Implant technology - unit 1 deck 4

Question 1

Describe what fretting corrosion is and how it occurs and give an example of implants prone to this type of corrosion

Answer 1

• This is corrosion which occurs when there is repetitive rubbing together, under a load, of components that are designed to be joined together without movement. The rubbing together can remove the protective metal oxide layer and corrosion will occur
• Fretting also causes surface damage to implants, reducing their fatigue life. Careful design is essential to minimise fretting

It is known to occur between screws and plates and in interference fits such as morse tapers, which rely on the friction between two tapered components to prevent motion

Question 2

Describe what crevice corrosion is, how it occurs and what implants are particularly prone to this type of corrosion

Answer 2

This is where body fluid can become trapped in crevices between implants, the body fluid loses its normal supply of dissolved oxygen resulting in high concentration acids forming which corrode the metals

Edges of bone plates and between screws and plates are particularly prone to this ==> Careful surgical assembly of components to ensure good screw-plate contact is therefore important.

Question 3

What are the 2 main methods which can be used for improving corrosion resistance of all metal alloys/ metals used in implants ?

Answer 3

Nitric acid immersion treatment and titanium nitride coating

Question 4

Describe how nitric acid immersion works to improve corrosion resistance

Answer 4

It improves the natural passivation (oxide) layer, the mechanism by which is not understood.

In the case of stainless steel and cobalt chrome alloys (both of which contain chromium) it is thought to be related to the increased amount of chromium in the passivation layer, which improves corrosion resistance.

Question 5

Describe how Titanium nitride coating works to improve corrosion resistance and what does it stop the release of into the body?

Answer 5

It significantly decreases corrosion

It is effective in reducing the release of vanadium and aluminium from titanium alloys, both of which are particularly harmful to the body. The release of titanium from titanium alloys is, however, not decreased by coating with titanium nitride, but titanium is regarded at present as being the least harmful orthopaedic implant metal.

Question 6

The products of corrosion of metallic and polymeric implants appear in small quantities in the blood, the urine, some tissues, storage organs such as the liver and in the nails and hair.

What are the main biological reactions to implant materials?

Answer 6

• Growth of a thin fibrous tissue layer between the implant and body tissue (usually bone), particularly if there is any micromotion at the interface. The fibrous layer is the body‟s means of isolating itself from the foreign body.
• Local infection, which may be due to ingress of bacteria into a wound before or during surgery. Implant materials tend to cause local suppression of the body’s defence mechanisms to infection
• Sensitisation of the body to metals. It has been estimated that 50% of patients may develop metal sensitivity to chromium, cobalt or nickel, although the longer term clinical effects are not known
• Inflammation in regions of metal corrosion, where the protective oxide layer is lost and the small wear particles of the material react with the body tissues
• Tissue necrosis (death) in the region of the bone cement used in joint replacements, which generates considerable heat as it cures. There is also tissue damage due to leaching of monomer (the material before it reacts and sets) and to additives present in the cement
• Immunological reaction due to wear particles from the surfaces of joint replacements. This can lead to cell mediated bone resorption, a process which is not well understood and is being researched. It occurs both with cemented and non- cemented prostheses
• Tumours occurring near the sites of long term implants, but only in a very few cases. The development of tumours in skeletal tissues can take many years so it will be some time in the future before the extent of the risk of tumour due to an implant is properly understood.

Question 7

Name three important features required of an orthopaedic material.

Answer 7

1. Good biocompatability
2. Suitable structural properties
3. Ease of manufacture

Question 8

Under what circumstances can an implant made of one alloy behave as two electrodes?

Answer 8

If it contains impurities or the alloy components are not uniformly mixed.

Question 9

Write down 5 adverse effects of implanted materials on the human body.

Answer 9

1. Growth of a fibrous layer between the bone and the implant
2. Iocal infection
3. Body sensitisation to metals
4. Tissue inflammation in regions of implant corrosion
5. Tissue necrosis in the region of bone cement
6. Immunological reaction to wear particles
7. Tumours

Question 10

In this set of flashcards we will confine our discussion to the main metallic implant materials that are used for structural support in both joint replacements and fracture fixation devices

Note :

• There is no satisfactory alternative material to metals, except for fibre reinforced plastics which will also be mentioned briefly. Ceramic components have been tried but they fail in a brittle manner, giving no advanced warning of failure. Metals are more ductile and frequently deform before they fail.
• Other materials used in orthopaedic implants are specifically either for joint bearing surfaces or for fixation.

Question 11

What is the most common type of stainless steel used for structural support in implants ?

Answer 11

Stainless steel 316L grade, which is a low carbon steel (0.03% carbon by weight).

Question 12

What is the purpose of the low carbon content in 316L stainless steel?

Answer 12

To minimise sensitisation of tissues and makes it more resistant to corrosion by the body.

Question 13

What are the main elements contained within stainless steel 316L?

Answer 13

1. Iron (about 63%)
2. Chromium (about 18%)
3. Nickel (about 13%)
4. Molybdenum (about 3%)
5. Manganese (about 2%).

Question 14

Why is stainless steel not an ideal material for permanent implants such as hip replacements but is suitable for use in more temporary implants, such as those used in fracture fixation (e.g. screws and plates)?

Answer 14

• Because despite having a high corrosion resistance it may corrode and crack when highly stressed and it is prone to crevice corrosion.
• Therefore in temporary implants, such as those used in fracture fixation (e.g. screws and plates), where the load on the implant decreases as the bones heal and the implant can subsequently be removed it is more suitable

Question 15

Why is forged stainless steel used over casted in orthopaedic implants ?

Answer 15

Because forged stainless steel can be as high as 4x’s stronger than casted stainless steel and manufacturing costs are relatively lower

Question 16

What are the disadvantages of stainless steel compared to cobalt chrome alloy and titanium/ titanium alloy?

Answer 16

It suffers more from local pitting corrosion (due to the passivation layer failing) and its fatigue strength is lower.

Question 17

Which metal alloy is more resistant to corrosion when inside the body - cobalt chrome alloy or stainless steel?

Answer 17

Cobalt chrome alloy

Question 18

What 2 metals in cobalt chrome alloy provide it with its corrosion reistant properties ?

Answer 18

• Chromium (main one providing it)
• Cobalt (this helps)

Question 19

What implants is cobalt chrome generally perferred for compared to stainless steel, what is the drawback of cobalt chrome though compared to stainless steel?

Answer 19

It is therefore generally preferred to stainless steel for permanent implants, although it is not as strong but is as stiff

Question 20

What are the main elements which comprise cobalt chrome alloy?

Answer 20

Called Stellite 21, is typically 65% cobalt (Co), 25 to 30% chromium (Cr) and 6% molybedum (Mo),

Question 21

What is the other cobalt chrome alloy which is not as corrosion resistant but is stronger?

Answer 21

MP35N

Question 22

By what method is cobalt chrome alloy (CoCrMo) such as satellite 21 made compared to cobalt chrome alloy MP35N?

Answer 22

• CoCrMo is casted
• MP35N (which contains 35% nickel and only 20% cobalt) is forged hence it is stronger but recall has worse corrosion resistance

Question 23

How does the strength of casted cobalt chrome alloy compared to stainless steel affect what type of implants it is used in?

Answer 23

• In joints such as the hip this tends not to be a problem because the replacement part is massive enough to have sufficient strength. However, in devices such as fracture fixation plates the cross sections are small so stainless steel and titanium are preferred because they are stronger.
• Therefore alloys such as MP35N, which is fabricated by a forging process, are much stronger than the cast CoCrMo alloy and are used in hip joint stems.

Question 24

What are the advantages of Cast CoCrMo and what is it particularly suitable for?

Answer 24

• Complex shapes can be cast more easily than they can be made in a forging or machining process.
• The alloys are particularly suitable as bearing surfaces because of their low coefficient of friction with polyethylene, which is the plastic used almost exclusively in joint prosthesis bearings.

Question 25

What forms of titanium are used in orthopaedic implants ?

Answer 25

1. Pure titanium
2. Or Titanium alloy most commonly Ti6Al4V (titanium with about 6% aluminium and 4% vanadium).

Question 26

What process makes pure titanium very resistant to corrosion ?

Answer 26

Anodising process - this is where the thickness of the passivation layer is increased

Question 27

Which is more resistant to corrosion stainless steel or titanium ?

Answer 27

Titanium

Question 28

Are the the corrosion products of titanium more or less harmful to the body than those from stainless steel and cobalt chrome alloys?

Answer 28

Less

Question 29

What are the advantages of titanium ?

Answer 29

• It is less dense (lighter) and about half as stiff as steel and cobalt chrome, which makes it slightly more compatible structurally with bone.
• It has a higher fatigue strength than stainless steel

Question 30

What extra advantage does the forged and heat treated titanium alloy Ti6A14V also have?

Answer 30

A higher yield stress than that of stainless steel and cobalt chrome alloys

Question 31

What implants is titanium commonly used in ? and what implants is it not used in and why?

Answer 31

• Titanium is commonly used in bone fixation plates
• It is an unsuitable material for bearings in joint replacements due to having a low wear resistance

Question 32

What are Fibre reinforced polymers used in implants ?

Answer 32

These are a composite material which has high strength properties, stiffness and is not brittle. This is due to being constructed from very stiff, high strength, but brittle fibres embedded in a much more flexible resin material

Question 33

What is the main fibre reinforced polymers used in implants and what is it used in?

Answer 33

• Carbon fibre reinforced polymers because they are the most biocompatable
• The material has been used successfully for internal bone fixation plates and fracture plates are now manufactured commercially.

Question 34

What are the advantages of fibre reinforced polymers compared to metals used in implants?

Answer 34

• They have have a stiffness about 1/3rd that of stainless steel, so mechanically they are more compatible with bone
• Superior fatigue properties to many metals, such as stainless steel, which means that it can overcome the problem of fatigue in metal plates due to movement at the fracture site

Question 35

Which metal implant materials are thought to have the best corrosion resistance?

Answer 35

Titanium and its alloys.

Question 36

In what way is carbon fibre reinforced plastic more like bone than the metals used for orthopaedic implants?

Answer 36

It has lower material stiffness than metals - about three times that of cortical bone rather than ten times (steel and cobalt chrome) or five times (titanium and its alloys).