CHAPTER

3 Degradation of Materials in the Biological Environment

• The environment to which biomaterials are exposed during prolonged use (i.e., the internal milieu of the body) can be described as an aqueous medium containing various anions, cations, organic substances, and dissolved oxygen.

• The anions are mainly chloride, phosphate, and bicarbonate ions. The principal cations are Na+, K+, Ca2+, but with smaller amounts of many others.

• Table 1 (Ch.6.3) presents the range of values for the anion and cation concentrations in blood plasma and extracellular fluid.

• This represents an environment with a chloride concentration of approximately a third of that of sea water. The concentration of dissolved oxygen also influences the aggressive nature of the environment and in venous blood is approximately a quarter of that in air.

• The organic substances include low-molecular-weight species as well as relatively high-molecular-weight proteins and lipids.

• Table 2 (Ch.6.3) gives examples of the concentration of various organic components of blood plasma.

• The protein content of the environment is known to have a significant influence on the corrosive nature of body fluids.

3.3 DEGRANDATIVE EFFECTS OF THE BIOLOGICAL ENVIRONMENT ON METALS AND CERAMICS

• The pH in this well-buffered system is around 7.4, although because of inflammation it may change for short periods following surgery to as low as 4 or 5. The temperature remains constant around 37°C.

• On the basis of existing knowledge of the stability of materials in various environments, we should predict that metals, as a generic group, should be relatively susceptible to corrosion in this biological environment, whereas ceramics should display a varying susceptibility, depending on solubility.

• This correlates fairly well with experimental observations and clinical experience, since it is well known that all but the most corrosion-resistant metals will suffer significant and destructive attack upon prolonged implantation.

• Also, even the most noble of metals and those that are most strongly passivated (i.e., naturally protected by their own oxide layer) will still show some degree of interaction.

• The important passivating implant alloys and their compositions are presented in Ch.6.3: Table 3.

• There are some ceramics that have a combination of very strong partially ionic, partially covalent bonds that are sufficiently stable to resist breakdown within this environment, such as the pure simple oxide ceramics, and others in which certain of the bonds are readily destroyed in an aqueous medium so that the material essentially dissolves, for example certain calcium phosphates.

• With these general statements in mind, we have to consider the following questions in relation to the corrosion and degradation of metals and ceramics:

1. Within these groups, how does the susceptibility to corrosion and degradation vary; by what precise mechanisms do the interfacial reactions take place; and how is material selection (and treatment) governed by this knowledge?

2. Are there variables within this biological environment other than those described above that can influence these processes?

3. What are the consequences of such corrosion and degradation phenomena? • We review each of these questions in this chapter. It is particularly important

to bear in mind some general points as these questions are discussed. 1. Material selection cannot be governed solely by considerations of stability,

and mechanical and physical properties especially may be of considerable importance. Since corrosion is a surface phenomenon, however, it may be possible to optimize corrosion resistance by manipulation of the bulk chemistry. This offers the possibility of developing sufficient corrosion resistance in materials of excellent bulk mechanical and physical properties.

一. Thus, noble metals such as gold and platinum are rarely used for structural applications (apart from dental restoration) because of their inferior mechanical properties, even though they have excellent corrosion resistance; instead, base metal alloys with passivated or protected surfaces offer better all-around properties.

2. Medical devices are not necessarily used in mechanically stress-free conditions and indeed the vast majority of those using metals or ceramics are structurally loaded. It is well known that mechanical stress plays a very important role in the corrosion of metals and the degradation process in ceramics, both potentiating existing effects and initiating others. This has to be taken into consideration.

3. We cannot expect the biological environment to be constant. Within the overall characteristics described earlier, here are variations (with time, location, activity, health status, etc.) in, for example, oxygen levels, availability of free radicals, and cellular activity, all of which may cause variations in the corrosive nature of the environment. Most important, corrosion is not necessarily a progressive homogeneous reaction with zero-order kinetics. Corrosion processes can be quiescent but then become activated, or they can be active but then become passivated and localized, with transient fluctuations in conditions playing a part in these variations.

4. The effects of corrosion or degradation may be twofold. First, and in the conventional metallurgical sense, the most obvious, the problem can lead to loss of structural integrity of the material and function. This may be undesirable, as in the case of many long-term prostheses, or desirable, as in devices intended for short-term function (e.g., ceramics for drug delivery systems) or where the material is replaced by tissue during the degradation process, as with ceramic bone substitution. In addition to this, however, and usually of much greater significance with biomaterials, when released into the tissue, the corrosion or degradation products can have a significant and controlling effect on that tissue. Indeed, it is likely that the corrosion process is the most important mediator of the tissue response to metallic materials. It is therefore important that we know both the nature of the reaction products and their rate of generation. In this respect it is important to recognize that a very small release of certain metallic ions that cause adverse biological reactions may be more significant than a larger amount of a less stimulating by product of corrosion or degradation.

METALLIC CORROSION Basic Principles • The most pertinent form of corrosion related to metallic biomaterials is

aqueous corrosion. This occurs when electrochemical reactions take place on a metallic surface in an aqueous electrolyte.

• There are always two reactions that occur; the anodic reaction, which yields metallic ions, for example, involving the oxidation of the metal to its salt:

M→ M(n+) + n(electrons) (1) • and the cathodic reaction, in which the electrons so generated are consumed.

The precise cathodic reaction will depend on the nature of the electrolyte, but two of the most important in aqueous envitonments are the reduction of hydrogen:

2H+ + 2e- → H2 (2) • and the reduction of dissolved oxygen: O2 + 4H + + 4e- → 2H2O (3) in acidic solutions or: O2 + 2H2O + 4e- → 4OH - (4) in neutral or basic solutions.

• In all corrosion processes, the rate of the anodic or oxidation reaction must equal the rate of the cathodic or reduction reaction. This is a basic principle of electrochemically based metallic corrosion.

• It also explains how variations in the local environment can affect the overall rate of corrosion by influencing either the anodic or cathodic reactions.

• The whole corrosion process can be arrested by preventing either of these reactions. From a thermodynamic point of view, first consider the anodic dissolution of a pure metal isolated in a solution of its salt.

• The metal consists of positive ions closely surrounded by free electrons. When the metal is placed in a solution, there will be a net dissolution of metal ions since the Gibbs free energy (∆G) for the dissolution reaction is less than for the reverse reaction.

• This leaves the metal with a net negative charge, thus making it harder for the positive ions to leave the surface and increasing the ∆G for the dissolution reaction. There will come a point when the ∆G for the dissolution reaction will equal the ∆G for the reverse reaction.

• At this point, a dynamic equilibrium is reached and a potential difference will be set up across the charged double layer surrounding the metal.

• The potential difference will be characteristic of the metal and can be measured against a standard reference electrode. When this is done against a standard hydrogen electrode in a 1 N solution of its salt at 25°C, it is defined as the standard electrode potential for that metal (Ch.6.3: Table 5).

• The position of a metal in the electrochemical series primarily indicates the order with which metals displace each other from compounds, but it also gives a general guide to reactivity in aqueous solutions.

• Those at the top are the noble, relatively unreactive metals, whereas those at the bottom are the more reactive. This is the first guide to corrosion resistance, but, as we shall see, there are major difficulties related to the use and interpretation of reactions from this simple analysis.

• Now consider a system in which the metal is in an aqueous solution that does not contain its ions. In this situation, the electrode potential at equilibrium (i.e., when the rate of the anodic reaction equals the rate of the cathodic reaction) will be shifted from the standard electrode potential and can be defined by the Nernst equation:

E = E0 + (RT/nF In (αanod /αcath) • Where E0 is the standard electrode potential, RT/F is a constant, n is the

number of electrons transferred, and α is the activity of the anodic and cathodic reactants.

• At low concentrations, the activity can be approximated to the concentration. In this situation, there is a net dissolution of the metal and a current will flow.

• At equilibrium, the rate of the metal dissolution is equal to the rate of the cathodic reaction, and the rate of the reaction is directly proportional to the current density by Faraday’s law; therefore

i anodic = i cathodic = i corrosion And the Nernst equation can be rewritten: E – E0 = ±β In(i corr / io) Where β is a constant and io is the exchange current density, which is defined as the anodic (or cathodic) current density at the standard electrode potential. • Current density is the current, measured in amperes, normalized to the surface

area of the metal. These conditions represent convenient models for the basic mechanisms of corrosion, but they are hardly realistic.

• Indeed, in this situation of a homogeneous pure metal existing within an unchanging environment, an equilibrium is reached in which no further net movement of ions takes place.

• In other words, the corrosion process takes place only transiently, but is effectively stopped once this equilibrium is reached.

• In reality, we usually have neither entirely homogeneous surfaces or solutions, nor complete isolation of the metal from other parts of the environment, and this equilibrium is easily upset. If the conditions are such that the equilibrium is displaced, the metal is said to be polarized.

• There are several ways in which this can happen. Two main factors control the behavior of metals in this respect and determine the extent of corrosion in practice.

• The first concerns the driving force for continued corrosion (i.e., the reasons why the equilibrium is upset and the nature of the polarization), and the second concerns the ability of the metal to respond to this driving force. It is self-evident that if either the accumulating positive metal ions in the surrounding media or the accumulating electrons in the metal are removed, the net balance between the dissolution and the replacement of the ions will be disturbed.

• This will occur in the biological environment surrounding implanted alloys due to the interaction of the proteins with the metal ions.

• Metal ions can form complexes with proteins and these complexes can be transported away from the immediate vicinity.

• This removes the metal ions from the charged double layer at the interface allowing further release of metal ions to reestablish the equilibrium.

• Similarly, relative movement between the implant and the tissue, for example, at a bearing surface or on a cyclically loaded implant, will cause mixing at the interface and will modify the composition of the electrolyte and may modify the surface of the alloy.

• The equilibrium is established precisely because of the imbalance of charge, so that if the charge balance is disturbed, further corrosion will occur to attempt to reestablish the balance.

• The result will be continued dissolution as the system attempts to achieve this equilibrium, in other words, sustained corrosion. An environment that allows the removal of electrons in contact with the metal or stirring of the electrolyte will achieve this. The process of galvanic corrosion may be used to demonstrate this effect.

• Consider a single homogeneous pure metal, A, existing within an electrolyte. The metal will develop its own potential, VA, with respect to the electrolyte. If a different metal electrode, B, is placed into the same electrolyte, but without contacting A, it will develop its own potential VB. If VA is not equal to VB, there will be a difference in the numbers of excess free electrons in each. This is of no consequence if A and B are isolated from each other, but should they be placed in electrical contact, electrons will flow from that metal with the greater potential in an attempt to make the two electrodes equipotential.

• This upsets the equilibrium and causes continued and accelerated corrosion of the more active metal (anodic dissolution) and protects the less active (cathodic protection).

• Galvanic corrosion may be seen whenever two different metals are placed in contact in an electrolyte. It has been frequently observed with complex, multicomponent surgical implants such as modular total joint designs consisting of titanium alloy femoral stems and cobalt alloy femoral heads.

• It is not necessary for the components to be macroscopic, monolithic electrodes for this to happen, and the same effect can be seen when there are different microstructural features within one alloy, such as the multiphase microstructure evident in implants of sensitized stainless steel where the grain boundaries become depleted in chromium and corrode preferentially to the remaining surface.

• In practice, it is the regional variations in electrode potential over an alloy surface that are responsible for much of the generalized surface corrosion that takes place in metallic components.

• Many of the commonly used surgical alloys contain highly reactive metals (i.e., with high negative electrode potentials), such as titanium, aluminum, and chromium. Because of this high reactivity, they will react with oxygen upon initial exposure to the atmosphere.

• This initial oxidation leaves an impervious oxide layer firmly adherent to the metal surface; thus all other forms of corrosion may be significantly reduced because the oxide layer acts as a protective barrier, passivating the metal.

• The manufacturing process for implant alloys may include a passivating step to enhance the oxide layer prior to implantation, for example nitric acid treatment of 316L stainless steel.

• In summary, the basic principles of corrosion determine that: 1. In theory, corrosion resistance can be predicted from standard electrode

potentials. This explains the nobility of some metals and the considerable reactivity of others, but is not useful for predicting the occurrence of corrosion of most alloy systems in practice.

2. Irrespective of standard electrode potentials, the corrosion resistance of many materials is determined by their ability to become passivated by an oxide layer that protects the underlying metal.

3. Corrosion processes in practice are influenced by variations in surface microstructural features and in the environment that disrupt the charge transfer equilibrium.

INFLUENCE OF THE BIOLOGICAL ENVIRONMENT • It is reasonable to assume that the presence of biological macromolecules

will not cause a completely new corrosion mechanism. However, they can influence the rate of corrosion by interfering in some way with the anodic of cathodic reactions discussed earlier.

• Four ways in which this could occur are discussed next: 1. The biological molecules could upset the equilibrium of the corrosion

reactions by consuming one or other of the products of the anodic or cathodic reaction. For example, proteins can bind to metal ions and transport them away from the implant surface. This will upset the equilibrium across the charged double layer and allow further dissolution of the metal; in other words, it will decrease ∆G for the dissolution reaction.

2. The stability of the oxide layer depends on the electrode potential and the pH of the solution. Proteins and cells can be electrically active and interact with the charges formed at the interface and thus affect the electrode potential can alter the pH of the local environment through the generation of acidic metabolic products that can shift the equilibrium.

3. The stability of the oxide layer is also dependent on the availability of oxygen. The adsorption of proteins and cells onto the surface of materials could limit the diffusion of oxygen to certain regions of the surface. This could cause preferential corrosion of the oxygen-deficient regions and lead to the breakdown of the passive layer. Alternatively the biomolecule adsorption layer could act as a capacitor preventing the diffusion of molecules from the surface.

4. The cathodic reaction often results in the formation of hydrogen, as shown earlier. In a confined locality, the buildup of hydrogen tends to inhibit the cathodic reaction and thus restricts the corrosion process. If the hydrogen can be eliminated, then the active corrosion can proceed. It is possible that bacteria in the vicinity of an implant could utilize the hydrogen and thus play a crucial role in the corrosion process.

• There is sufficient evidence to support the premise that the presence of proteins and cells can influence the rate of corrosion of some metals.

• Studies have examined these interactions electrochemically and have found very few differences in many of the parameters measured (e.g., electrode potential, polarization behavior, and current density at a fixed potential).

• However, analysis of the amount of corrosion through weight loss or chemical analysis of the electrolyte has shown significant effects from the presence of relatively low concentrations of proteins.

• These effects have varied from several fold increases for some metals under certain conditions, to slight decreases under other conditions.

• It has been shown that proteins adsorb onto metal surfaces and that the amount adsorbed appears to be different on a range of metals. Similarly, proteins have been shown to bind to metal ions and it is suggested that they are transported away from the local site as a protein-metal complex and distributed systemically in the body.

• It is therefore likely that proteins will influence the corrosion reactions that occur when a metal is implanted, although there is no direct evidence to explain the mechanism of the interaction at this time.

CORROSION AND CORROSION CONTROL IN THE BIOLOGICAL ENVIRONMENT

• The need to ensure minimal corrosion has been the major determining factor in the selection of metals and alloys for use in the body.

• Two broad approaches have been adopted. The first has involved the use of noble metals, that is, those metals and their alloys for which the electrochemical series indicates excellent corrosion resistance. Examples are gold, silver, and the platinum group of metals.

• Because of cost and relatively poor mechanical properties, these are not used for major structural applications, although it should be noted that gold and its alloys are extensively used in dentistry; silver is sometimes used for its antibacterial activity; and platinum-group metals (Pt, Pd, Ir, Rh) are used in electrodes.

• The second approach involves the use of the passivated metals. Of the three elements that are strongly passivated (i.e., aluminum, chromium, and titanium), aluminum cannot be used on its own for biomedical purposes because of toxicity problems; however, it has an important role in several Ti alloys, Chromium is very effectively protected but cannot be used in bulk.

• It is, however, widely used in alloys, especially in stainless steels and in the cobalt-chromium-based alloys, where it is normally considered that a level of above 12% gives good corrosion resistance and about 18% provides excellent resistance.

• Titanium is the best in this respect and is used as a pure metal or as the major constituent of alloys.

• In alloys the passivating layer promoting the corrosion resistance is predominantly composed of one of these metal oxides. For example, chromium oxide passivates 316L stainless steel and Co-Cr-based alloys and Ti oxide in Ti alloys. The other alloying elements may be present in the surface oxide and this can influence the passivity of the layer.

• Careful pretreatment of the alloys can be used to control the passivity of these alloys. In particular, production procedures need to be controlled because of their influence on the surface oxides, for example, the cleaning and sterilization procedures.

• Although these metals and alloys have been selected for their corrosion resistance, corrosion will still take place when they are implanted in the body.

• Two important points have to be remembered. First, whether noble or passivated, all metals will suffer a slow removal of ions from the surface, largely because of local and temporal variations in microstructure and environment.

• This need not necessarily be continuous and the rate may either increase or decrease with time, but metal ions will be released into that environment.

• This is particularly important with biomaterials, since it is the effect of these potentially toxic or irritant ions that is the most important consequence of their use.

• Even with a strongly passivated metal, there will be a finite rate of diffusion of ions through the oxide layer, and possibly a dissolution of the layer itself.

• It is well known that titanium is steadily released into the tissue from titanium implants.

• Second, some specific mechanisms of corrosion may be superimposed on this general behavior; some examples are given in the next section.

Pitting Corrosion • The stainless steels used in implantable devices are passivated by the

chromium oxide that forms on the surface. It has been shown, however, that in a physiological saline environment, the driving force for repassivation of the surface is not high.

• Thus, if the passive layer is broken down, it may not repassivate and active corrosion can occur.

• Localized corrosion can occur as a result of imperfections in the oxide layer, producing small areas in which the protective surface is removed.

• These localized spots will actively corrode and pits will form in the surface of the material. This can result in a large degree of localized damage because the small areas of active corrosion become the anode and the entire remaining surface becomes the cathode.

• Since the rate of the anodic and cathodic reactions must be equal, it follows that a relatively large amount of metal dissolution will be initiated by a small area of the surface, and large pits may form.

Fretting Corrosion • The passive layer may be removed by a mechanical process. This can be a

scratch that does not repassivate, resulting in the formation of a pit, or a continuous cyclic process in which any reformed passive layer is removed.

• This is known as fretting corrosion, and it is suggested that this can contribute to the corrosion observed between a fracture fixation plate and the bone screws attaching the plate to the bone.

• There are three reasons why fretting can affect the corrosion rate. • The first is due to the removal of the oxide film as just discussed. • The second is due to plastic deformation of the contact area; this can subject

the area to high strain fatigue and may cause fatigue corrosion. • The third is due to stirring of the electrolyte, which can increase the limited

current density of the cathodic reaction.

Crevice Corrosion • The area between the head of the bone screw and countersink on the fracture

fixation plate can also be influenced by the crevice conditions that the geometry creates.

• Porous coated implants may also demonstrate crevice corrosion. Accelerated corrosion can be initiated in a crevice by restricted diffusion of oxygen into the crevice.

• Initially, the anodic and cathodic reactions occur uniformly over the surface, including within the crevice. As the crevice becomes depleted of oxygen, the reaction is limited to metal oxidation balanced by the cathodic reaction on the remainder of the surface.

• In an aqueous sodium chloride solution, the buildup of metal ions within the crevice causes the influx of choride ions to balance the charge by forming the metal chloride.

• In the presence of water, the chloride will dissociate to its insoluble hydroxide and acid.

• This is a rapidly accelerating process since the decrease in pH causes further metal oxidation.

Intergranular Corrosion • As mentioned earlier, stainless steels rely on the formation of chromium oxides

to passivate the surface. • If some areas of the alloy become depleted in chromium, as can happen if

carbides are formed at the grain boundaries, the regions adjacent to the grain boundaries become depleted in chromium. The passivity of the surface in these regions is therefore affected and preferential corrosion can occur.

• Although this problem can easily be overcome by heat treating the alloys, it has been observed on retrieved implants and can cause severe problems since once initiated it will proceed rapidly and may well cause fracture of the implant and the release of large quantities of corrosion products into the tissue.

Stress Corrosion Cracking • Stress corrosion cracking is an insidious form of corrosion since an applied

stress and a corrosive environment can work together and cause complete failure of a component, when neither the stress nor the environment would be a problem on their own.

• The stress level may be very low, possibly only residual, and the corrosion may be initiated at a microscopic crack tip that does not repassivate rapidly.

• Incremental crack growth may then occur, resulting in fracture of the implant. Industrial uses of stainless steels in saline environments have shown susceptibility to stress corrosion cracking and therefore it is a potential source of failure for implanted devices.

Galvanic Corrosion • If two metals are independently placed within the same solution, each will

establish its own electrode potential with respect to the solution. If these two metals are placed in electrical contact, then a potential difference will be established between them, electrons passing from the more anodic to the more cathodic metal. Thus equilibrium is upset and a continuous process of dissolution from the more anodic metal will take place.

• This accelerated corrosion process is galvanic corrosion. It is important if two different alloys are used in an implantable device when the more reactive may corrode freely. Whenever stainless steel is joined to another implant alloy, it will suffer from galvanic corrosion. If both alloys remain within their passive region when coupled in this way, the additional corrosion may be minimal.

• Some modular orthopedic systems are made of titanium alloys and cobalt-based alloys on the basis that both should remain passive, but evidence of corrosion has been reported. Certainly, as shown in Ch.6.3: Fig. 5, titanium stems of modular prostheses can exhibit extensive corrosion.

• Galvanic corrosion may also take place on a microscopic scale in multiphase alloys where phases are of considerably different electronegativity.

• In dentistry, some amalgams may show extensive corrosion because of this mechanism

CERAMIC DEGRANDATION • The rate of degradation of ceramics within the body can vary considerably from

that of metals in that they can be either highly corrosion resistant or highly soluble.

• As a general rule, we should expect to see a very significant resistance to degradation with ceramics and glasses. Since the corrosion process in metals is one a conversion of a metal to ceramic structure (i.e., metal to a metal oxide, hydroxide, chloride, etc.) we must intuitively conclude that the ceramic structure represents a lower energy state in which there would be less driving force for further structural degradation.

• The interatomic bonds in a ceramic, being largely ionic but partly covalent, are strong directional bonds and large amounts of energy are required for their disruption.

• As extraction metallurgists know, it takes a great deal of energy to extract aluminum metal from the ore aluminum oxide, but as we have seen, the reverse process takes place readily by surface oxidation. Thus, we should expect ceramics such as Al2O3, ZrO2, TiO2, SiO2, and TiN to be stable under normal conditions.

• This is what is observed in clinical practice. There is limited evidence to show that some of these ceramics (e.g., polycrystalline Al2O3 and ZrO2) do show “aging” phenomena, with reductions in some mechanical properties, but the significance of this is unclear.

• Alternatively, there will be many ceramic structures that, although stable in the air, will dissolve in aqueous environments.

• Consideration of the classic fully ionic ceramic structure NaCl and its dissolution in water demonstrate this point. It is possible, therefore, on the basis of the chemical structure, to identify ceramics that will dissolve or degrade in the body, and the opportunity exists for the production of structural materials with controlled degradation.

• Since any material that degrades in the body will release its constituents into the tissue it is necessary to select anions and cations that are readily and harmlessly incorporated into metabolic processes and utilized or eliminated.

• For this reason, it is compounds of sodium, and especially calcium, including calcium phosphates and calcium carbonates, that are primarily used. The degradation of such compounds will depend on chemical composition and microstructure. For example, tricalcium phosphate [Ca3(PO4)2] is degraded fairly rapidly while calcium hydroxyapatite [Ca10(PO4)6 (OH)2] is relatively stable.

• Within this general behavior, however, porosity will influence the rates so that a fully dense material will degrade slowly, while a microporous material will be susceptible to more rapid degradation.

• In general, dissolution rates of these ceramics in vivo can be predicted from behavior in simple aqueous solution. However, there will be some differences in detail within the body, especially with variations in degradation rate seen with different implantation sites.

• It is possible that cellular activity, either by phagocytosis or the release of free radicals, could be responsible for such variations.

• In between the extremes of stability and intentional degradability lie a small group of materials in which there may be limited activity. This is particularly seen with a number of glasses and glass ceramics, based on Ca, Si, Na, P, and O, in which there is selective dissolution on the surface involving the release of Ca and P, but in which the reaction then ceases because of the stable SiO2-rich layer that remains on the surface.

• This is of considerable interest because of the ability of such surfaces to bond to bone, and this subject is dealt with elsewhere in this book.

• On the basis of this behavior, bioceramics are normally classified under three headings:

• Inert, or “nearly inert” ceramics • Resorbable ceramics • Ceramics of controlled surface reactivity

SUMMARY • This chapter has attempted to demonstrate that metals are inherently

susceptible to corrosion and that the greatest care is needed in using them within the human body. In general, ceramics have much less tendency to degrade, but care still has to be taken over aging phenomena. The human body is very aggressive toward all of these materials.

QUESTIONS 1. (a) What are the three most common implant alloys used in

structural applications? For each one, state which element in the alloy is chosen to enhance corrosion resistance, and how do they do it.

2. (b) Describe the mechanisms of intergranular corrosion and fretting corrosion.

3. (c) If you had an orthodontic appliance where Ni-Ti wire was placed in the groove of a stainless steel bracket, what corrosion problems might you encounter?

2. Consider a situation in which a 316L stainless steel fracture fixation plate has been used to treat a tibial fracture. Discuss the possible mechanisms of corrosion of the device with reference to the alloy composition, the geometry of the device, and the mechanical environment. Include a discussion on the fate of any corrosion products and their possible effect on the patient.

3. Discuss the potential disadvantages of using two different alloys for the components of modular orthopedic prostheses.