Mechanical Properties and Surface Characteristics of Three Archwire Alloys

Vinod Krishnan; K. Jyothindra Kumar

doi:10.1043/0003-3219(2004)074<0825:MPASCO>2.0.CO;2

Abstract

Recent developments in material science have presented newer archwire materials as well as improvements in the properties of existing ones. Proper selection and understanding of the biomechanical requirement of each case requires proper characterization studies on archwire alloys. The present study characterizes and compares three orthodontic archwire alloys, stainless steel, beta titanium alloy (TMA™), and a newly introduced titanium alloy (TiMolium™), for the parameters (1) ultimate tensile strength (UTS), 0.02% offset yield strength (YS), and modulus of elasticity (E); (2) load deflection characteristics; (3) frictional properties; (4) surface characteristics and (5) elemental analysis for TiMolium™. Seven specimens of each archwire alloy were used for evaluating each parameter. An instron universal testing machine was used for tensile testing, three-point bend testing, and evaluation of frictional characteristics. Scanning electron microscope was used for surface evaluation and X-ray fluorescence for elemental analysis of TiMolium™ wire specimens. Stainless steel was the strongest archwire alloy with high UTS, E, 0.02% offset YS, and less friction at the archwire-bracket interface. TMA™ wires exhibited better load deflection characteristics with less stiffness than the other two wires. The surface of TMA™ appeared rough and exhibited very high values for friction at the archwire-bracket interface. TiMolium™ appeared to be an alpha-beta titanium alloy composed of titanium, aluminum, and vanadium and intermediate in nature for all the parameters evaluated.

INTRODUCTION

Over the past 100 years, improvements in both mechanotherapy and treatment philosophy have led to major advancements in orthodontic patient care. Changes in the field of mechanotherapy have largely been made possible with the emergence of newer orthodontic materials. Archwire materials form a large part of this change, and selecting the appropriate archwire requires a thorough knowledge of archwire biomechanical and clinical applications. This knowledge requires proper characterization of archwire alloys to predict their outcome when used clinically.

In the past few decades, a number of wire alloys with a wide spectrum of mechanical properties have been introduced, adding versatility to orthodontic treatment. Two articles reviewing the mechanical properties as well as the clinical applications of archwire alloys have been published recently.1 2 These articles stressed evaluating tensile properties, bending characteristics, frictional characteristics, and surface properties as part of archwire alloy characterization. They agree that austenitic stainless steel with its good balance of environmental stability, stiffness, resilience, and formability has continued to be the mainstay archwire alloy since its introduction.1–3

Titanium-based alloys are slowly gaining popularity in recent years, mainly with the introduction of nickel-titanium4 and beta titanium,5 with titanium's superior properties of biocompatibility, corrosion resistance, and low stiffness. TiMolium™ is a recent introduction in the field of titanium-based orthodontic alloy archwire materials,6 and proper literature regarding the properties of this archwire alloy is lacking. The present study was aimed at characterizing and comparing the mechanical properties and surface characteristics of the two archwire alloys used commonly in orthodontics, ie, stainless steel and TMA™, with the newly introduced TiMolium™.

MATERIALS AND METHODS

Preformed 0.43 × 0.64-mm (0.017 × 0.025 inches) archwires were obtained as stainless steel (Ormco Corporation, Glendora, Calif), TMA™ (Ormco), and TiMolium™ (TP Labs, Indianapolis, Ill).

The specimens of these three archwires were coded as:

group I: stainless steel
group II: TMA™
group III: TiMolium™.

Seven specimens from each group were evaluated for the parameters:

ultimate tensile strength (UTS), yield strength (YS), and modulus of elasticity (E);
load deflection characteristics;
frictional characteristics;
surface characteristics;
elemental analysis for TiMolium™ (as it is a recent introduction).

Ultimate tensile strength, yield strength and modulus of elasticity

A standard tensile test7 (Figure 1) using each of the archwire alloys from groups I–III were performed in an Instron Universal Testing Machine (Model No. 1195, Instron Corporation, Canton, Mass). A full-scale load of 1000 N was set in the machine with a crosshead speed of one mm/ minute. The span of the wire between crossheads was standardized as 40 mm. The load taken to break the wire divided by the cross-sectional area of the wire gave the value for UTS. The load deflection data obtained from the tensile testing were plotted as stress-strain curves from which the E as well as 0.2% offset YS were calculated.

FIGURE 1. Setup for tensile evaluation with specimen in place — **FIGURE 1.** Setup for tensile evaluation with specimen in place
Citation: The Angle Orthodontist 74, 6; 10.1043/0003-3219(2004)074<0825:MPASCO>2.0.CO;2

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.8 The test was performed on a specially designed fixture (Figure 2a) comprising two poles 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 brackets fixed on the poles using 0.012-inch elastomeric ligatures. The stage was attached to the upper movable head of the Instron machine. A single pole, fixed to a stage was attached to the lower head of machine in such a manner that the tip of the pole was on the center of the test-wire span (Figure 2b). The mid portion of the wire was then deflected one mm with a crosshead speed of one mm/minute and a full-scale load of 10 N. The load values were noted at 0.5 mm loading (0.5 L), one mm loading (1 L), and 0.5 mm unloading (0.5 UL).

FIGURE 2. (a) Fixture for three-point bend test with wire in place. (b) Setup for three-point bend test — **FIGURE 2.** (a) Fixture for three-point bend test with wire in place. (b) Setup for three-point bend test
Citation: The Angle Orthodontist 74, 6; 10.1043/0003-3219(2004)074<0825:MPASCO>2.0.CO;2

Frictional characteristics

Evaluation of the friction produced at the archwire-bracket interface was done following a test protocol described by Tidy.9 This test consisted of a simulated half-arch fixed appliance with archwire ligated in position (Figure 3). Four 0.022- × 0.028-inch edgewise brackets with zero torque and zero angulations were bonded into a rigid Perspex sheet at eight-mm intervals. A 16-mm space was left at the center for sliding the canine bracket to simulate canine retraction. The archwires were secured using 0.012-inch elastomeric ligatures. 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 center 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 a base plate (Perspex sheet) was mounted on the crosshead below. The full-scale load was set at 5 N with a crosshead speed of 10 mm/minute. At the start of each test, a trial 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 of 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 appropriate formulae.9

Statistical analysis

All the data obtained from the three parameters described above were tabulated and entered into SPSS (ver. 10), a computer-based statistical program. Means and standard deviations were calculated, and analysis of variance (ANOVA) test was performed to find out the level of significance between the values obtained from archwire alloy specimens in groups I–III. In addition, repeated-measures ANOVA was performed on values obtained through the three-point bend test. This was performed to verify whether there is any significant difference when the same archwire alloy is subjected to continuous loading and unloading and measured repeatedly.

Surface characteristics

Surface characteristics of each of the specimens of wires from groups I–III were studied with the help of a scanning electron microscope (SEM) (JEOL JSM 5600LV). A one-cm-long specimen of each alloy wire was mounted on studs, which were later placed in the vacuum chamber of the SEM. The accelerating voltage, angle of fit, and the aperture was adjusted to optimize the quality of the micrograph. The surface was scanned and viewed on the monitor at different magnifications and representative micrographs (500×) of each alloy were obtained.

Elemental analysis

The group III wire specimens were examined using X-ray fluorescence (XRF) to evaluate the composition of the alloy because it forms a new introduction to orthodontic specialty. The wire was wound over the stud, and fluorescent X-ray was passed over it to identify the alloying components.

RESULTS

Ultimate tensile strength, yield strength and modulus of elasticity

The value obtained through tensile testing of archwire alloys indicated clearly superior strength for stainless steel (group I), followed by TiMolium™ (group III), and TMA™ (group II). The group I wire was significantly greater than group II and III wires (P < .05), but the difference between groups II and III was statistically insignificant.

The load deflection curves obtained through tensile tests were plotted as stress-strain curves (Figure 4) for each archwire alloy. The slope of the curves, representative of 0.2% offset YS and E, were determined. The values for the mechanical properties of alloy archwires obtained through tensile testing are summarized in Table 1.

FIGURE 4. Stress-strain curve for the archwire alloys evaluated — **FIGURE 4.** Stress-strain curve for the archwire alloys evaluated
Citation: The Angle Orthodontist 74, 6; 10.1043/0003-3219(2004)074<0825:MPASCO>2.0.CO;2