An overview of the corrosion aspect of dental implants (titanium and its alloys)

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1 REVIEW ARTICLE An overview of the corrosion aspect of dental implants (titanium and its alloys) TP Professor, Division of Orthodontics and General Dentistry, Faculty of Dental Sciences, Institute of Medical Sciences, 4GF Jodhpur Colony, Banaras Hindu University, Varanasi , Uttar Pradesh, India Received : Review completed : Accepted : PubMed ID : DOI: / ABSTRACT 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. Key words: Biocompatibility, corrosion, dental implants, titanium, titanium alloys 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 Þve 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 ßuid. 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 inßuence corrosion. Corrosion, the graded degradation of materials by electrochemical attack is of concern particularly when a metallic implant, metallic Þlling, or orthodontic appliances are placed in the hostile electrolytic environment provided by the human mouth. [1,2] For dental implants, biocompatibility depends on mechanical and Address for correspondence: Dr. TP, tpchaturvedi@rediffmail.com corrosion/degradation properties of the material, tissue, and host factors. Biomaterial surface chemistry, topography (roughness), and type of tissue integration (osseous, Þbrous, 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 signiþcant 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. [3] the pathomechanism of the impaired wound healing is modulated by speciþc metal ions released by corrosion. 91

2 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. [4] 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 Þlm. 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 sufþciently 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 indeþnitely 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 [5] 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 crownmetal, a Ni-Cr-Mo alloy, exhibited corrosion. Yokoyama, et al. [6] concluded that titanium in a biological environment absorbs hydrogen and this may be the reason for delayed fracture of a titanium implant. CELLULAR RESPONSES Hexavalent chromium ions are released from implant materials. [7] 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., [8] marginal bone loss around implants showed the worst results with titanium sprayed implants. Olmedo, et al. [9] 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 calciþed 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. [10] Corrosion products have been implicated in causing local pain or swelling in the region 92

3 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, inßammation, bone modeling, and bactericidal characteristics. CORROSION 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 inßuence 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: [11] 1. Oxidation and reduction reactions. 2. Factors that physically impede or prevent corrosion from taking place (process of passivation or the formation of a metal oxide passive Þlm 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 ßuid 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. Galvanic corrosion 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. [12] 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 ßow 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 Table 1: Different types of corrosion commonly occuring in the oral cavity Types of corrosion Description Uniform corrosion A uniform, regular removal of metal from the surface is the usually expected mode of corrosion. Pitting corrosion A form of localized, symmetric corrosion in which pits form on the metal surface. It usually occurs on base metals, which are protected by a naturally forming, thin Þ lm of an oxide. Crevice corrosion Crevice corrosion occurs between two close surfaces or in constricted places where oxygen exchange is not available. Galvanic corrosion Galvanic corrosion occurs when dissimilar alloys are placed in direct contact within the oral cavity or within the tissues. Stress corrosion Stress corrosion occurs because of fatigue of metal when it is associated with a corrosive environment. Fretting and erosion-corrosion The combination of a corrosive ß uid and high ß ow velocity results in erosion-corrosion. Fretting corrosion is responsible for most of the metal release into tissues. Intergranular corrosion Reactive impurities may segregate, or passivate elements such as chromium may be depleted at the grain boundaries. Microbial corrosion Microorganisms affect the corrosion of metal and alloys immersed in an aqueous environment. The formation of organic acids during glucolysis pathways from sugars by bacteria may reduce ph levels. A low ph level creates a favorable environment for aerobic bacteria for corrosion. 93

4 Uniform Intergranular Galvanic Surface cracks Internal voids Crevice Pitting Hydrogen damage Stress corrosion Corrosion fatigue Hydrogen induced cracking Fretting Erosion Cavitation Cavitation, erosion and fretting Figure 1: Diagrammatic summary of the various types of corrosion galvanic current passes through the metal/metal junction and also through tissues, which causes pain. The current ßows through two electrolytes, saliva, or other liquids in the mouth and the bone and tissue ßuids. The differential surface of a metallic restoration may have small pits/crevices. Consequently, stress and pit corrosion occurs. The mechanical and notched sensitivity, [13] stress corrosion cracking, torsional, [14] and smooth and notched corrosion fatigue [15] 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. MICROBIAL CORROSION 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. [16] 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. 94

5 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 bioþlm, the metal surface beneath the bioþlm 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. [2] 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. [2] studied the electrochemical behavior of microbes on orthodontic wires in artiþcial 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, ßuoride 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 ßuoride ions on the corrosion resistance of Ti or Ti alloys has been extensively reported. Fluoride ions are very aggressive on the protective TiO 2 Þlm formed on Ti and Ti alloys. Odontogenic ßuoride 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 [17] demonstrated the presence of galvanic currents in the oral cavity probably for the Þrst time in detail. Approximate values for the magnitude were established. Burse, et al. [18] 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. [19] described a highly sensitive analytical technique that showed the release of individual elements over a 1 month period, which appeared to be correlated 95 with micro structural phases in the alloys. Notable changes due to galvanic coupling have been reported in literature. Pourbaix [20] reviewed the methods of electrochemical thermodynamics (electrode potentialph equilibrium diagrams) and electrochemical kinetics (polarization curves) to understand and predict the corrosion behavior of metals and alloys in the presence of body ßuids. Sutow, et al. [21] 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. [22,23] 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. [24] 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 ßow that results from galvanic corrosion may lead to bone destruction. In another study, Reclaru and Meyer [25] examined the corrosion behavior of different dental alloys, which may potentially be used for superstructures in galvanic coupling with titanium. Cortada, et al. [7] had reported that metallic ions are released in the artiþcial 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. [26] 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. [27] 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 Þxation. Oh and Kim [28] 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

6 Table 2: Summary of the experimental work on the corrosion of implant materials References Implant alloys Medium, temperature, ph, period, method, etc Sutow, et al. [21] Ravnholt and Jensen [22] Reclaru and Meyer [25] Cortada, et al. [7] Aparicio, et al. [27] Oh and Kim [28] Yamazoe, et al. [33] Type 316 LVM stainless steel, cast Co-Cr-Mo, wrought Co-Cr-W-Ni, non-nitrided and nitrided Ti-6Al-4V ELI, and CP- Ti, Grades I and 4, crevice cell Titanium in contact with amalgam and cast prosthodontic alloys Co-Cr alloy, silver-palladium, gold, ternary titanium. With Ti implant abutment material Titanium oral implants coupled with different metal superstructures (cast-titanium, machined-titanium, gold alloy, silver-palladium alloy and chromium-nickel alloy), Commercially pure titanium shot blasted with different materials Gold, silver-palladium, cobalt-chromium, and nickelchromium suprastructures were used in combination with titanium (Ti) implants Ti, Ti-6Al-4V, Ti-6Al-7Nb Ti-0.5Pt, Ti-6Al-4V-.5Pt, Ti-6Al-7Nb-.5Pt Ringer s solution, ph =7, 37 o C. Anodic polarization was conducted potentiostatically at pre-selected levels, and resultant currents were monitored: 0.9% NaCl Solution Potentiostat Scanning potentiostat, artiþ cial saliva, Room temperature ArtiÞ cial saliva at 37 o C. Inductively coupled plasma mass spectrometry technique Electrochemical behavior (open circuit potential, electrochemical impedance spectroscopy and cyclic polarization) Potentiostat, artiþ cial saliva, 37 o C Electrochemical analyzer, artiþcial saliva containing 0.1 and 0.2 % NaF, ph-4 Huang, et al. [34] Ti-6Al-4V alloy ArtiÞ cial saliva with 0.5% NaF, with 0.1%NaF % Bovine albumin (BA), 37 o C, ph-5, X-ray photoelectron spectroscopy Jose, et al. [32] Manaranche, et al. [35] Zavanelli, et al. [36] Nickel based alloys, one noble alloy, one high noble alloy, and two copper aluminum alloys. Au, Pd, Ag, Cu, Zn, Ti. Precious alloys Pd-base, Au-Pt, Au-Pt-Pd, Au-Pd, Au-Ag-Cu alloys. Pure titanium, titanium alloy (Ti-6Al-4V) ArtiÞ cial Saliva, 15 days, Metal casts were subjected to continuous ß ow of saliva thrice daily lasting 30 minutes each, consisting ph decreases and salinity increases Electrochemical test; NaCl solution, 37 o C. ph 7.4, Potentiometer, 2 h. Chemical corrosion; 37 o C, sodium chloride and lactic acid. ph-2.3, 7 days, ICP spectroscopy Air, synthetic saliva, Flouride+synthetic saliva. At room temperature. Scanning electron microscope Remark Results for stainless steel demonstrated that a HNO 3 passivation treatment reduced its crevice corrosion susceptibility, although a dulling and discoloration of CP-Ti was evident recognizing that 600 mv is in excess of the O 2 reduction potential. No current ph or change in ph was registered when gold, cobalt chromium, stainless steel, carbon composite, or silver palladium alloys were in metallic contact with titanium. Changes occurred when amalgam was in contact with titanium. 1. In coupling, the titanium must have weak anodic polarization 2. The current generated by the galvanic cell must also be weak. 3. The crevice potential must be much higher than the common potential. The titanium oral implant coupled with a chromium-nickel alloy releases a high quantity of ions and the implant coupled with the titanium superstructure presents low values of ions release. Increased surface area of the material because of the increasing surface roughness is cause for the differences found in the electrochemical behavior and corrosion resistance of the blasted CP-Ti. Suprastructure/Ti implant couples produced passive current densities in the range of microa/cm 2 ; Ti abutment/ti implant and gold / Ti implant couples exhibited relatively low passive current densities; Co-Cr/Ti implant couples the highest. Co-Cr and Ni-Cr/Ti implant couples showed breakdown potentials of 700 and 570 mv (SCE), respectively. Surface of Ti, Ti-6Al-4V, Ti-6Al-7Nb were markedly rough, 0.5%Pt were not affected in ß uoride containing environment. (a) The electrochemical mechanism of Ti-6Al- 4V alloy in artiþ cial saliva is related to the ß uoride and bovine albumin concentration. (b) Electrochemical impedance spectroscopy (EIS) is suitable for the study of the electrochemical behaviour of dental alloys. (a) Cu-Al alloys released Cu, Al, Ni, Mn & Fe. Ni-based alloys released Ni, Cr. (b) Beryllium containing alloys released beryllium and Ni. (c) Noble and High noble alloys were very resistant to corrosion. (d) Ions released to be far below the tolerable upper intake levels for each ion. The Pd-base and Au-Pt-Pd dental alloys are the most resistant to chemical and electrochemical corrosion, even higher than gold. Ti-6Al-4V alloy had the highest value of fatigue; however, there were no signiþ cant differences when compared with commercially pure titanium. Contd... 96

7 Yukyo, et al. [37] Johanson [38] Taher, et al. [39] Chang, et al. [16] Dental magnetic attachment (ferric and austenitic stainless steel), Au-Ag-Pd alloy, type 4 gold alloys, titanium Cast and machined titanium in contact with conventional and high copper amalgams Co-Cr alloys, Ni-Cr, silver palladium. Gold and Ternary Ti, coupled with endosseous Ti implant abutment material. amalgam alloy and commercially pure Ti cylinders (SSTi) were coupled with endosseous Ti implants Commercially pure titanium (CPT), Ti-6Al-4V (TAV), Ti-Ni (TN), Co-Cr-Mo alloy (CCM), 316L stainless steel (SSL), 17Cr-4Ni PH-type stainless steel (PH), and Ni-Cr alloy (NC) 0.9% NaCl solution or 1% lactic acid solution at 37 o C. 7 days Saline solutions with and without ß uoride ions, surface treatment Scanning potentiostat, artiþ cial saliva solution, time was 24 h for each couple Gamry corrosion test system, (1) sterilized Ringer s solution as a control for (2), (2) S. mutans mixed with sterilized Ringer s solution; (3) sterilized tryptic soy broth as a control for (4), and (4) byproducts of S. mutans mixed with sterilized tryptic soy broth The contact of the stainless steel and the dental metals increased the amount of ions released from the stainless steel. Corrosion resistance of type 316L is inferior to that of ferric stainless steel in contact with precious alloys. Conventional amalgams corroded more than high copper amalgams in contact with titanium in saline solutions. Surface preparations and ß uoride affect the electrochemical activity of titanium. The best couples were Ti/Gold, Ti/Ter Ti, Ti/ Co-Cr and Ti/Ag-Pd. The least acceptable couples were Ti/amalgam, SSTi/SSTi 00, while the Ti/Ni-Cr couple showed unstable galvanic corrosion behavior. S. mutans causes different types of corrosion reactions. 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 signiþcant. Kasemo and Lausmaa [29] 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 inßuence 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 [30] studied the effects of topical ßuoride 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 ßuorides should be avoided. Nakagawa, et al. [31] studied the relationship between ßuoride concentrations and ph values at which Ti corrosion occurred in the presence of ßuoride 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., [32] it is difþcult 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 97 toothpaste can all inßuence 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, ßuids, and toothbrushes remove ions. Thus, corrosion of the restorations will continue. SUMMARY 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. REFERENCES 1. Anusavice KJ editors. Phillips science of dental materials. 11 th ed. Saunders-Elsevier; p Maruthamuthu S. Electrochemical behavior of microbes on orthodontic wires. Curr Sci 2005;89:

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