Mechanical properties of a new thermoplastic polymer orthodontic archwire Juan Carlos Varela, Marcos Velo, Eduardo Espinar, Jose Maria Llamas, Elisa R´uperez, Jose Maria Manero, F. Javier Gil PII: DOI: Reference:
S0928-4931(14)00268-9 doi: 10.1016/j.msec.2014.05.008 MSC 4632
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
17 June 2013 23 March 2014 6 May 2014
Please cite this article as: Juan Carlos Varela, Marcos Velo, Eduardo Espinar, Jose Maria Llamas, Elisa R´ uperez, Jose Maria Manero, F. Javier Gil, Mechanical properties of a new thermoplastic polymer orthodontic archwire, Materials Science & Engineering C (2014), doi: 10.1016/j.msec.2014.05.008
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MECHANICAL PROPERTIES OF A NEW THERMOPLASTIC
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POLYMER ORTHODONTIC ARCHWIRE
Juan Carlos Varela, Marcos Velo, Eduardo Espinar*, Jose Maria
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Llamas*, Elisa Rúperez**, Jose Maria Manero** and F.Javier Gil**
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Grupo de investigación en Ortodoncia. Facultad de Odontología. Universidad Santiago de Compostela. Santiago de Compostela. Spain. Grupo de investigación en Ortodoncia. Facultad de Odontología.
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*
Dept. C. Materiales e Ing. Metalúrgica. Universitat Politècnica de
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**
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Universidad de Sevilla. Spain.
Catalunya. Centre de Recerca Nanoenginyeria. Member of Biomedical
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Research Networking Center in Bioengineering, Biomaterials and Nanomedicine. CIBER-BBN. Spain
CORRESPONDANCE ADDRESS: F.J.Gil Dept. C. Materials i Eng. Metalúrgica. E.T.S.E.I.B. Universidad Politécnica de Cataluña. Av. Diagonal 647. 08028-Barcelona. Spain Tel:
34
93
4016708
[email protected]
Fax:
34
93
4016706
E-mail:
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ABSTRACT
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A new thermoplastic polymer for orthodontic applications was obtained and extruded into wires with round and rectangular cross sections. We evaluated the potential of new aesthetic archwire:
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tensile, three point bending, friction and stress relaxation behavior,
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formability characteristics were assessed. Stresses delivered were generally slightly lower than typical beta-titanium and nickel-titanium
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archwires. The polymer wire has good instantaneous mechanical
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properties; tensile stress decayed about 2% over 2 hours depending
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on the initial stress relaxation for up to 120 hours. High formability allowed shape bending similar to that associated with stainless steel wires.
The
friction
coefficients
were
lower
than
the
metallic
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conventional archwires improving the slipping with the brackets. This new polymer could be a good candidate for aesthetic orthodontic archwires.
Key words: Polymer archwire, Orthodontic applications, Mechanical properties, friction.
ACCEPTED MANUSCRIPT INTRODUCTION
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The interest in aesthetic orthodontic appliances is increasing: labial
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placed brackets, aesthetically coated archwires, ceramic brackets are
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different examples. The latest development is a polymeric orthodontic archwire, translucent, with high springback and high ductility [1].
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Marroco ML et al [2] and Goldberg et al [3] studied a new polymer wire based on polyphenylene, a novel polymer whose rigid molecular
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leads to a high yield strength and modulus of elasticity [1]. The clinical evaluation demonstrated the efficiency of tooth movement
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during the first stage of levelling phase of treatment [4].
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The clinical plateau of the unloading curve (deactivation curve) is the main one of interest in relation to the moving teeth [5]. This plateau allows the orthodontist to apply an almost continuous light force with
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larger activations that results in the reduction of tissue trauma and the patient discomfort, thus facilitating enhanced tooth movement. In contrast, forces that are high in magnitude encourage hyalinisation of the periodontal ligament and may cause irreversible tissue damage such as root resorption.
Besides the aesthetic reasons, the oral cavity represents a harsh environment for a metallic orthodontic appliance of any kind [6]. Corrosion of orthodontic appliances has been thoroughly studied [7-
ACCEPTED MANUSCRIPT 12]. Two main concerns are directly related to the effects of corrosion: biocompatibility and appliance performance [13]. The most
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important aspect is the interaction that the appliance may have with
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the patient in terms of absorption of corrosion products and the
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systemic reactions that may arise. Attention has been focused on Nickel [14-23] as being an element able to induce allergenic reactions
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since it is one of the most used constituents of the alloys commonly used in orthodontics like Stainless Steel, Nickel-Titanium and Copper-
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Nickel-Titanium. Recently, Ni-free Ti alloys with superelastic behavior have been studied for orthodontic applications [24-27].These alloys
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are beta-titanium and more specifically the family Ti-Nb with different
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additional elements (Ta, Hf). These alloys have superelastic features
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due to the presence of austenite in the microstructure at 37ºC [25,26-27]. The alloy shows a superelastic effect with physiological critical stress (low and continuous) and a minimal loss of the recovery
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around 150 mechanical cycles. The increase of 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 has an excellent in-vitro response. These new alloy Ni-free superelastic alloys are being studied for the first time in orthodontic applications. Because of the above, TiNb alloys
ACCEPTED MANUSCRIPT can be a good candidates for orthodontic applications since they avoid allergic problems. They do not, however, solve aesthetic
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problems.
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For polymeric archwires activated, there is an instantaneous applied stress proportional to the activation. With time, the extended
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molecular conformations relax towards their equilibrium position causing a gradual decrease in the applied force even though the
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deflection is constant: stress relaxation. For metallic archwires, the
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stress remains constant with a constant activation.
polystyrene-g-polyisoprene
copolymers,
which
are
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multigraft
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Recently, Hadjichristidis et al. [28, 29] have studied a series of
thermoplastics elastomers with high strains at break, up to about 1550% [29]. The mechanical properties can be controlled by both
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molecular architecture and morphology. Due to the high strains at break and low residual strains in hysteresis tests, these materials have a behaviour similar to superelastic materials.
The low tensile
stress is the main problem in order to apply these polymers as orthodontic archwires.
The aim of this work is to obtain and determine the mechanical properties (tensile, three-point bending, stress relaxation and friction behaviour) of a new thermoplastic polymer orthodontic archwire and
ACCEPTED MANUSCRIPT to compare its properties with the metallic archwires. These new polymer archwires present the aesthetic advantages; avoid the
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corrosion and the ion release.
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MATERIALS AND METHODS
Material resin
was
obtained
by
mixing
a
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The
liquid
phase
(methyl-
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methacrylate monomer and butyl acrylate) and a solid component (benzoyl peroxide, tricresyl phosphate and dichloromethyl silane).
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Once mixed, the material was initially cured at 40° C for 2 hours and
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afterwards at 60° C for 14 hours. The material was then subjected to
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a post-curing heat treatment of one hour at 130° C.
The thermoplastic was extruded by using specially designed profile
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dies to achieve wires with clinically relevant cross sections of either 0.508x0.762 mm2 rectangular or 0.508 mm round. A total of approximately 60 m of wire were extruded. The wire was visually examined,
and
cross-sectional
dimensions
were
inspected
continuously along the length. At several locations, samples were cut and examined with optical and scanning electron microscopy to evaluate the outer surface and cross-sectional shape.
Calorimetry
ACCEPTED MANUSCRIPT The transformation temperature (Tg) was measured by means of a calorimeter. The calorimetric system used has already been described
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in previous papers [9] and it is based on a flow calorimeter which
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measures differential signals (T) by means of thermobatteries.
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Temperature was measured by means of a standard Pt-100 probe. All signals were digitized through a multichannel recorder and linked to a
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microcomputer. The transformation temperatures occur when there is a sudden increment in calorimetric signal. In the same way, the final
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temperatures are determined when the calorimetric signal returns to
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the base line.
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Thermal cycling effects.
Five orthodontic archwires were thermally cycled (n=100). The samples were treated at 100ºC to 0ºC both for 10 minutes followed
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by cooling in water at 37ºC. Transformation temperatures were determined at different number of thermal cycles.
Tensile properties. Tensile tests were carried out in an electromechanical universal testing machine working (MTS-Adamel) at a cross-bar speed of 10mm/min. Laser extensometer was used in these tests.
The
specimens tested were of 150 mm of height in rectangular section
ACCEPTED MANUSCRIPT (0.508x0.762 mm2) rectangular and the same height in round archwires (0.508 mm) and the gauge length of the specimens or the
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distance between the grips was 77 mm. The tests were carried out in
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artificial saliva medium at 37ºC. The chemical composition of the
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artificial saliva is shown in Table 1. Elastic modulus, ultimate strength, elongation at yield at break were determined for each
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orthodontic archwire (n=40) at different temperatures.
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Three-point bending
A three-point bending tests were conducted using the universal
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testing machine (MTS-Adamel). The load frame was equipped with a
37ºC.
The
bending
tests
were
investigated
using
a
laser
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of
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1 kN static cell The measurements were taken in a body temperature
extensometer. Ten measurements were recorded for each specimen. The dimensions of the specimens were the same used in tensile tests.
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The mid portion of the wire segment was deflected at the speed of 1 mm/min under the pressure from a metal pole of 5 mm in diameter. Each sample was loaded until a deflection of 4 mm was located. The samples were unloaded at the same cross-head speed until the force became zero [5].
Friction Coefficients. Friction coefficients were performed in a CSM pin-on-disk tribometer, in accordance to the ASTM G99 standard. The underlying principle on
ACCEPTED MANUSCRIPT this test could be called wire-on-disk because of its analogy with the pin-on-disk test. The samples studied were 10 archwires for each
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material: polymer, stainless steel, TiMo, cp Ti, NiTi and NiTiCu. The
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chemical composition of the alloys is shown in Table 2.
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The orthodontic archwires were carefully affixed with cyanoacrylate adhesive in a bakelite holder, without adhesive residues, on the wire
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surface to be tested. The contact wire plane and the disc were in the
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longitudinal direction to simulate full-arc contact bracket. An angular velocity of the disks and the normal load of 0.5236 rad/s
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and 10N were used respectively. Ideally, the normal load on the wire
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should have been around 1N to simulate the load in service (in which typical values ranging from 20 to 100 g force). However, 10N were
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employed to ensure that there was a full contact between both surfaces that could influence the determination of the coefficients of
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friction.
The static and dynamic friction coefficients µ (the proportionality constant between the friction force and the normal force) were determined for the orthodontic archwires against the materials commonly used for the brackets (manufactured in 316 stainless steel and
Ti-6Al-4V).
These
coefficients
were
measured
in
an
environmental chamber which contained artificial saliva at 37ºC. The dynamic friction coefficients µ (the proportionality constant
ACCEPTED MANUSCRIPT between the friction force and the normal force) were determined and the wear rates (volume loss) for the orthodontic archwires against
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the materials commonly used for the brackets (manufactured in 316
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Stainless steel and Ti-6Al-4V) were also measured.
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As the wear test was being performed, gravimetric measures were controlled in a Sartorius Micro Balance CPA26P, in order to determine
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the weight loss over time by means of a high-precision set of scales. The sensitivity of these measures was ±0.001 mg. With uniform
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density, the weight lost can be stated as volume loss.
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Stress relaxation: Creep tests. Ten archwires were strained at values of 5, 10, 20, 40, 60 and 80%,
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which are the values at which the archwires are strained in clinical use for tooth movement [7-8, 10]. The mechanical tests were carried
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out with an MTS-Adamel electromechanical testing machine. The rate of application of the load to reach the target strain was 1 mm/minute and the rate of data acquisition by the Autotrack software was 25 points/second; this enabled us to control automatically the stress relaxation that was produced over time. A load cell of 1KN was used because it has greater sensitivity in the force values that must be applied to maintain these levels of strain. The force values were determined at different time intervals up to 120 hours.
To control the constant strain a laser extensometer was used. This
ACCEPTED MANUSCRIPT type of extensometer was used with the aim that the pressure should not cause cuts in the polymer and therefore affect the values of the
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mechanical forces measured. With the laser extensometer fluorescent
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tapes were placed on the sample to be measured in order to allow the
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laser to measure the displacement values remotely.
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With the aim of carrying out the mechanical tests in conditions similar to those of the mouth, a physiological chamber was designed with
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EXPERIMENTAL RESULTS
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artificial saliva at a temperature of 37ºC.
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The polymer was successfully extruded into round and rectangular orthodontic wires with cross-sectional dimensions that fit existing brackets. These translucent wires potentially offer an aesthetic
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alternative to current metal orthodontic wires, as can be seen in Figure 1.
From the thermograms obtained, the Tg was 39.12 ºC. The start temperature was 35.83ºC and the finish temperature 40.57ºC. In Figure 2 can be observed a calorimetric thermogram of a polymeric archwire tested. These temperatures are compatible with the mouth temperature 37ºC. The thermal cycling did not show statistically differences on the transformation temperatures in relation to the un-
ACCEPTED MANUSCRIPT treated polymeric archwires.
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Tensile properties of the round and rectangular extruded wires are
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reported in Table 3 at different test temperatures (35, 37 and 39ºC).
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The results of the elastic modulus are higher at temperatures near Tg temperature. Besides, the maximum stress decreases and the
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deformation increases slightly with increasing test temperature from
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35 to 39 ° C. This is due to the proximity of the Tg temperature.
Three-point bending activation and deactivation forces of the wires
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measured at a deflection of 5 mm are listed in Table 4. Statistically
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differences were not found in any of the force levels (activation and
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deactivation). The results are very similar to the beta-titanium orthodontic archwires [5].
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The results of the frictional tests performed on the achwires under scrutiny on the two different types of brackets (316L stainless steel and the Ti6Al4V alloy) provided the values for the static and dynamic friction coefficients (µ) as it can be seen in Table 5. It can be observed that the polymer static and dynamic friction coefficients are lower than beta-Ti and cp-Ti, NiTi and NiTiCu orthodontic archwires in both bracket materials tested. From the results shown in Table 6, we can see that the wear rate on polymeric wires is much lower than on metallic wires.
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One limitation of most polymers is the phenomenon of stress
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relaxation or creep. Over time, stress below the yield strength can
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lower the force and cause permanent deformation. Figure 3 shows
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the decrease in load for the 0.508 x 0.762 mm2 after being initially strained from 5 to 80% of its initial length at 20ºC. The pattern of
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stress relaxation was typical for a viscoelastic material. There was decreasing stress at a decreasing rate asymptotically reaching a
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constant stress level. At the end of the test, the samples retained 94.2%, 85.5%, 79.6%, 77.5%, 75.1% and 74.8 % of their original
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stress level when were strained to 5%, 10%, 20%, 40%, 60%, and
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80% of their original shape, respectively.
DISCUSSION
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An important advantage of the polymer orthodontic archwire is the excellent formability. Beta-Ti and cp-Ti present also inherent a good formability. However, the metallic wire processing can be problematic because of the reactivity of titanium that can result in some batches of wires being susceptible to fracture during clinical manipulation. For NiTi and NiTiCu alloys, small changes in the chemical composition produced by surface oxidation can modify the corrective stresses [29, 30].
ACCEPTED MANUSCRIPT The polymer wires were highly ductile, as evidenced by their high elongations at yield and break of about 200%. This has two important
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clinical advantages. First, the wires are highly ductile and, hence,
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they are unlikely to fracture in the mouth under high-stress
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conditions. Second, it is possible to make secondary bends in the wire; in this respect, handling is similar to working with a metal wire.
thermoplastic
polymer
wire
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More complicated configurations such as loops could be formed in the without
the
use
of
heat.
The
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thermoplastic polymer wires that were formed into archwires by heating demonstrated good shape control in 3 dimensions and also in
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their cross sections. Moreover, thermal cycles produced in the mouth
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do not effect the transformation temperatures, which give an the archwire. However, the stress of the
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important stability to
polymer (around 20-23MPa) is lower than the NiTi (45-60MPa) producing a longer period of archwire application on the teeth [9-11].
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Therefore, these polymers can be very suitable for certain stages of orthodontic treatment in which the metals do not have suitable properties.
Orthodontic treatment can be divided into three stages. Each stage of treatment requires wires with specific properties tailored for certain functions: ● Initial stage. Levelling and alignment of teeth in mandibular (lower) and Maxillofacial (upper) arches. During the initial
ACCEPTED MANUSCRIPT stage of treatment teeth are generally crowed, particularly in the anterior sections of the jaws. Highly resilient (resilence
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of the wire refers to the elastic deformation with full
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recovery after deformation) wires having a relatively low
required
to
weave
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modulus of elasticity (low stress and high elastic strain) are through
the
bracket
slots
without
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permanent deformation and, at the same time, the wire must provide low and constant force to gently move the
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teeth to their new positions.
● Intermediate stage -root and bite corrections. Teeth have
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been substantially levelled and aligned during the first stage
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of treatment and the resilence of the wire is less critical.
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However, the wire is required to have greater force for torque and root corrections as well as the manipulation of the arch shapes.
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● Finishing stage. The final stage of treatment involves fine adjustments
and
minor
corrections of teeth positions,
torques, tips, rotations,… At the initial stage of orthodontics treatment, where there are significant tooth-to-tooth discrepancies, polymeric archwires are most suitable. They provide very high resilence and relatively
low
stiffness
compared
with
other
orthodontic
materials such as stainless steel, titanium alloys or NiTi alloys.
ACCEPTED MANUSCRIPT The
tensile,
demonstrated
elasticity, that
spring-back,
these
polymer
and
wires
formability potentially
results
have
the
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necessary properties to produce effective tooth movement.
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The polymer wires tested at three-point bending exhibited results very similar under different forces for the same amount of deflection
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that beta-titanium [5, 31-32]. These values are sufficient for optimum biological dental movement; the archwires tested projected
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an optimal force to promote molar distalization with normal tissue reaction. Cantilever and three-piece archwires are other useful clinical
force
has
an
important
consideration
in
orthodontic
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Frictional
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applications of polymer orthodontic archwire.
mechanotherapy. This contribution evaluated static as well as dynamic friction, and the results indicated lower friction at archwire-
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bracket interface when polymer and Ti-6Al-4V or 316 stainless steel are used in comparison with the other 4 alloy archwires.
Clinically, this means that the net force required for translator movement will be lower for polymer and higher when Ti-based and superelastic alloys (NiTi and NiTiCu) wires are used [33].
The decrease in stress while being held at a constant strain exhibited polymeric viscoelastic behaviour.
The stress decreased about 2%
ACCEPTED MANUSCRIPT depending on the initial deformation level over a period of up 2 hours. The rate of relaxation decreased over the first 24-48 hours,
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and then asymptotically approached a constant value. The magnitude
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of the relaxation increased with increasing initial strain. The time
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period for the time-dependent response increased with initial strain
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level.
An increase of stress can be seen after 80 hours, as can be observed
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in Figure 3. This phenomenon is due to stretching of the polymer chains, as occurs in viscoelastic materials, which requires more stress
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for each strain percentage [6].
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The more relevant clinical situation is where, under constant strain, the archwires overtime exhibit
a loss of stress (stress relaxation).
Then, as the teeth move to a new position, strain is reduced allowing
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a possible recovery of part of the stress.
Clinicians should be aware of stress relaxation and recognize that force is still being exerted, even if stress relaxation occurs. Furthermore, part of any observed deformation is viscoelastic (time dependent) and recoverable. The clinically relevant aspect is that the mechanical behaviour can be modelled and is predictable. The new polymer orthodontic archwire requires the orthodontist to understand and apply important viscoelastic concepts to optimize its clinical use.
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The absence of variation in the transformation temperatures of the polymer by
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thermal cycling prevents changes in mechanical behavior in the ingestion of
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food or drinks hot or cold.
CONCLUSIONS
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A thermoplastic polymer was extruded into archwires with clinically
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relevant cross sections. The wires were visually smooth, the crosssectional dimensions were consistent along the length, and the cross-
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sectional profiles were well defined. The polymer archwires presented
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transformation temperatures adequate for oral applications and these temperatures did not change with respect to the thermal cycles. The
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mechanical strength was lower than metallic alloys but its value is adequate for the first stage in the orthodontic therapy-low and
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continuous stress and high ductility-. The friction coefficients and wear rates better than metallic alloys used and consequently the slipping (wire-bracket) is favoured. These polymer materials present a moderate stress relaxation, that must be taken into account for orthodontic treatment. The wires were readily formed into arches and other loop configurations. The application of the thermoplastic archwires as orthodontic archwires is warranted.
Acknowledgements The authors are grateful to the CICYT MAT 13547 and Andorra
ACCEPTED MANUSCRIPT Government and Generalitat de Catalunya (CTP project) for funding the present work. The authors do not have any conflict of interest with respect to this work.
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27. M. Arciniegas, J. Peña, J.M. Manero, J.C. Paniagua, F.J Gil. Quantum parameters for guiding the design of Ti alloys with shape memory and/or low elastic modulus. Phil Mag 88 (2008) 25292548. 28. M. Arciniegas, J.M. Manero, J. Peña, F.J Gil, J.A. Planell. Study of new multifunctional shape memory and low elastic modulus Nifree Ti alloys. Mater Sci Eng A. 39 (2008) 742-751.
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29. Y. Duan, E. Rettler, K. Schneider, R. Schlegel, M. Thunga, R.Weidisch, H.W. Siesler, M. Stam, J.N. Mays, N. Hadjidristidis. Deformation behaviour of sphere-forming trifunctional multigraft copolymer. Macromolecules, 2008, 42: 4565-4568. 30. R.Schlegel, D. Wilkin, Y. Duan, R. Weidish, G. Heinrich, D. Uhrig, J.N.Mays, H. Jatrou, N. Hadjidristidis. Stress softening of multigraft copolymer. Polymer 2009, 50: 6297-6304. 31. W.A. Brantley, T. Eliades. Orthodontic Materials. Scientific and Clinical aspects. New York NY, Thieme 2001: 78-103. 32. M. Barrabés, A. Michiardi, C. Aparicio, P. Sevilla, J.A. Planell, F.J. Gil. Oxidized nickel-titanium foams for bone reconstructions: chemical and mechanical characterization J. Mater.Sci. Mat. Med. 2007; 18:2123-2129. 33. J.A.
Gurgel,
R.M.
Pinzan-Vercelino,
J.
Powers.
Mechanical
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properties of beta-titanium wires. Angle Orthod. 2011 ; 81 : 471473.
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Figure 1
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Figure 2
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Figure 3
ACCEPTED MANUSCRIPT Figure 1. Superelastic orthodontic archwires studied.
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Figure 2. Thermogram obtained for the polymeric archwires.
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Figure 3. Force relaxation curves at different strains at 20ºC and artificial
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saliva environment.
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Composition (g/dm3)
K2HPO4
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0.20
KCl
1.20 0.33
0.70 1.50
Urea
1.50
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NaHCO3
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Lactic acid
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0.26
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Na2HPO4
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KSCN
NaCl
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Chemical product
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TABLE 1. Chemical composition of the artificial saliva.
up to pH=6.7
ACCEPTED MANUSCRIPT Table 2. Chemical compositions of the orthodontic archwires studied. (% in weight)
material
Ti
Cu
14.8
3.0
87.0
Ti
99.9 55.8
44.1
NiTiCu
49.1
45.2
MA D TE CE P
5.7
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NiTi
Cr 18.0
C
Fe
0.02
64.2
13.0
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Ti-Mo
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Mo
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Stainless Steel
Ni
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Orthodontic archwire
0.1
ACCEPTED MANUSCRIPT Table 3. Elastic modulus (E), maximum strength (max) and ductility (A) for each section at 35, 37 and 39ºC.
MA D TE CE P AC
() 198.112.5 188.713.8 228.615.5 208.718.3 244.613.5 238.611.3
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max(MPa) 25.61.4 26.62.3 23.21.7 21.64.3 19.31.9 18.62.3
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E(MPa) 129.54.9 128.34.5 121.33.9 100.22.7 101.33.9 95.22.7
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T (ºC) 35 35 37 37 39 39
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Shape Round Rectangular Round Rectangular Round Rectangular
ACCEPTED MANUSCRIPT Table 4. Forces for activation and deactivation for a deflection of 5 mm Activation force g 115.3 9.0 108.2 7.2
Deactivation force g
28.7 1.7 29.6 4.3
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Shape Round Rectangular
ACCEPTED MANUSCRIPT Table 5. Static and Dynamic friction coefficients of orthodontic archwires obtained against two types of brackets: Ti6Al4V and 316L stainless steel.
Stainless Steel ASI304
TiMo
Bracket
0,29 0,08
0,36 0,05
0,33 0,10
0,42 0,03
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(353 1 HVN)
Acero (316L)
0,38 0,07
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Ti-6Al-4V
Ti (cp)
0,45 0,09
NiTi
NiTiCu
0,43 0,11
0,59 0,07
0,49 0,13
0,51 0,13
0,66 0,02
0,57 0,11
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(435 3 HVN)
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Superelastic Polymer
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Archwire
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STATIC
DYNAMIC
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Bracket
Superelastic Polymer
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Archwire
Ti-6Al-4V
Stainless Steel ASI304
TiMo
Ti (cp)
NiTi
NiTiCu
0,20 0,02
0,26 0,01
0,31 0,02
0,33 0,01
0,51 0,05
0,44 0,03
0,27 0,08
0,32 0,02
0,39 0,02
0,41 0,03
0,56 0,02
0,47 0,01
(353 1 HVN)
Acero (316L) (435 3 HVN)
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Ti-6Al-4V
(353 1 HVN)
TiMo
NiTiCu
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NiTi
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(435 3 HVN)
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Stainless steel 316L
Ti (cp)
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Stainless Steel
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Polymeric Material
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Table 6. Wear rate of orthodontic archwires (Volume worn in mm3/h) obtained against two reference materials Ti6Al4V and 316L stainless steel with its hardness values.
ACCEPTED MANUSCRIPT Highlights
- A new thermoplastic polymer for orthodontic applications was
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obtained.
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- This polymer could be a good candidate for aesthetic orthodontic
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archwires.
- The polymer has good mechanical properties as orthodontic wire
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coating.
- The friction coefficients were lower than the metallic archwires
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improving the slipping with the brackets. - High formability allowed shape bending similar to that associated
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with stainless steel wires.
S0928-4931(14)00268-9 doi: 10.1016/j.msec.2014.05.008 MSC 4632
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
17 June 2013 23 March 2014 6 May 2014
Please cite this article as: Juan Carlos Varela, Marcos Velo, Eduardo Espinar, Jose Maria Llamas, Elisa R´ uperez, Jose Maria Manero, F. Javier Gil, Mechanical properties of a new thermoplastic polymer orthodontic archwire, Materials Science & Engineering C (2014), doi: 10.1016/j.msec.2014.05.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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MECHANICAL PROPERTIES OF A NEW THERMOPLASTIC
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POLYMER ORTHODONTIC ARCHWIRE
Juan Carlos Varela, Marcos Velo, Eduardo Espinar*, Jose Maria
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Llamas*, Elisa Rúperez**, Jose Maria Manero** and F.Javier Gil**
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Grupo de investigación en Ortodoncia. Facultad de Odontología. Universidad Santiago de Compostela. Santiago de Compostela. Spain. Grupo de investigación en Ortodoncia. Facultad de Odontología.
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*
Dept. C. Materiales e Ing. Metalúrgica. Universitat Politècnica de
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**
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Universidad de Sevilla. Spain.
Catalunya. Centre de Recerca Nanoenginyeria. Member of Biomedical
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Research Networking Center in Bioengineering, Biomaterials and Nanomedicine. CIBER-BBN. Spain
CORRESPONDANCE ADDRESS: F.J.Gil Dept. C. Materials i Eng. Metalúrgica. E.T.S.E.I.B. Universidad Politécnica de Cataluña. Av. Diagonal 647. 08028-Barcelona. Spain Tel:
34
93
4016708
[email protected]
Fax:
34
93
4016706
E-mail:
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ABSTRACT
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A new thermoplastic polymer for orthodontic applications was obtained and extruded into wires with round and rectangular cross sections. We evaluated the potential of new aesthetic archwire:
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tensile, three point bending, friction and stress relaxation behavior,
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formability characteristics were assessed. Stresses delivered were generally slightly lower than typical beta-titanium and nickel-titanium
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archwires. The polymer wire has good instantaneous mechanical
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properties; tensile stress decayed about 2% over 2 hours depending
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on the initial stress relaxation for up to 120 hours. High formability allowed shape bending similar to that associated with stainless steel wires.
The
friction
coefficients
were
lower
than
the
metallic
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conventional archwires improving the slipping with the brackets. This new polymer could be a good candidate for aesthetic orthodontic archwires.
Key words: Polymer archwire, Orthodontic applications, Mechanical properties, friction.
ACCEPTED MANUSCRIPT INTRODUCTION
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The interest in aesthetic orthodontic appliances is increasing: labial
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placed brackets, aesthetically coated archwires, ceramic brackets are
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different examples. The latest development is a polymeric orthodontic archwire, translucent, with high springback and high ductility [1].
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Marroco ML et al [2] and Goldberg et al [3] studied a new polymer wire based on polyphenylene, a novel polymer whose rigid molecular
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leads to a high yield strength and modulus of elasticity [1]. The clinical evaluation demonstrated the efficiency of tooth movement
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during the first stage of levelling phase of treatment [4].
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The clinical plateau of the unloading curve (deactivation curve) is the main one of interest in relation to the moving teeth [5]. This plateau allows the orthodontist to apply an almost continuous light force with
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larger activations that results in the reduction of tissue trauma and the patient discomfort, thus facilitating enhanced tooth movement. In contrast, forces that are high in magnitude encourage hyalinisation of the periodontal ligament and may cause irreversible tissue damage such as root resorption.
Besides the aesthetic reasons, the oral cavity represents a harsh environment for a metallic orthodontic appliance of any kind [6]. Corrosion of orthodontic appliances has been thoroughly studied [7-
ACCEPTED MANUSCRIPT 12]. Two main concerns are directly related to the effects of corrosion: biocompatibility and appliance performance [13]. The most
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important aspect is the interaction that the appliance may have with
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the patient in terms of absorption of corrosion products and the
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systemic reactions that may arise. Attention has been focused on Nickel [14-23] as being an element able to induce allergenic reactions
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since it is one of the most used constituents of the alloys commonly used in orthodontics like Stainless Steel, Nickel-Titanium and Copper-
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Nickel-Titanium. Recently, Ni-free Ti alloys with superelastic behavior have been studied for orthodontic applications [24-27].These alloys
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are beta-titanium and more specifically the family Ti-Nb with different
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additional elements (Ta, Hf). These alloys have superelastic features
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due to the presence of austenite in the microstructure at 37ºC [25,26-27]. The alloy shows a superelastic effect with physiological critical stress (low and continuous) and a minimal loss of the recovery
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around 150 mechanical cycles. The increase of 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 has an excellent in-vitro response. These new alloy Ni-free superelastic alloys are being studied for the first time in orthodontic applications. Because of the above, TiNb alloys
ACCEPTED MANUSCRIPT can be a good candidates for orthodontic applications since they avoid allergic problems. They do not, however, solve aesthetic
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problems.
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For polymeric archwires activated, there is an instantaneous applied stress proportional to the activation. With time, the extended
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molecular conformations relax towards their equilibrium position causing a gradual decrease in the applied force even though the
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deflection is constant: stress relaxation. For metallic archwires, the
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stress remains constant with a constant activation.
polystyrene-g-polyisoprene
copolymers,
which
are
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multigraft
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Recently, Hadjichristidis et al. [28, 29] have studied a series of
thermoplastics elastomers with high strains at break, up to about 1550% [29]. The mechanical properties can be controlled by both
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molecular architecture and morphology. Due to the high strains at break and low residual strains in hysteresis tests, these materials have a behaviour similar to superelastic materials.
The low tensile
stress is the main problem in order to apply these polymers as orthodontic archwires.
The aim of this work is to obtain and determine the mechanical properties (tensile, three-point bending, stress relaxation and friction behaviour) of a new thermoplastic polymer orthodontic archwire and
ACCEPTED MANUSCRIPT to compare its properties with the metallic archwires. These new polymer archwires present the aesthetic advantages; avoid the
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corrosion and the ion release.
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MATERIALS AND METHODS
Material resin
was
obtained
by
mixing
a
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The
liquid
phase
(methyl-
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methacrylate monomer and butyl acrylate) and a solid component (benzoyl peroxide, tricresyl phosphate and dichloromethyl silane).
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Once mixed, the material was initially cured at 40° C for 2 hours and
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afterwards at 60° C for 14 hours. The material was then subjected to
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a post-curing heat treatment of one hour at 130° C.
The thermoplastic was extruded by using specially designed profile
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dies to achieve wires with clinically relevant cross sections of either 0.508x0.762 mm2 rectangular or 0.508 mm round. A total of approximately 60 m of wire were extruded. The wire was visually examined,
and
cross-sectional
dimensions
were
inspected
continuously along the length. At several locations, samples were cut and examined with optical and scanning electron microscopy to evaluate the outer surface and cross-sectional shape.
Calorimetry
ACCEPTED MANUSCRIPT The transformation temperature (Tg) was measured by means of a calorimeter. The calorimetric system used has already been described
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in previous papers [9] and it is based on a flow calorimeter which
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measures differential signals (T) by means of thermobatteries.
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Temperature was measured by means of a standard Pt-100 probe. All signals were digitized through a multichannel recorder and linked to a
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microcomputer. The transformation temperatures occur when there is a sudden increment in calorimetric signal. In the same way, the final
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temperatures are determined when the calorimetric signal returns to
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the base line.
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Thermal cycling effects.
Five orthodontic archwires were thermally cycled (n=100). The samples were treated at 100ºC to 0ºC both for 10 minutes followed
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by cooling in water at 37ºC. Transformation temperatures were determined at different number of thermal cycles.
Tensile properties. Tensile tests were carried out in an electromechanical universal testing machine working (MTS-Adamel) at a cross-bar speed of 10mm/min. Laser extensometer was used in these tests.
The
specimens tested were of 150 mm of height in rectangular section
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distance between the grips was 77 mm. The tests were carried out in
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artificial saliva medium at 37ºC. The chemical composition of the
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artificial saliva is shown in Table 1. Elastic modulus, ultimate strength, elongation at yield at break were determined for each
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orthodontic archwire (n=40) at different temperatures.
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Three-point bending
A three-point bending tests were conducted using the universal
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testing machine (MTS-Adamel). The load frame was equipped with a
37ºC.
The
bending
tests
were
investigated
using
a
laser
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of
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1 kN static cell The measurements were taken in a body temperature
extensometer. Ten measurements were recorded for each specimen. The dimensions of the specimens were the same used in tensile tests.
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The mid portion of the wire segment was deflected at the speed of 1 mm/min under the pressure from a metal pole of 5 mm in diameter. Each sample was loaded until a deflection of 4 mm was located. The samples were unloaded at the same cross-head speed until the force became zero [5].
Friction Coefficients. Friction coefficients were performed in a CSM pin-on-disk tribometer, in accordance to the ASTM G99 standard. The underlying principle on
ACCEPTED MANUSCRIPT this test could be called wire-on-disk because of its analogy with the pin-on-disk test. The samples studied were 10 archwires for each
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material: polymer, stainless steel, TiMo, cp Ti, NiTi and NiTiCu. The
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chemical composition of the alloys is shown in Table 2.
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The orthodontic archwires were carefully affixed with cyanoacrylate adhesive in a bakelite holder, without adhesive residues, on the wire
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surface to be tested. The contact wire plane and the disc were in the
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longitudinal direction to simulate full-arc contact bracket. An angular velocity of the disks and the normal load of 0.5236 rad/s
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and 10N were used respectively. Ideally, the normal load on the wire
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should have been around 1N to simulate the load in service (in which typical values ranging from 20 to 100 g force). However, 10N were
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employed to ensure that there was a full contact between both surfaces that could influence the determination of the coefficients of
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friction.
The static and dynamic friction coefficients µ (the proportionality constant between the friction force and the normal force) were determined for the orthodontic archwires against the materials commonly used for the brackets (manufactured in 316 stainless steel and
Ti-6Al-4V).
These
coefficients
were
measured
in
an
environmental chamber which contained artificial saliva at 37ºC. The dynamic friction coefficients µ (the proportionality constant
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the materials commonly used for the brackets (manufactured in 316
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Stainless steel and Ti-6Al-4V) were also measured.
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As the wear test was being performed, gravimetric measures were controlled in a Sartorius Micro Balance CPA26P, in order to determine
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the weight loss over time by means of a high-precision set of scales. The sensitivity of these measures was ±0.001 mg. With uniform
MA
density, the weight lost can be stated as volume loss.
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Stress relaxation: Creep tests. Ten archwires were strained at values of 5, 10, 20, 40, 60 and 80%,
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which are the values at which the archwires are strained in clinical use for tooth movement [7-8, 10]. The mechanical tests were carried
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out with an MTS-Adamel electromechanical testing machine. The rate of application of the load to reach the target strain was 1 mm/minute and the rate of data acquisition by the Autotrack software was 25 points/second; this enabled us to control automatically the stress relaxation that was produced over time. A load cell of 1KN was used because it has greater sensitivity in the force values that must be applied to maintain these levels of strain. The force values were determined at different time intervals up to 120 hours.
To control the constant strain a laser extensometer was used. This
ACCEPTED MANUSCRIPT type of extensometer was used with the aim that the pressure should not cause cuts in the polymer and therefore affect the values of the
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mechanical forces measured. With the laser extensometer fluorescent
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tapes were placed on the sample to be measured in order to allow the
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laser to measure the displacement values remotely.
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With the aim of carrying out the mechanical tests in conditions similar to those of the mouth, a physiological chamber was designed with
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EXPERIMENTAL RESULTS
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artificial saliva at a temperature of 37ºC.
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The polymer was successfully extruded into round and rectangular orthodontic wires with cross-sectional dimensions that fit existing brackets. These translucent wires potentially offer an aesthetic
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alternative to current metal orthodontic wires, as can be seen in Figure 1.
From the thermograms obtained, the Tg was 39.12 ºC. The start temperature was 35.83ºC and the finish temperature 40.57ºC. In Figure 2 can be observed a calorimetric thermogram of a polymeric archwire tested. These temperatures are compatible with the mouth temperature 37ºC. The thermal cycling did not show statistically differences on the transformation temperatures in relation to the un-
ACCEPTED MANUSCRIPT treated polymeric archwires.
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Tensile properties of the round and rectangular extruded wires are
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reported in Table 3 at different test temperatures (35, 37 and 39ºC).
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The results of the elastic modulus are higher at temperatures near Tg temperature. Besides, the maximum stress decreases and the
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deformation increases slightly with increasing test temperature from
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35 to 39 ° C. This is due to the proximity of the Tg temperature.
Three-point bending activation and deactivation forces of the wires
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measured at a deflection of 5 mm are listed in Table 4. Statistically
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differences were not found in any of the force levels (activation and
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deactivation). The results are very similar to the beta-titanium orthodontic archwires [5].
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The results of the frictional tests performed on the achwires under scrutiny on the two different types of brackets (316L stainless steel and the Ti6Al4V alloy) provided the values for the static and dynamic friction coefficients (µ) as it can be seen in Table 5. It can be observed that the polymer static and dynamic friction coefficients are lower than beta-Ti and cp-Ti, NiTi and NiTiCu orthodontic archwires in both bracket materials tested. From the results shown in Table 6, we can see that the wear rate on polymeric wires is much lower than on metallic wires.
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One limitation of most polymers is the phenomenon of stress
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relaxation or creep. Over time, stress below the yield strength can
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lower the force and cause permanent deformation. Figure 3 shows
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the decrease in load for the 0.508 x 0.762 mm2 after being initially strained from 5 to 80% of its initial length at 20ºC. The pattern of
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stress relaxation was typical for a viscoelastic material. There was decreasing stress at a decreasing rate asymptotically reaching a
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constant stress level. At the end of the test, the samples retained 94.2%, 85.5%, 79.6%, 77.5%, 75.1% and 74.8 % of their original
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stress level when were strained to 5%, 10%, 20%, 40%, 60%, and
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80% of their original shape, respectively.
DISCUSSION
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An important advantage of the polymer orthodontic archwire is the excellent formability. Beta-Ti and cp-Ti present also inherent a good formability. However, the metallic wire processing can be problematic because of the reactivity of titanium that can result in some batches of wires being susceptible to fracture during clinical manipulation. For NiTi and NiTiCu alloys, small changes in the chemical composition produced by surface oxidation can modify the corrective stresses [29, 30].
ACCEPTED MANUSCRIPT The polymer wires were highly ductile, as evidenced by their high elongations at yield and break of about 200%. This has two important
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clinical advantages. First, the wires are highly ductile and, hence,
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they are unlikely to fracture in the mouth under high-stress
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conditions. Second, it is possible to make secondary bends in the wire; in this respect, handling is similar to working with a metal wire.
thermoplastic
polymer
wire
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More complicated configurations such as loops could be formed in the without
the
use
of
heat.
The
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thermoplastic polymer wires that were formed into archwires by heating demonstrated good shape control in 3 dimensions and also in
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their cross sections. Moreover, thermal cycles produced in the mouth
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do not effect the transformation temperatures, which give an the archwire. However, the stress of the
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important stability to
polymer (around 20-23MPa) is lower than the NiTi (45-60MPa) producing a longer period of archwire application on the teeth [9-11].
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Therefore, these polymers can be very suitable for certain stages of orthodontic treatment in which the metals do not have suitable properties.
Orthodontic treatment can be divided into three stages. Each stage of treatment requires wires with specific properties tailored for certain functions: ● Initial stage. Levelling and alignment of teeth in mandibular (lower) and Maxillofacial (upper) arches. During the initial
ACCEPTED MANUSCRIPT stage of treatment teeth are generally crowed, particularly in the anterior sections of the jaws. Highly resilient (resilence
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of the wire refers to the elastic deformation with full
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recovery after deformation) wires having a relatively low
required
to
weave
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modulus of elasticity (low stress and high elastic strain) are through
the
bracket
slots
without
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permanent deformation and, at the same time, the wire must provide low and constant force to gently move the
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teeth to their new positions.
● Intermediate stage -root and bite corrections. Teeth have
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been substantially levelled and aligned during the first stage
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of treatment and the resilence of the wire is less critical.
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However, the wire is required to have greater force for torque and root corrections as well as the manipulation of the arch shapes.
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● Finishing stage. The final stage of treatment involves fine adjustments
and
minor
corrections of teeth positions,
torques, tips, rotations,… At the initial stage of orthodontics treatment, where there are significant tooth-to-tooth discrepancies, polymeric archwires are most suitable. They provide very high resilence and relatively
low
stiffness
compared
with
other
orthodontic
materials such as stainless steel, titanium alloys or NiTi alloys.
ACCEPTED MANUSCRIPT The
tensile,
demonstrated
elasticity, that
spring-back,
these
polymer
and
wires
formability potentially
results
have
the
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necessary properties to produce effective tooth movement.
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The polymer wires tested at three-point bending exhibited results very similar under different forces for the same amount of deflection
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that beta-titanium [5, 31-32]. These values are sufficient for optimum biological dental movement; the archwires tested projected
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an optimal force to promote molar distalization with normal tissue reaction. Cantilever and three-piece archwires are other useful clinical
force
has
an
important
consideration
in
orthodontic
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Frictional
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applications of polymer orthodontic archwire.
mechanotherapy. This contribution evaluated static as well as dynamic friction, and the results indicated lower friction at archwire-
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bracket interface when polymer and Ti-6Al-4V or 316 stainless steel are used in comparison with the other 4 alloy archwires.
Clinically, this means that the net force required for translator movement will be lower for polymer and higher when Ti-based and superelastic alloys (NiTi and NiTiCu) wires are used [33].
The decrease in stress while being held at a constant strain exhibited polymeric viscoelastic behaviour.
The stress decreased about 2%
ACCEPTED MANUSCRIPT depending on the initial deformation level over a period of up 2 hours. The rate of relaxation decreased over the first 24-48 hours,
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and then asymptotically approached a constant value. The magnitude
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of the relaxation increased with increasing initial strain. The time
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period for the time-dependent response increased with initial strain
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level.
An increase of stress can be seen after 80 hours, as can be observed
MA
in Figure 3. This phenomenon is due to stretching of the polymer chains, as occurs in viscoelastic materials, which requires more stress
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for each strain percentage [6].
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The more relevant clinical situation is where, under constant strain, the archwires overtime exhibit
a loss of stress (stress relaxation).
Then, as the teeth move to a new position, strain is reduced allowing
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a possible recovery of part of the stress.
Clinicians should be aware of stress relaxation and recognize that force is still being exerted, even if stress relaxation occurs. Furthermore, part of any observed deformation is viscoelastic (time dependent) and recoverable. The clinically relevant aspect is that the mechanical behaviour can be modelled and is predictable. The new polymer orthodontic archwire requires the orthodontist to understand and apply important viscoelastic concepts to optimize its clinical use.
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The absence of variation in the transformation temperatures of the polymer by
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thermal cycling prevents changes in mechanical behavior in the ingestion of
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food or drinks hot or cold.
CONCLUSIONS
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A thermoplastic polymer was extruded into archwires with clinically
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relevant cross sections. The wires were visually smooth, the crosssectional dimensions were consistent along the length, and the cross-
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sectional profiles were well defined. The polymer archwires presented
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transformation temperatures adequate for oral applications and these temperatures did not change with respect to the thermal cycles. The
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mechanical strength was lower than metallic alloys but its value is adequate for the first stage in the orthodontic therapy-low and
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continuous stress and high ductility-. The friction coefficients and wear rates better than metallic alloys used and consequently the slipping (wire-bracket) is favoured. These polymer materials present a moderate stress relaxation, that must be taken into account for orthodontic treatment. The wires were readily formed into arches and other loop configurations. The application of the thermoplastic archwires as orthodontic archwires is warranted.
Acknowledgements The authors are grateful to the CICYT MAT 13547 and Andorra
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Gurgel,
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Pinzan-Vercelino,
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Powers.
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properties of beta-titanium wires. Angle Orthod. 2011 ; 81 : 471473.
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Figure 1
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Figure 2
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Figure 3
ACCEPTED MANUSCRIPT Figure 1. Superelastic orthodontic archwires studied.
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Figure 2. Thermogram obtained for the polymeric archwires.
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Figure 3. Force relaxation curves at different strains at 20ºC and artificial
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saliva environment.
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Composition (g/dm3)
K2HPO4
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0.20
KCl
1.20 0.33
0.70 1.50
Urea
1.50
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NaHCO3
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Lactic acid
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0.26
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Na2HPO4
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KSCN
NaCl
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Chemical product
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TABLE 1. Chemical composition of the artificial saliva.
up to pH=6.7
ACCEPTED MANUSCRIPT Table 2. Chemical compositions of the orthodontic archwires studied. (% in weight)
material
Ti
Cu
14.8
3.0
87.0
Ti
99.9 55.8
44.1
NiTiCu
49.1
45.2
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5.7
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NiTi
Cr 18.0
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Fe
0.02
64.2
13.0
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Ti-Mo
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Mo
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Stainless Steel
Ni
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Orthodontic archwire
0.1
ACCEPTED MANUSCRIPT Table 3. Elastic modulus (E), maximum strength (max) and ductility (A) for each section at 35, 37 and 39ºC.
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() 198.112.5 188.713.8 228.615.5 208.718.3 244.613.5 238.611.3
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max(MPa) 25.61.4 26.62.3 23.21.7 21.64.3 19.31.9 18.62.3
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E(MPa) 129.54.9 128.34.5 121.33.9 100.22.7 101.33.9 95.22.7
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Shape Round Rectangular Round Rectangular Round Rectangular
ACCEPTED MANUSCRIPT Table 4. Forces for activation and deactivation for a deflection of 5 mm Activation force g 115.3 9.0 108.2 7.2
Deactivation force g
28.7 1.7 29.6 4.3
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Shape Round Rectangular
ACCEPTED MANUSCRIPT Table 5. Static and Dynamic friction coefficients of orthodontic archwires obtained against two types of brackets: Ti6Al4V and 316L stainless steel.
Stainless Steel ASI304
TiMo
Bracket
0,29 0,08
0,36 0,05
0,33 0,10
0,42 0,03
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(353 1 HVN)
Acero (316L)
0,38 0,07
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Ti-6Al-4V
Ti (cp)
0,45 0,09
NiTi
NiTiCu
0,43 0,11
0,59 0,07
0,49 0,13
0,51 0,13
0,66 0,02
0,57 0,11
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(435 3 HVN)
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Superelastic Polymer
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Archwire
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STATIC
DYNAMIC
AC
Bracket
Superelastic Polymer
CE P
Archwire
Ti-6Al-4V
Stainless Steel ASI304
TiMo
Ti (cp)
NiTi
NiTiCu
0,20 0,02
0,26 0,01
0,31 0,02
0,33 0,01
0,51 0,05
0,44 0,03
0,27 0,08
0,32 0,02
0,39 0,02
0,41 0,03
0,56 0,02
0,47 0,01
(353 1 HVN)
Acero (316L) (435 3 HVN)
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Ti-6Al-4V
(353 1 HVN)
TiMo
NiTiCu
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NiTi
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(435 3 HVN)
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Stainless steel 316L
Ti (cp)
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Stainless Steel
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Polymeric Material
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Table 6. Wear rate of orthodontic archwires (Volume worn in mm3/h) obtained against two reference materials Ti6Al4V and 316L stainless steel with its hardness values.
ACCEPTED MANUSCRIPT Highlights
- A new thermoplastic polymer for orthodontic applications was
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obtained.
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- This polymer could be a good candidate for aesthetic orthodontic
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archwires.
- The polymer has good mechanical properties as orthodontic wire
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coating.
- The friction coefficients were lower than the metallic archwires
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improving the slipping with the brackets. - High formability allowed shape bending similar to that associated
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with stainless steel wires.
