Design of Joint Surfaces

Question 1

What 2 factors is the frictional force dependent on?

Answer 1

The surface properties of the 2 materials in contact (coefficient of friction)
the magnitude of the load pressing them together

Question 2

Why is HDP used over other materials even tho they have a lower coefficient of friction?

Answer 2

Other materials are either toxic or have poorer wear properties

Question 3

What is a bad consequence of replacement joints having a higher contact friction than the normal joint?

Answer 3

Much higher shear force transmitted to the acetabulum (can cause loosening at fixation interface)

Question 4

Why do most hip replacements nowadays use small heads?

Answer 4

Less interface force

Question 5

How does the frictional force related to the are of contact?

Answer 5

Frictional force is independent of the area of contact

Question 6

What are the 2 main types of joint wear?

Answer 6

Abrasive wear

Adhesive wear

Question 7

What is adhesive wear?

Answer 7

When 2 bearing surfaces stick to each other when they are pressed together and one, usually the softer one, is torn off by the harder one

Question 8

How can adhesive wear be reduced?

Answer 8

Bearing surfaces should be made of materials that have a low level of adhesion
Lubrication

Question 9

What causes abrasive wear?

Answer 9

When surfaces are not perfectly smooth

Question 10

Why is a good circulation of lubricant important in terms of abrasive wear?

Answer 10

to remove wear particles

Question 11

Equation to calculate volume of wear

Answer 11

V = c.N.s / p

v = volume of wear 
c = coefficient of wear 
N = applied load across the bearing surfaces 
s = distance the bearing slides 
p = hardness of the surface being worn

Question 12

What problems can wear particles cause in a replacement joint?

Answer 12

Cause intense inflammatory tissue reactions

Cause aseptic loosening

Question 13

What is the process by which wear particles cause aseptic loosening?

Answer 13

Particles cause adverse tissue reactions
Results in local inflammation
Over a number of years bone resorption and loosening

Question 14

What are the 3 methods for reducing wear?

Answer 14

Reduce loading o the joint
Keep sliding distance as small as poss
find alternative materials (reduce coefficient of wear)

Question 15

Why does a femoral head with a small radius wear less than a head with a large radius?

Answer 15

Bearing surface moves less as it slides

Question 16

Which materials have been found to reduce HDP wear?

Answer 16

Ceramics

Question 17

Examples of ceramics

Answer 17

Alumina

Zirconia

Question 18

What are the disadvantages of alumina?

Answer 18

More brittle and prone to fracture than metals

Question 19

What are the advantages of Zirconia over Alumina?

Answer 19

Tougher
More fracture resistant
Harder
More scratch resistant

Question 20

What are the disadvantages of a smaller diameter head?

Answer 20

Greater rate of depth of wear

Increased likelihood of dislocation in post op period

Question 21

Why is the rate of depth of wear greater for a smaller head?

Answer 21

Contact area is less

Question 22

Why is the increased rate of depth of wear often not an issue?

Answer 22

Prosthesis fails for other reasons first

Question 23

What problem is caused as the head wears into the cup?

Answer 23

Reduced range of motion (it is necessary to reduce the diameter of the neck as the head diameter is reduced to maintain motion)

Question 24

What are two advantages of small diameter head?

Answer 24

reduce cup-bone interface shear stresses (lessens risk of loosening)
Produce less volume of HDP wear particles