Orthopaedic Implant materials

Question 1

What is the most common stainless steel used for implants?

Answer 1

316L grade

Question 2

What are the benefits of the low carbon content of 316L stainless steel?

Answer 2

Minimise sensitisation of tissues

Makes it more resistant to corrosion by the body

Question 3

What makes up stainless steel?

Answer 3

Iron (63%) 
Chromium (18%)
Nickel (13%) 
Molybdenum (3%) 
Manganese (2%)
Also small amounts of silicon, sulphur and phosphorus

Question 4

Why is stainless steel not an ideal material for permanent implants such as hip replacements?

Answer 4

Although has high corrosion resistance it may corrode and crack when highly stressed and it is prone to crevice corrosion

Question 5

In what implants is stainless steel most useful?

Answer 5

Temporary - such as fracture fixation

Question 6

How are stainless steel implants usually made?

Answer 6

Forging (heated metal hammered into shape - wor involved causes metal to harden and increases its yield stress)

Question 7

How does the strength of stainless steel that is forged compared to cast stainless steel?

Answer 7

Forged is 4x stronger

Question 8

What are the disadvantages of stainless steel over cobalt chrome and titanium?

Answer 8

Stainless steel suffers more from local pitting corrosion (due to passivation layer failing) and its fatigue strength is lower

Question 9

Why is cobalt chrome preferred to stainless steel for permanent implants?

Answer 9

More corrosion resistant even though it is not as strong

Question 10

What is the rough composition of Stellite 21?

Answer 10

Cobalt (65%)
Chromium (25-35%)
Molybedum (6%)

Question 11

Why is the reduced strength of cobalt chrome not an issue in hip replacements?

Answer 11

Replacement part is massive enough to have sufficient strength

Question 12

How does cobalt chrome manufactured by casting cokmpare to that of stainless steel?

Answer 12

Not as strong but stiffness is similar

Question 13

Why are titanium and stainless steel preferred over cobalt chrome in fracture fixation plates?

Answer 13

They are stronger

Question 14

Which type of cobalt chrome is used in hip joint stems and why?

Answer 14

MP35N (made by forging process) as is much stronger than cast CoCrMo

Question 15

What makes cobalt chrome allows particiularly suitable as bearing surfaces?

Answer 15

Low coefficient of friction with polyethylee

Question 16

Why is pure titanium very resistant to corrosion within the body?

Answer 16

It is anodised (a process which increases the thickness of an anti-corrosive protective layer on a metal’s surface)

Question 17

How does the density and stiffness of titanium compare to that of steel and cobalt chrome?

Answer 17

Titanium is less dense (lighter) and about half as stiff as steel and cobalt chrome - makes it more compatible structurally with bone

Question 18

Why is titanium not suitable for bearings in joint replacements

Answer 18

Low wear resistance

Question 19

What are fibre reinforced polymers?

Answer 19

Very stiff, high strength but brittle fibres embedded in a much more flexible resin material

Question 20

What are the advantages of fibre reinforced polymers?

Answer 20

Most biocompatible
Stiffness can be selected by number of fibres used
Superior fatigue properties to many metals