Trends Biomater. Artif. Organs, 25(1), 34-46 (2011) http://www.sbaoi.org

Corrosion in Titanium Dental Implants/Prostheses - A Review

Rahul Bhola*, Shaily M. Bhola, Brajendra Mishra and David L. Olson

Dept of Metallurgical & Materials Engineering, 1500 Illinois Street, Golden, Colorado 80401, USA*Corresponding author (rbhola@mines.edu) - Rahul Bhola

Received 31 July 2010; Accepted 3 August 2010; Available online 1 March 2011

The corrosion of biomaterials primarily dental implants/prostheses has a significant clinical relevance. In spite of several recentadvances in metallurgical and materials science and remarkable improvements in the design and development of surgical andprosthetic materials, failures do occur sometimes. The current paper focuses the problem of corrosion in titanium based implantsand prostheses with its clinical significance and emphasis on galvanism induced in-vitro and in-vivo.

Introduction

The general consensus about the most corrosion resistantbiocompatible metallic biomaterials are the special metalsnamely-titanium, niobium, tantalum and their alloys,followed by cobalt based alloys and finally the stainlesssteel grades [1-3]. The most commonly used implant/prostheses material used today has been summarizedbelow in Table 1 with their common names, UNS, ASTM,ISO and alloy designations [4-6]. Commercially puretitanium and its alloys are known for their use in medicalapplication owing to their good corrosion resistance,biocompatibility and bioactivity in the human body[7].Titanium and its alloys have been used as prostheticmaterial in several reconstructive and resectiveprocedures in human body since many years [8]. Theuse of dental implants in the partial and completeedentulism has become the primary treatment regimenin the modern dentistry [9]. These implants appeared asearly as late 1920’s and gained widespread usage duringthe last 2-3 decades.

Although these biomaterials have good mechanical andbiological properties there corrosion resistance is stillcritical for the overall success of the treatment procedure.It has been long recognized that the corrosion productsformed as a result of metal-environment interactions havea significant bearing on the biocompatibility and long termstability of the prostheses/implant. The material usedmust not cause any biological adverse reaction and mustretain its form and properties [10-12] during function.

Human stomatognathus is subjected to varying changesin pH and temperature owing to differences in local,systemic, environmental, economic and social conditionsfor each individual. Corrosion can result from thepresence of a number of corrosive species like hydrogenion (H+), sulfide compounds (S2-), dissolved oxygen, freeradicals (O2, O

-), and chloride ion (Cl-) resulting in themetal surface breakdown and a consequent adversetissue reactions [13].

Figure 1: Prerequisites for an implant biomaterial [17]

Corrosion in Titanium Dental Implants/Prostheses - A Review 35

Nowadays, osteo-integrated implants/prostheses can beregarded as well proven safe alternatives on a large scaleto functionally rehabilitate both partially and completelyedentulous patients. One of the chief success factorsrelated to modern titanium implant system is theachievement of a fast and tight interconnection betweenthe implant surface and the bone tissue owing to its lowthermal conductivity, low density, lower elastic modulusmismatch compared to bone, high hardness, outstandingbiocompatibility and remarkable corrosion resistance [14-16]. The prerequisites of a successful implant system havebeen summarized in Figure.1.

Physical metallurgy of titanium alloys

Titanium is a transition metal with an incomplete d-shellin its electronic structure that enables it to form solidsolutions with most substitutional elements having a sizefactor within 20% (Hume-Rothery’s principles forsubstitutional and interstitial solutions). In its elementalform titanium has a high melting point (1678°C), exhibitinga hexagonal close packed crystal structure (a/alpha) upto the beta transus temperature (882.5°C), transformingto a body centered cubic structure (b/beta) above it[18].

Titanium alloys may be classified accordingly either a ,near a or a + b, metastable b or stable b depending upontheir room temperature microstriucture.[18-19]. In thisregard substitutional alloying elements for titanium fall intothree basic categories

1. Alpha stabilizers like Al, O, N, C

2. Beta stabilizers like Mo, V, Nb, Ta (isomorphous), Fe,W, Cr, Si, Ni, Co, Mn, H (eutectoid)

3. Neutral stabilizing elements like Zr.

Alpha and near alpha titanium alloys exhibit superiorcorrosion resistance but lower strength. On the other handa + b alloys exhibit higher strength due to presence ofboth phases in the microstructure.Their properties dependupon composition, the relative proportions of each phasepresent with their prior thermal treatment and thermomechanical processing conditions. Beta alloys in contrasthave high strength, good formability and hardenability.Beta alloys also offer lower elastic modulus and highercorrosion properties compared to other alpha and mixedalloys there by suggesting lower coefficient of elastic

Table 1: Metallic biomaterials for surgical implant and prostheses. [4-6]

Metallic Biomaterials for Implants/Prostheses Material Designation

CommonName/ Trade Name

UNS Designation

ASTM Designation

ISO Designation

Alloy #

Speciality Metallic Biomaterials Ta, Unalloyed, cast Unalloyed Tantalum (α) R05200 F 560 − 0.0 Zr−2.5 Nb Zircadyne R60705 F 04.12.45 − 2.5 Ni−45Ti Nitinol N01555 F 2063 − 55.0

Titanium Base Biomaterials Ti CP−1 CP−1 (α) R50250 F 67 ISO 5832−2 0.1 Ti CP−2 CP−2 (α) R50400 F 67 ISO 5832−2 0.2 Ti CP−3 CP−3 (α) R50550 F 67 ISO 5832−2 0.3 Ti CP−4 CP−4 (α) R50700 F 67 ISO 5832−2 0.4 Ti−3Al−2.5V Ti−3Al−2.5V (α/β) R56320 F 2146 − 5.5 Ti−5Al−2.5Fe Tikrutan (α/β) Unassigned − ISO 5832−10 7.5 Ti−6Al−4V Ti−6Al−4V (α/β) R56400 F 1472 ISO 5832−3 10.0 Ti−6Al−4V cast Ti−6Al−4V (α/β) R56406 F 1108 − 10.1 Ti−6Al−4V ELI Ti−6Al−4V ELI (α/β) R56401 F 136 ISO 5832−3 10.2 Ti−6Al−7Nb Ti−6Al−7Nb (α/β) R56700 F 1295 ISO 5832−11 13.0 Ti−15Mo Ti−15Mo (Metastable β) R58150 F 2066 − 15.0 Ti−12Mo−6Zr−2Fe TMZF (Metastable β) R58120 F 1813 − 20.0 Ti−11.5Mo−6Zr−4.5Sn Beta 3 (Metastable β) R58030 F 9046 − 22.0 Ti−13Nb−13Zr Ti−13Nb−13Zr (Metastable β) R58130 F 1713 − 26.0 Ti−45Nb Ti−45Nb (Metastable β) R58450 F 04.12.44 − 45.0 Ti−35Nb−7Zr−5Ta TiOsteum (Metastable β) R58350 F 04.12.23 − 47.0

Cobalt Base Biomaterials Co−28Cr−6Mo casting alloy Cast CoCrMo R30075 F 75 ISO 5832−4 34.0 Co−28Cr−6Mo wrought alloy #1 Alloy 1 R31537 F1537 ISO 5832− 12 34.1 Co−28Cr−6Mo wrought alloy #2 Alloy 2 R31538 F 1537 ISO 5832−12 34.2 Co−28Cr−6Mo wrought alloy #3 GADS R31539 F 1537 − 34.3 Co−20Cr−15W−10Ni−1.5Mn L−605 R30605 F 90 ISO 5832−5 46.5 Co−20Cr−20Ni−5Fe−3.5Mo−3.5W−2Ti Syncoben R30563 F 563 ISO 5832−8 49.0 Co−19Cr−17Ni−14Fe−7Mo−1.5Mn Grade 2− Phynox R30008 F 1058 ISO 5832−7 58.5 Co−20Cr−15Ni−15Fe−7Mo−2Mn Grade 1 – Elgiloy R30003 F 1058 ISO 5832−7 59.0 Co−35Ni−20Cr−10Mo 35N R30035 F 562 ISO 5832−6 65.0

Steel Base Biomaterials Fe−18Cr−14Ni−2.5Mo 316 L Stainless Steel S31673 F 138 ISO 5832−1 34.5 Fe−18Cr−12.5Ni−2.5Mo cast 316 L Stainless Steel Unassigned F745 − 34.6 Fe−21Cr−10Ni−3.5Mn−2.5Mo REX 734 S31675 F 1586 ISO 5832−9 37.0 Fe−22Cr−12.5Ni−5Mn−2.5Mo XM 19 S20910 F 1314 − 42.0 Fe−23Mn−21Cr−1Mo−1N 108 Unassigned F 04.12.35 − 46.0

 

36 R. Bhola, S.M. Bhola, B. Mishra and D.L. Olson

mismatch with the bone and favorable stress distributionand outcome.[20].

A beta alloy is fundamentally defined as an alloy whosechemical composition lies above bc as shown in Figure 2that is, it contains sufficient amount of beta stabilizingagents to retain 100% body centered cubic microstructureupon quenching from above the beta transus temperature[20-21]. Alloys lying above this critical minimum level ofbeta stabilizing content may still lie within the two-phaseregion, making the as-quenched beta phase beingmetastable with the vulnerability of precipitating a secondphase upon aging.

Titanium alloys with increasing alloying content, exceedinga critical beta value are considered stable beta alloyswhere no precipitation of the second phase takes placeduring practical long-time thermal exposure. Processvariations are traditionally used to control the alloymicrostructure and therefore to optimize titanium alloyproperties such as ductility, strength, fatigue resistanceand fracture toughness. The effects of variousmicrostructures are then correlated with engineeringproperties and the most common microstructural featuresstudied are the metastable beta alloys, beta grain size,size and age distribution of aged alpha phase. Apart fromthe alpha phase precipitation transient b’ or w phase orseveral other intermetallic compounds may also beobserved in the metastable beta region depending uponalloy composition, heat treatment, processing history andservice conditions [22-23].

General Concepts related to corrosion

There are two essential features determining corrosionof an implant

1. Corrosion Thermodynamics – The application ofchemical thermodynamics are primarily related to theoxidation-reduction reactions for the given electrochemicalsystem. The system tends to attain the lowest energy byreleasing energy during the reaction [24]. It is representedby the potential (voltage) axis on the Evan’s diagram andis given by equation

DG = DG° + RT lnK for the chemical reaction

DE = DE° - lnK Nernst Equation for theelectrochemical reaction

Where DG, DE are the energy and potential change, DG°and DE° are the standard free energy and potentialchange, R is universal gas constant, T is the temperatureunder study, n is the number of valence electrons involvedin the reaction and K is the equilibrium constant of thegiven reaction at a specified temperature.

2. Corrosion Kinetics – The electrochemical kineticsinclude factors that physically, obstruct corrosion fromtaking place [24]. These factors tend to hinder the variouselectrochemical reactions going in the system eitherthrough concentration, temperature or velocity kinetics inthe system. It is represented by log current density axison Evan’s diagram and is majorly driven by theoverpotential equation

hconc = log[1 -

Where, hconc is the concentration overpotential, R isuniversal gas constant, T is the temperature under study,n is the number of valence electrons involved in theelectrochemical reaction, ic is the cathodic current densityand iL is the limiting current density.

Types of corrosion

Uniform corrosion (Generalized corrosion)

A uniform regular removal of the metal from the surfaceis usually the most common mode of corrosion. Thecorrosive environment must have same access to all partsof the metal surface and the metal itself must bemetallurgically and compositionally uniform. At timesgeneral corrosion in aqueous body fluids like phosphatebuffer saline (PBS), ringer’s lactate (RL), normal saline(NS) etc may take the mottled form, severely roughenedmetal surface that resembles localized attack. This unevenlocalized attack results from variations in the corrosionrate of localized surface patches due to localized maskingof metal surfaces by process scales, corrosion products,food lodgment and surrounding and adjacent superstructures. When titanium is in the fully passive condition,corrosion rates are typically less than 0.02 mm/yr (0.8mil/yr) and well below the 0.13 mm/yr (5 mils/yr) maximumcorrosion rate commonly accepted for biomaterial designand application. This minimal acceptable corrosion rateis primarily due to the finite +4 oxidation of titanium alloysowing to the formation of adherent TiO2 film although thesurface oxide is more complex than a single TiO2 oxideover their surface.

General corrosion becomes a concern at hightemperatures in highly acidic environments owing toconsumption of hot, spicy and sticky foods. In strong and/or hot reducing acids (plaque deposits) the oxide film oftitanium can deteriorate and dissolve, and the unprotectedmetal is oxidized to the violet colored soluble trivalent ion(Ti3+) in acid solutions which is further converted to paleyellow Ti4+ ion in presence of oxidizing species which onfurther hydrolysis may form insoluble TiO2 precipitates/scales and inhibiting subsequent corrosion.

Uniform corrosion for titanium implants can be determinedfrom weight loss data (increase or decrease in weight

Figure 2: Pseudo binary phase diagram for titanium beta stabilizer

Corrosion in Titanium Dental Implants/Prostheses - A Review 37

depending upon the environment and by products inaccordance to ASTM G1 & G31), dimensional changes(shape, size, appearance and texture) andelectrochemical methods (anodic and cathodicpolarization, cyclic voltammetry and electrochemicalimpedance measurements).

Corrosion rates in millimeters per year for titanium alloyscan be calculated from weight loss data as under:

Corrosion rate (mm/yr) =

where d is the titanium implant alloy density (in grams percubic centimeter which is approximately 4.51g/cc forc.p.Ti), A is the sample surface area (in squarecentimeters), t is the exposure time (in hours), and W isthe weight change (in grams).

Corrosion rates in millimeters per year can be calculatedfrom electrochemical measurements on the other handusing the equation:

Corrosion rate (mm/yr) =

where icorr is the measured corrosion current density (inmilliamps per square centimeter), d is titanium alloy density(in grams per cubic centimeter), and EW is the equivalentweight for titanium. The equivalent weight for titanium isapproximately 16 under reducing acid conditions and 12under oxidizing conditions depending upon the numberof valence electrons involved. The value of icorr is typicallydetermined from Tafel slope extrapolation or linearpolarization methods [25-26]

Pitting corrosion

Localized corrosion attack in an otherwise resistantsurface produces pitting corrosion. It usually occurs onbase metals which are protected by a naturally formingthin film of an oxide (for instance the firmly adherent TiO2over the Ti surface) when the potential of the film exceedsthe breakdown potential of the oxide in a givenenvironment. In the presence of certain ions like chloridesand sulphides the film locally break downs and rapiddissolution of underlying metal occurs in the form of pits.When the anodic breakdown (pitting) potential of the metalis equal to or less than the corrosion potential under agiven set of conditions, spontaneous pitting can beexpected.

Because of the protective oxide films the titanium implantsurface exhibits anodic pitting potential that are very high(»1V) compared to other biomaterials used (iron, steel,cobalt-chromium alloys etc). Thus pitting corrosion is notof much concern in the oral environment for titanium alloys.For example pitting potentials exceed +5 to +10 V versusthe saturated calomel electrode (SCE) in body fluids likechlorides (NS) and lactates(RL) and typically +60 to +80V in sulfates and phosphates(PBS).Although pittingpotential for titanium dental implants is not of much clinicalsignificance but this property of protection potential orrepassivation can still be used to define the minimumpotential at which pitting can be maintained [27].

Guiding principles under the American Standards forTesting of Materials G3 & G5, electrochemical techniquesinvolving potentiostatic and potentiodynamic testing are

used similar to uniform corrosion measurements to studypitting behavior of titanium dental implants [28-29]. Anodicpitting potential requires slow scan rates (d–0.5 mV/s) andsurface condition of the implant surface under test. Forexample, abraded or sandblasted implants will exhibitsignificantly lower pitting potentials compared to acidpickled implant surfaces. Repassivation potentials on theother hand can be measured using the galvanostatictechnique [27,30] by impressing an anodic current densityof approximately 200 mA/cm2 on the specimen for at leastseveral minutes before measuring the repassivationpotential of the sample.

Weber and his coworkers [31] studied the aspects ofdental corrosion on titanium system using variouselectrochemical techniques on titanium and its alloys withiron as an important constituent in dental media. Thesusceptibility to localized pitting corrosion of titanium andits alloys were evaluated by the breakdown potential,pitting potential, corrosion current density and thecorresponding anodic polarization curves and tafel slopes.

Crevice corrosion

Localized crevice corrosion occurs from the geometry ofthe implant/prostheses assembly. Corrosion of an alloyis greater in the small sheltered volume of the crevicecreated by contact with another material. The other metalcould be part of the fastener of the same or different alloy,a sheltered crown, cement packing or implant prosthesesjoint [32-33]. Crevice corrosion seems to prefer (metal-metal) Ti implant superstructure and base metal crowncrevice with constricted space and oxygen gradient.Titanium alloy implants may be subjected to localizedcrevice attack exposed to short time periods of hot (>70°C, or 160 °F) chloride, bromide, iodide, fluoride or sulfatecontaining solutions during electrosurgery, electrocauteryor thermocautery procedures. The reduction in pH andincrease in crevicular chloride ion concentration are theessential factors in the initiation and propagation of thepits. When the acidity of the micro-environment aroundcrevice increases with time it dissolves the passive oxidelayer thereby causing localized destruction and crevicecorrosion [34].

The mechanism for crevice corrosion in titanium is verysimilar to that seen in stainless steel, in which oxygen-depleted reducing acid conditions develop within tightcrevices [32]. The model for crevice corrosion is illustratedin Figure 3. Dissolved oxygen or other oxidizing speciesin the bulk solution are depleted in the restricted volumeof solution in the crevice. Finite surface oxidation increvices consumes these species faster than diffusionfrom the bulk solution can replenish them as a result metalpotentials within crevices become more active comparedto metal surface exposed to the bulk body tissues. Thissituation creates a micro electrochemical cell in the bonysocket where the shielded crevices become anode andcorrode, and the surrounding more noble metal surfacebecomes cathode and is protected.

Titanium chlorides formed within the crevice are unstableand tend to hydrolyze, forming hydrochloric acid (HCl)and titanium oxide/hydroxide corrosion products. Becauseof the small, restricted volumes of solution within tightimplant-superstructure crevice, low crevice pH levels (0-

38 R. Bhola, S.M. Bhola, B. Mishra and D.L. Olson

1) with high H+ ions concentration can develop. Theselocal reducing acidic conditions can result in severe andrapid localized active corrosion within crevices, dependingon alloy resistance, temperature, and redox potential ofthe surrounding crevicular fluid.

Several oxidizing species such as oxygen, chlorine, ferricion (Fe3+), and cupric ion (Cu2+)from food and saliva, tendto effectively inhibit the general corrosion of exposedtitanium implant surfaces, but most of these tend toaggravate the onset and propagation of titanium alloycrevice corrosion. These ionic species are excellentcathodic depolarizers and rate controlling, thus acceleratecathodic reduction kinetics involved in the electrochemicalprocess. These cationic oxidizing species will not diffuseinto the active crevice to inhibit attack compared to certainanionic oxidizing species such as NO3

-, ClO3-, OCl-, CrO3

2-

, ClO4-, and MnO4

-, which can migrate into the implant-superstructure crevice inhibiting corrosion under halidesolutions.

Crevice corrosion testing of titanium implants in functionis insidious and very rapid, and may leach several ionsinto the crevicular space activating the host complementresponse and causing an adverse reaction that may ormaynot be tolerated.

Several crevice test assemblies are employed includingthe multiple-crevice serrated washer and the sandwich-type crevice test assembly (consisting of 25 to 38 mmsquare flat sheet or plate specimen, with thinpolytetrafloroethylene gaskets interspersed to provide thedesired number of metal-to-metal and metal-to-gasketcrevices) for the laboratory testing of crevice corrosion intitanium implants over varying time duration. In additionto visual and weight change measurements, monitoringof creviced implant potential and current has been usedto a limited extent to identify the initiation of titanium alloycrevice corrosion.[35]

Environmentally induced cracking

Titanium alloy implants/prostheses processed underhydrogen-containing environments and under conditions

in which galvanic couples or cathodic charging (impressedcurrent) causes hydrogen to be evolved on metal surfacesmay cause hydrogen induced cracking (HIC) [36-37] andthe impairment of ductility of implant/prosthesespermanently and irreversibly. The tenacious TiO2 surfacefilm over the implant surface is a highly effective barrierto hydrogen penetration. Small amounts of moisture oroxygen in hydrogen-gas-containing environmentseffectively maintain this protective film, thus avoiding orlimiting hydrogen uptake compared to anhydroushydrogen atmospheres at higher temperatures andpressures causing increased uptake [38-39]. Hydrogenattack is the reaction of hydrogen with carbides in titaniumto form methane, resulting in decarburization, surfacevoids and blisters. These voids are formed when theatomic hydrogen migrates from the surface to internaldefects and inclusions, where molecular hydrogen gascan nucleate, generating internal stresses to deform andrupture the metal locally. In á and á-â alloys, excessivehydrogen uptake can induce the precipitation of titaniumhydride in the á phase. These acicular-appearing hydrideplatelets, as shown in Figure 4 are brittle and have beenwell characterized in literature in detail [38].

Biaxial and triaxial mechanical properties such as ductility,cold-drawing/formability and impact toughness in á andnear-á alloys are very sensitive to hydrogen levels [40-42]. Hydrogen contents above critical levels can result insustained load cracking which drastically reduce theservice life of cracked/notched or etched surfaces ofimplants under functional load of mastication andocclusion [40-42].Beta titanium alloys on the other handhave high solubility for hydrogen, therefore embrittlementis generally not associated with hydride precipitation.Severe loss in ductility and formability may not occur belowseveral thousand parts per million of hydrogen [43]. Thisenhanced tolerance of beta alloys to hydrogen decreaseswith aging (tempering/ normalizing) as more of the á phaseis precipitated. This increased â alloy tolerance must beweighed against higher hydrogen uptake rates that resultfrom the higher hydrogen diffusion coefficient in â titaniumalloys [43]. Covington et al. [39] have characterized thethree conditions that must coexist to cause embrittlement

Figure 3: Illustration showing the mechanism of crevicecorrosion for titanium in aqueous chloride media (NS)

Figure 4: Micrograph of hydrogen embrittled unalloyed titaniumat a magnification of 200×

Corrosion in Titanium Dental Implants/Prostheses - A Review 39

of commercially pure titanium á implant alloys for dentalapplications in aqueous media.

1. A mechanism for generating nascent (atomic) hydrogenon a titanium surface. This hydrogen generation may befrom a galvanic couple, an impressed cathodic current,corrosion of titanium, or severe continuous abrasion ofthe titanium surface in an aqueous medium.

2. Metal temperature above approximately 80 °C (175 °F),above which the diffusion rate of hydrogen into á titaniumbecomes significant [44]

3. Solution pH less than 3 or greater than 12, or impressedpotentials more negative than -0.75 V (versus Ag/AgClreference electrode) [45-47]

No hydrogen uptake and embrittlement problems occurwhen titanium implant is galvanically coupled to fullypassive materials like titanium alloys, resistant (passive)stainless steels, copper alloys, and nickel-base alloys,depending on surrounding environments.

Very high cathodic charging of dental implant titaniumalloys may cause enhanced hydride film formation andpenetration over the implant surface and HIC at roomtemperature. Hot alkaline (>80 °C, and pHe–12),conditions may also result in increase hydrogen uptakeand embrittlement of titanium alloys.

Hydrogen analysis for the titanium implants can be doneusing the electrochemical impedance spectroscopy andmeasuring the relative impedance of the retainedhydrogen as a function of impedance or by the hot vacuumextraction method, where the implant samples are heatedto 1100 to 1400 °C for a stipulated time to reversiblyrelease the absorbed hydrogen, followed by evolved gasmeasurements.

Stress induced cracking

Stress-corrosion cracking (SCC) is a fracture phenomenoncaused by the combined factors of tensile stress, asusceptible alloy, and a corrosive environment. The metalnormally shows no evidence of general corrosion attack,although slight localized attack in the form of pitting maybe visible in certain metals. Usually specific combinationsof metallurgical and environmental conditions cause SCC.This combination of conditions is important because it isoften possible to eliminate or reduce SCC susceptibilityby modifying either the metallurgical characteristics of themetal and/or the environment. Another important aspectof SCC is the requirement for the tensile stress to bepresent, such as those developing from cold work, residualstresses during fabrication/machining, and externallyapplied functional/occlusal loads. Different surfaces of ametallic restoration (implant or crown structure) may havesmall pits and crevices and may be differentially exposedto different stresses consequently leading to stresscorrosion cracking [36-37].The primary idea behind,titanium alloy SCC is the observation that no apparentcorrosion, either uniform or localized, usually occursbefore the cracking process [48-49] as a result it is difficultto translate the real oral situation into a laboratoryexperiment [49].

Over decades several models [49-54] have been proposedto explain the mechanism behind SCC in titanium alloysand broadly fall under two large categories.

1. Anodic-assisted cracking may begin where localizedcorrosion has occurred in the presence of a tensile stress.If corrosion is not rapid to impede the advancing cracktip, the crack will continue to advance into the metal andeventually lead to failure. Once a crack initiates, thebalance among the crack tip corrosion rate, the crack tipenvironment, and the crack tip stress state is critical tocrack propagation.

2. Hydrogen-assisted cracking (HIC) is said to occur byabsorption of hydrogen near the crack tip. Hydrogenabsorption leads to embrittlement of the metal ahead ofthe crack tip and promotes crack formation. The sourceof hydrogen is normally associated with anodic dissolution(that is, from the concurrent cathodic hydrogen-reductionreaction) at freshly exposed metal at the crack tip. As aresult, anodic dissolution in the vicinity of the crack tip isnormally required for this mechanism to operate.

ASTM has formulated the standard practice for testingSCC of titanium alloys in three basic categories [49]:

1. Use of smooth and statically loaded specimens suchas U-bend, C-ring, bent beam, and dead-loaded tensilespecimens.

2. Use of notched and pre-cracked specimens that arestatically or dynamically loaded such as cantilever beambend specimen, compact tension specimens and double-cantilever beam specimens.

3. Use of smooth or notched tensile specimens that aredynamically loaded at relatively low strain rates.

Fretting Corrosion / erosion corrosion

The combination of corrosive fluid (saliva with severalenzymes and food particles) and high velocity in the oralenvironment results in erosion-corrosion or fretting. It isresponsible for most of the metal release in tissue. Conjointaction of chemical (enzymes and proteins) and mechanicalwear (mastication) during function further aggravates theattack [36-37,55-56].In general during the passiveenvironments, the hard and tenacious TiO2 surface filmover the metal surface provides a superb barrier toerosion-corrosion. For this reason titanium alloys canwithstand flowing water velocity as high as 30 m/s withlittle or no metal loss. The ability of the oxide film to repairitself when damaged and the intrinsic hardness of titaniumalloys both contribute to their excellent resistance toerosion-corrosion.

Titanium alloys exhibit relatively high resistance to fluidscontaining suspended solids. Critical velocities forexcessive metal removal depend upon the concentration,shape, size, hardness of the suspended particles, fluidimpingement angle,[57] local turbulence, and titaniumalloy properties.

The typically low concentrations of organic material in oralcavity is of little importance but continuous exposures tolocal changes around the implant during function can leadto finite removal of the metal as well as the cementing

40 R. Bhola, S.M. Bhola, B. Mishra and D.L. Olson

material between the implant and superstructure there bynot only promoting erosion corrosion but crevice andgalvanic corrosion as well.

Intergranular corrosion/cracking

For intergranular cracking to occur reactive impurities maysegregate, or passivating elements may deplete at grainboundaries. This results in grain boundaries as apreferential region for corrosion owing to its highsusceptibility and so the grains might fall out of the surfaceleading to material cracking [36-37]. This type of corrosionis not usually observed in titanium oral implants.

Galvanic corrosion of dental implants

The most common form of corrosion occurring in titaniumimplants is the galvanic corrosion or dissimilar corrosion.Titanium has been chosen as the material of choice forseveral trans-osseous and end-osseous implantations.Long term studies and clinical observations haveestablished the fact that titanium is noble (due to thepresence of adherent oxide) and does not corrode inhuman tissues however galvanic coupling of implant toseveral other metallic restorations may induce one of theseveral forms of corrosion. Thus coupling remains a greatconcern for the metallic superstructures covering theimplant body.

Gold alloys have widely been used as a super structureowing to their excellent biocompatibility, corrosionresistance and mechanical properties. Owing to highercost of the precious metal alloys (noble alloys)used inprosthodontics, it has led to the development of costeffective semi-precious metallic alternatives(non-noblealloys) [58-59] like gold-palladium, nickel-chromium,cobalt-chromium, nickel-titanium and several othertitanium alloys.

Galvanic coupling occurs when two dissimilar metals areplaced in direct contact within the oral cavity (adjacent orcontralateral) or within the tissues. The complex corrosionprocess occurring involves the electrochemical reactionsoccurring at the dissimilar metals interface in presence ofelectrolyte (saliva or oral fluids or body fluids) resulting inthe flow of electric current between them [37,60]. An in-vivo (in the living tissue) electrochemical cell is formedand galvanic current causes the corrosion of active metaland the noble metal is protected. The current also passesthrough the cellular junctions and tissues (desmosomes,hemi-desmosomes and cellular attachments) therebyactivating the proprioceptors causing pain.

As already discussed in the section on hydrogen damageabove attention should be given to possible excessivehydrogen uptake by titanium implant when it is galvanicallycoupled to active metal superstructures. This situation isof great concern in á titanium alloys when temperaturesexceed 80°C in aqueous electrolytes during implantprocessing especially when hydrogen recombinationpoisons, such as sulfides, arsenides and cyanides arepresent.

Phenomena of galvanic corrosion

When two dissimilar metals with different electrodepotentials come in contact in the presence of a corrosive

electrolyte, a potential is generated. The net result is achemical reaction with oxidation occurring at the anodeand reduction occurring at the cathode, the electronicexchange occurs through the contact and ionic exchangeoccurs through the electrolyte.

The electrochemical cell reactions occurring at thedifferent electrodes, depending upon the pH and aerationconditions as well the addition of oxidizers are [37]:

1) Anode (Oxidation)

M ® Mn+ + ne-

2) Cathode (Reduction)

2H+ + 2e- ® H2

4H+ + 4e- + O2 ® 2H2O

2H2O + 2e- ® H2 +2OH-

2H2O + 4e- + O2 ® 4OH-

Mn+ + e- ® M(n-1)+

Thus if a base metal alloy superstructure is provided overthe titanium implant, the less noble metal alloy forms theanode and the more noble titanium alloy is protected as acathode. The electronic exchange occurs through themetallic contact and ionic contact occurs through saliva,mucosa, tissue and bone fluids. Though the hydrogenevolution at the implant surface may further complicatethe situation by pressure under the prosthesis andhydrogen embrittlement of the implant surface.

Mechanisms of Corrosion resistance

The excellent corrosion resistance of titanium and its alloysused for implants is due the formation of athermodynamically stable, continuous, highly adherent,and protective surface oxide film. Since titanium metalitself is highly reactive and has an extremely high affinityfor oxygen, this beneficial surface oxide film is formedspontaneously and instantly when fresh metal surface isexposed to air and/or moisture. In fact, a damaged oxidefilm can generally reheal itself instantaneously if at leasttraces (that is, a few parts per million) of oxygen or waterare present in the environment. Within a millisecond ofexposure to air, a 10´ oxide layer will be formed on thecut surface of the exposed pure Ti which will grow to about100´ thick within a minute.

The nature, composition, and thickness of the protectivesurface oxides that form on titanium alloys depend onenvironmental conditions. In most oral environments theoxide is typically TiO2 but may consist of mixtures of othertitanium oxides as well including TiO2, Ti2O3, and TiO[61].High-temperature oxidation promotes the formation ofdenser, more chemically resistant form of TiO2 known asrutile, whereas lower temperatures often generate a lesscrystalline and protective form of TiO2, anatase, or amixture of rutile and anatase [61].

In dilute reducing acids, a 20 to 100 Å multiplex filmconsisting of a hydrated titanium sesquioxide (Ti2O3) innerlayer and a TiO2 outer layer has been shown to form [62].Increasing redox potential favors TiO2 formation, and

Corrosion in Titanium Dental Implants/Prostheses - A Review 41

increasing exposure temperature and/or time motivateconversion to the more stable crystalline form of TiO2.These naturally formed films are typically less than 10nm thick[63] and are not visible to the human eye. TiO2oxide is highly resistant chemically and is attacked by veryfew chemicals like hot concentrated HCl, H2SO4, NaOH,and HF. This thin surface oxide is also a highly effectivebarrier to hydrogen penetration of the alloy, as is discussedin a later section of this article. Furthermore, the TiO2 filmbeing an n-type semiconductor exhibits increasingelectronic conductivity with increasing temperature. As acathode, titanium readily passes current and permitselectrochemical reduction of ions in an aqueouselectrolyte. On the other hand, very high resistance toanodic current flow (anodic polarization) across thepassive oxide film can be seen in most aqueous solutions.Because the passivity of titanium occurs by the formationof a stable oxide film the corrosion behavior of implantmetal interface can be understood by recognizing theconditions under which this oxide is thermodynamicallystable.

The Pourbaix (potential-pH) diagram [64] for the titanium-water system at 25°C has been shown in Figure 5 anddepicts the broad range around which the passive TiO2film is stable, based on thermodynamic (free-energy)considerations. Oxide stability over the full pH scale isindicated over a wide range of highly oxidizing to mildlyreducing potentials. Oxide film breaks and the corrosionof titanium implant surface occurs under reducing acidicconditions. Under strongly reducing (cathodic) conditions,titanium hydride formation is predicted. This range of oxidefilm stability and passivation is relatively insensitive to thepresence of chlorides, explaining the high innateresistance of titanium to aqueous chloride environments[65].

The nature of the oxide film on titanium alloys remainsunaltered in the presence of minor alloying constituents.Small additions (<2 to 3%) of most commercially usedalloying elements or variations in interstitial impurities havelittle effect on the corrosion resistance of titanium innormally passive environments. Increasing the alloyingiron and sulfur content may increases corrosion ratesexceeding ~0.10 mm/yr (3.9 mils/yr) [66]. Thus minorvariations in alloy chemistry may be of concern only underconditions in which the passivity of titanium is borderlineor when the metal is actively corroding.

Clinical significance of corrosion

Although titanium alloys have better corrosion propertiescompared to Co-Cr and stainless steel (other implantmaterials) their corrosion leads to dissolution of titaniumand other alloying elements like aluminum, vanadium,niobium, molybdenum etc [67] causing localized togeneralized host response as illustrated in Figure 6. Theleached ions may induce potentially osteolytic cytokinesinto tissues leading to implant loosening [67] and mayeven cause severe allergic reactions or hypersensitivity[67]. Incidence of tumors and malignancies has also beenreported in the literature but are few.

Fracture of Dental Implant

Fracture of dental implant/prostheses is a very rarephenomenon more often associated with mechanicalfunction and previously use of screw preload systems to

Figure 6: Host tissue response to an implant material

Figure 5: Pourbaix (potential-pH) diagram for the titanium-water system at 25 °C

42 R. Bhola, S.M. Bhola, B. Mishra and D.L. Olson

clamp flat to flat abutment implant junctions. They canhave serious clinical complications. Corrosion canseverely limit the fatigue strength and ultimate tensilestrength of the material leading to its mechanical failure.

As reported by Green[68], the end-osseous implantsuperstructures, leached metal ions into the surroundingtissues as an event to corrosion, leading to fatigue fracture,following four years of functional loading into the oralcavity. Yokoyama et al.[ 69] on the other hand investigatedthe delayed fracture of titanium implant into the oralenvironment owing to hydrogen embritlement andenvironmentally induced cracking (EIC).

Bone loss and osteolysis

Olmedo, Fernandez and Guglidomotti [70] havespeculated the corrosion induced ionic leaching to beresponsible for peri-implantitis and treatment failure.

As already explained in Figure 6. the particles that areleached as a result of corrosion process arephagocytosized by macrophages (under the influence ofhost response) and release mediators of inflammation inthe form of cytokines (host defense) [67] through theinflammatory cascade which inhibit the production ofosteoblasts causing increased activity of the bonedestroying cells leading to peripheral osteolysis andloosening of the implant.

Localized pain and inflammation (swelling)

Watterhehn et al .[71] have studied the effect of ionicrelease as a result of corrosion, causing localized painand swelling with or without infection in the region ofimplant insertion. Figure 7 shows the localizedinflammatory response to a freshly inserted implant in theright lateral incisor region (12 region). [71]

Cytotoxic tissue response

Corrosion release of the several substitutional alloyingelements from various titanium alloys used in dentistryhave been widely known in literature. Watterhehn et al.[71-76] have reported these metal ion release to beassociated with carcinogenic and mutagenic activity ofthe oral cavity as shown in Figure 8.

Several studies have further shown that the cellular uptakeof hexavalent chromium is many folds greater than thetrivalent chromium ion and its increased uptake causesreduction of the alkaline phosphatase activity of theosteoblastic cells [77-79].

In-Vitro Studies

Several clinical cases of galvanic corrosion have beenreported in the literature. Ravnholt et al.[ 80] studied thegalvanic corrosion of titanium implants in contact withamalgam and cast metal alloys. As a continuation of theirwork Ravnholt and Jensen et al. [ 80-81] reported nochanges in current or pH when titanium superstructureswere in metallic contact with gold, cobalt-chromium, silverpalladium and composite carbon alloys. Geis-Gerstorferet al.[31] stated the importance of galvanic corrosion inimplant system in two ways; first the possibility of biologicaleffects like discoloration, pigmentation, localized reactionoccurring from alloy components and secondly theosteolysis occurring as a mediation to phagocyte cascadeactivation.

Recalru and Meyer [60] studied the potential alloys thatcould be used as implant superstructure and concludedthat an alloy that is potentially usable for superstructuresin galvanic coupling to titanium must fill the followingrequisites

1. During coupling the titanium should have weak anodicpolarization

2. The current generated during galvanism should be weak

3. The crevice potential must be much higher incomparison to common potential

Recently, Venugopalan and Lucas [82] defined theparameters for an acceptable couple combination to fulfillthe following

1. The difference in the OCP (open circuit potential) andthe corrosion current density (coupled materials) shouldbe as small as possible

2. The breakdown potential of the anodic componentshould be significantly noble than the coupled corrosiondensity of the combination

Figure 8: Malignancy formation as a response to adjacentimplant leach (corrosion)

Figure 7: Localized inflammatory response in a 17 year oldadult as a result of ionic leaching from an implant in 12region.

Corrosion in Titanium Dental Implants/Prostheses - A Review 43

3. The repassivation potential of the anodic componentof the couple should be as similar as possible with minimalhysteresis

Johanson et al. [83] studied the effect of fluoride oncorrosion behavior of machined and cast titanium withvarying surface treatments and exposed area in contactwith conventional and high copper amalgams. Heobserved higher current density for conventional amalgamin contact with titanium and concluded that surfacepreparations and anionic inclusions affect corrosionbehavior of titanium implants.

In-Vivo Studies

Despite the use of titanium and its alloys as implants inhuman body increasing evidence is found that titaniumand various substitutional alloying elements leach into thecrevicular space around the implant [11-12,84-86].

Titanium like the other active-passive metals is coveredby a protective air formed oxide of TiO2 which is adherentand stable. Although the oxide coating isthermodynamically stable, any form of chemical,electrochemical and mechanical trauma can lead torelease of alloying elements through passive dissolutionprocess.

Ferguson and coworkers [87] first documented theelevated titanium levels locally around the implantperiphery. Bumgardner et al. [ 88] reported the increasedrelease of metal ions through a gallium titanium galvaniccouple causing increased implant and tissuedeterioration.Cortada et al.[89] used the ICP-AES(inductively coupled plasma- atomic emissionspectroscopy ) technique to confirm the release of metallicions during titanium implant galvanism.

Nakagawa et al. [90] studied the effect of fluoridationduring prophylactic therapy on the corrosion behavior ofan implant and reported higher corrosion current densitywith increasing anionic concentrations. Extended work hasbeen done by Bhola et al. [ 91] to study the effect ofKnutson fluoridation on the corrosion behavior of titanium-niobium dental implant alloys in normal saline.

Kirkpatrick et al. [ 92] highlighted the patho-physiologyand patho-mechanics behind impaired wound and tissuehealing under metallic ions release during corrosion.

Dental corrosion Mitigation

With the development of corrosion science and advancesin material engineering and design several strategies havebeen employed today to prevent in-use (within the oralcavity) and out-use (outside oral cavity) corrosion oftitanium implants and prostheses.

1. Alloying titanium with noble metals (platinum-groupmetals) which facilitates cathodic depolarization bycatalyzing the hydrogen reduction step in acid solutionsused during active processing of dental implants in theindustry for out-use prevention.

2. Alloying titanium with more thermodynamically stableacid-resistant elements such as molybdenum, zirconium,tantalum, niobium used during processing of dental

implants, also makes the implant surface morebiocompatible and cell adherent for in use prevention.

3. Addition or presence of soluble oxidizing ions orcompounds (cathodic depolarizers) in the oral media (e.g.,Fe3+, O2, Cl2, NO3

-), can be used during implant insertionand placement using NS, RL, PBS etc, for in-useprevention.

4. Noble metal surface treatments (platinum-group metalcoatings or enriched/modified surfaces) can be usedduring implant surface processing for in-use corrosionprevention.

5. Anodic protection via impressed positive potentials froman external power source or from direct galvanic couplingwith a noble metal (platinum-group metal) can be usedduring processing for out-use prevention.

6. Thermal oxidation or nitriding of titanium surfaces canbe used during implant manufacturing and processing tofacilitate in-use prevention.

To summarize several techniques do exist to effectivelyreduce the corrosion in dental implants, but we can mainlyrely on material selection and coating the surface for aneffective in-use clinical performance into a patient’s oralcavity.

Conclusion

The metallic titanium dental implants/prostheses used indentistry today derive their biocompatibility from thealloying elements responsible for the formation of acontinuous stable TiO2 passive film on its surface.

There is a significantly small release of alloying ions evenunder the ideal conditions of passivity and with no damageto the implant surface. Corrosion of these implants mayoccur when the oral conditions are unfavorable as undermechanical trauma to the implant surface (duringplacement, subject induced, and trauma to assault) or theuse of inappropriate metal combination as auxiliaryprostheses (galvanism).

The potential adverse effects of metal ion release intoliving tissues can be proposed based on information fromliterature and various clinical, preclinical and animal trialstudies in-vivo and in-vitro. The results of in-vivo and in-vitro testing do not necessarily take into account all of theprotection mechanisms and physiological host responsecharacteristics of the actual implant environment in oralcavity.

It is clear that corrosion is bound to occur and itsconsequences can be quite severe. Our current researchstandards, regulations for biocompatibility testing, materialand design understanding and composition of metals andits alloys suitable to a special application have drasticallyreduced the incidence of implant failures in oralphysiological environment under corrosion and its adverseevents.

44 R. Bhola, S.M. Bhola, B. Mishra and D.L. Olson

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