Trends Biomater. Artif. Organs, 25(2), 67-74 (2011)
http://www.sbaoi.org
A Comparative Study of Physical and Mechanical Properties of the Different Grades of Australian Stainless Steel Wires Singaraju Gowri Sankar*, Surendra Shetty V. 1, Diwakar Karanth H.S.2 Departmentof Orthodontics, Narayana Dental College, Nellore 517501, Andhra Pradesh 1 Departmentof Orthodontics, College of Dental Surgery, Mangalore, Karnataka 2 Departmentof Orthodontics, Maratha Mandal‘s Nathajirao G. Halgekar Institute of Dental Sciences & Research Centre, Belgaum, Karnataka *Corresponding author: Dr. Singaraju Gowri Sankar, e-mail:
[email protected] Received 31 July 2010; Accepted 3 August 2010; Available online 4 May 2011
Introduction Archwires are the components of an orthodontic fixed appliance through which forces are generated and consequently tooth movement is achieved. Austenitic stainless steel with its greater strength, higher modulus of elasticity, good resistance to corrosion and moderate cost was introduced as an orthodontic wire in 1929 and shortly gained popularity (1). Recent developments in material science have lent themselves to improving the properties of orthodontic stainless steel arch wire technology. The A.J.Wilcock arch wire also called as the Australian arch wire is basically a high tensile stainless steel wire that is heat treated to yield its familiar and excellent clinical properties (2). The A.J. Wilcock Scientific and Engineering Company, the manufacturers of the Australian wire, have been conducting Research and Development studies to further improve the wire. Traditionally available grades (tempers) of wires are Regular, Regular plus, Special, Special Plus and Premium. Further two new grades of Australian wires have been introduced recently. They are Premium plus and Supreme which are two grades above that of Special plus. Australian wire is available in sizes ranging from 0.0123 to 0.0243 round wire .The Supreme grade is available in the sizes of 0.008 to 0.011 inch wires. They are used for aligning crowded teeth and used to form mini-uprightning springs that produce very gentle forces (3). A technique of manufacture has been developed called pulse straightening, which produces a far superior wire in terms of physical properties (4). Pulse straightened wires are available in Premium, Premium Plus and Supreme grades. They are supposed to have a smoother surface finish, and theoretically, therefore, less frictional resistance. Mollenhaeur has stated that some of the physical properties of this wires may be increased by an order of 300% (3). Although Australian wire, a distinctive type of steel used in various techniques and treatment
philosophies, has been included in the orthodontic armamentarium for quite some time, a review of the published literature reveals a lack of information on fundamental physical and mechanical properties. Presently an orthodontist may select from all the available grades of Wilcock arch wires one that best meets the demands of a particular clinical situation and the efficiency of the operator. The selection of appropriate grade of wire in turn would provide the benefit of optimum and predictable treatment results. The clinician must therefore be conversant with the difference in the mechanical properties and clinical application of these various grades of wires. A review of the literature revealed that most of the literature has addressed the study of the mechanical and structural properties of traditional stainless steel orthodontic wires. Very few studies are available exclusively on Australian wires. None of the studies compared the properties of all the available grades of A.J. Wilcock wire. Comprehensive Studies on the comparative properties of different grades of the Australian wire are also not available. There is also scarcity of data regarding the properties of pulse straightened and spinner straightened wires (5-14). The purpose of the present study is to evaluate and compare the mechanical structural properties of all grades of 0.016’’ Wilcock Australian Stainless Steel round arch wires and to compare the properties of Spinner straightened and pulse straightened wires. The Wilcock wires included in this study are Regular, Regular plus, Special, Special plus, Premium, Premium Plus (Spinner straightened), Premium plus (pulse straightened), Supreme (Spinner straightened) and Supreme (pulse straightened). The properties that are evaluated are: Tensile properties, Maximum load via three point bending, Working range Kinetic friction, Stress relaxation,
S.G. Sankar, V.S. Shetty, H.S.D. Karanth
0.404 0.392 0.372 0.368 0.370 0.384 0.352 0.350 0.322
Microhardness, Scanning Electron Microscopic (SEM), Evaluation of surface topography and Elemental analysis.
42.04 ± 1.24 40.24 ± 2.48 40.14 ± 1.26 37.24 ± 1.44 36.33 ± 1.54 35.98 ± 0.94 33.24 ± 1.23 34.98 ± 1.28 33.14 ± 1.54 1870.24 ± 25.44 1940.35 ± 34.25 2324.14 ± 33.23 2480.29 ± 54.24 2496.31 ± 48.24 2602.23 ± 37.28 2998.94 ± 41.24 2968.35 ± 44.29 3291.28 ± 37.54
(gms) 822.33 ± 23.24 908.63 ± 34.24 1018.24 ± 37.24 1074.28 ± 42.98 1191.34 ± 41.94 1280.23 ± 35.23 1393.14 ± 25.24 423.14 ± 15.93 480.94 ± 26.38
Percentage elongation Yield strength (MPa)
Materials and Methods The objective of the study is to evaluate and compare the structural and mechanical properties of all grades of Australian and Wilcock Stainless round wires. All the grades (tempers) of wires are 0.016" size except for the Supreme grade wire. The Wilcock wires included in this study are 0.016’’ Regular, Regular plus, Special, Special plus, Premium, Premium Plus (Spinner straightened), Premium plus (pulse straightened), and 0.11’’ Supreme (Spinner straightened) and Supreme (pulse straightened) Since 0.016" inch size wires is not available in Supreme grade, 0.011" wire is taken for comparison. All the samples were coded separately in order to avoid any observer bias. Five wires of each sample were taken for each experiment. Only one sampling of each wire was done for elemental analysis. Long specimens were chosen to minimize stress incorporation. These segments were carefully handled as they were uncoiled from the round wound spools and were subjected to the following tests. Tensile Test A tensile test (15) was used to evaluate the tensile properties of all the grades of wire. Five straight sections of each wire grade were subjected to tensile loading on an Instron universal testing machine (Model 4204, Instron) having a load cell capacity of 100 kg. A load range at 100 kg was used for this test. An inter cross-head distance of 50 mm was used and the speed at which the cross-head moved was 0.5 mm/minute. All tests are continued until the failure due to fracture occurred. The force - elongation data are replotted as stress- strain graphs. The ultimate tensile strength (U.T.S.) is equated to the braking strength of the wire. The yield strength was determined by offsetting 0.1 % deviation from the linearity. Strain percentage gives the percentage elongation of the wire.
Regular Regular plus Special Special plus Premium Premium plus(S.S.) Premium plus(P.S.) Supreme (S.S.) Supreme (P.S.) 1 2 3 4 5 6 7 8 9
Wire Sl.no
0.016 0.016 0.016 0.016 0.016 0.016 0.016 0.011 0.011
Ultimate Tensile Strength (MPa) 2221.33 ± 124.23 2398.24 ± 25.38 2633.84 ± 137.24 2820.72 ± 28.23 2870.54 ± 34.27 2985.71 ± 87.24 3124.85 ± 35.24 3178.72 ± 24.98 3417.29 ± 85.98
Three Point bend test
Diameter (inches)
Table 1: Measured properties of different grades of A.J. Wilcock stainless steel archwire
Load At 2mm deflection
Coefficient of frictional resistance At 50mg load 0.0371 ± 0.0020 0.0375 ± 0.0025 0.0397 ± 0.0018 0.0406 ± 0.0016 0.0417 ± 0.0021 0.0423 ± 0.0054 0.0540 ± 0.0013 0.0324 ± 0.0045 0.0410 ± 0.0015
Vhn
620 ± 24 622 ± 32 634 ± 36 640 ± 28 648 ± 24 652 ± 28 684 ± 32 650 ± 28 678 ± 24
Surface area roughness Sv
68
A three point bend test is done with a slight modification as described by Miura et al (16). A two unit male-female fixtures were designed to test the amount of force required for a given deflection. The female part is attached to the lower head of the Instron machine. It consists of two vertical plates separated by a distance of 14 mm. The male part is attached to the upper movable head and consists of a single vertical plate. When upper head is depressed fully, this vertical plate coincides with the centre of the inter distance of the female vertical plates. The female vertical plates have grooves in which the wire specimen is placed. The upper head is depressed at a rate of 0.5 mm/min and the force required to produce a deflection of 2 mm is noted. Five specimens from each grade of the wire are tested and the average force needed for a given deflection of 2 mm is calculated. Working range A five unit test jig was constructed for the purpose of this study with slight modification proposed by wong et al 10. The test jig represents a three bracket system with the middle bracket simulating as a malaligned tooth in the labiolingual direction (Fig 1). With this design no ligation
Physical and Mechanical Properties of Australian Stainless Steel Wires
69
Table 2: Measurement of working ranges of different grades of A.J. Wilcock stainless steel archwire
Sl. No. 1 2 3 4 5 6 7 8 9
Wire Regular Regular Plus Special Special Plus Premium Premium Plus (S.S) Premium Plus (P.S) Supreme (S.S) Supreme (P.S)
Diameter (inches ) 0.016 0.016 0.016 0.016 0.016 0.016 0.016 0.011 0.011
Percentage recovery for 1mm 2mm 87.6 92.75 98.8 93.75 96.5 94.60 99.7 95.10 99.5 90.25 97.6 94.50 96.6 93.75 100 98.35 99.9 97.30
different amounts of deflection 3mm 4mm 5mm 82.50 66.12 63.70 82.16 64.62 64.72 81.5 61.37 65.30 84.66 60.80 69.94 81.16 58.61 69.88 81.83 61.37 66.10 78.16 55.60 56.50 91.16 73.95 79.68 88.50 71.00 73.48
Table 3: Elemental analysis of different grades of A.J. Wilcock stainless steel archwire
was needed to secure the wire in the slot. Hence it eliminates the influence of ligation method on the deformation of wire. Each test jig consists of 1) two fixed side bars acting as supports 2) a movable middle bar also with slots to deflect the wires 3) a base to which these bars were fixed. The middle bar can deflect the wire to a maximum of 10 mm. Deflections for 1mm, 2mm, 3 mm, 4mm and 5mm were tested separately in each unit. Permanent deformation was measured with Travelling microscopic (Olympus, DF plan IX, Model 52H 100, Stereo Zoom Microscope). Fine ink marks were made on the wire precisely where it emerged from the slot. These marks were used as reference points to measure the amount of deflection with an accuracy of 0.001mm. Wires were unloaded after 1 hour .The amount of permanent deformation was measured at the middle of the span after unloading the wires. The absolute magnitude and percentage of creep deformation (Time dependent deformation) were calculated for all wires for given deflection. Creep deformation % = (creep deformation/deflection) x 100 The percentage recovery is calculated by using the formula Percentage recovery = 100 - creep deformation percentage.
Frictional resistance The measurement of friction was done for all the grades of the wire according to the experimental design proposed by Tidy (17). The apparatus setup included non-torqued and non angulated edgewise brackets (Dentaurum, Pfrozheim, Germany) having an 0.018"x0.025" slot, bonded on to a rigid persplex sheet at 8 mm intervals with a space of 10mm at the centre for including a movable bracket. The wires were secured in place with 0.010" elastomeric ligatures. The movable bracket was fitted with
a 10 mm power arm from which weights could be hung to represent the single equivalent force acting at the center of resistance of the tooth root. All tests were conducted under dry conditions with an Instron universal testing machine. The movable bracket was suspended from the load cell of the testing machine while the base plate was mounted on the cross-head below. The cross head, connected with the suspended bracket moved upward at the speed of 2mm/minute. At the start of each test, a trial run was performed with no load on the power arm to check that bracket was not binding on the arch wire. Weights of 50 and 100 gms were suspended from the power arm and the load needed to move the bracket across the central span in the apparatus was recorded on chart paper. Five representative readings were then taken with each load with each grade of wire. The load cell readings represented the clinical force of retraction that would be applied to the teeth, part of which would be lost in friction whilst the remainder would be transmitted to the tooth root and constitute the translational force. The difference between the load cell readings and the load on the power arm represents frictional resistance. The coefficient of friction between the two surfaces in contact was calculated using the formula. P = 2 Fhm / W OR m = P x W / 2Fh m = Coefficient of friction ; P = Frictional resistance; W = Bracket slot width ; F = Equivalent force acting at a distance: h = 10 mm
Stress relaxation An experimental set up has been designed and constructed as described by Hazel8 to measure the stress relaxation. The arch wire specimens are fabricated along the following lines. For the purpose of the study an arch shape was determined from measurement of each of
70
S.G. Sankar, V.S. Shetty, H.S.D. Karanth
twenty randomly selected maxillary casts of ideal occlusion. The measurements made were inter canine width, intermolar width and the anterior posterior distance from the centre point of the arch to inter molar axis. The arch form and dimensions are presented in Figure 2. The apparatus consists of an acrylic plate form attached to a specially constructed fixture attached to a lower head of the instron machine (Fig 2). Two round molar tubes were fixed separated by a distance of 55mm. Begg bracket is fixed to the anterior portion of the acrylic plate such that it lies in the centre of arch when the wires in depressed. A tip back bend of 45° was used .The tip back bends projected 2 mm mesial to the molar tube and the distal ends of arch wire were cinched. The anterior vertical depressant force required to straighten the wire is measured by a special fixture attached to upper head of Instron machine. A load cell capacity of 100 kg with sensitivity of 0.001 kg is used. The initial load required to straighten the wire and the resultant strain is noted. The load reading is arbitrarily taken when the arch wire first touches the acrylic plate. The anterior portion of the arch wire is ligated in the bracket. The ligatures were relieved after one week. The final load required to straighten the wire for the given strain is noted. From the difference between the two readings stress relaxation is calculated. Microhardness Five wire specimens of each type were assessed for Micro hardness. The Vickers hardness (HV) of wires was assessed by using a microhardness tester (DHV-3000; Chroma Systems India) under a 500 g load and testing time of 15 seconds. The area of the sloping surface of the indentation is calculated. The Vickers hardness (HV) is the quotient obtained by dividing the Kilogram Force load by square mm of indentation. Structure and elemental analysis The X-ray diffraction XRD studies were carried out using a Philips X’Pert Powder Diffractometer (PW 3040, PANalytical, Almelo, the Netherlands) for structural analysis using CuKá radiation (40 kV, 50 mA), with the programmable divergence slit and receiving slit kept at 1
degree and 0.1 mm, respectively. Elemental analysis is done Neutron activation analysis (NAA) technique. Surface topography The surface topography of the wire was evaluated with the aid of a Scanning Electron Microscope ( JSM -5410 LL, Scanning Microscope, Jeol, Datum Ltd., Tokyo, Japan). The specimens, of about an inch long, were mounted on specimen studs using silver paste which is an electrically conductive adhesive. The studs with the specimens were later placed inside the vacuum chamber of the S.E.M. An accelerating voltage of 20 kv, a current of 3 amperes were used. The surface was scanned and observed on the screen at 20x, 200x, 500x and 1000x magnifications. Photographs of the specimen were then taken at 500 X magnification. A square mm grid inbuilt into the microscope was superimposed up on the photomicrographs at 1000 X magnification. The surface porosity was analyzed by the method described by De Hoff R.T (18). The horizontal lines of the grid were scanned and the number of intersections made by the surface porosities and elevations were counted. The total length of the lines scanned was estimated since the length of each line is known (100 mm x 10lines = 1000mm). The line intersect count (P L) was then obtained. (PL denotes the number of intersections counted divided by the length of test line samples). The surface area in unit volume (S v) was then calculated by utilizing the following relations: PL = 1/2 Sv OR Sv = 2 PL
Results and Discussion The readings and Values obtained from all tests with the exception of Elemental analysis, surface topography, and micro hardness were statistically analyzed. The ultimate Tensile strength, yield strength and percentage elongation are calculated from stress-strain graphs for all grades of wires. All data are expressed as Mean ± SD. Analysis of the difference between each corresponding grade of wire is done by using the students‘t’ test of significance between means at confidence interval of 95%. ANOVA test was carried for the categorical data of working range. A search of the orthodontic literature yielded very little information has been presented on Australian orthodontic
Figure 1: A five unit test jig. Each unit has 1. A movable middle bar with slot and 2. Fixed side bars with slots. The middle bar moves to and fro simulating labiolingual displacement of teeth. Deflections and elastic recovery are tested for deflections of 1mm, 2mm,3mm, 4mm and 5mm
Figure 2: Figure showing the representative arch form and measurement of stress relaxation
Physical and Mechanical Properties of Australian Stainless Steel Wires wires. Only a few studies pertinent to the analysis of surface characteristics, elemental composition, and mechanical properties of the Australian stainless steel orthodontic wires were available. Meanwhile, the introduction of different sizes and grades (tempers) makes application of the full range of these wires a highly empirical task, with lack of justification of specific selection. The present investigation relates to the extent of differentiation of wire properties with regard to grade and process of manufacturing. A tensile test for determination of ultimate tensile strength, yield strength and percentage elongation is conducted. (Table 1) The wires tested in this study exhibited a range of values for the above three parameters. Ultimate tensile strength is a material property and is expected to have constant value which is independent of the dimension of shape of the material. However, in the present study different values are obtained for UTS ranging from the lowest value of 2221.33 MPA for Regular to the highest value of 3417.29 MPA for Supreme (P.S.) 0.11" wires. Comparing the wires of same dimensions, Regular 0.016 showed least tensile strength of 2221.33 MPA followed by Regular plus, Special, Special plus, Premium, Premium plus (S.S) and Premium plus (P. S) in increasing order with Premium plus (P.S.) showing an ultimate tensile strength of 3124.85 MPA. The difference in the values is found to be statistically significant. The pulse straightened wires of 0.016 Premium plus (P.S.) and 0.011" Supreme showed correspondingly higher values than their spinner straightened (S.S.) counterparts. The difference between means is very highly significant (t=3.30 for 0.016’’ and t=5.96 for 0.011’’ wires). It is also noted that with the Yield strength and Ultimate Tensile Strength of the 0.011" diameter wires are higher than that of 0.016" counterparts. The present values obtained for different grades of wire are consistent with the earlier findings (7-9,13) while the values obtained in the present study are slightly less than that of another study(14). Yield strength (Table 1) of a material represents the stress below which deformation is entirely elastic. It indicates the amount of energy stored in an orthodontic wire before it is plastically deformed. The same trend in the values observed for ultimate tensile strength are also noticed in the determination of yield strength .Regular 0.016" wire with 1870.24 MPA has lowest value with the highest value being that of Premium plus (P.S.) with a yield strength of 2998.94 MPA. The spinner straightened Supreme 0.011" and 0.016" wires have lower values compared to their pulse straightened counterparts. Premium plus pulse straightened wire seems to have increased value of yield strength by an average of 15% than their corresponding spinner straightened wires. The percentage elongation (Table 1) is an indication of the ductility of the material. It refers to the capacity of a material to undergo deformation under tension without rupture. It is a measure of the degree of plastic deformation that has been sustained fracture. Lower values of percentage elongation indicate the brittleness of the material. Knowledge of the ductility of material is important for at least two reasons. First, it indicates to a designer the degree to which the wire will deform plastically before the fracture. Secondly it specifies the degree of allowable
71
deformation during fabrication of the appliance. In the present study Premium plus (P.S.) exhibited lowest percentage elongation of 33.24% and highest values for percentage elongation is displayed by Regular with value of 42.04% in 0.016 size wires. Spinner straightened wires possess greater percentage elongation than the corresponding diameters of pulse straightened wires. Thus improvements in the properties of these wires is accompanied by certain unavoidable drawbacks, one among this is the increased brittleness due to less percentage elongation and consequent occasional fracture of the wire during bending with orthodontic pliers. However as Dr. Begg states: This is a small price to pay for wires of such superior quality (4). The load deflection rate is an important determinant of the biologic response seen in tooth movement. There is an increasing realisation that light continuous forces are preferable in tooth movements. A three point bend test is used to measure the load deflection rate of wires when subjected to 2 mm deflection. This indirectly indicates the stiffness of wire and its resistance to permanent deformation. It is also an indication amount of forces exerted by an archwire for a given deflection. A comparison of the load values at 2 mm deflection revealed the force values being increased from Regular to Premium plus pulse straightened wires in 0.016" dimensions. The lowest value noted for Regular grade is 822.24 grams where as higher values of 1393.14 grams are being observed for Premium plus pulse straightened wires. Pulse straightened wires displayed a greater force requirement than their corresponding spinner straightened wires. The Premium plus 0.016" wires produced higher force levels of an average 8.8% than spinners straightened wire of same grade. Comparison of Supreme 0.011" pulse straightened wire revealed a 13% increase in the force values delivered than their corresponding counter parts of spinner straightened wires. The findings in the study indicate that the newer pulse straightened wire would have greater resistance to deformation and stiffest of all grades of wires. But equally important is the greater amount of force it would exert while aligning displaced teeth. Whether these forces are beneficial or harmful for tooth movement is yet to be tested. Working range is a measure of how far a wire can be deformed without exceeding the limits of the material. It is a measure of distance without regard to the force that is required to accomplish the deflection. Clinically, it indicates how far tooth will be moved in a single activation. The greater the working range, the lesser the appliance activations required and at low force levels this approaches the concept of optimal tooth movement. The test jig is used in the study to measure the increase in permanent deformation. The amount of permanent deformation experienced by the wire gives an indication of its working range. The larger the permanent deformation the smaller the working range. With this design no ligation was needed to secure the wire in the slot. Hence it eliminated the influence of ligation method on the deformation of wire. In the present study it is observed that there is no consistent pattern of working range from Regular to Premium plus pulse straightened wire in the 0.016 dimensions (Table 2). Of all the wire specimens tested the Special plus displayed superior quality of elastic
72
S.G. Sankar, V.S. Shetty, H.S.D. Karanth
recovery for a given deflection. It is also seen that spinner straightened wires have superior working range compared to pulse straightened wires of 0.016" wire dimension. The percentage recovery of Premium plus spinner straightened showed an average superior recovery rate from 97.6% to 66.10% depending upon the amount of deflection where as pulse straightened wire exhibited a working range from 96.6% to 55.6%. However the difference is not statistically significant. It is also noticed that as the deflection increased from 1mm to 5 mm, larger diametre (0.016") wires displayed a poorer working range which is as low as 55%. But in the smaller diametre (0.011") Supreme grade wire, elastic recovery was excellent and minimum working range it exhibited at 5 mm deflection is not even less than 70%. Considering that the larger diametre wires are bound to exert greater forces on the teeth, it may be more efficacious if the Supreme grade of wires are utilised to effect tooth alignment particularly so when tooth displacements are larger. These wires can be used piggy back in initial aligning phase along with larger diameter arch wires. The gentler forces developed by these auxiliary wires may facilitate alignment of teeth rapidly without unduly taxing the anchorage. Frictional resistance is an important consideration in orthodontic mechanotherapy because forces needed to retract teeth necessarily has to overcome the frictional force. The greater the frictional resistance, greater is the force needed for tooth movement. This implies that anchorage requirements will be proportionately higher. Reduction of frictional resistance is one of the objectives of contemporary orthodontic mechanotherapy. In the present study the results obtained are quite contrary to the expectations. According to manufacturing process pulse straightened wires have smooth finish. But in present study Premium plus pulse straightened 0.016" wire showed the highest frictional resistance. In 0.011" dimension also pulse straightened wires showed greater frictional resistance (Table 1). The mean coefficient of friction for pulse straightened wires in 0.016" dimension seems to be 1.7 times greater than the corresponding spinner straightened wire. The Supreme pulse straightened 0.011" wire exhibited frictional resistance which is 1.2 times greater than spinner straightened wire. Among the 0.016" Spinner straightened wires, Premium plus exhibited greater frictional resistance while the Regular exhibited lowest frictional resistance. On the basis of this study it can be inferred that situations which require translatory tooth movement may tax anchorage more in the event of using pulse straightened wires. In the light of the surface topography analysis which revealed a better smoothness for pulse straightened wires, further studies are warranted to determine whether straightening and heat treatment procedures in the pulse straightened wires produce some surface product which may be responsible for nearly the double coefficient of friction values obtained in this study. Stress relaxation Force for intrusion is achieved in the arch form by including the anchor bends or reverse curve of spee which become activated when the arch wire is pinned to the teeth i.e. stress are produced in the wire and these generate forces which are transmitted to the teeth. In order to achieve
intrusion it is necessary that the orthodontic wire to exert consistent force over a period of time. However in the clinical situation, forces exerted by the wire may vary with time because of tooth movement and stress relaxation. Stress relaxation occurs when stress decreases over time at a constant amount of strain. This results in increase in permanent deformation and a decrease in stored energy in the wire. Clinically a decrease in force and stored energy may lead to a decrease in the amount of tooth movement. In the present study a predetermined arch form with 45° anchor bends is used to determine the amount of initial intrusive forces generated by the wire and the stress relaxation in the wire after one week. Initial forces developed by Premium plus pulse straightening 0.016" wire is greater than other grade of same dimensions. The Regular grade marked a low force values of 82.23gms compared to Premium plus (P.S.) with highest value of 100.24 gm. (Table 1) The forces generated by pulse straightened wires seems to be higher than spinner straightened wire by 1.9%. The stress relaxation noted after 1 week is lowest for Premium plus pulse straightened with 0.49% where as highest force degradation occurred in Regular plus wires with a stress relaxation of 1.82%. These findings indicate that while correction of deep bite by newer pulse straightened wires may create greater intrusive loads on anterior teeth and extrusive loads on posterior molars. This may affect anchorage requirements. The higher forces developed by Premium plus pulse straightened wire can effectively over come the extrusive component of inter maxillary elastics. However whether tooth movement is facilitated or retarded by the higher forces is to be further evaluated by clinical studies. Earlier studies of Twelftree 7 and Hazel et al ( 8) evaluated certain grades of Wilcock wire and reported that negligible stress relaxation occurs even when assessed over a period of 28 days. They found that the Wilcock wires are superior to other stainless steel wires. Thus in spite of different forms of specimen used and the methods of testing there is general agreement with regard to the relaxation of these wires. The advantage of present study is the form of sample which is much close to that used in clinical practise and the value of the forces and the rates of relaxation can be applied more directly. Microhardness This increased hardness may lead to fracture during clinical bending and possibly to adverse affects during orthodontic mechanotherapy The Vickers hardness test is used to measure the hardness of these wires. The hardness value of the wires is found in between 620 to 684 for 0.016’’ wires. (Table 1) The Premium plus wires has maximum hardness value of 684 and minimum is found for Regular grade wires. The Pulse straightened wires are comparatively harder than spinner straightened wires. No significant difference in hardness is found in different sizes of same grade (Table 1). The Vickers hardness of Australian wire obtained in this study are higher compared to the traditional stainless steel alloys but of the same range to that of obtained by Brian M. Pelsue (14) Surface hardness is influenced by the hardness of a material are its strength, proportional limit, ductility, malleability and resistance to abrasion and cutting. The higher hardness values could be attributed to increased
Physical and Mechanical Properties of Australian Stainless Steel Wires
73
Figure 3: The SEM pictures (500 x) of different grades of wire. All the wires irrespective of their grades and manufacturing process exhibited surface roughness
carbon content in the Australian wire, along with the manufacturing process. The high range of hardness values of this wires make them incompatible with brackets that have less hardness brackets. This will have more effect on retraction mechanics specifically that use sliding techniques. The large difference in hardness between Australian wire and less harder brackets may cause the wire to bind and may result in loss of anchorage. Elemental analysis Powdered x-ray diffraction analysis showed that this wire is mainly austenitic phase and contains other phases. Elemental analysis show that the carbon content of the steels is around 0.1% in higher grades of wire. (Table 3) The chromium content is found to be around 17-18 % and nickel around 12%. Australian wires reported in the present study indicates that the carbon content is well above the values reported for typical 18/8 stainless steel wire which is around 0.02%. The increased carbon content may contribute to the increased hardness of higher grades of wire. This increased hardness may cause Australian wire to be more brittle than traditional stainless steel wire and consequently may adversely affect the ability of the wire to withstand bending. Surface Topography An evaluation of surface topography is important because an irregular surface is associated with greater friction and a rough surface may be more susceptible to corrosion. All the wires irrespective of their grades, size and method of manufacture exhibited surface irregularities. Tempering
of high carbon stainless steel alloys is associated with precipitation of carbides on surface and formation of rough, irregular, and excessively porous surfaces noted in SEM images ( Fig 3). It is noticed that among 0.016" dimension wires, Premium plus pulse straightened has smoothest finish with PL count of 0.176 and the roughest surface texture is exhibited by Regular wire with a P L count of 0.202. (Table 1) The pulse straightened wires exhibited less surface irregularities when compared to spinner straightened wires. The surface area of irregularities is also greater in spinner straightened wires compared to the pulse straightened wires. It is worth while to investigate whether this surface texture alters the chances of corrosion and tarnish as there is no data available on it. The surface topography obtained in present study is in contradiction with those obtained by Brian M. Pelsue etal (14).
Conclusion The present investigation supports that the temper, size and manufacturing process effects the physical and mechanical properties of Australian wires. Australian Wilcock newer pulse straightened wire possesses superior strength and stiffness compared to older grades of 0.016" wires. It can be assumed that the same tendency of proportion can be observed in other range of sizes for these grades of wires until proven otherwise. Thus this pulse straightened Premium plus wires can effectively overcome the deflective forces from mastication and can resist deformation in the oral cavity. Despite the superior surface topography exhibited by this wires it is found that
74
S.G. Sankar, V.S. Shetty, H.S.D. Karanth
they show a Kinetic friction of 1.5 times greater than spinner straightened wires. This need to be evaluated critically by further studies. The smaller size supreme grade wires can be effectively used in initial aligning phase. The Supreme material will remind the clinician of nickel titanium wires, with the added advantage of formability and significant reduction in cost. Its size allows it to be placed in the same slot with basal arch wires without having to use specially designed deep slotted brackets. The force relaxation of this wires seems to be negligible. This is a favourable situation because the orthodontic wire should apply the intrusive forces even
after 1 1/2 months. This shortens the treatment time and number of appointments. However, the experimental set ups used in the present study does not simulate the force delivery system in the clinical situation. Further more the wires are tested in a controlled environment devoid of saliva and mechanical insults. Therefore, the possible effects of corrosion, mastication and oral hygiene measures were avoided. The wires require to be evaluated in invivo clinical conditions or experimental models simulating oral environment.
References 1. J.V. Wilkinson, “Some metallurgical aspects of Orthodontic stainless steel“, Angle Orthod, 48, 192-206(1962) 2. P.R. Begg and P.C.Kesling, Begg orthodontic theory and technique, Ed.3, W.B. Saunders, Phildelphia, 94-95 (1977) 3. B. Mollenhauer, “Update to new approaches to the Begg technique“, Aust Orthod,10, 67 “ 87(1987) 4. P.C.Kesling, “J.C.O. Interview with Arthur J. Wilcock Jr. On orthodontic wires,“ J Clin Orthod, 22, 241 - 249 (1987) 5. G.V. New man, “Biomechanical analysis of the Begg light arch technique“ , Am J Orthod, 49,721 “ 740 (1963) 6. G.S Ware, L. Masson, “ Physical properties of orthodontic wires“, Aust J Dent,4,53 “ 61(1975) 7. C.C. Twelftree, G.J Cocks, M.R Sims, “Tensile properties of orthodontic wire“, Am J Orthod, 72, 682-687(1977) 8. R. J. Hazel, G.J Rohan, Y.C West, “Force relaxation in orthodontic archwires“, Am J Orthod, 86, 396 “ 402 (1984) 9. K.Jyotindra Kumar, “Working range characteristics of Newer Arch wire materials used in stage of Begg technique - A study“, J Ind Orthod Soc, 20, 251 “ 264(1989) 10. E.K.F. Wong, D.W. Borland, V.C. West. “Deformation of arch wires over time“, Aust. Orthod J, 15, 152 - 157(1994) 11. R.P. Kusy “A review of contemporary archwires: their properties and characteristics“, Angle Orthod, 67,197“207(1997) 12. V. Krishnan, K. Jyontindra Kumar, “ Mechanical properties and surface characteristics of three archwire alloys“, Angle Orthod, 74,825“831(2004) 13. K .Anuradha Acharya, V.P. Jayade, “ Metallurgic properties of stainless steel orthodontic archwires: a comparative study “, Trends Biomater. Artif. Organs,18,125 “ 135( 2005) 14. M. Brian Pelsue, Spiros Zinelis, T.Gerard Bradley, W. David Berzins, Theodore Eliades, George Eliades, “ Structure, Composition, and Mechanical Properties of Australian Orthodontic Wires“, Angle Orthod , 79, 97-101(2009) 15. O.P. Khanna , properties of engineering materials: A text book of material science and metallurgy, Ed.2, Dhanpat Rai publications, New Delhi , 5 “ 15(1998) 16. F. Miura , M. Mogi, Y .Ohura, H .Hamanaka, “. The super-elastic property of the Japanese NiTi alloy wire for use in orthodontics“, Am J Orthod Dentofacial Orthop, 90, 1-10 (1986). 17. D.C. Tidy , “Frictional forces in fixed appliances“, Am J Orthod Dentofac orthop, 96, 249 - 54 (1989) 18. G.F Vander Voort, Applied metallography, Van Nostrand Reinhold Co, New York,53-56 (1975)
