delivery has always fascinated the clinicians. This made the researchers develop an archwire alloy with superior weld characteristics, which led to the introduction of newer archwire alloys like Beta-Titanium,  timolium and titanium niobium. ,
Timolium  archwire has almost the same frictional resistance and half the stiffness of stainless steel making it an ideal choice for finishing, aligning as well as leveling and Torquing throughout all phases of treatment. Titanium niobium  is an innovative archwire designed for precise tooth-tooth finishing. This wire has the advantage of not having the range of action of Titanium Molybdenum alloy (TMA) but has the stiffness of TMA. This study aims to evaluate and compare the mechanical and metallurgical properties of stainless steel and TMA archwires with recently introduced timolium and titanium niobium arch wires.
| Materials and Methods|| |
In this study, four different archwire materials have been evaluated for tensile strength, yield strength, modulus of elasticity, and load deflections which have been grouped as shown in [Table 1].
Tensile strength, yield strength and modulus of elasticity
Tensile strength, yield strength and modulus of elasticity were measured using the Instron Universal testing machine (Model 4301, Instron Corp, Canton and Mass). A full scale load of 1000 N was set in the machine and operated in tensile mode with a crosshead speed of 1 mm/min. The span of the wire between the crossheads was standardized as 40 mm and the load taken to break the wire was noted. The deflections obtained for the respective loads were plotted on an X-Y plotter. The data obtained was converted into stress-strain curves and the yield load values, modulus of elasticity and ultimate tensile load values were calculated from the slopes of the graphs. The formulae used to calculate the yield strength, modulus of elasticity and ultimate tensile strength were
Load deflection rate
The load deflection characteristics of specimens from each group were evaluated with the help of the three-point bend test as described by Miura et al.  using the Instron universal testing machine (Model 4301, Instron Corp, Canton, Mass) performed on a specially designed jig made of acrylic [Figure 1]. Edgewise brackets with 0.018 × 0.025 inch slot were placed 14 mm apart on a stage to obtain the values at loading as well as unloading (0.5 mm and one mm of loading and 0.5 mm of unloading). The test wire specimen was secured on bracket fixed on the acrylic jig using 0.012-inch elastomeric ligatures. The stage was attached to the lower head of the Instron machine. A Bard Parker (B.P) handle with a horizontal groove was attached to the upper head of machine in such a manner that the tip of the B.P handle was on the center of the test-wire span [Figure 2]. The mid portion of the wire was then deflected one mm with a crosshead speed of one mm/minute and full-scale load of 10 N. The load values were noted at 0.5 mm unloading.
Evaluation of the friction produced at the archwire-bracket interface was done following a test protocol described by Tidy.  This test consisted of a simulated half arch fixed appliance with archwire ligated in portion. Four 0.018 × 0.025- inch edgewise brackets with zero torque and zero angulations were bonded into a rigid Perspex sheet at eight-mm interval [Figure 3]. Space of about 16 mm space left at the center for sliding the canine bracket to simulate canine retraction. The archwires were secured using 0.012-inch elastomeric ligature. The movable canine bracket was soldered with a 12-mm power arm from which weights of 0.05 N/0.1 N were hung to represent the single equivalent force acting at the centre of resistance of the tooth root.
All tests were conducted in dry condition with an Instron universal testing machine. The movable bracket was suspended from the load cell of the testing machine; whereas the base plate (Perspex sheet) was mounted on the crosshead below [Figure 4]. The full scale load was set at 5 N with a crosshead speed of 10 mm/minute. At the start of each test, a trail run was performed with no load on the power arm to check whether there was any binding between the archwire and bracket.
Then a 0.05 N followed by 0.1 N weight was suspended from the power arm, and the load needed to move the bracket across the central span in apparatus was recorded separately. The load cell reading represented the clinical force for retraction that would be applied to a canine, part of which would be critical friction whereas the rest would be the translation force on the tooth. The difference between the load cell reading and load on the power arm represented frictional resistance. The coefficient of friction, both static and kinetic, at the archwire-bracket interface was calculated using the formula given by Tidy. 
P = Frictional resistance (difference between load cell reading and load on the power arm)
F = Equivalent force acting at distance 'h'
h = 10 mm
W = Width of the slot
µ = Coefficient of friction
Tensile-shear test for weld joint strength
Ten archwire specimens from each of the archwire alloy materials were welded in straight lengths using a spot welder and subjected to tensile-shear loading in an Instron universal testing machine. The wire length between the crossheads of the machine was standardized as 40 mm. The full scale load was set at 1000 N with a crosshead speed of one mm/minute. The load taken to break the weld joint divided by the cross-sectional area of the weld surface gave the value for joint strength in N/mm 2 .
| Results|| |
Ultimate tensile, yield strength and modulus of elasticity of plain wires
The values of tensile strength, yield strength and modulus of elasticity indicate a superior strength of the stainless steel (group I) followed by the timolium (group III), TMA (group II) and titanium niobium (group IV) arch wires, as summarized in [Table 2].
Load deflection rate
The load deflection values obtained through three-point bend testing of wire specimens from group I to group IV [Table 3] were analyzed statistically and one-way ANOVA at 95% confidence level was used to calculate the P value. Multiple-range test by Tukey-HSD procedure was employed to identify the significant groups at 5% level and were highly significant for 0.5, one mm loading and 0.5 mm unloading. The rank order of wires according to load deflection characteristics can be summarized as:
[0.5 mm loading: Stainless steel > Timolium > Titanium Niobium > TMA
One mm loading: Stainless steel > Timolium > Titanium Niobium > TMA
0.5 mm Unloading: TMA > Titanium Niobium > Timolium > stainless steel.]
The load values for frictional resistance obtained were substituted in the equation for determining the coefficient of friction (µ). The values were tabulated, and the means and standard deviations calculated [Table 4]. One-way ANOVA at 95% confidence level was used to calculate the P value. Multiple-range test by Tukey-HSD procedure was employed to identify the significant groups at 5% level. The rank order of the wires in descending order of frictional properties can be summarized as:
Static friction (0.5N and 1N) titanium niobium > TMA > Timolium > Stainless steel.
Kinetic friction (0.5N and 1N) titanium niobium > TMA> Timolium > Stainless steel.
Ultimate tensile, yield strength and modulus of elasticity of welded wires
The values indicate a superior strength for TMA (group II) followed by titanium niobuim (group IV), Stainless steel (group I) and timolium (group III) which are summarized in [Table 5].
| Discussion|| |
An archwire for a given clinical situation is selected taking into account the mechanical properties and manipulative characteristics of the alloy. Ideal archwires should possess a good balance of environmental stability, stiffness, resilience and formability. Archwire of 0.016 × 0.022 inch dimension was considered in this study since it is the most commonly used archwire in all the prescriptions and slot dimensions. A modified version of the three-point bend test by Miura et al.  was performed to evaluate the load deflection property which is one of the most important parameter in determining the biologic nature of the tooth movement.
The results of the present study clearly indicated the kinder nature of beta titanium archwires to tooth as well as supporting tissues. Due to its low load deflection value, the engagement of the archwire into the bracket becomes easier in a malaligned tooth. Evaluation of loading characteristics revealed a resilient as well as consistent nature of TMA wires when compared with the other three archwire alloys.
Stainless steel was the more rigid among four archwire alloys with very high loading values and less spring back properties. Stainless steel was followed by timolium and titanium niobium. This indicates that the hysteresis (energy loss upon loading) is associated with all four archwire alloys and is higher with stainless steel followed by timolium, titanium niobium and TMA. Frictional force has long been an important consideration in orthodontic mechanotherapy. It is a well known fact that any force needed to retract teeth must overcome friction.
The friction encountered during tooth movement can be divided into static and kinetic. Static friction is defined as the force required initiating tooth movement, where as kinetic friction is the force that resists motion. Static as well as dynamic frictions have been evaluated in this study by the method proposed by Tidy;  the results clearly indicate greater friction at the archwire-bracket interface when titanium niobium wires are used, in comparison with the other three archwire alloys. The least archwire-bracket interface friction was observed with stainless steel archwires, which was similar to the study by Cash et al.  Frictional characteristics of TMA were secondary to timolium but superior to titanium niobium. Timolium can be considered superior to TMA but inferior to stainless steel in its frictional characteristics.
Clinically, this means that the net force required for translatory movement will be lower for stainless steel followed by timolium and TMA. Higher forces are required when titanium niobium wires are used. Spot welding involves joining of metal with simultaneous application of pressure and current. Resistance to the flow of current at the weld mate interface results in nugget formation, which helps to join the material. Strength testing is an important aspect of any weldability study, as effective joints should be produced without weakening the mechanical properties of the wire.
Static tensile-shear test is the most preferred laboratory method to evaluate the joint strength at a weld area because of its simplicity and it clearly demonstrate the comparative mechanical properties of arch wires. ,,
The present study used the tensile-shear test to evaluate four archwire alloy materials-stainless steel, TMA, timolium and titanium niobium-for weld joint strength. The results of this study indicated a higher strength for Beta titanium archwires with the capability of tolerating a greater amount of orthodontic force before failure and are in accordance with earlier studies. ,,,.
Titanium niobium was next in weld strength values, followed by stainless steel. Timolium exhibited very low values on the tensile-shear test when compared with the other three alloys.
| Conclusion|| |
The following conclusions were drawn:
- Tensile testing of the plain archwire alloys indicate clearly superior strength for stainless steel followed by timolium, TMA and titanium niobium.
- Three-point testing for load deflection indicates that TMA generates low and consistent force followed by titanium niobium, timolium and stainless steel.
- Frictional resistance values show that titanium niobium archwire generated highest frictional force compared to other archwires and stainless steel had the lowest followed by timolium.
- Tensile testing of the welded archwire alloys indicates higher strength for TMA archwires with the capability of tolerating a greater amount of orthodontic force before failure, followed by titanium niobium and Stainless steel; timolium exhibits the least value.
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R Devaki Vijayalakshmi
Department of Orthodontics, Meenakshi Ammal Dental College, Maduravoyal, Chennai - 600 095
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]