January, 2008

Corrosion Behavior of Orthodontic Alloys

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T.P. Chaturvedi, BDS, MDS

Abstract

Various types of metallic orthodontic wires and brackets e.g. stainless steel, cobalt-chromium-nickel alloys, nickel-titanium alloys, beta-titanium alloys etc. are used in the treatment of malocclusion. These metals undergo chemical or electrochemical reactions with the oral environment resulting in dissolution or formation of chemical compounds. Under several situations the oral environment is highly aggressive and leads to corrosion. The most favorable archwire material/bracket is the one which is capable of resisting the most extreme conditions that could possibly be encountered in the mouth. Orthodontic alloys must have excellent corrosion resistance to the oral environment, which is highly important for biocompatibility as well as for orthodontic appliance durability. The present article reviews the various aspects of corrosion as well as the biocompatibility of orthodontic wires and devices. It will explore emerging research strategies for probing the biological properties of materials. During orthodontic treatment the use of nickel-free alloys, better corrosion resistance alloys and less use of fluoride containing toothpaste or gel is expected.

Introduction

The oral cavity provides an ideal and unique environment for studying the biological processes involving metallic dental aids. Dental materials within the mouth interact continuously with physiological fluids. Oral tissues are exposed to a veritable bombardment of both chemical and physical stimuli, as well as the metabolism of about 30 species of bacteria (the total salivary bacterial count is said to be five thousand million/ml of saliva). Yet, for the most part, oral tissues other than dental tissues remain healthy. Saliva is a hypotonic solution containing bioactonate, chloride, potassium, sodium, nitrogenous compounds and proteins. [1] The pH of saliva varies from 5.2 to 7.8. Teeth function in one of the most inhospitable environments in the human body. They are subject to larger temperature variations than most other parts, coping with ice cold temperatures (0°C) through to hot coffee and soup (60°C). Corrosion, the graded degradation of materials by electrochemical attack, is of concern particularly when orthodontic appliances are placed in the hostile electrolytic environment provided by the human mouth. [2,3] Factors such as temperature, quantity and quality of saliva, plaque, pH, proteins, physical/chemical properties of solids/liquids food and oral conditions may influence corrosion processes.

Orthodontic wires are formed into various configurations or appliances to apply forces to teeth and move them into more desirable alignment. Various types of wires and brackets are used in the treatment of malocclusion e.g. stainless steel, cobalt-chromium-nickel alloys, nickel-titanium alloys, β-titanium alloys, etc (Table-1). [4] A ductile wire can be formed into various shapes. As a manufacturing process, the industry uses brazing alloys to join the base and wing components of brackets. Silver based brazing alloys form a galavanic couple that can lead to ionic release of mainly copper and zinc. The main purpose of the present article is to briefly review the corrosion aspects and its effect on orthodontic wires and brackets in oral environments; effect of fluoride gel/toothpaste on orthodontic appliances; nickel sensitivity. Summary of the a few experimental works on the corrosion of orthodontic alloys (in vitro) is given in Table-2, which can explore the future research strategies for the corrosion study of orthodontic materials.

Clinical Significance of Corrosion on Orthodontic Wires

It has been proven that small galvanic currents associated with a corrosive environment are continually present in the oral cavity. As long as metallic dental restorative materials or orthodontic wires are employed, there seems to be little possibility that these galvanic currents can be eliminated. [5] For all practical purposes, the metallic restoration/orthodontic wires cannot be isolated electrically from the tooth. Hence resistance to corrosion is critically important for orthodontic wires because corrosion can lead to roughening of the surface, weakening of the appliances, and liberation of elements from the metal or alloy. [6] Release of elements can produce discoloration of adjacent soft tissues and allergic reactions in susceptible patients. [7,8,9] Corrosion can severely limit the fatigue life and ultimate strength of the material leading to mechanical failure of the dental materials. [10] Some alloys and metals are resistant to corrosion because of inherent nobility or the formation of a protective surface layer. High noble alloys used in dentistry are so stable chemically that they do not undergo significant corrosion in the oral environment.

Weakening of Appliances

Fracture of orthodontic appliances is a rare phenomenon, which may affect clinical results. Stainless steel becomes susceptible to intergranular corrosion, which may ultimately weaken the alloy. Tensile strength of the orthodontic silver-soldered stainless steel joints will be affected by corrosion process. Despite the good corrosion resistance of stressed NiTi, breakage of orthodontic wires has frequently been found in clinical studies and subjected to degradation caused by corrosion in the oral environment. [11,12,13] According to Zinelis et al. [14] Ag-based soldering alloys introduce a galvanic couple with stainless steel alloys, inducing release of metallic ions like Cu++ and Zn++, the elements most readily leached out from silver solder alloys. This indicates that orthodontic appliances do fail in the short term and require remedial action prior to the completion of the patient treatment cycle. Anisa Vahed et al [15] reported that prolonged exposure in simulated saliva lead to significant reduction in the tensile failure load of silver-soldered stainless joints. The reduction in tensile properties is brought about by a weakness induced by localized corrosion of the solder metal at the solder/wire interface. The preponderance of Cu-rich particles that form in the solder metal at the interface provides a microgalvanic effect that leads to selective dissolution of these particles and corresponding weakening of the interface. Corrosion is the main reason for the progressive dissolution of brazing filler metal, leading to detachment of the wing from the bracket base during orthodontic therapy or at the debonding stage. [16]

Sliding Friction

Corrosion increases orthodontic friction force between the archwire/bracket interfaces due to increase in the surface roughness. It results in an inappropriate force distribution on orthodontic appliances and, consequently, the effectiveness of arch guided tooth movement decreases. [17,18]

Local Pain / Swelling

Corrosion products have been implicated in causing local pain or swelling in the region of the orthodontic appliances in the absence of infection, which can lead to secondary infection. [19]

Cytotoxic Responses

Nickel and chromium induce a type-IV hypersensitivity reaction in the body, and act as a hapten, carcinogen and mutagen. These metals cause several cytotoxic responses including decrease in some enzyme activities, interference with biochemical pathways, carcinogenicity, and mutagenicity. [20,21] Titanium wires containing nickel may cause localized tissue irritation in some patients. Manganese from the alloy is also consumed with saliva which produces toxicity leading to nervous, skeletal disorders, etc. Recent publications have suggested that long term exposure to nickel containing dental materials affect both human monocytes and oral mucosal cells. [22,23]

Decalcification of Teeth

Various acids are formed during the microbial attack on metallic orthodontic appliances in the oral environment. Biofilm forms on the tooth surface with the help of food debris and metabolic products of microbes. Aerobic microbes utilize the simple sugars into the glycolysis process releasing carbon dioxide (CO2). Generally, the facultative chemoorganotrophs enter into the fermentative pathway utilizing the sugars and produced organic acids, alcohols and CO2. Chemoorganotrophs in the anaerobic condition, viz Sulphate Reducing Bacteria (SRB), utilize the lactate as carbon source and reduce sulphate into sulphide. Finally, sulphide combines with ferrous ion to form ferrous sulphide as the corrosion product. Ultimately, in the presence of Sulphate Oxiding Bacteria (SOB), sulphide oxidizes into sulphate. In the oral cavity, hydrogen combines with sulphate to form sulphuric acid which is more corrosive in nature. Reduced pH, due to produced acid, influences the decalcification of teeth and corrosion of metallic appliances. [24]

Effect of Corrosion on Orthodontic Wires

Corrosion of Stainless Steel

Orthodontic stainless steel wires are generally made of austenitic stainless steel containing approximately 18% chromium and 8% nickel. This provides a good combination of strength, corrosion resistance and moderate cost. It owes its corrosion resistance property to chromium, a highly reactive base metal. The corrosion resistance of alloy depends on the passive film, which spontaneously forms (passivation) and reforms (repassivation) in air and under most fluid conditions. Oxygen is necessary to form and maintain the film, whereas acidity and chloride ions can be particularly detrimental to it. [25] The presences of soldered joints increase corrosion susceptibility since they have a tendency to emit electrogalvanic currents with saliva and consequently release metal ions. Austenitic stainless steel may loose its resistance to corrosion if it is heated between approximately 400°C and 900°C. Such temperatures are within the range used by the orthodontist for soldering and welding. The decrease in corrosion resistance is caused by the precipitation of chromium-iron carbide at the grain boundaries at these high temperatures. [26] Below these temperatures precipitation does not occur and chances of corrosion is less. Corrosion of stainless steel may result due to galvanic cell forming in one or more of the following ways:

  1. Surface roughness of stainless wires may cause localized corrosion attack.
  2. Any cut or abrasion of stainless steel by carbon steel pliers/carbon steel bur may act as galvanic cell, and
  3. Brazed or soldered joints in orthodontic appliances can also form galvanic couples in vivo.

Titanium and its Alloys (cobalt chromium nickel alloys, nickel-titanium alloys, β-titanium alloys)

Titanium is highly corrosion resistant as a result of the passivating effect afforded by a thin layer of titanium oxide that is formed on its surface. The surface roughness of titanium containing alloy is more as compared to stainless steel wires, which may act as galvanic cell in the mouth. [27,28,29]

Corrosion

Definition of corrosion

The term corrosion is defined as the process of interaction between a solid material and its chemical environment, which leads to loss of substance from the material, change in its structural characteristics, or loss of structural integrity. Many types of electrochemical corrosion are possible in the oral environment because saliva is a weak electrolyte. The electrochemical properties of saliva depend on the concentrations of its components, pH, surface tension, and buffering capacity. Each of these factors may influence the strength of any electrolyte. Thus, the magnitude of the resulting corrosion process will be controlled by these variables. The features that determine how and why dental materials corrode are oxidation and reduction reactions as well as passivation or the formation of a metal oxide passive film on a metal surface. [30]

Types of Corrosion in Orthodontic Wires

Various forms of corrosion are shown in Figure 1. and described as follows.

Uniform Corrosion

A uniform, regular removal of metal from the surface is the usually expected mode of corrosion. It is the most common type of corrosion, occurring with all metals at different rates. The process arises from the interaction of metals with the environment and the subsequent formation of hydroxides or organometallic compounds. For uniform corrosion, the corrosive environment must have the same access to all parts of the surface, and metal itself must be metallurgically and compositionally uniform. Uniform attack may not be detectable before large amounts of metal are dissolved.

Pitting Corrosion

It is a form of localized, symmetrical corrosion in which pits form on the metal surface. It usually occurs on base metals, which are protected by a naturally forming thin film of an oxide. It has been identified in brackets and wires. In the presence of chlorides in the environment, the film locally breaks down and rapid dissolution of the underlying metal occurs in the form of pits. Surfaces of stainless steel and NiTi wire may exhibit crevices and pores which may give rise to attack since they represent sites susceptible to corrosion. Potentiodynamic polarization experiments and scanning electron microscopic oberservations of archwires composed of stainless steel, CoCr, NiCr, NiTi and Beta-Ti exposed to electrochemical corrosion in artificial saliva have shown evidence of pitting corrosion formed on the wire surfaces. [31] Liu [32] studied mechanical characteristics and corrosion behavior of titanium aluminum nitride coating on dental alloys especially in nickel based and chromium based dental materials. They tested corrosion behaviors in 0.9% NaCl solution and observed a higher positive corrosion potential and a lower corrosion current density in alloy having coated with titanium aluminum nitride as compared with uncoated dental alloys. In addition, the pitting corrosion was substantially reduced by the employment of TiAlN coating. Es Souni et al [33] found that Cr-Co alloy (Elgiloy) showed high pitting corrosion compared with NiTi alloy, lower repassivation potentials and increase in current density once pitting had occurred. Hera Kim [34] showed that nickel titanium and stainless steel wires were susceptible to pitting and localized corrosion.

Crevice Corrosion

Crevice corrosion occurs between two close surfaces or in constricted places where oxygen exchange is not available. It often occurs through the application of nonmetallic parts of metal (ie, elastomeric ligatures on a bracket). The reduction in pH and increase in the concentration of chloride ions are two essential factors in the initiation and propagation of the crevice corrosion phenomenon. When the acidity of the medium increases with time the passive layer of the alloy dissolves and it accelerates local corrosion process. Crevice corrosion of stainless steels in aerated salt solutions is widely known. Corrosion products of Fe, Cr, and Ni, the main components of stainless steel, accumulate in the crevice and form highly acidic chloride solutions in which corrosions rates are very high. [35,36] Platt et al [37] reported that 2205 duplex stainless is an improved alternative to 316L for orthodontic bracket fabrication when used in conjunction with Ti, its alloys, or stainless steel archwires. Rogers [38] showed that by electroplating with gold before and after silver soldering, the stainless steel prevented crevice corrosion.

Fretting and Erosion-Corrosion

The combination of a corrosive fluid and high flow velocity results in erosion-corrosion. The same stagnant or slow-flowing fluids will cause a low or modest corrosion rate, but rapid movement of the corrosive fluid physically erodes and removes the protective corrosion product film, exposes the reactive alloy beneath and accelerates corrosion. Fretting corrosion (type of erosion-corrosion) is responsible for most of the metal release into tissues. [39,40] Conjoint action of chemical and mechanical attack results in fretting corrosion. It occurs in contact areas of materials under load and finds its analogue in the slot-archwire interface of the bracket.

Intergranular Corrosion

Reactive impurities may segregate, or passivating elements such as chromium may get depleted at the grain boundaries. As a result, the grain boundary and adjacent regions are often less corrosion resistant and preferential corrosion at the grain boundary may be severe enough to drop grains out of the surface. Stainless steel brackets subjected to a range of temperatures, known as sensitization temperatures, undergo an alteration in their microstructure. The phenomenon is due to a precipitation of carbide at the boundaries of the grains. [41] Berge et al [42] reported that austenitic stainless steel wires release nickel and chromium in higher amounts than cobalt- chromium wires, resulting in discolouration, rust, or even breakages.

Galvanic Corrosion of Orthodontic Alloys

It is the dissolution of metals driven by macroscopic differences in electrochemical potentials, usually as a result of dissimilar metals in proximity. When two or more metals or dissimilar alloys ( or even the same alloy, subjected to different treatments) come into contact while exposed to oral fluids, the difference between their corrosion potentials results in a flow of electric current between them. In a clinical situation, two dissimilar alloys having different corrosion potentials are often placed in contact such as in orthodontic brackets and archwires. This can cause galvanic corrosion that leads to preferential release of metal ions from the anodic metal or alloy. Surface area ratio of the two dissimilar alloys is a very important factor because it affects the galvanic corrosion behavior. An unfavourable area ratio which consists of large cathode and a small anode, might lead to greater corrosion rate from the anodic alloy. Reed, and Willman [43] demonstrated the presence of galvanic currents in the oral cavity probably first time in detail. Approximate values for the magnitude were established. Burse et al [44] described an experimental protocol for in vivo tarnish evaluation and showed the importance of proper elemental ratio in gold alloys. Masahir et al [45] showed the NiTi alloy coupled with SUS 304 or Ti exhibited a relatively large galvanic current density even after 72 hours. It is suggested that coupling SUS 304-NiTi and Ti-NiTi may remarkably accelerate the corrosion of NiTi alloy, which serves as the anode. The different anode-cathode area ratios used in this study had little effect on galvanic corrosion behavior. Tufekci et al [46] described highly sensitive analytical technique which showed the release of individual elements over a month period which appeared to be correlated with micro structural phases in the alloys. Several forms of electrochemical corrosion are based on the mechanism that produces inhomogeneous areas. Failures could be minimized in orthodontic solder joints by employing material couples that minimize galvanic and microgalvanic effects.

Stress Corrosion of Orthodontic Wires

Stress corrosion occurs because of fatigue of metal when it is associated with a corrosive environment. When arch wire engaged to brackets bonded to crowded teeth, the reactivity status of the alloy increases. The increased reactivity results from the generation of tensile and compressive stresses developed locally because of the multiaxial, three dimensional loading of wire. Thus, an electrochemical potential difference occurs with specific sites acting as anodes and other surfaces acting as cathodes. NiTi orthodontic wires remain in the oral environment for several months and suffer a large number of small loads during mastication. Despite good corrosion resistance of stressed NiTi, the breakage of NiTi orthodontic wires has frequently been found in clinical studies. [47] Wang et al [48] studied stress corrosion cracking of NiTi in artificial saliva and demonstrated that the orthodontic NiTi wires were broken by stress corrosion cracking during service. The slight change in temperature will cause the dynamic phase transformation which causes the change in surface state.

Hydrogen damage

Hydrogen attack is the reaction of the hydrogen with carbides in steel to form methane, resulting in decarburization voids, and surface blisters. It can embrittle reactive metals such as titanium, vanadium, niobium etc.

Microbial Corrosion in Orthodontic Appliances

Microbiology-related corrosion has been noted in industry for many years. It is widely recognized that microorganisms affect the corrosion of metal and alloys immersed in aqueous environment. Under similar conditions, the effect of bacteria in the oral environment on the corrosion of dental metallic materials remains unknown. Matasa [49] was the first to show evidence of microbial attack on adhesives in the orthodontic field. The effect of enzymatic activity and degradation of composite resins has been reported earlier. Occurrence of these phenomena in brackets results in the formation of craters in the bracket base. [50] Large surface area provide by wire surface provide favorable environment for growth of bacteria. Brushing and attachment of microbes on wire may disturb the passivity of passive metal. The formation of organic acids during glucolysis pathways from sugars by bacteria may reduce pH. A low pH creates favorable environment for aerobic bacteria for corrosion. Microbes oxidize manganese and iron and reaction products viz. MnO2 FeO, Fe2O3, MnCl2, FeCl2 favour corrosion of orthodontic wires. A complex mechanism of interaction occurs amongst anaerobic and aerobic bacteria in various zones, favoring corrosion products. Due to deposition of the biofilm, the metal surface beneath the biofilm and the other areas are exposed to different amount of oxygen, which leads to creation of differential aeration cells. Less aerated zone act as an anode, which undergoes corrosion, releasing metal ions into the saliva. These metal ions combine with the end- products of the bacteria, along with chloride ion in the electrolyte (saliva) to form more corrosive products like MnCl2, FeCl2 etc. favouring further corrosion. [51] Microbial corrosion occurs when the acidic waste products of microbes and bacteria corrode metal surfaces. The incidence and severity of microbial corrosion can be reduced by keeping the area as clean as possible and by using anti-biotic sprays and dips to control the population of microbes. Chang et al [50] showed that corrosion behavior of dental metallic materials in the presence of Streptococcus mutans and its growth byproducts is increased. Maruthamuthu et al [51] studied electrochemical behavior of microbes on orthodontic wires in artificial saliva with or without saliva. According to him bacteria slightly reduce the resistance and increase the corrosion current. Leaching of manganese, chromium, nickel and iron from the wires may be due to the availability of manganese oxidizers, iron oxidizers and heterotrophic bacteria in the saliva.

Enhancing oral environment

In oral environment, fluoride containing commercial mouthwashes, toothpaste and prophylactic gels are widely used to prevent dental caries or relieve dental sensitivity are proper oral cleaning after application of normal brushes with tooth paste. The detrimental effect of fluoride ions on the corrosion resistance of Ti or Ti alloys has been extensively reported. Fluoride ions are very aggressive on the protective TiO2 film formed on Ti and Ti alloys. Since outermost surface of NiTi archwire contains mainly TiO2 film with trace amount of NiO, fluoride enhanced corrosion of the NiTi archwires in fluoride containing environment has been considered. [52,53] Fluoride-containing environments can penetrate into the narrow crevices between the orthodontic archwire and bracket in the mouth which is not cleaned out thoroughly. Topical high fluoride concentrations will stay in place and attack the arch wire/bracket interface depending on the fluoride concentration. This may increase friction force between archwire and bracket. Using topical fluoride agents with NiTi wire could decrease the functional unloading mechanical properties of wires and contribute to prolonged orthodontic treatment. [54] Orthodontic patients are required to maintain a high level of oral hygiene, which include regular tooth brushing. In vitro studies of effect of tooth brushing showed significant increase in elemental release from nickel alloys when toothpaste was used, however, without toothpaste there is no significant increase in elemental release. There is also evidence to suggest that some mouth rinses may also increase ionic release from silver soldered joints in orthodontic appliances. Schiff et al [55] studied corrosion resistance of three types of brackets (cobalt-chromium, iron chromium-nickel and titanium based brackets) in a three fluoride mouthwashes. The result showed that the bracket materials could be divided into two groups: Ti and FeCrNi in one and CoCr, which has properties close to those of Pt. Many studies have shown that fluoride ions can destroy the protectiveness of the surface TiO2 passive film on Ti or Ti alloy, leading to attacked corrosion morphology, decreased polarization resistance and an increased anodic current density or metal ion release. [56] Furthermore, the corrosion resistance of NiTi decreases on increasing NaF concentration in the artificial saliva. Schiff et al [57] studied the corrosion resistance of orthodontic wires in three different commercial mouthwashes and found that the NiTi wires were subject to severe corrosion in Na2FPO4 containing mouthwashes. Huang [58] studied surface topography variations of different nickel-titanium orthodontic archwires in different commercial fluoride containing environments. Four tested NiTi archwires had different surface topography variations, depending on the fluoride ion concentration. The archwire manufacturer and emersion environment had a statistically significant influence on surface roughness variation. The increase in surface roughness of NiTi orthodontic archwires in the commercial fluoride containing environments should be taken into account when considering the effectiveness of orthodontic appliances.

Nickel Containing Orthodontic Wires

Nickel containing alloys find extensive application in orthodontics, including metallic brackets, archwires, bands, springs and ligature wires. For most materials, a rough surface promotes corrosion. Doubts remain about biocompatibility of Ni-based alloys when used in dentistry. The use of nickel is of particularly concern since it is the most allergenic of all metallic materials. Not all nickel-allergic individuals will react to intraoral nickel, and it is currently not possible to predict which individuals will react. Because the frequency of nickel allergy is high, it is possible that individuals will become sensitized after placement of nickel containing alloys in the mouth. Nickel is a known allergen. [59] In a study of Finnish adolescents, the prevalence of nickel allergy was found to be 30% in girls and 3% in boys. [60] This was thought to be related to sensitization to nickel by ear piercing as the prevalence in adolescents, with ear piercing it was found to be 31% and only 2% otherwise. Allergic responses are mediated through the immune system. The majority of dental allergies, including responses to nickel containing dental alloys, comprise type IV hypersensitivity reactions, cell mediated by T-lymphocytes. Nickel containing dental alloys can undergo corrosion with release of metal ions. [61,62,63] High content nickel-titanium wires should be avoided in nickel sensitive patients, nickel free alternatives being available for use in such cases. [64] Bishara et al [65] studied biodegradation of orthodontic appliances in vitro and showed that nickel ions released from orthodontic appliances of nickel- titanium and stainless steel increased over the first week then diminished over time. Gjerdt et al [66] studied metal release from heat treated orthodontic wires and demonstrated that heat treatment of the alloys under laboratory conditions increased the release of metal ions-15-60 times. They showed significant initial increase in the concentration and mass of nickel in saliva sample of patient with fixed orthodontic appliances as compared to sampled saliva of patients without orthodontic appliances. Other studies have shown that the release of nickel ions is not proportional to the nickel content of orthodontic wires, but to the nature of the alloys and the method of construction of the appliance. Kerosuo et al [67] studied in vitro release of nickel and chromium ions from different type of simulated orthodontic appliances. Metal appliances immersed in 0.9% sodium chloride solution showed significantly higher cumulative release of nickel under dynamic (simulated function) compared to static condition. It should be noted that nickel ions released from metallic restorations and intraoral appliances will normally be swallowed and will not accumulate in the oral environment furthermore. The amount of nickel released from dental alloys is significantly less than that consumed orally as part of the dietary intake, although the ingested ions will obviously add to the overall burden of previously nickel sensitized patients. According to Kim [33] for patients allergic to nickel, the use of titanium or epoxy-coated wires during orthodontic treatment is recommended.

Clinical signs and symptoms seen in allergic reactions to nickel include oral oedema, perioral stomatitis, gingivitis, and extraoral manifestations such as eczematous rashes. [68,69,70] The mechanisms of high allergy frequency to nickel are not known, but there is probably genetic component. [71,72] In addition, the tendency of nickel containing alloys to release relatively large amounts of nickel ions probably contributes to their allerginicity. Nickel ions are a documented mutagen in humans, but there is no evidence that nickel ions cause any carcinogenesis intraorally. [73] Galvanic Current or release of ions could account for many types of dyscrasias, such as lichenoid lesions, ulcers, leukoplakia, cancer and kidney disorder, although research has failed to find any correlation between dissimilar metals and tissue irritation. Future steps would be to find correlation between the problems observed in the mouth due to corrosion products and the results of corrosion tests in vitro. [74] According to Cioffi et al [75] thin layer activation (TLA) method in the biomedical field appears a suitable technique to monitor in real time the corrosion behaviour of medical devices.

Summary

The corrosion of dental materials is a pertinent clinical issue. A primary requisite of any metal alloys used in the mouth is that they must not produce corrosion products that will be harmful to the body. In spite of recent innovative metallurgical and technological advances and remarkable progress in the design and development of surgical and dental materials, failures do occur. One of the reasons for these failures can be corrosion of orthodontic appliances. Corrosion causes severe and catastrophic disintegration of the metal body. Its attack may be extremely localized and cause rapid mechanical failure of a structure, even though the actual volume loss of material is quite small. Surface roughening and deposit build up may have adverse effect on the efficiency of relative wire/bracket movement in orthodontic treatment. Application of fluoride gel containing gel/toothpaste may affect the efficiency of orthodontic appliances. In the future nickel free materials should be expected to be used. Future research is needed regarding material composition influencing corrosion, manufacturing of metallic brackets, influence of various dietary patterns as well as diet substance on corrosion, use of topical fluoride treatment during orthodontic treatment for oral hygiene maintenance.

Table 1. Different Types of Wires Used for Orthodontic Treatment and their Composition.

Different Types of Wires Used for Orthodontic Treatment and their Composition

Table 2. Summary of the some experimental work on the corrosion of orthodontic alloys (in vitro).

Reference PDF Document (link at top of article)

Legend

Figure 1. Diagrammatic summary of the various types of corrosion

Diagrammatic summary of the various types of corrosion

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Contributed by:

Dr. T.P. Chaturvedi
Division of Orthodontics, Faculty of Dental Sciences, Institute of Medical Sciences, Banaras Hindu University, Varanasi – 221005, U.P. (India)

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