Year : 2009 | Volume
: 20 | Issue : 1 | Page : 91--98
An overview of the corrosion aspect of dental implants (titanium and its alloys)
Professor, Division of Orthodontics and General Dentistry, Faculty of Dental Sciences, Institute of Medical Sciences, 4GF Jodhpur Colony, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India
T P Chaturvedi
Professor, Division of Orthodontics and General Dentistry, Faculty of Dental Sciences, Institute of Medical Sciences, 4GF Jodhpur Colony, Banaras Hindu University, Varanasi 221005, Uttar Pradesh
Titanium and its alloys are used in dentistry for implants because of its unique combination of chemical, physical, and biological properties. They are used in dentistry in cast and wrought form. The long term presence of corrosion reaction products and ongoing corrosion lead to fractures of the alloy-abutment interface, abutment, or implant body. The combination of stress, corrosion, and bacteria contribute to implant failure. This article highlights a review of the various aspects of corrosion and biocompatibility of dental titanium implants as well as suprastructures. This knowledge will also be helpful in exploring possible research strategies for probing the biological properties of materials.
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Chaturvedi T P. An overview of the corrosion aspect of dental implants (titanium and its alloys).Indian J Dent Res 2009;20:91-98
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Chaturvedi T P. An overview of the corrosion aspect of dental implants (titanium and its alloys). Indian J Dent Res [serial online] 2009 [cited 2019 Jul 22 ];20:91-98
Available from: http://www.ijdr.in/text.asp?2009/20/1/91/49068
The mouth is the portal entry of the human body. It is also the habitat of microbial species that are kept wet by saliva. Oral tissues are exposed to a veritable bombardment of both chemical and physical stimuli as well as 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 remain healthy. Saliva has several viruses, bacteria, yeast and fungi and their products, such as organic acids and enzymes, epithelial cells, food debris, and components from gingival crevicular fluid. Moreover, saliva is a hypotonic solution containing bioactonate, chloride, potassium, sodium, nitrogenous compounds, and proteins. The pH of saliva varies from 5.2 to 7.8. Many gram-negative and gram-positive bacterial species form a major part of dental plaque around the teeth and also colonize the mucosal surfaces. Teeth function in one of the most inhospitable environments in the body. They are subject to larger temperature variation than most other parts, coping with cold of ice (0°C) to hot coffee and soup. Factors such as temperature, quantity and quality of saliva, plaque, pH, protein, and the physical and chemical properties of food and liquids as well as oral health conditions may influence corrosion. Corrosion, the graded degradation of materials by electrochemical attack is of concern particularly when a metallic implant, metallic filling, or orthodontic appliances are placed in the hostile electrolytic environment provided by the human mouth. , For dental implants, biocompatibility depends on mechanical and corrosion/degradation properties of the material, tissue, and host factors. Biomaterial surface chemistry, topography (roughness), and type of tissue integration (osseous, fibrous, and mixed) correlate with host response. Biocompatibility of the implants and its associated structure is important for proper function of the prosthesis in the mouth. Corrosion can severely limit the fatigue life and ultimate strength of the material leading to mechanical failure of the dental materials. High noble alloys used in dentistry are so stable chemically that they do not undergo significant corrosion in the oral environment, the major component of these alloys are gold, palladium, and platinum.
Clinical Significance of Corrosion
It has been proven that small galvanic currents associated with electrogalvanism are continually present in the oral cavity. As long as metallic dental restorative materials are employed, there seems to be little possibility that these galvanic currents can be eliminated. Post operative pain caused by galvanic shock can be a source of discomfort in the metallic restoration to an occasional patient. Resistance to corrosion is critically important for dental materials because corrosion can lead to roughening of the surface, weakening of the restoration, liberation of elements from the metal or alloy, and toxic reactions. The liberation of elements can produce discoloration of adjacent soft tissues and allergic reactions such as oral edema, perioral stomatitis, gingivitis, and extraoral manifestation such as eczematous rashes in susceptible patients. According to Kirkpatric, et al.  the pathomechanism of the impaired wound healing is modulated by specific metal ions released by corrosion.
The Effect of Corrosion on Dental Implants
Dental implant treatment has been one of the most recent success stories of dentistry. The use of dental implants in the treatment of complete and partial edentulisms has become an integral treatment modality in dentistry. Dental implants are made of biocompatible materials and they are surgically inserted into the jaw bone primarily as a prosthetic foundation. Titanium and titanium alloys are commonly used as dental implant materials. The process of integration of titanium with bone has been termed as "osseointegration" by Branemark.  Presently, most of the commercially available implant systems are made of pure titanium (CP-Ti) or titanium alloy Ti-6Al-4V. Titanium and its alloys provide strength, rigidity, and ductility similar to those of other dental alloys. Whereas, pure titanium castings have mechanical properties similar to Type III and Type IV gold alloys, some titanium alloy castings, such as Ti-6Al-4V and Ti-15V have properties closer to Ni-Cr and Co-Cr castings with the exception of lower modulus. Titanium and its alloys give greater resistance to corrosion in saline and acidic environments. Even though titanium alloys were exceptionally corrosion-resistant because of the stability of the TiO 2 oxide layer, they are not inert to corrosive attack. When the stable oxide layer is broken down or removed and is unable to reform on parts of the surface, titanium can be as corrosive as many other base metals.
The oral cavity can simulate an electrochemical cell under certain circumstances. Although titanium shows better corrosion resistance, it may interact with living tissue in several years. This interaction results in a release of small quantities of corrosion products even though they are covered by thermodynamically stable oxide film. If a base metal alloy superstructure is provided over a Ti implant, then also an electrochemical cell is formed. The less noble metal alloy forms anode and the more noble titanium forms cathode. Electrons are transferred through metallic contact, and the anode is the surface or sites on a surface where positive ions are formed (i.e., the metal surface that is undergoing an oxidation reaction and corroding) with the production of the free electrons.
Fracture of Dental Implant
Although a fracture of dental implants is not a frequent phenomenon, it can cause unfavorable clinical results. Corrosion can severely limit the fatigue life and ultimate strength of the material leading to mechanical failure of the implant. It has been found that metal fatigue can lead to implant fracture. Titanium is not sufficiently stable to prevent wear and tear in bearings under load. Under static conditions, Ti and Ti alloy are able to withstand exposure to physiologic chlorine solutions at body temperature indefinitely but are susceptible to oxide changes caused by mechanical micromotion. For example, stainless steel and Ti alloy demonstrated crack-like features when loaded to yield stress. Therefore, repeated oxide breakdown such as sustained abrasion is likely to damage corrosion resistance. The superstructures also cause a release of metal ions. Corrosion sets in and results in the leaking of ions into surrounding tissues. Green  reported a fracture of a dental implant 4 years after loading. The failure analysis of the implant revealed that the fracture was caused by metal fatigue and that the crown-metal, a Ni-Cr-Mo alloy, exhibited corrosion. Yokoyama, et al. concluded that titanium in a biological environment absorbs hydrogen and this may be the reason for delayed fracture of a titanium implant.
Hexavalent chromium ions are released from implant materials.  Nickel and chromium induce Type-IV hypersensitivity reactions in the body and act as haptens, carcinogens, and mutagens. They can cause several cytotoxic responses including a decrease in some enzyme activities, interference with biochemical pathways, carcinogenicity, and mutagenicity. Long-term exposure to nickel containing dental materials may adversely affect both human monocytes and oral mucosal cells. Titanium containing nickel may cause localized tissue irritation in some patients. Manganese from the alloy is also consumed with saliva, which produces toxicity leading to skeletal and nervous, system disorders.
Bone Loss and Osteolysis
Ti alloys have shown integration with bone and soft tissue environments. However, there is concern that Ti alloys contain significant amounts of alloying elements that exhibit different morphology and crystallization, which may affect osseointegration especially due to corrosion products containing aluminium and vanadium. According to Roynesdal, et al.,  marginal bone loss around implants showed the worst results with titanium sprayed implants. Olmedo, et al.  reported that the presence of macrophages in peri-implant soft tissue induced by a corrosion process plays an important role in implant failure. Free titanium ions inhibit growth of hydroxyapatite crystals (mineralization of calcified tissues at the interface). These processes lead to local osteolysis and loss of clinical stability of the implant.
Local Reactions (Pain/Swelling)
Although titanium exhibits better corrosion resistance, it may interact with living tissues over several years. An increased level of calcium and phosphorous have been found in oxide surface layers indicating an exchange of ions at the interface.  Corrosion products have been implicated in causing local pain or swelling in the region of the implant in the absence of infection and it can cause secondary infection. A hydrogen peroxide environmental condition has been shown to interact with titanium and is associated with low toxicity, inflammation, bone modeling, and bactericidal characteristics.
Corrosion behavior in the oral cavity
Many types of electrochemical corrosion are possible in the oral environment because saliva, with salt, acts as 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 as follows: 
Oxidation and reduction reactions. Factors that physically impede or prevent corrosion from taking place (process of passivation or the formation of a metal oxide passive film on a metal surface).
Types of corrosion
There are two types of corrosive reactions: chemical and electrochemical. In chemical corrosion (dry corrosion), there is a direct combination of metallic and non metallic elements to yield a chemical compound through processes such as oxidation, halogenation, or sulfurization reactions. Electrochemical corrosion (wet corrosion) requires the presence of water or some other fluid electrolytes. This general mode of corrosion is important for dental restorations. Various forms of corrosion that may occur with the above types of reactions are mentioned in [Figure 1] and [Table 1].
The complexity of the electrochemical process involved in the implant-superstructure joint is linked to the phenomenon of galvanic coupling and pitted corrosion. The reduction in pH and the 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 accelerates the 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.
The most common form of corrosion, which is generally present in dental implants, is galvanic corrosion. Titanium has been chosen as the material of choice for endosseous implantation. Even though titanium alloys are exceptionally corrosion resistant because of the stability of the TiO 2 layer, they are not inert to corrosive attack. When the stable oxide layer is broken down or removed and is unable to reform on part of surface, titanium can be as corrosive as many other base metals.  Galvanic coupling of titanium to other metallic restorative materials may also generate corrosion. Hence, there is a great concern regarding the materials for suprastructures over the implants.
Gold alloys are generally chosen as the superstructures because of their excellent biocompatibility, corrosion resistance, and mechanical properties. However, these are quite expensive. Therefore, new alloys such as Ni-Cr, Ag-Pd, and Co-Cr alloys are generally used. They have good mechanical properties and are cost effective. But their biocompatibility and corrosion resistance are of concern.
When two or more dental prosthetic devices/restorations made of dissimilar alloys come into contact while exposed to oral fluids, the difference between their corrosion potential results in a flow of electric current between them. An in vivo galvanic cell is formed and the galvanic current causes acceleration of corrosion of the less noble metal. The galvanic current passes through the metal/metal junction and also through tissues, which causes pain. The current flows through two electrolytes, saliva, or other liquids in the mouth and the bone and tissue fluids.
The differential surface of a metallic restoration may have small pits/crevices. Consequently, stress and pit corrosion occurs. The mechanical and notched sensitivity,  stress corrosion cracking, torsional,  and smooth and notched corrosion fatigue  are properties of titanium materials used for implant.
The conjoint action of chemical and mechanical attack results in fretting corrosion. Fretting is another type of erosion-corrosion, but in a vapor phase.
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.
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 an aqueous environment. Under similar conditions, the effect of bacteria in the oral environment on the corrosion of dental metallic materials remains unknown. The effect of enzymatic activity and degradation of composite resins has been reported earlier. Chang, et al. showed that the corrosion behavior of dental metallic materials in the presence of Streptococcus mutans and its growth byproducts is increased. Brushing and the attachment of microbes on implants 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 a favorable environment for aerobic bacteria for corrosion. Microbes oxidize manganese and iron and reaction products viz. MnO 2, FeO, Fe 2 O 3 , MnCl 2 , FeCl 2 favor corrosion of the implant. A complex mechanism of interaction occurs among anaerobic and aerobic bacteria in various zones, favoring corrosion products. Due to the deposition of the biofilm, the metal surface beneath the biofilm and the other areas are exposed to different amounts of oxygen, which leads to the creation of differential aeration cells. Less aerated zones 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 MnCl 2 , FeCl 2, etc. favoring further corrosion.  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 antibiotic sprays and dips to control the population of microbes. Maruthamuthu, et al. studied the 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.
The effect of fluoride ion concentration
In the oral environment, fluoride contained in commercial mouthwashes, toothpaste, 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 toothpaste. 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. Odontogenic fluoride gels should be avoided because they create an acidic environment that leads to the degradation of the titanium oxide layer and possibly inhibits osseointegration.
In vitro and in vivo studies
A primary requisite of any metal used in the mouth is that it must not produce corrosion products that will be harmful to the body. Reed and Willman  demonstrated the presence of galvanic currents in the oral cavity probably for the first time in detail. Approximate values for the magnitude were established. Burse, et al. described an experimental protocol for in vivo tarnish evaluations and showed the importance of the proper elemental ratio in gold alloys compositions. Various experimental in vitro studies regarding corrosion are shown in [Table 2], which can explore the future research strategies for the corrosion study of implant materials.
Tufekci, et al.  described a highly sensitive analytical technique that showed the release of individual elements over a 1 month period, which appeared to be correlated with micro structural phases in the alloys.
Notable changes due to galvanic coupling have been reported in literature. Pourbaix  reviewed the methods of electrochemical thermodynamics (electrode potential-pH equilibrium diagrams) and electrochemical kinetics (polarization curves) to understand and predict the corrosion behavior of metals and alloys in the presence of body fluids.
Sutow, et al. studied the in vitro crevice corrosion behavior of implant materials. The galvanic corrosion of titanium in contact with amalgam and cast prosthodontic alloys has been studied in vitro. , No current or change in pH was registered when gold, cobalt chromium, stainless steel, carbon composite, or silver palladium alloys came in metallic contact with titanium. Changes occurred when amalgam was in contact with titanium.
Geis-Gerstorfer, et al. stated that the galvanic corrosion of implant/superstructure systems is important in two aspects: (i) the possibility of biological effects that may result from the dissolution of alloy components and (ii) the current flow that results from galvanic corrosion may lead to bone destruction.
In another study, Reclaru and Meyer  examined the corrosion behavior of different dental alloys, which may potentially be used for superstructures in galvanic coupling with titanium. Cortada, et al.  had reported that metallic ions are released in the artificial saliva of titanium oral implants coupled with different metal superstructures. In this work, metallic ion release in oral implants with superstructures of different metals and alloys used in clinical dentistry was determined.
The study regarding the measurement and evaluation of galvanic corrosion between titanium and dental alloys was also carried out by Grosgogeal, et al. using electrochemical techniques and auger spectrometry. The results showed that the intensity of the corrosion process is low in case of Ti/dental alloys. Other types of corrosion, e.g., pitting corrosion and crevice corrosion should also be considered.
Aparicio, et al.  studied the corrosion behavior of commercially pure titanium shot blasted with different materials and sizes of shot particles for dental implant applications. It is well known that the osseointegration of the commercially pure titanium (CP-Ti) dental implant is improved when the metal is shot blasted to increase its surface roughness. This roughness is colonized by bone, which improves implant fixation.
Oh and Kim  carried out a study regarding the electrochemical properties of suprastructures galvanically coupled to a titanium implant. Photomicrographs after electrochemical testing showed crevice or pitting corrosion in the marginal gap and at the suprastructure surface. Tested samples of Co-Cr/Ti implant couples showed the possibility of galvanic corrosion, but its degree was not significant.
Kasemo and Lausmaa  demonstrated the dissolution of corrosion products into the bioliquid and adjacent tissues. Thus, the outermost atomic layers of an implant are critical regions associated with biochemical interactions of the implant-tissue interface. This should have a tremendous influence on a high degree of standardization and surface control in the production of dental implants. The response of bone to different implant materials is the principal factor on which an implant material is selected as suitable or unsuitable for osseointegration.
Siiril and Knnen  studied the effects of topical fluoride on commercially pure titanium and concluded that toothbrushes used in contact with titanium surfaces should be as nonabrasive as possible, and that long lasting contamination with topical fluorides should be avoided. Nakagawa, et al. studied the relationship between fluoride concentrations and pH values at which Ti corrosion occurred in the presence of fluoride ions.
From the above brief review of literature, it is evident that monitoring of corrosion potential is helpful in indicating the existence and the extent of galvanic corrosion occurring in dental implants. According to Jose, et al.,  it is difficult to predict the clinical behavior of an alloy from in vitro studies, since such factors as changes in the quantity and quality of saliva, diet, oral hygiene, polishing of alloy, the amount and distribution of occlusal forces, or brushing with toothpaste can all influence corrosion to varying degrees. The increase in metal ion content in the environment may eventually prevent further corrosion. Sometimes a metal ceases corroding because its ions have saturated the immediate environment. This situation does not usually occur in dental restorations because dissolving food, fluids, and toothbrushes remove ions. Thus, corrosion of the restorations will continue.
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 dental implants. The most favorable suprastructure/implant couple is the one which is capable of resisting the most extreme conditions that could possibly be encountered in the mouth. The choice of the materials used for the implant as well as implant borne suprastructures become crucial, and can be made by way of evaluating their galvanic corrosion behaviors. When the mechanisms that ensure implant bioacceptance and structural stabilization are fully understood, implant failures will become a rare occurrence, provided that they are used properly and placed in sites for which they are indicated.
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