Indian Journal of Dental Research

: 2010  |  Volume : 21  |  Issue : 2  |  Page : 275--284

An overview of orthodontic material degradation in oral cavity

TP Chaturvedi1, SN Upadhayay2,  
1 Department of Orthodontics, Faculty of Dental Sciences, Institute of Medical Sciences, Varanasi, UP, India
2 Department of Chemical Engineering, IT, Banaras Hindu University, Varanasi, UP, India

Correspondence Address:
T P Chaturvedi
Department of Orthodontics, Faculty of Dental Sciences, Institute of Medical Sciences, Varanasi, UP


Various types of metallic orthodontic appliances are used in the management of malocclusion. These appliances are placed in oral environnent under many stresses and variations such as masticatory forces, appliance loading, temperature fluctuations, varieties of ingested food and saliva. These metals undergo electrochemical reactions with the oral environment resulting in dissolution or formation of chemical compounds. Various microorganisms and many aggressive ions containing oral environment can cause material degradation (corrosion) and its associated problems during long time exposure. Orthodontic alloys must have excellent corrosion resistance to the oral environment, which is highly important for biocompatibility as well as for orthodontic appliance durability. This article reviews various aspects of corrosion (surface degradation) of orthodontic alloys. It explores the emerging research strategies for probing the biocompatibility of materials. During orthodontic treatment, use of nickel free, better corrosion resistance alloys and less use of fluoride containing toothpaste or gel is expected.

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Chaturvedi T P, Upadhayay S N. An overview of orthodontic material degradation in oral cavity.Indian J Dent Res 2010;21:275-284

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Chaturvedi T P, Upadhayay S N. An overview of orthodontic material degradation in oral cavity. Indian J Dent Res [serial online] 2010 [cited 2021 May 10 ];21:275-284
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Biocompatibility of dental material is now a fundamental requirement of successful clinical behavior in oral cavity. It draws on knowledge from biology, patient risk factors, clinical experience and engineering. There are two key factors that appear to be important in determining biocompatibility of any dental material - some involve various types of corrosion or material degradation and others include surface characteristics. Dental materials within the mouth interact continually with physiological fluids. Saliva is a hypotonic solution containing bioactonate, chloride, potassium, sodium, nitrogenous compounds and proteins. [1] 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] 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]. 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 galvanic couple that can lead to ionic release mainly copper and zinc. Solder joints of removable appliances and face bows, and brazed joints of some stainless steel brackets may affect the mechanical properties. [4] The objective of the present article is to review briefly the corrosion aspects (material degradation) and its effect on orthodontic wires and brackets in oral environment, effect of fluoride gel/toothpaste on orthodontic appliances and nickel sensitivity. A Summary of the few experimental works on the corrosion of orthodontic alloys (in vitro) are given in [Table 2], which can explore the future research strategies for properties of orthodontic materials.

 Importance of Corrosion on Orthodontic Wires

For all practical purposes, the metallic restoration/orthodontic wires cannot be isolated electrically from the tooth. 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. [5],[6] Release of elements can produce discoloration of adjacent soft tissues and allergic reactions in susceptible patients. [7],[8],[9],[10] Corrosion can severely affect the ultimate strength of the material leading to mechanical failure of the orthodontic materials. [11] Some alloys and metals are resistant to corrosion because of inherent nobility or the formation of a protective surface layer. However, many ions containing oral environment can cause corrosion during long time exposure.

 Weakening of Appliances

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. [12],[13] 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. [14],[15] According to Zinelis et al.[16] Ag-based soldering alloys introduce a galvanic couple with stainless steel alloys, inducing release of metallic ions like Cu ++ and Zn ++ , the elements mostly leached out from silver solder alloys. Vahed A et al.[17] report that prolonged exposure in stimulated saliva leads 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 forms in the solder metal at the interface provides a micro galvanic effect that leads to selective dissolution of these particles and corresponding weakening of the interface. Corrosion is the main cause of the progressive dissolution of brazing filler metal, leading to detachment of the wing from the bracket base during orthodontic therapy or at debonding stage. [18]

Sliding friction

Friction is a phenomenon that generally affects the clinical efficacy of orthodontic appliances. The frictional forces that oppose the tooth movement during sliding are effects of different arch wire bracket combinations (e.g. material, size, shape and angulations), influence of ligation e.g. material and contact force and interaction of appliances with surrounding environment e.g. (interbracket distances and oral fluids). Corrosion increases orthodontic friction force between the arch wire/bracket interfaces due to an increase in the surface roughness. [17],[18],[19]

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. [20],[21]

Cytotoxic responses

Iron, nickel and chromium are major corrosive products of stainless steel. Nickel and chromium induce Type-IV hypersensitivity reaction in the body. [22] These metals cause several cytotoxic responses including decrease in some enzyme activities, interference with biochemical pathways, carcinogenicity, and mutagenicity. [23],[24] 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. It has been suggested that long term exposure to nickel containing dental materials affect both human monocytes and oral mucosal cells. [25],[26]

Decalcification of teeth

Various acids are formed during the microbial attack on metallic orthodontic appliances in oral environment. Biofilm for med on the tooth surface with the help of food debris and metabolic products of microbes. Acids cause reduction of pH and influence the decalcification of teeth and corrosion of metallic appliances. [27]

 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. In general, these wires have good biocompatibility and high corrosion resistance in oral environment. 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 wet conditions. Oxygen is necessary to form and maintain the film, whereas acidity and chloride ions can be particularly detrimental to it. [28] The presence 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 lose its resistance to corrosion if it is heated between approximately 400 o C and 900 o 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. [29] Precipitation does not occur below these temperatures and chances of corrosion are less. Corrosion of stainless steel may result due to galvanic cell forming in one or more of the following ways:

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

Titanium and its Alloys (nickel-titanium alloys, β-titanium alloys, titanium- aluminum-vanadium alloy)

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. When the stable oxide layer is broken down or removed and is unable to reform on parts of the surface, titanium and its alloy can be as corrosive as many other base metals. The surface roughness of titanium containing alloy is more as compared to stainless steel wires, which may act as galvanic cell in the mouth. [30],[31],[32]

 Types of Corrosion in Orthodontic Wires

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

Uniform corrosion

A uniform corrosion is any chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or over a large area. 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

Pitting is a form of extreme attack that results in holes on 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 chloride 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 observations 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. [34] Liu [35] studied mechanical characteristics and corrosion behavior of titanium aluminum nitride coating on dental alloys, especially in nickel- 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 alloys 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.[36] 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. Kim et al.[37] 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 (i.e., 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 corrosion rates are very high. [38],[39] Platt et al.[40] report that 2205 duplex stainless steel is an improved alternative to 316L for orthodontic bracket fabrication when used in conjunction with Ti, its alloys, or stainless steel arch wires. Rogers [41] showed that by electroplating with gold before and after silver soldering, the stainless steel prevented crevice corrosion. Recycling orthodontic wires and brackets was once common in many countries, but it is not recommended in many countries now. Recycling can comprise heat chemical and mechanical process which could lead to accelerated crevice corrosion of brazed joints.

Fretting and erosion-corrosion

Erosion-corrosion is increase in rate of deterioration or attack on material because of relative movement between corrosive fluid and material surface. The 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 released into tissues. [42],[43] 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-arch wire interface of the bracket.

Intergranular corrosion

Due to more reactive nature of grain boundaries, intergranular corrosion occurs adjacent to grain boundaries with little corrosion of grains. 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. [44] Berge et al.[28] reported that austenitic stainless steel wires release nickel and chromium in higher amounts than cobalt- chromium wires, resulting in discoloration, rust, or even breakages.

Galvanic corrosion

This type of corrosion occurs due to galvanic coupling of dissimilar metals involved in which less corrosion resistant metals become an anode and usually corrode. In a clinical situation, two dissimilar alloys having different corrosion potentials are often placed in contact such as in orthodontic brackets and arch wires. 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 unfavorable area ratio, which consists of a large cathode and a small anode, might lead to greater corrosion rate from the anodic alloy. Reed and Willman [45] demonstrated the presence of galvanic currents in the oral cavity probably first time in detail. Approximate values for magnitude were established. Burse [46] described an experimental protocol for in vivo tarnish evaluation and showed the importance of proper elemental ratio in gold alloys. Masahir et al.[47] 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.[48] 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 micro galvanic effects.

Stress corrosion

Stress corrosion cracking refers to cracking caused by the simultaneous presence of tensile stress in corrosive medium. This process can dangerously impair mechanical integrity of orthodontic wires. When arch wire engaged to brackets are 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. [49],[50] Wang et al.[51] 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 causes the dynamic phase transformation, which causes the change in surface state.

Hydrogen damage

Under certain circumstances, orthodontic wires can absorb hydrogen under cathodic condition. The presence of absorbed hydrogen decreases ductility of metals. It can embrittle reactive metals such as titanium, vanadium, niobium etc. [1],[6]

 Effect of Oral Environment

In the oral environment, fluoride-containing commercial mouthwashes, toothpastes and prophylactic gels are widely used to prevent dental caries or relieve dental sensitivity or for proper oral cleaning after application of normal brushes with tooth paste. [52],[53] 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 TiO 2 film formed on Ti and Ti alloys. Since outermost surface of NiTi arch wire contains mainly TiO 2 film with trace amount of NiO, fluoride enhanced corrosion of the NiTi arch wires in fluoride containing environment has been considered. [54],[55] Fluoride-containing environments can penetrate into the narrow crevices between the orthodontic arch wire 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 arch wire and bracket. Using topical fluoride agents with NiTi wire could decrease the functional unloading mechanical properties of wires and contribute to prolonged orthodontic treatment. [56],[57] Orthodontic patients are required to maintain a high level of oral hygiene, which include regular tooth brushing. In vitro studies [58],[59] on effects 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.[60] studied corrosion resistance of three types of brackets (cobalt-chromium, iron chromium-nickel and titanium based brackets) in three fluoride mouthwashes. The results 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 TiO 2 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. [61]

Further, the corrosion resistance of NiTi decreases on increasing NaF concentration in the artificial saliva. Schiff et al.[62] studied the corrosion resistance of orthodontic wires in three different commercial mouthwashes and found that the NiTi wires were subject to severe corrosion in Na 2 FPO 4 containing mouthwashes. Huang [63] studied surface topography variations of different nickel-titanium orthodontic arch wires in different commercial fluoride containing environments. Four tested NiTi arch wires had different surface topography variations, depending on the fluoride ion concentration. The arch wire manufacturer and emersion environment had a statistically significant influence on surface roughness variation. The increase in surface roughness of NiTi orthodontic arch wires 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, arch wires, 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 particular 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. [64] In a study of Finnish adolescents, the prevalence of nickel allergy was found to be 30% in girls and 3% in boys. [65],[66] 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. [67],[68] High content nickel-titanium wires should be avoided in nickel sensitive patients, nickel free alternatives being available for use in such cases. [69] Bishara et al.[70] 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.[71] 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.[72] studied the in vitro release of nickel and chromium ions from different types 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. Kim et al.[37] opine that 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 edema, perioral stomatitis, gingivitis, and extra oral manifestations such as eczematous rashes. [73],[74],[75] The mechanisms of high allergy frequency to nickel are not known, but there is probably genetic component. [76],[77] In addition, the tendency of nickel containing alloys to release relatively large amount 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. [78] 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. [78], According to Cioffi et al. thin layer activation (TLA) method in the biomedical field appears to be a suitable technique to monitor in real time the corrosion behavior of medical devices.

 Protective Measure from Corrosion

To reduce corrosion, patient should avoid eating too much salty food. Salt provides chlorides ions which combines with hydrogen and produces acid which may cause corrosion. Chloride ion is detrimental for formation of passive layer on metal surface. Elastics should be clean or should be change frequently. Empty pockets at the bracket/adhesive interface should be prevented to avoid acceleration of corrosion process. Clinician should follow proper storage, maintenance and sterilization procedures and avoid intermetal contacts. Selection of attachment should be made that are less susceptible to corrosion. Indiscriminate use of heat or disinfection as well as thermal recycling should be avoided. Orthodontic arch wires and brackets can be coated with either titanium nitride or an epoxy resin. The former is used to improve hardness and reduce friction, the latter improves esthetics. Epoxy coating improves corrosion resistance by preventing attack by corrosive fluid. [37] Another method of reducing corrosion of metals is to add a corrosion inhibitor to a solution into which the material is placed but it very difficult to follow in oral cavity.


A primary requisite of any alloy metal used in the mouth is that it must not produce corrosion products that will be harmful to body. In spite of recent innovative metallurgical and technological advances and remarkable progress related to orthodontic materials, failures do occur. One of the reasons for these failures could be corrosion (material degradation) of orthodontic appliances. It causes severe and catastrophic disintegration of the metal body. Corrosion (material degradation) attack may be extremely localized and causes 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 effects on the efficiency of relative wire/bracket function in orthodontic treatment. Application of fluoride containing gel/toothpaste may affect efficiency of orthodontic appliances. In future nickel free materials should be expected in use. Future research is needed regarding material composition influencing corrosion, manufacturing of metallic brackets, influence of various diet pattern as well as diet substance on corrosion, use of topical fluoride treatment during orthodontic treatment for oral hygiene maintenance.


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