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 Table of Contents  
Year : 2023  |  Volume : 12  |  Issue : 1  |  Page : 50

Metallurgy in orthodontic—A systematic review and meta-analysis on the types of metals used

1 Department of Clinical Sciences, Center of Medical and Bio-Allied Health Sciences Research, College of Dentistry, Ajman University, Ajman, United Arab Emirates
2 Orthodontic Division, Preventive Dentistry Department, Orthodontic Division, College of Dentistry, Jouf University, Sakaka 72345, Saudi Arabia; Department of Dental Research Cell, Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India; Department of Public Health, Faculty of Allied Health Sciences, Daffodil lnternational University, Dhaka, Bangladesh
3 Specialist, Practicing in Dental Clinic, AlBaha, Saudi Arabia
4 School of Dental Sciences, University Sains Malaysia, Gelugor 11800, Malaysia
5 Consultant Orthodontist, Aseer Specialty Dental Center, Abha, Saudi Arabia

Date of Submission10-Apr-2023
Date of Decision27-Apr-2023
Date of Acceptance27-Apr-2023
Date of Web Publication04-Sep-2023

Correspondence Address:
Mohammad Khursheed Alam
Orthodontic Division, Preventive Dentistry Department, Orthodontic Division, College of Dentistry, Jouf University, Sakaka 72345

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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jos.jos_52_23

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A variety of metals and alloys are employed in the field of orthodontics, primary of which happen to be the construction of wires. Through this systematic review, we aimed to assess the various metallurgical characteristics of the said metals and alloys. Four hundred and eighty-two documents in total were found after a thorough search of the online journals, and 169 of the papers were initially chosen. Ultimately, 16 documents were selected that satisfied the necessary inclusion and exclusion criteria, primarily in vitro studies, literature reviews, and comparative analyses. NiTi alloy was found to be the most commonly used alloy in construction of orthodontic wires across all the studies that we had selected for our review. It also had better performance and consistency in terms of its usage as depicted by the meta-analysis performed, with stainless steel wires being a close second primarily due to its lesser cost compared to the former. Metallurgy and orthodontics are inextricably linked with one another. The various components of orthodontics such as wires, pliers, and other instruments utilize the metallurgical characteristics of metals and alloys that are specially prepared for the challenges of this field.
PROSPERO Registration Number: CRD42022378444.

Keywords: Alloys, Metallurgy, Nickel-titanium, Orthodontic wires, Orthodontics

How to cite this article:
Abutayyem H, Alam MK, Kanwal B, Alswairki HJ, Alogaibi YA. Metallurgy in orthodontic—A systematic review and meta-analysis on the types of metals used. J Orthodont Sci 2023;12:50

How to cite this URL:
Abutayyem H, Alam MK, Kanwal B, Alswairki HJ, Alogaibi YA. Metallurgy in orthodontic—A systematic review and meta-analysis on the types of metals used. J Orthodont Sci [serial online] 2023 [cited 2023 Sep 21];12:50. Available from: https://www.jorthodsci.org/text.asp?2023/12/1/50/385074

  Introduction Top

Orthodontic archwires have evolved since they were first manufactured in the 1970s, becoming increasingly sophisticated and useful in a variety of clinical situations. The teeth are moved by the orthodontic archwires continuously and gently. The orthodontic archwire must exhibit elastic behavior when a force is applied over a few weeks to many months. Additionally, different orthodontic archwires are needed for the initial, intermediate, and ultimate phases of orthodontic therapy.[1],[2]

Due to their special qualities—the shape memory effect and superelasticity—NiTi orthodontic archwires are the most often utilized archwires at the earliest stages of orthodontic treatment (leveling and alignment).[1],[2],[3],[4],[5] A martensitic transition is thought to have produced these functional characteristics. This transformation can be caused by heat or stress and can occur either directly from austenite (the parent phase with B2 cubic symmetry; space group) to martensite (the product phase with B19' monoclinic symmetry; space group), or it can travel through the R-phase, an intermediary phase (trigonal symmetry; space group).[6]

Superelasticity is encouraged by stress-induced martensite (SIM), and the shape memory effect is encouraged by thermally induced martensite (TIM). Martensitic changes in NiTi alloys therefore exhibit both thermal and mechanical hysteresis.[7] As is common knowledge, Ni content can regulate these practical qualities: Ti-rich NiTi alloys exhibit the form memory effect at room temperature, while Ni-rich and equiatomic NiTi alloys exhibit the superelastic effect close to and above it.

At a certain temperature, martensite (M) begins to develop as the material cools from the austenite (A) domain (Ms temperature). Direct transformation, which culminates at martensite finish temperature, is the process that converts austenite to martensite (Mf temperature). The austenite phase begins to form when the material in the low-temperature phase (M) is heated to a specific temperature; this value is known as the As temperature, and the transformation is complete when the Af temperature is reached. Reverse transformation is the name given to this transition.[6]

As long as the martensitic stability temperature range is maintained, the material can sustain this deformation even when it is deformed in the martensitic domain up to a specific level (often up to 10%). When heated above As temperature, the material begins to take on its former shape. When Af is reached, thermally induced martensite recovery by the shape memory effect ought to be complete.[6]

However, when a stress is applied to the material within a specific range of temperature where austenite is thermally stable, the stress-induced martensitic transformation takes place, which promotes the superelastic effect. After all applied stress has been removed, the deformation caused by loading may be recovered up to 10% strain.[6],[8],[9]

Orthodontic archwires made of ordinary superelastic NiTi exhibit a uniform composition all along the wire. The forces necessary to move the incisor, premolar, and molar teeth, however, differ from one another. It is well known that a wire must have a structural gradient along its length to be subject to various actuating forces. By altering the Ni/Ti ratio in the austenite matrix, primarily through heat treatment, such as annealing, which encourages the Ni4Ti3 precipitate production, it is feasible to create this functional gradient. The range of transformation temperatures and the loading and unloading plateaus of the stress–strain curves during the superelastic regime are altered as a result of this precipitation, which also causes a local shift in the Ni/Ti ratio in the surrounding matrix.[6],[10]

Some manufacturers use localized heat treatments to create this effect and make archwires that exhibit various actuation forces throughout the same archwire while taking into account the structural gradient.[11],[12] Functionally graded materials have been employed for engineering applications in thermal, structural, optical, and electronic materials because of their high capacity for tackling complicated problems, such as those requiring a wider controllable range of temperature or stress.[13],[14]

Hence, by the means of this systematic review and meta-analysis, we aimed to analyze studies available in orthodontic literature that mentioned the metallurgical characteristics of orthodontic wires and the various chemical, physiological, and physical interactions of the metals/alloys used in their construction (both inside and outside the oral cavity) as well as their performance compared to one another.

  Materials and Methods Top

Protocol employed

This systematic review was performed as per the Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) strategy and rules from the Cochrane group and the book Orderly Reviews in Health Care: Meta examination.

Review hypotheses

Through this systematic review, our primary objective was to review studies that were published in the orthodontic literature and that discussed the metallurgical properties of orthodontic wires, as well as the various chemical, physiological, and physical interactions of the metals and alloys used in their production (both inside and outside the oral cavity) and their performance in comparison with one another.

Study selection process

There were a total of 482 documents discovered after extensive search on the online journals, and 169 of the papers were selected initially. Following that, 115 similar/duplicate articles were eliminated, which resultantly made 58 separate papers available at first. The abstracts and titles of submissions were then reviewed, and a further 42 papers were eliminated. Finally, 16 documents that met the requisite inclusion and exclusion criteria were chosen, which primarily included in vitro studies, literature reviews, and comparative assessments [Figure 1].
Figure 1: Representation of selection of articles through PRISMA framework

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Inclusion criterion

Articles that contained relevant data for our review objectives were selected for full-text screening. Studies that reported clinical trials, in vitro studies, randomized/non-randomized studies, systematic/literature reviews containing substantial sample volume, and detailed case reports were considered for inclusion in our review. We also monitored studies that possessed higher methodological quality.

Exclusion criteria

The following were excluded from the scope of our systematic review: incomplete data, seminar presentations, scholarly articles, placebo-controlled studies, and opinion articles.

Since the literature available on this topic was quite scant in volume, we did not limit our search in terms of the time period when the studies were published; that is, we took into account all the papers that were published in context to our topic (where the number of papers itself was found to be quite sparse in number). Also, excluded were literature reviews and cases published in languages other than English.

Search strategy

Using relevant keywords, reference searches, and citation searches, the databases PubMed-MEDLINE, Web of Science, Cochrane, and Scopus were all searched. “Alloys,” “metallurgy,” “nickel-titanium,” “orthodontics,” and “orthodontic wire” were the search terms used to access the database.

Data selection and coding

Two independent reviewers located the relevant papers by using the right keywords in various databases and online search tools. The chosen articles were compared, and a third reviewer was brought in if there was a dispute.

After choosing the articles, the same two reviewers independently extracted the following data: author, year of publication, country, kind of publication, study topic, population demographics (n, age), outcome measure(s), relevant result(s), and conclusion(s). The data were compared, and many differences were discussed with the third reviewer.

Risk of bias assessment

The AMSTAR-2 technique[15] was used to evaluate the risk of bias in the studies we chose. AMSTAR-2 joins a number of other instruments that have been released for this purpose as a critical evaluation tool for systematic reviews [Table 1]. As seen in [Table 2] below, it is a 16-point checklist. Two instruments that have drawn a lot of attention served as the foundation for the creation of the original AMSTAR tool. The original AMSTAR was duplicated in two newly produced instruments. The AMSTAR-2 risk of bias items identifies the domains specified in the Cochrane risk of bias instruments for systematic reviews. In each case, these indicate an agreement that was achieved after input from more than 30 methodology experts.
Table 1: AMSTAR-2 16-point checklist of risk of bias assessment in studies selected for the systematic review

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Table 2: Description and outcomes as observed in the studies selected for the systematic review

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Statistical analysis

After selecting data on the sample size, variables analyzed, and various elements of the investigations, the data were then entered into the Revman 5 program for meta-analysis. Forest plots illustrating the odds ratio for different study methodologies were obtained as part of the meta-analysis for our study as shown in [Figure 2], [Figure 3], [Figure 4], [Figure 5].
Figure 2: Odds ratio of in vitro studies selected in this systematic review which assessed the performance of NiTi wires represented on a forest plot after meta-analysis

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Figure 3: Odds ratio of in vitro studies selected in this systematic review where the metals/alloys were subjected to saliva, ordinary condiments, and corrosive liquids represented on a forest plot after meta-analysis

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Figure 4: Odds ratio of literature reviews selected in this systematic review and their metallurgical analysis of orthodontic wires represented on a forest plot after meta-analysis

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Figure 5: Odds ratio of comparative studies selected in this systematic review where the metals/alloys were compared in terms of their metallurgical profile against one another represented on a forest plot after meta-analysis

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  Results Top

The study design, methodology employed, description, and outcome are mentioned in [Table 2]. The results of the meta-analysis are provided in [Figure 2], [Figure 3], [Figure 4], [Figure 5].

  Discussion Top

The pseudo-elasticity is enhanced, neighbor grain orientation is made easier with smaller grains, and the stiffness is correlated with the quantity of martensite plates created. According to the interaction between the transformational strain and each newly formed plate, these plates can be found in a variety of orientations.[32],[33] Shape memory alloys (SMA) support pseudo-elastic and critical strain that is primarily caused by transformational strain, while superelastic unloading recovery is caused by the disappearance of martensite plates in conjunction with a decrease in transformational strain.[33] The range of transition temperatures and variations in the number of electrons accessible for bonding are both caused by the composition of NiTi alloys. The temperature of transformation (TTR) can be lowered, and the permanent yield strength of the austenite phase can be increased by about three times with a very modest excess of nickel in the structure. Additionally, it is possible to regulate the alloy's nickel content even during the melting and casting of ingots. In contrast, titanium-rich alloys have greater transformation temperatures than nickel less-rich or equiatomic NiTi alloys because they feature a second phase, Ti2Ni, in their matrix.

In this systematic review, we assessed studies with different (but high-quality) methodological approaches to analyze various outcomes with respect to the metallurgical profiling of different types of metals/alloys used in the construction of orthodontic wires. For example, three of the studies that we selected in our review[17],[20],[28] analyzed the interactions of salivary fluids with the metals used in the orthodontic wires, and it was found that all the metals used in the wires interacted with the salivary constituents, indicating the potential hazards that can be associated with long-term usage of such appliances. These observations are similar to the investigations conducted by Babaei et al.,[34] where they had investigated potential hazards with respect to ions leaching in the oral cavity due to orthodontic appliance usage.

When the surface of wire was mechanically polished, Shabalovskaya[35] discovered that the Ti: Ni ratio was 5.5, indicating that there was five times as much Ti on the surface. The Ti: Ni ratio climbed to 23.4–33.1 and the Ni content reduced when the wire was autoclaved or brought to a boil in water. The Ti: Ni ratio in polished samples was 5.8, but after 30 days of immersion in a neutral electrolyte solution, it rose to 91. The titanium-aluminum vanadium alloy (Ti6Al4V) surface in the study by Hanawa et al.[36] had aluminum quantities comparable to the quantity of nickel in NiTi, despite the fact that Ti6Al4V's bulk material only contained 6% Al and NiTi had 50% Ni. On the surface of SS, Cr and Fe were detected in small concentrations, but Ni was absent.

One of the most biocompatible materials is pure titanium, as well as several of its alloys.[37] The stable titanium oxide layer is assumed to be responsible for their good biocompatibility. The oxide layer that forms on a titanium implant during implantation expands and absorbs minerals and other components of tissue fluids, and these reactions lead to surface modification. Hanawa[36] discovered that the calcium phosphate and titanium dioxide layers make up the oxide layer on the implants. In other words, a layer of inert oxide was covered in calcium phosphate. The Ca:P ratio of the film was similar to that of hydroxyapatite, and it was thicker on pure titanium than titanium alloys (including NiTi). Less hydroxyapatite-like calcium phosphates were produced on NiTi or Ti6Al4V. These results could have been influenced by the presence of Ni on the surface of NiTi alloy and aluminum on the surface of Ti6Al4V. This form of calcium phosphate layer is also present in SS. However, compared to NiTi, this layer forms more slowly.[36],[38]

The corrosion resistance of Ti and related alloys in corrosive settings was increased by the TiN layer, as demonstrated by Shenhar et al.[39] and Huang et al.[40] According to Liu et al.,[41] nitriding NiTi instruments' surfaces at various temperatures improved their cutting effectiveness and corrosion resistance when they came into contact with sodium hypochlorite (NaOCl). According to a different investigation by Lin et al.,[30] adding a TiN layer to commercial rotary NiTi instruments at temperatures of 200°C, 250°C, and 300°C greatly improved the corrosion resistance of files in contact with 5.25% NaOCl. Despite the fact that the files nitrided at 300°C had the highest polarization resistance and lowest passive current, it is not advised to use this approach in clinical settings because at this temperature the instrument's superelasticity character may be lost. Therefore, for clinical use, equipment nitrided at 250°C is preferred.[41] Studies on the cryogenic treatment (CT) of NiTi rotary instruments are scarce. The effects of cryogenic treatment on the composition, microhardness, or cutting effectiveness of NiTi rotary instruments were examined by Kim et al.[42] They discovered that the microhardness of the cryogenically treated instruments was much higher than that of the controls. Additionally, the austenite phase was predominant in both the test and control groups, which were consisted of 56% wt Ni, 44% wt Ti, and 0% N. Deep dry CT greatly improved the cutting effectiveness of NiTi instruments, but it had little effect on wear resistance, according to a different study by Vinothkumar et al.[43] Deep CT considerably increased the cyclic fatigue resistance of NiTi rotary files, according to George et al.[44]

NiTi wires are manufactured using rolling and intermediate annealing techniques, which breaks the tiny crystal and necessitates recrystallization to produce predictable average grain size and orientation. Uniform grains or precipitates of some chemical components of the matrix may develop as a result of composition and thermal processing. These components, which have different properties from the matrix and are not thermodynamically stable at the heat treatment temperature, can be mixed with additional components and evenly distributed throughout the matrix. Precipitates of Ni2Ti and Ni3Ti allow more nickel to leave the crystal matrix, which lowers the Ni concentration and increases TTR. Other precipitates, such as Ti3Ni4, may have an impact on the mechanical properties of the matrix austenite phase, enhancing the shape memory effect's recoverability.[20],[36]

Chemical metallurgy and physical metallurgy are two additional divisions of the vast and complex science of metallurgy. The reduction, oxidation, and chemical behavior of metals, mineral processing, metal extraction, thermodynamics, electrochemistry, chemical degradation, mechanical, physical, and performance characteristics of metals are only some of the major areas of study in metallurgical assessments. Further, crystallography, material characterization, mechanical metallurgy, phase transitions, and failure mechanisms constitute only a few of the practical applications of metallurgical sciences. Hence, we believe that the number of investigations that we selected for our systematic review and meta-analysis might be a little less than what would be considered ideal for a topic as boundless as metallurgy. However, we only selected studies that we deemed to have quite a high methodological quality and possessed results that were of a sound statistical value. We also ensured the selection of a variety of studies that carried different methodologies (in vitro studies, literature reviews, comparative assessments, part in vivo studies) which further reduced the ambiguity of the results that we obtained through the meta-analysis. However, it is imperative that more studies concerning the metallurgy of different metals and alloys be performed to keep pace with the rapidly evolving domain of orthodontics and ensure proper patient compliance in terms of both convenience and safety.

  Conclusions Top

Metallurgy and orthodontics are inextricably linked with one another. The various components of orthodontics such as wires, pliers, and other instruments utilize the metallurgical characteristics of metals and alloys that are specially prepared for the challenges of this field. By the means of this systematic review, we shed light upon the different types of reactions that metals and alloys exhibit, in different solutions as well as compared to one another, and it was clear that among all the alloys utilized, NiTi was the most consistent in terms of its performance and consistency in usage.


The authors acknowledge the Saudi Orthodontic Society for their support.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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