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Table of Contents
Year : 2019  |  Volume : 9  |  Issue : 1  |  Page : 19-24

Force magnitude of nickel-titanium orthodontic wires

1 Department of Orthodontics, Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru, São Paulo, Brazil
2 Department of Orthodontics, Bauru Dental School, University of São Paulo, Bauru, São Paulo, Brazil
3 Department of Mechanical Engineering, College of Engineering – Universidade Estadual Paulista, Bauru, São Paulo, Brazil
4 Department of Stomatology and Oral Biology, Bauru Dental School, University of São Paulo, Bauru, São Paulo, Brazil
5 Department of Orthodontics, University of North Parana, Londrina, Paraná, Brazil
6 Department of Orthodontics, Bauru Dental School, Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru, São Paulo, Brazil

Date of Web Publication18-Feb-2019

Correspondence Address:
Renata Sathler
Hospital for Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru, São Paulo
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jid.jid_14_18

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Objective: The purpose of this study is to evaluate the magnitude of forces released by nickel-titanium (NiTi) orthodontic wires used for leveling and alignment. Materials and Methods: Eleven groups of 0.014” NiTi 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 was followed. Kolmogorov–Smirnov tests were used to verify normality and ANOVA followed by Tukey's tests were used for intergroup comparisons. Results: Abzil conventional, GAC conventional, Morelli heat-activated (HA), Ormco CuNiTi, and Orthometric conventional released load within the optimum range of 50–100 cN. HA wires released lower forces compared to conventional wire of the same brand. Conclusions: Heat-activated Morelli and CuNiTi Ormco presented homogeneous loads within an optimal range.

Keywords: Materials, nickel, orthodontics, orthodontic wire

How to cite this article:
Sathler R, de Freitas MR, Soufen CA, Zanda M, Freire Fernandes TM, Wagner MC, Vieira Maranhão OB, Garib DG, Janson G. Force magnitude of nickel-titanium orthodontic wires. J Interdiscip Dentistry 2019;9:19-24

How to cite this URL:
Sathler R, de Freitas MR, Soufen CA, Zanda M, Freire Fernandes TM, Wagner MC, Vieira Maranhão OB, Garib DG, Janson G. Force magnitude of nickel-titanium orthodontic wires. J Interdiscip Dentistry [serial online] 2019 [cited 2023 Mar 23];9:19-24. Available from: https://www.jidonline.com/text.asp?2019/9/1/19/252523

   Clinical Relevance to Interdisciplinary Dentistry Top

In Orthodontics, it is important to know the force magnitude and its homogeneity provided by the different wires used. This is important to perform more predictable tooth movement during orthodontic treatment. This article shows which wire types and brands provide better performance regarding these issues.

   Introduction Top

Introduction of nickel-titanium (NiTi) orthodontic wire occurred in the early 1970s, by George Andreasen.[1] Thereafter, new studies emerged, the alloy was improved, and several studies have shown its new and exciting features.[2] Concurrently, in a very aggressive manner, massive advertisements made competition for attention of the orthodontist increasingly fierce.[3] Therefore, deep knowledge of the mechanical characteristics of orthodontic wires is essential and their selection must be made according to their behavior.[4],[5]

In order to achieve this, one of the most reliable and clinically relevant assessments of orthodontic wires is provided by evaluation of elastic deflection using the 3-point bending test. This test mimics quite satisfactorily what occurs in clinical practice, when the orthodontist inserts a wire in the bracket slot.[5],[6] Because it is essential to know the magnitude of force released by these wires to select them,[7],[8] the objective of this investigation is to evaluate the magnitude of force released by NiTi orthodontic wires with a systematic testing protocol following the ISO 15841 regulation.

   Materials and Methods Top

The sample consisted of 165 0.014” (0.36 mm) conventional (not heat-activated [HA]) and HA NiTi 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 mouth temperature.
Table 1: Sample details

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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 ≤±0.02 mm or ± 0.05% of reading, whatever is greater.[9] The load cell has an accuracy of ± 0.5% of reading, at 25°C, which was maintained.[9]
Figure 1: 3342 Instron universal testing machine and the 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.[10] 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 [Figure 2]. The tests were performed at the same temperature for all groups tested (36°C ± 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).[3],[7],[11] The load cell was kept at room temperature, conditioned at 25°C [Figure 3].
Figure 2: Figure representing the 3-point test, ISO 15841

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Figure 3: Acrylic container adapted to the Instron machine

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Before each test, the load cell was adapted and calibrated using the Instron Bluehill Lite software (v.2.25, 2005), Norwood, MA, USA. 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 of 3-point test (a) and unloading at 3 (b) and 0 mm (c)

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Statistical analysis

The ISO 15841 regulation recommends six specimens of each sample. However, to minimize the chances of some technical error and increase reliability of the results, 15 specimens were selected for each group, according to sample calculation. Normal distribution was evaluated with Kolmogorov–Smirnov tests. Then, ANOVA followed by Tukey's tests was applied to compare the performance of each group with respect to the magnitude of forces released at different deflections.

   Results Top

Force variability

[Table 2] shows the descriptive analysis of the data. Some groups presented standard deviation values below 10 cN (≈grf): Morelli Conventional, Morelli HA, and Orthosource HA. Values between 10 and 20 cN were presented by these groups: Abzil HA and Orthosource conventional. Values between 20 and 30 cN were presented by these groups: GAC HA, Ormco, Orthometric conventional, and Orthometric HA. Values above 30 cN were presented by Abzil conventional and GAC conventional groups.
Table 2: Descriptive analysis of 3-point bending test during unloading at 0.5, 1, 2 and 3 mm (cN)

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[Table 3] shows the percentage of wires that released a magnitude of force within its group mean, in a range of ± 20 cN.
Table 3: Percentage of specimens that released magnitude of force within its group mean±20 cN

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Force magnitude

The groups presented different magnitudes of force during the test and where divided accordingly: Abzil conventional, GAC conventional, Morelli HA, Ormco, and Orthometric conventional released forces between 50 and 100 cN. Abzil HA, GAC HA, Orthometric HA, and Orthosource HA released forces below 50 cN, mostly. Morelli conventional and Orthosource conventional released forces above 100 cN, mostly [Table 4].
Table 4: Unloading comparison of 3-point bending tests between different groups. (ANOVA followed by Tukey's test)

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Conventional versus heat-activated

When comparing conventional and HA wires of the same brand, conventional wires released forces greater than their HA analogs except for Abzil at 3 mm, GAC at 0.5 and 3 mm, and Orthometric at 3 mm [Table 4].

   Discussion Top

The choice of conducting laboratory investigation is because external factors are less likely to influence the results as in a clinical research.[12] Although these results may be different from a clinical study, the patterns obtained in both laboratory and clinical environment are quite similar, allowing comparison bases.[13]

The wire diameter of choice was the 0.014” (0.36 mm).[11] This choice was based on the general knowledge that NiTi wires are more suitable to begin leveling and alignment. The intention is to minimize pain, that sometimes can be described as very intense,[14] discouraging orthodontic treatment, affecting patient compliance, and worsening oral hygiene.[15] Besides these facts, because bending test simulates a condition of crowding, it would be more coherent to evaluate wires that undergo this situation, such as small diameter round wires.

To endorse the use of NiTi alloys, a 2008 study on trends of practice among clinical orthodontists showed that 87% of orthodontists use NiTi wire for the initial treatment and 13.9% at the end.[16]

The selected wire brands consisted of the conventional and HA variations. Nevertheless, not all brands show the temperature of Austenite finish (Af) of their product, contrary to the ISO 15841 regulation [Table 1]. The Af temperature is that in which the wire passes from the martensitic, which is more flexible, to the austenitic phase, which is stiffer.

The 3342 Instron universal testing machine (Norwood, MA, USA) was used with a 10 N load cell. The choice of the load cell should be carefully made in accordance with the expected force delivered by the specimen to be studied, to provide precision of results. The 3342 Instron load cell has an accuracy of ± 0.5% of reading. Thus, a reading of 60 cN (≈ grf) of force, for example, has a maximum error of ± 0.3 cN.

The 3-point test was chosen because it is recommended by ISO 15841 regulation and because of its clinical similarity and reproducibility.[5],[17] This test is most useful for the study of elastic deflection because it eliminates many of the variables found in a clinical simulation device such as friction between components, interbracket distance and the influence of materials outside the interest of the study such as elastic or metallic ligatures.[13]

From a practical point of view, loading represents the force applied to the tooth when the clinician deflects the archwire toward the slot of the bracket. The unloading represents the potential tooth movement while the archwire returns to its original shape. It is therefore of major clinical interest to know the forces released by the unloading test than the forces delivered by the loading test.[8],[18] Additionally, the ISO 15841 regulation recommends evaluation during unloading.

The temperature selected for the tests of this study was also based on the ISO 15841 regulation that indicates 36°C ± 1°C which is the average mouth temperature.[19]

Force variability

Clinically, the standard deviation reveals the brand consistency about the quality and uniformity of specimens. [Table 3] shows the percentage of wires that released forces within the group mean ± 20 cN. This percentage represents the actual chance to use a wire with the characteristics found for its group mean. The groups with average percentage above 80% were as follows: Abzil HA, Morelli conventional and HA, Ormco, and Orthosource conventional and HA. The groups with percentage between 60% and 40% were GAC conventional and HA and Orthometric conventional and HA, respectively. Abzil conventional group presented average percentage of 25%. Other studies also reported large standard deviations,[5] which may be caused by the manufacturing process and/or thermal treatment.[20] Moreover, significant differences were found between batches of the same brand[18] and different mechanical behavior than advertised by the manufacturer.[21] Accordingly, it is possible to establish other criteria for assessing brand quality. Therefore, the most uniform brands were Abzil HA, Morelli conventional and HA, Ormco, and Orthosource conventional and HA.

Force magnitude

Another objective of this investigation was to compare the forces released by elastic deflection between the groups. For a fairer and more applicable comparison, it would be ideal to have a range of optimum force and from it, sort the wires with the best results. There is a theory that the force required to move a tooth is very low, about 0.025 cN/cm2, which is the pressure of the blood capillaries. Much larger forces may cause hyalinization and necrosis of the surrounding tissues, which take 7–14 days to reorganize. Furthermore, these forces cause greater pain. Another situation that can happen after these events is root resorption, which is an irreversible damage to dental tissues. During this time, tooth movement slows and can even stop, delaying treatment.[3] There are many studies reporting that continuous low forces are more effective for tooth movement.[17],[22],[23] However, it is necessary to establish a guidance range to abandon the term “light force” that is subjective.[24] A wire that releases very low forces may not be the best choice, promoting very slow tooth movement,[25] or even no movement at all.[5] That is why it may be risky to simply consider the best wire to be those that release the lowest forces.

Despite the difficulty in defining the optimal force, some authors have ventured to quantify it. One of these authors stated that forces >100 cN result in a lag phase of about 21 days before tooth movement occurs and with low forces, such as 60 cN, tooth movement begins without a lag phase and occurs in a clinically significant rate. Application of 60 cN for canine retraction promoted faster tooth movement than that produced with 18 cN.[22] Another study compared two magnitudes of force for canine retraction: 200 cN and 20 cN. Both forces were able to effectively move teeth, however, there was greater reporting of pain in the group using 200 cN of force.[26] In a similar comparison, it was observed that 50 cN was most suitable to prevent anchorage loss than 300 cN. In another survey, it was observed that 50 cN can effectively induce tooth movement, similar to the application of 150 cN of force. However, application of 50 cN of force generated less pain and less inflammation.

There were also authors that suggested a magnitude for the optimum force based on their experience. According to Thurow, light forces are those below 100 cN. According to Proffite et al., the mean optimum force indicated for extrusion and rotational correction of teeth is in the range of 35–60 cN.[27] Based on history and on current literature, an unloading force ranging from 50 to 150 cN was accepted as clinically useful for tooth movement.[28] More specifically for anterior teeth, a force of 50 cN[26] within the range of 50–75 cN was accepted as optimal. Therefore, it seems reasonable to propose an optimum force range of 50–100 cN as suitable for orthodontic movement.[22],[23],[28]

According to this rationale, the groups that showed forces within this range throughout the test were: Abzil conventional, GAC conventional, Morelli HA, Ormco and Orthometric conventional [Table 4].

Conventional versus heat-activated

Based on this optimum force range, it was observed that the HA wires released lower forces during almost the entire test, with the exception of Morelli HA and Ormco groups. Some studies have suggested that these wires may clinically release even lower forces than those detected in the laboratory. This is because friction with the brackets increases the force released during loading, but decreases the force released by the wire during unloading. According to these findings, the current results, obtained during unloading, could be even clinically lower.[4] Although there is no evidence of the optimum force, it is known that suboptimal forces can slow tooth movement.[29]

All these issues raise a question on the correct indication of HA wires. These wires have the advantage of facilitating insertion into the brackets slots,[17] but would this apply to small diameter wires that are naturally easy to be inserted? Would the forces be below the expected level for optimal tooth movement?

The best application of HA wires would be to initiate the treatment with a square or rectangular wire. The purpose of this type of treatment is to accelerate torque correction, increasing clinical stability.[30] In this case, it makes sense to use a wire that releases lower forces than the conventional one of the same diameter and that can be adapted more easily in the brackets slot, after reducing its temperature, since a square or rectangular wire is naturally difficult to insert into misaligned teeth. The shape memory of HA wire allows an easier adaptation at low temperatures, while the wire is in the martensitic phase. Once adjusted, the wire recovers its original form when it returns to the austenitic phase, at mouth temperature.[30]

Clinical applications

It is possible to state that consensus in the orthodontic literature indicates low and continuous forces for tooth movement.[17],[22],[23] Nevertheless, there is also a suggestion that virtually any force is capable of causing tooth movement.[22] It is therefore possible that all the evaluated wires are equally able to initiate movement.

Thus, analysis of [Table 4] allows choosing the most appropriate wire in view of a fast tooth movement, but without excessive pain or severe root resorption. Therefore, five groups were within the range considered optimal: Abzil conventional, GAC conventional, Morelli HA, Ormco, and Orthometric conventional.

Another important factor that must be considered is the force variation. Calculated in percentages, the results showed that, among these groups, those with homogeneity above 80% were Morelli HA and Ormco.

   Conclusions Top

  • The groups that showed forces within an optimal range for induction of tooth movement were Abzil conventional, GAC conventional, Morelli HA, Ormco and Orthometric conventional
  • Among these, the groups that showed smaller force variability were Morelli HA and Ormco CuNiTi.

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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Kotian R. Shape memory effect and super elasticity. Its dental applications. Indian J Dent Res 2001;12:101-4.  Back to cited text no. 2
Parvizi F, Rock WP. The load/deflection characteristics of thermally activated orthodontic archwires. Eur J Orthod 2003;25:417-21.  Back to cited text no. 3
Bartzela TN, Senn C, Wichelhaus A. Load-deflection characteristics of superelastic nickel-titanium wires. Angle Orthod 2007;77:991-8.  Back to cited text no. 4
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Burstone CJ, Qin B, Morton JY. Chinese NiTi wire – A new orthodontic alloy. Am J Orthod 1985;87:445-52.  Back to cited text no. 6
Garrec P, Tavernier B, Jordan L. Evolution of flexural rigidity according to the cross-sectional dimension of a superelastic nickel titanium orthodontic wire. Eur J Orthod 2005;27:402-7.  Back to cited text no. 7
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Lombardo L, Marafioti M, Stefanoni F, Mollica F, Siciliani G. Load deflection characteristics and force level of nickel titanium initial archwires. Angle Orthod 2012;82:507-21.  Back to cited text no. 11
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Gurgel JA, Kerr S, Powers JM, LeCrone V. Force-deflection properties of superelastic nickel-titanium archwires. Am J Orthod Dentofacial Orthop 2001;120:378-82.  Back to cited text no. 13
Fuck LM, Drescher D. Force systems in the initial phase of orthodontic treatment – A comparison of different leveling arch wires. J Orofac Orthop 2006;67:6-18.  Back to cited text no. 14
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Berger J, Waram T. Force levels of nickel titanium initial archwires. J Clin Orthod 2007;41:286-92.  Back to cited text no. 17
Waters NE. Superelastic nickel-titanium wires. Br J Orthod 1992;19:319-22.  Back to cited text no. 18
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Waters NE. A rationale for the selection of orthodontic wires. Eur J Orthod 1992;14:240-5.  Back to cited text no. 23
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Iijima M, Ohta M, Brantley WA, Naganishi A, Murakami T, Muguruma T, et al. Transformation behavior of nickel-titanium orthodontic wires under tensile load. Dent Mater J 2011;30:398-403.  Back to cited text no. 29
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4]


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