Mechanical factors affecting surface shape and degree of motion

Question 1

What are the 3 important mechanical factors relating to the surface shape of a knee prosthesis?

Answer 1

The effect of constraint on load transmission adn the generation of high shear forces

The effect of surface contact on wear of the HDP tibial component

The effect of surface contact area on the stresses in the HDP tibial component

Question 2

Why does HDP become stiffer?

Answer 2

Increase in density after sterilisation with gamma radiation
Over time after implantation due to oxidisation

Question 3

Why doe the HDP wear first rather than CoCr?

Answer 3

It is the softer material

Question 4

Which two motions are most knee replacements designed to provide constraint in?

Answer 4

Axial rotation

Latral motion

Question 5

How does constraint affect the length of stem required?

Answer 5

The more constrained the motion the greater the length of stem required

Question 6

Formula for calculating volume of HDP wear

Answer 6

v = c.N.s / p

v = volume of wear 
c = constant (coefficient of wear) 
N = applied load 
s = distance the beaing slides 
p = hardness of the surface being worn

Question 7

What is the simplified formula used for comparing the wear rate of different surface geometries?

Answer 7

v = K x s

K = some constant

Question 8

What does the formula v -= Ks explain about wear?

Answer 8

It is proportional to the sliding distance moved

Question 9

In a rotating bearing what is the sliding distane proportional to?

Answer 9

the radius of the bearing

Question 10

What effect will a smaller diameter of bearing have on the volume of wear material?

Answer 10

Smller diameter will reduce the volume of wear material

Question 11

Is it the rate of depth of wear or the rate of volume of wear that determines the life of the HDP bearing?

Answer 11

Rate of depth of wear

Question 12

how does rate of depth of wear relate to rate of volume of wear?

Answer 12

Volume of wear (v) = area of contact (A) x depth of wear

Question 13

What should be done to minimise the rate of production of wear particles?

Answer 13

Minimise sliding distance

Question 14

What should be done to reduce the rate of depth of wear?

Answer 14

Increase contact area (have a wide bearing)

Question 15

What causes lamination cracking?

Answer 15

Combo of both high contact stress and a cyclical sliding motion between the surfaces

Question 16

How do the type of stresses vary across the tibial bearing surfaces?

Answer 16

Central part of contact area = always in compression

Periphery = always in tension

Question 17

Why should low contact designs be avoided?

Answer 17

They produce stresses in the HDP that are high enough to cause fatigue failure