Bioinert Ceramics Part 2

Question 1

Parts of total hip replacement

Answer 1

CoCr alloy outer cup
UHMWPE acetabular cup
CoCr alloy or alumina femoral head
CoCr/Ti alloy femoral stem

Question 2

Analysis of wear equation after initial wear rate period

Answer 2

V = kFx
k = constant characteristic of material pair
F = contact force
x = distance of relative travel

Question 3

Compare zirconia, PSZ, and alumina in the application of femoral heads

Answer 3

Can use pure zirconia, or yittria doped zirconia (PSZ)

PSZ has higher fracture toughness and strength than alumina, allowing for smaller, less sensitive femoral heads

Hundreds of thousands have been removed from the market due to failure or accelerated aging

Question 4

How and why is partially stable zirconia made? When is it stable? What are its properties?

Answer 4

Monocyclic phase of zirconia is stable at room temp, less favourable mech properties compared to tetragonal phase

Tetragonal phase metastable at room temp, so it is stabilized with Y2O3 (making PSZ)

Stress induced phase trans of PSZ induces crack propagation resistance

PSZ prone to degradation in hydrated environment (steam sterilization banned for it)

Question 5

Surface degradation of zirconia femoral heads due to its metastable nature?

Answer 5

1. Nucleation of water on a surface grain
2. Growth of phase transformation zone
3. Extensive cracking and surface roughening, surface uplift
4. Grain pullout due to wear

Question 6

Conditions that impact zirconia degradation

Answer 6

Grain size and yittria content
Surface conditions
More water and higher temperatures together
Tetragonal to monoclinic phase transformation which creates microcracks

Question 7

Which heat treatment of zirconia results in more tetragonal phase stability?

Answer 7

Batch (kiln)

Question 8

Which heat treatment of zirconia results in more monocyclic phase stability?

Answer 8

Belt (Tunnel)

Continuous converter belt moves items through multiple heating chambers

Question 9

Thermal profile of batch and tunnel/belt furnaces

Answer 9

Both reach the same peak temperature
3-fold more rapid decrease in temperature in tunnel furnace
As a result, more rapid degradation and failure

Batch: tetra
Belt/Funnel: mono

Question 10

What difference did fractured zirconia femoral heads have over non fractured ones?

Answer 10

Larger grain sizes

Larger size of monocyclics

Question 11

How many years of clinical service are equivalent to 1 hour of steam autoclaving for zirconia femoral heads?

Answer 11

3-4 years

Why steam is not used for femoral head sterilization

Question 12

Chevaliers personal recommendations for yttrium stabilized tetragonal zirconia

Answer 12

In vitro ageing test in steam
Appropriate grain size distribution
less than 20% monocyclic content