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Corrosion in the Human Body: The Ultimate Bio-Corrosion Scenario by Douglas C. Hansen

t is not something one usually thinks about until it becomes a personal Ihealth issue for most of us. That is, the need for the implantation of a medical device (Fig. 1) to improve our existing medical condition. The majority of these devices are made of a metal (Table I), and for most of us, any concerns about corrosion implications from these implants are secondary to the relief we feel from the anticipated beneficial effect these devices are designed to have. As the global population increases in age, there is a parallel increase in the number of implantation procedures. One study reports that world-wide sales of orthopedic implants alone in 2003 was $8.7 billion and projected to increase at an annual growth rate of 12.5% and reach $17.9 billion by 2009.1 Clearly, as new devices and technologies are developed, there will be a continuing need for the Fig. 1. Knee and hip implant components. (Photo courtesy of Medcast Inc.) understanding and characterization of how metal surfaces of implants interact resulting in the rejection of the implant heat treatment, cold working, and surface with their surrounding physiological by the surrounding tissue, or both.3 In finishing, where surface treatments are environment. either case, explantation of the device is particularly important for corrosion Implant alloys are typically derived usually required to correct the situation. and properties. Since are from three materials systems: stainless The human body is not an inherently susceptible to corrosion, , - based alloys and environment that one would consider implants are routinely pre-passivated alloys.2 The question “Does hospitable for an implanted metal alloy: prior to final packaging using an corrosion of a metallic implant cause a highly oxygenated saline bath or some other electrochemical a clinically relevant problem?” is one at a pH of around 7.4 and a process (titanium alloys),4 or that probably only an electrochemist of 98.6°F (37°C). While it is well known an method (stainless or materials engineer will ask when that solutions are among the and cobalt alloys).5 Alloys specific confronted with the prospect of having most aggressive and corrosive to metals, for the intended uses of the implant are a metal device implanted into his or her the ionic composition and protein determined based upon whether they body. While numerous issues may arise concentration in body fluids complicate will be load bearing (wear and fretting with the implant following surgery, one the nascent understanding of biomedical resistant) or not. Finally, galvanic couples of the most fundamentally important is corrosion even further. Variations in are routinely encountered in static (i.e., the interaction between the surrounding alloy compositions can to subtle no relative motion) situations where the physiological environment and the sur- differences in mechanical, physical, or consideration of the potential difference face of the implant itself. This interaction electrochemical properties. However, between the metals involved is secondary can lead to either the failure of the these differences are minor compared to the required strength and implant to function as it was intended, with the potential variability caused by strength: weight ratio of the implant or have an adverse effect on the patient differences in fabrication methodology, device, such as screws anchoring a bone fixation plate. Table I. Major biomedical metals and alloys and their applications. The aim of this article, therefore, is to give the reader a broad overview of Material Major Application the different types of metals and alloys used, the corrosion of metals in the 316L Stainless Steel cranial plates, orthopedic fracture plates, human body, the different environments dental implants, spinal rods, joint encountered and how well these ma- replacement prostheses, stents, catheters terials resist degradation in the body. Cobalt-Chromium alloys orbit reconstruction, dental implants, orthopedic fracture plates, heart valves, Implant Materials spinal rods, joint replacement prostheses The fundamental requirement for Titanium, cranial plates, orbit reconstruction, choosing a metallic implant material is Nitinol, maxillofacial reconstruction, dental that it be biocompatible, that is, not Titanium alloys implants, dental wires, orthopedic fracture exhibiting any toxicity to the surroun- (Ti-6Al-4V, Ti-5AL-2.5 Fe, Ti-6Al-7Nb) plates, joint replacement prostheses, ding biological system. For more than stents, ablation catheters a hundred years, various metals have been investigated for implantation into

The Electrochemical Society Interface • Summer 2008 31 Hansen (continued from previous page)

Table II. Mechanical properties of implant alloys and human bone.

Material Tensile Strength Yield Strength Vickers Hardness Young’s Modulus Limit 2 2 2 2 (MN/m) (MN/m) (Hν) (GN/m) (GN/m) 316L SS 650 280 190 211 0.28 Wrought 1540 1050 450 541 0.49 Co-Cr Alloy

Cast 690 490 300 241 0.30 Co-Cr Alloy

Ti-6Al-4V 1000 970 --- 121 ---

Human Bone 137.3 --- 26.3 30 --- the human body, such as aluminum, quantitative corrosion measurements of , , and carbon steels, implants are made in vivo. However, to silver, nickel, and .3 All of maintain reproducibility and minimize these were discarded as being too reactive variables, very few in vitro studies involve in the body for long term implantation. simulated body fluids that contain When stainless steel was introduced into amino , proteins and at the general engineering as a new corrosion- proper temperature and pH, simply resistant material in the early 1900s, it due to the complexity of the system and was soon utilized in surgical applications. the inherent difficulty of reproducing However, the 18-8 stainless steel that that system in the laboratory. While this was initially used was found to exhibit approach may appear to be flawed, the due to high overall ranking of biomaterials in terms (0.08%) carbon content and gross pitting of corrosion resistance tested in vitro due to low content. Of all does not change when compared to the the stainless steels, only the austenitic measurement of the same biomaterials molybdenum-bearing 316 was of any Fig. 2. Dental implants showing anchors and in vivo (although in quantitative terms, use, even though it was described as dental prostheses. (Image courtesy of BioHorizons, corrosion rates for each specific alloy may inherently corrodible.6 Movement toward Inc.) rise or fall).3 316L alloy, having a much lower carbon of the human body, this environment content (0.03%), greatly reduced the risk can contain water, complex organic Implant Corrosion Mechanisms of intergranular attack. compounds, dissolved , sodium, During the same period of time, chloride, bicarbonate, potassium, cal- The types of corrosion that are cobalt-chromium and cobalt-chromium- cium, magnesium, , amino pertinent to the currently used alloys are: molybdenum alloys were first introduced acids, proteins, plasma, lymph, saliva pitting, crevice, galvanic, intergranular, and utilized in dental and orthopedic etc. Upon implantation, the tissue stress-corrosion cracking, corrosion applications due to their corrosion environment is disturbed, disrupting fatigue, and fretting corrosion. These resistance. The most corrosion resistant blood supply to the surrounding tissue corrosion types will be discussed in of the implant materials presently em- and the ionic equilibrium. The initiation relation to the specific alloys and their ployed is titanium and its alloys. Titanium of corrosion can be the result of various occurrence. alloys were first used in the 1960s and conditions existing along the implant Titanium alloys.—The shape memory their use has been growing steadily since surface, whether it is the formation of alloy, Nitinol, is composed of near the mid-1970s and continues to increase. localized electrochemical cells resulting equi-atomic amounts of Nickel and Several titanium alloys (α & β phases), in pitting attack, or crevice corrosion Titanium. Since the early 1970s it such as Ti-6Al-4V, Ti-5Al-2.5Fe, and at the interface between a plate and a has found widespread clinical use as Ti-6Al-7Nb provide ideal strength and locking screw, or any one of the other corrosion resistance characteristics. The forms of corrosion that main advantage of titanium and its alloys can occur, which will be is the non-reactivity of the passive film discussed later. that is formed; the main disadvantages are its susceptibility to fretting as well Corrosion Testing as oxygen during fabrication, 7 causing embrittlement. The mechanical Numerous methods properties of the alloys discussed here are 8 have been used to evaluate presented in Table II. the corrosion resistance of implant materials in Biological Environment the laboratory, with the majority involving either When a metal device is implanted qualitative measurements of into the human body, it is continually implantation of devices into exposed to extracellular tissue fluid experimental animals (in (Fig. 2, for example). The exposed metal vivo) or quantitative electro- surface of the implant undergoes an chemical measurements electrochemical dissolution of material at in simulated body fluid (in 9 a finite rate, due to interactions with the vitro) or a combination of Fig. 3. Fully expanded endovascular Nitinol stent. (Image courtesy surrounding environment. In the case both where qualitative and of FDA)

32 The Electrochemical Society Interface • Summer 2008 an orthodontic material10 and more elevated cobalt levels in the blood recently as vascular stents due to its due to corrosion in patients having exceptional mechanical characteristics mixed-alloy modular metal-on- and its high biocompatibility11 (Fig. 3). polyethylene hip implants.16 Several studies have highlighted the 316L stainless steel.—Surgical grade variation in the corrosion performance 316L implants (Fig. 5) corrode in the of Nitinol depending upon the surface human body environment and release condition of the test specimens used Fe, Cr and Ni ions and these ions and the surface condition given.12, 13 are found to be powerful allergens Since heat treatment is involved and carcinogens.24 Studies on retrieved during the manufacturing process, implants show that more than 90% of the passivating present on the failure of implants of 316L stainless Nitinol is polycrystalline in nature, steel are due to pitting and crevice and has been found to exhibit severe corrosion attack.25 This fact alone pitting and crevice corrosion, whereas deems that a better material be used for surface treatment to form amorphous even temporary implant devices. oxide results in excellent corrosion The corrosion of 316L in the human resistance.14 Other surface treatments, body can take many forms and the such as electrochemical polishing, following are the more important Fig. 4. Orthopedic hip replacement implant showing has also been found to be a good corrosion mechanisms that have been femoral stem and head. (Image courtesy of Wines identified.3 surface treatment prior to implantation, Medical) resulting in significantly increased Intergranular corrosion.—More than 30 years ago, heterogeneous inter- corrosion resistance and extremely low the effect that carbide inclusions granular distribution of carbon levels of Ni dissolution, well below the have on the corrosion behavior of the was observed in surgical grade 316, estimated average dietary intake levels alloy. Results indicate that while the 11, 15 resulting in intergranular corrosion due of 200–300 μg per day. inclusions were significant features on to the formation of chromium carbides. Titanium-aluminum- the alloy surface, they did not affect the Since then, surgical specifications alloys have exhibited very good corrosion or dissolution mechanisms, have demanded lower and lower corrosion resistance, but are subject rather the presence of proteins caused carbon content. It is only when the to fretting and wear, with particles of ligand-induced dissolution thereby carbon content of austentitic stainless the alloy found in surrounding tissue, increasing the Cr concentration in the steel is below 0.03% are the carbides rather than precipitated corrosion surrounding extracellular tissue fluid.21 reproducibly absent, thus greatly products due to uniform or localized Laboratory studies where Co-Cr-Mo 16, 17 reducing the risk of corrosion.26 corrosion. A current problem related alloy has been immersed in a simulated Pitting.—Pitting is the most common to orthopedic alloys is corrosion at body fluid (Hank’s solution) form of corrosion arising from the the taper connections of modular joint showed that cobalt dissolved from the breakdown of the passivating oxide replacement components. With the surface and the remaining surface oxide film, which can be enhanced by the large and increasing number of total consisted of chromium oxide (Cr+3) presence of proteins in the tissue fluid joint designs that include metal-on- containing molybdenum oxide (Mo+4, and serum.27, 28 metal conical taper connections, the Mo+5, and Mo+6).22 XPS analysis of the Fretting.—Corrosion products effect of crevices, stress and motion take samples in that study revealed that due to fretting on 316L immersed in on increasing importance. Retrieval chromium and molybdenum were more extracellular tissue fluid are studies have shown severe corrosion widely distributed in the inner layer containing chromic chloride and attack can occur in the crevices formed than in the outer layer of the oxide 18, 19 by these tapers in vivo. Gilbert, et al. film. In body fluids, cobalt reported that approximately 16– 35% is completely dissolved, of 148 retrieved total hip implants and the surface oxide showed signs of moderate to severe changes into chromium corrosive attack in the head-neck taper oxide containing a small 18 connection. Dental implants and root amount of molybdenum pins made of this alloy are used by oxide. dental surgeons due to its low corrosion There is little infor- rate, however particular care must mation in the literature be taken to avoid galvanic couples, about cobalt levels and particularly between pure titanium and metal-on-metal bearing 20 the Ti-6Al-4V alloy. for total hip replacements. Cobalt-chromium-colybdenum alloys.— Coleman, et al. reported These alloys are being used in ortho- an increase in the level pedic implants due to their hardness, of cobalt in the blood strength and resistance to corrosion and in the first year after wear (Fig. 4). There are three different implantation of an all types of Co-Cr-Mo material currently metal cast Co - Cr - Mo in use: cast (low carbon), wrought, and hip prostheses.23 Wear wrought (high carbon) alloys. Each at the bearing surfaces variety has a different microstructure seems to be responsible and different properties optimized for for generating release of a specific design or function. The cast the cobalt, but corrosion alloy is used for complicated shapes that of the implant materials cannot be machined (such as the stem or of the wear particles of a total hip replacement) whereas may also contribute to the the femoral head can be machined release of cobalt into the from the harder wrought (high carbon) surrounding tissue fluid. alloy. High and low carbon Co-Cr-Mo However, there has been alloys have been studied to determine Fig. 5. 316L Stainless steel bone fracture fixation plate and screw. a correlation between (Image courtesy of Synthes, Inc.)

The Electrochemical Society Interface • Summer 2008 33 potassium dichromate29 as well as practice. That is, the of the alloy 15. J. Ryhanen, E. Niemi, W. Serlo, E. variable amounts of calcium, chloride, with hydroxyapatite plays a dual role: Niemela, P. Sandvik, H. Pernu, and and phosphorous, with nickel and minimizing the release of metal ions T. Salo, J. Biomed. Mater. Res., 35, 451 manganese being absent, indicating by making it more corrosion resistant, (1998). preferential release of these metal ions as well as making the surface more 16. J. J. Jacobs, J. L. Gilbert, and R. M. into the surrounding solution.30 These bioactive and stimulating bone growth. Urban, J. Bone and Joint Surg., 80-A, results indicate that for 316L implant Other surface modification techniques, 268 (1998). surfaces, nickel and manganese are such as hard , laser nitriding, 17. J. Black, H. Sherk, H. Bonini, W. R. depleted in the oxide film and that the bioceramics, -implantation, and Rostoker, F. Schajowicz, and J. O. surface oxide composition changes to biomimetic coatings and materials Galante, J. Bone and Joint Surg., 72-A, mostly chromium and iron oxide with all have great potential to improve 126 (1990). a small percentage of molybdenum the performance characteristics of 18. J. L. Gilbert, C. A. Buckley, and J. J. oxide in the human body. biomedical implants and improving Jacobs, J. Biomed. Mater. Res., 27, 1533 Crevice corrosion.—316L is highly the lives of their recipients. It is (1993). susceptible to crevice corrosion attack as becoming clear that there are real risks 19. E. B. Mathiesen, J. U. Lindgren, G. compared to the other implant alloys.31 associated with the use of metals as G. A. Blomgren, and F. P. Reinholt, The occurrence of corrosion on the long term chronic implant devices, J. 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Herrington, and J. load conditions (i.e., relative motion). 517 (1988). T. Scales, Br. Med. J., 1, 527 (1973). In cases where there are galvanic 3. D. F. Williams, Annu. Rev. Mater. Sci., 24. F. Silver and C. Doillon, in couples arising from the combination 6, 237 (1976). Biocompatibility: Interactions of of dissimilar metals, such as 316L 4. ASTM F-86 04, Standard Practice Biological and Implantable Materials, stainless steel and the Co - Cr - M alloy or for Surface Preparation and Marking VCH Publishers, New York, Vol.1 Ti-6Al-4V alloy, the stainless steel will of Metallic Surgical Implants, ASTM (1989). be attacked and these combinations International, West Conshohocken, 25. M. Sivakumar and S. Rajeswari, J. should be avoided. Galvanic effects can PA (2004). Mater. Sci. Lett., 11, 1039 (1992). also occur by using metal alloys that 5. ASTM A 967-01, Standard Specification 26. D. Williams and R. Roaf, Implants in have undergone slightly different metal for Chemical Treatments Surgery, Saunders, London (1973). processing (cast vs. wrought Co - Cr - Mo). for Stainless Steel Parts, ASTM 27. R. L. Williams, S. A. Brown, and K. The placement of polymeric inserts International, West Conshohocken, Merritt, Biomaterials, 9, 181 (1988). between metal-metal interfaces will PA (2001). 28. A. Kocijan and I. Milosev, J. Mater. eliminate galvanic corrosion and would 6. W. W. Tennese and J. R. Calhoon, Sci. Mater. Medicine, 14, 69 (2003). considerably reduce fretting corrosion, Biomater. Med. Devices Artif. Organs, 29. J. Walczak, F. Shahgaldi, and F. however alloy selection is critical since a 1, 635 (1974). Heatley, Biomaterials, 19, 229 (1998). crevice situation would certainly be the 7. U. K. Mudali, T. M. Sridhar, and B. 30. J. E. Sundgren, P. Bodo, A. Berggen, result and a more aggressive corrosion Raj, Sadhana, 28, 601 (2003). and S. Hellem, J. Biomed. Mater. 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Res., 15, corrosion had weakened the device, Susceptibility of Small Implant 731 (1981). thus facilitating the fracture.37 Devices, ASTM International, West 35. P. Sury, Corrosion Sci., 17, 155 (1977). Conshohocken, PA (2006). 36. R. J. Gray, J. Biomed. Mater. Res. Symp., Conclusions 10. G. F. Andreasen and T. B. Hilleman, 5, 22 (1974). J. Am. Dent. Assoc., 82, 1373 (1971). 37. J. Brettle, Injury, 2, 26 (1970). 11. Carroll, W. M. and M. J. Kelly, Corrosion is one of the major issues J. Biomed. Mater. Res., 67A, 1123 resulting in the failure of biomedical (2003). About the Authors implant devices. The nature of the 12. D. J. Wever, A. G. Veldhuizen, J. de passive oxide films formed, and the Vries, H. J. Busscher, D. R. Uges, and Douglas C. Hansen is a Senior Research mechanical properties of the materials J. R. van Horn, Biomater., 19, 761 Scientist at the University of Dayton Research Institute and holds a joint appointment as a form some of the essential criteria for (1998). Professor of Graduate Materials Engineering selection of alternative or development 13. R. Venugopalan and C. Trepanier, and Chemical Materials Engineering at the of new materials. In clinical terms, the Min Invas. Ther. Allied Technol., 9, 67 biggest improvements could be made University of Dayton School of Engineering. (2000). His research interests are in the areas of by better material selection, design, and 14. C.-C. Shih, S-J. Lin, K-H. Chung, Y-L. quality control to reduce, or possibly biological and coatings as corrosion Chen, and Y-Y. Su, J. Biomed. Mater. inhibitors, biomaterials, and biomedical eliminate corrosion in implant devices. Res., 52, 323 (2000). Surface modification of 316L stainless corrosion of implant devices. He may be steel is one alternative that is already in reached via email at: douglas.hansen@udri. udayton.edu. 34 The Electrochemical Society Interface • Summer 2008