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Table of Contents
Year : 2020  |  Volume : 10  |  Issue : 1  |  Page : 29-34

Elastic deflection study of nickel-titanium orthodontic wires: 3-point bending test X clinical simulation device

1 Department of Dentistry, Hospital for Rehabilitation of Craniofacial Anomalies, São Paulo, Brazil
2 Department of Orthodontics, Bauru Dental School, University of São Paulo, São Paulo, Brazil
3 Department of Mechanical Engineering, Faculty of Engineering, Universidade Estadual Paulista Júlio De Mesquita Filho, São Paulo, Brazil
4 Department of Orthodontics, University of North Parana, Parana, Brazil

Date of Submission23-Oct-2018
Date of Acceptance28-Feb-2020
Date of Web Publication30-Apr-2020

Correspondence Address:
Dr. Renata Sathler
Hospital for Rehabilitation of Craniofacial Anomalies, R. Silvio Marchione, 3 20, Vila Universitaria, Bauru, Sao Paulo 17012-900
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jid.jid_78_18

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Introduction: The purpose of this study was to compare the most usual types of bending tests used to evaluate nickel-titanium orthodontic wires: the 3-point bending test and the clinical simulation device (CSD). Materials and Methods: Eleven groups of 0.014-inch nickel-titanium orthodontic wires of six different brands were tested. A 3342 Instron universal testing machine with a 10 N load cell was used, and the ISO 15.841 regulation was followed. T-tests were used to compare the evaluations. Results: At a deflection of 0.5 mm, the device released significantly lighter forces then the 3-point test. However, at deflections of 1; 2 and 3 mm, the device released significantly heavier forces. Conclusion: Because of the several differences between the tests and the inconsistency of the CSD, the 3-point bending test was elected the most reliable method to evaluate the elastic deflection of nickel-titanium orthodontic wires.

Keywords: Copper-nickel-titanium, orthodontic wires, titanium nickelide

How to cite this article:
Sathler R, de Freitas MR, Soufen CA, Zanda M, Fernandes TM, Maranhão OB, Garib DG, Janson G. Elastic deflection study of nickel-titanium orthodontic wires: 3-point bending test X clinical simulation device. J Interdiscip Dentistry 2020;10:29-34

How to cite this URL:
Sathler R, de Freitas MR, Soufen CA, Zanda M, Fernandes TM, Maranhão OB, Garib DG, Janson G. Elastic deflection study of nickel-titanium orthodontic wires: 3-point bending test X clinical simulation device. J Interdiscip Dentistry [serial online] 2020 [cited 2023 Apr 1];10:29-34. Available from: https://www.jidonline.com/text.asp?2020/10/1/29/283539

   Clinical Relevance to Interdisciplinary Dentistry Top

  • To compare the most usual types of bending tests;
  • To show the most reliable tool for addressing the elastic deflection;
  • To show possible differences between 3-point test and clinical simulation device.

   Introduction Top

Introduction of nickel-titanium orthodontic wire occurred in the early 70s by George Andreasen.[1] Thereafter, new studies were conducted, the alloy was improved, and new phenomena were discovered.[2] These wires replaced, at least partially, the stainless steel wires in the process of leveling and alignment, and several studies have emerged showing its new and exciting features.[3],[4]

In order to study the mechanical characteristics of orthodontic wires and to select the best materials according to their behavior,[5],[6] one of the most reliable and clinically relevant assessments of orthodontic wires is provided by the evaluation of the elastic deflection using bending tests. This test mimics quite satisfactorily what occurs in the clinical practice when the orthodontist inserts a wire in the bracket slot.[6],[7],[8] However, two different ways of conducting the bending test have been proposed. One is performed with the use of a clinical simulation device (CSD).[9] The other, which is most commonly used in Engineering, is the 3-point test. This test is much simpler to evaluate the relationship between the deflection and loading and is also less influenced by factors other than those investigated in the study.[2],[6],[10],[11] Because of this ambiguity, there is still a need to clarify whether there are differences in the results obtained by these two ways of checking the elastic deflection of orthodontic wires. An ISO (International Organization for Standardization) regulation, specific for laboratory tests of orthodontic wires, should be followed to obtain reliable and comparable results.[12]

Therefore, the objective of this research was to compare the results obtained by the 3-point test with those obtained by a CSD following a systematic protocol for these tests, indicated by the ISO 15841 regulation.

   Materials and Methods Top

The sample size was based on ISO 15841 standard[13] which recommends 10 wires per group. However, to avoid technical errors, 15 wires were used in each of the 11 groups, resulting in a sample of 165 0.014-inch (0.36 mm) round conventional (not heat-activated) and heat-activated (HA) nickel-titanium orthodontic wires, of six different manufacturers: Abzil (São José do Rio Preto, SP, Brazil), GAC (Bohemia, NY, USA), Morelli (Sorocaba, SP, Brazil), Ormco (Glendora, CA, USA), Orthometric (Marília, SP, Brazil) and Orthosource [Porto Alegre, RS, Brazil, [Table 1]. All wires were of a standard or medium format and the HA wires should have a transition temperature close to body temperature. Most labels of the HA wires selected did not describe the exact austenite finish temperature. Some only described the temperature as “close to body temperature,” which was included as the standard transition temperature.
Table 1: Sample details

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The study was performed in a Dental Materials testing laboratory at Bauru Dental School - University of Sao Paulo. To evaluate the elastic deflection of the selected wires, a 3342 Instron universal testing machine (Norwood, MA, USA) was used with a 10 N load cell [Figure 1]. In this machine series, the speed accuracy is ±0.2%, and the position accuracy (extension) is equal to or less than ±0.02 mm or ±0.05% of reading, whatever is greater.[14] The load cell has an accuracy of ±0.5% of reading at 25°C, which was maintained.[14]
Figure 1: 3342 Instron load cell used (10 N)

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To standardize the tests as adequately as possible, the methods used in this study followed the ISO 15841 regulation: Dentistry– Wires for use in orthodontics.[12] This regulation standardizes the laboratory tests for elastic deflection of orthodontic wires and recommends the 3-point test. The tested specimens were cut with at least 30 mm of length, taken from the straightest portion of the wire, and the distance between the supports for the 3-point test was of 10 mm, as described by the ISO 15841 standard[13] [Figure 2]. The tests were performed at the same temperature for all groups (36 ± 1)°C. Therefore, an acrylic container filled with water was adapted to the Instron machine at this temperature and maintained with support of a submersible heater with integrated thermostat (ATMAN Electronic Heater, China), confirmed by a thermometer of decimal precision, with a limit of error of ±0.2°C (INCOTERM, reference 5097, São Paulo, Brazil).[15],[16],[17],[18],[19],[20],[21],[22] The load cell was kept at room temperature, conditioned at 25°C [Figure 3].
Figure 2: Figure representing the 3-point test

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Figure 3: Acrylic container adapted to the Instron machine with heaters to maintain the temperature close to body temperature (A), a thermometer to control the temperature (B), one of the testing devices (C) and 3342 Instron load cell used (10 N) (D)

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Before each test, the load cell was adapted and calibrated using the Instron Bluehill Lite software (v. 2.25, 2005). The test speed was 2.0 mm/min and the software was programmed to begin testing at 3.1 mm of deflection and from this point, the forces released on unloading were measured at 3; 2; 1 and 0.5 mm, all in accordance to the ISO regulation [Figure 4].
Figure 4: Close-up view of 3-point test (a) and unload at 3 mm (b) and 0 mm (c)

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Because of divergence in the literature on the use of CSD, besides the 3-point test, the specimens were also tested with a CSD to enable comparison between these two methods.[23] Tests applied to the CSD were performed identically to that previously described. Acrylic structures that represented the maxillary teeth from the right second molar to the left second molar were positioned in an acrylic plate.[20],[22] To decrease the interference of metal or elastic ligatures, Damon 3MX™ self-ligation brackets were chosen.[23],[24] The distance between the brackets was kept constant at 6 mm[13] and the acrylic structure representative of the maxillary right central incisor served as support for deflection in the palatal-labial direction [Figure 5].
Figure 5: Clinical simulation device

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Statistical analyses

The ISO 15841 regulation recommends six specimens of each sample. However, to minimize the chances of some technical error and to increase the reliability of the results, 15 specimens were selected for each group, according to sample size calculation. Normal distribution was evaluated and confirmed with Kolmogorov–Smirnov tests. T-tests were used to compare the two types of elastic deflection tests carried out, namely the 3-point test and the CSD.

   Results Top

There were significant differences between the results of the 3-point test and the CSD in 35 of the 44 evaluations. At a deflection of 0.5 mm, the device released significantly lighter forces then the 3-point test. However, at deflections of 1; 2 and 3 mm, the device released significantly heavier forces [Table 2].
Table 2: Descriptive analysis of 3-point bending test and clinical simulation device during unloading at 0.5, 1, 2 and 3 mm (t-tests, Results in cN), mean±SD

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   Discussion Top

The 3342 Instron universal testing machine (Norwood, MA, USA) was used with a 10 N load cell. The choice of the cell should be carefully made in accordance with the expected force delivered by the specimen to be studied to provide the precision of results. This issue is sometimes neglected, which requires caution when interpreting results and evaluating conclusions. Although it is known that orthodontic forces are low, there are reports of studies using a load cell of 1,000 N,[25] 1000 pounds (4448.22 N),[26] 5000 N,[27] and up to 10,000 pounds (44,482.22 N).[28] It is quite logical that such load cells cannot provide reliable results for wires that release proportional low forces. Because this issue may be outside most orthodontists field of knowledge, it would be interesting if the ISO 15841 included some guidelines about the choice of the proper load cell. The 3342 Instron load cell has an accuracy of ±0.5% of reading.[13] Thus, a reading of 60 cN (≈ grf) of force, for example, has a maximum error of ±0.3 cN. Despite the lack of standardization related to the load cell, recent studies reported the use of 10 N or even 5 N load cell in 3-point tests, which provided more reliable results in these tests according to the literature.[29],[30],[31] Therefore, the accuracy of the forces reported in this research is very trustable.

There are several tests that can be applied to evaluate the physical characteristics of a metal alloy. The evaluation can be performed by studying the modulus of elasticity, springback, hysteresis, relaxation of the alloy, and others. Each of these assessments is important, but they are not considered the best by some authors.[2],[7] The elastic deflection test determines the force released by the wire when it is subjected to a load causing its deflection. This is the test that most closely matches the clinical orthodontic practice because it is precisely what the clinician will do when adapting an archwire to the bracket slot.[6],[7],[8] Burstone also states that while engineers work with parameters such as elastic modulus and yield strength, the clinician is more interested in knowing what is the force magnitude released by the wire with an increasing amount of deflection.[7] Besides, even those professionals more related to the research field have a great interest in the clinical nature of this type of information.[32]

There are some authors who advocate the use of arch simulating devices to obtain more reliable results.[9],[23] However, when these devices are used for clinical simulation, it is not possible to compare them, because any difference in the material used and in the disposition of their structures may influence the results.[11] The ISO 15841 regulation states that elastic deflection tests should be conducted with the 3-point test.[13] This test is most useful for the study of elastic deflection because it eliminates many of the variables found in a CSD such as friction between components, interbracket distance, and the influence of elastic or metallic ligatures.[29] Because of this divergence, testing of a CSD aimed to simulate deflection of the orthodontic archwire as compared to evaluations with a 3-point test.

Comparison between the 3-point test and the clinical simulation device

At a deflection of 0.5 mm, the device released lighter forces than the 3-point test. However, at deflections of 1, 2, and 3 mm, the CSD released heavier forces than the 3-point test [Table 2]. These findings confirmed other studies[11],[21],[22],[33],[34] and this behavior was explained by the higher friction on the CSD in comparison to the 3-point test.[11] Friction increases the force released by the wire during loading but decreases the force released during unloading.[5],[35],[36] The reason why the CSD often releases higher forces is probably because the force is evaluated in the anterior portion of the archwire, which is curved, unlike the 3-point test that evaluates the most straight wire region.[29] In the curved portion, the force released is greater than that released in the more straight portion.[9]

A greater force was already expected from the CSD device because the interbracket distance was of only 6 mm in comparison to the distance between the supports for the 3-point test, which was of 10 mm. However, the inconsistency between the forces released at 0.5 mm and those released at 1, 2, and 3 mm of deflection, for the CSD device was not. Due to this inter-support distance differences between the devices, one could question the need for this comparison. It was necessary to evaluate the behavior of the devices with the increasing amount of activations. The results showed greater consistency with the 3-point test.

Besides the diverging inter-support differences between the devices, the differences between the 3-point test and the CSD are especially the degree of friction and the portion of the wire submitted to deflection. Similarly to the CSD, in the clinical setting, most crowding is treated by the anterior portion of the wire, which is curved. Conversely, the 3-point test uses the straight portion, and its results are generally lower than those of the curved portion. Nevertheless, this lower force detected by the laboratory 3-point test may be offset by the increased friction observed clinically. The brackets, the inter-bracket distance, and especially crowding are factors that increase friction in the clinical setting.[24],[34] The higher the friction, the smaller is the force released during unloading.[34] Thus, this higher friction would be able to decrease the force released by the wire and compensates the fact that the 3-point test uses the straight portion instead of the curved portion of the wire. Thus, the results of clinical and laboratory 3-point test would be closest than previously estimated.

Besides, the proportion of increased force resulting from the curved portion may be calculated using mathematical formulas, and the approximate force released from every portion of the wire could be derived based on the results of the 3-point test.[11]

Finally, comparison of the CSD to the 3-point test showed that the devices present inconsistent results in different deflections,[11] large difference in force magnitude in comparison with the 3-point test[11],[22] and significant differences among the various device types.[21] Therefore, these differences show how CSD may have varying results. This reinforces the need to use a method less susceptible to these variations. Therefore, because of the above-highlighted issues, such as its precision, reproducibility, clinical significance,[2] and indication of the ISO 15841 regulation, the 3-point test should be chosen to evaluate the mechanical properties of elastic deflection of nickel-titanium orthodontic wires.

   Conclusions Top

  • Elastic deflection results showed differences between the 3-point test and the CSD
  • Because of its precision, clinical similarity, reproducibility, and its indication by the ISO 15841 regulation, the 3-point test was considered the most reliable tool for assessing the elastic deflection of nickel-titanium orthodontic wires.

Financial support and sponsorship

This investigation was supported by “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES”.

Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2]


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