Bioinert Ceramics 1

Question 1

What are ceramics and glasses?

Answer 1

Solids composed of inorganic non metallic substances

Question 2

What is the structure of ceramics?

Answer 2

Mostly polycrystalline in regular array - most are variants of FCC and HCP

Microstructure of individual crystals separated by boundary layers with glassy phase, phase boundaries, and open or closed pores

Question 3

What is the morphology of glasses?

Answer 3

Amorphous and high viscosity, behave like solids

Short range order of atomic arrangement

No microstructure features, don’t undergo crystallization
Ex: tetrahedral silicate glass (SiO2)

Question 4

Which has higher specific volume at below Tm: amorphous or crystalline structure?

Answer 4

Amorphous

Question 5

What is the structure of glass-ceramics?

Answer 5

Polycrystalline materials with fine ceramic crystallites in amorphous glassy matrix

Produced by controlled crystallization of glasses (heat treatments)
< 1 micrometer

Question 6

What are the general properties of ceramic materials?

Answer 6

Brittle - ionic and covalent bonds resist plastic deformation
High modulus and hardness
Difficult to process without flaws
Biological properties can be varied based on chemistry

Question 7

What kinds of flaws in ceramic materials can cause stress concentrations?

Answer 7

Process induced ex: inclusion or pores
Design induced ex: sharp corners or holes
Service induced ex: pH, wear

Question 8

Examples of bioceramics

Answer 8

Alumina
Bioactive glass
Glass-Ceramics
Hydroxyapatite (Ha) 
Tricalcium phosphate

First four used in coating for chemical bonding
Last one used for temporary bone fillers

Question 9

What are examples of bioinert oxide ceramics? What are they used for? How do they compare?

Answer 9

Single oxides of alumina and zirconia
Used in density for implants, and orthopaedic surgery
Alumina more chemically stable than zirconia, but latter has better mech properties

Question 10

Advantage of oxide ceramics

Answer 10

Superior degradation resistance and mech properties

Superior wear resistance

Question 11

Alumina applications

Answer 11

Hip ball and cup (femoral head THR)
Knee joint
Dental implants

Question 12

Zirconia Applications

Answer 12

Hip ball (femoral head THR) 
Dental implants

Question 13

Types of tissue interactions in bioinert ceramics

Answer 13

dense (i.e. morphological) and porous (i.e. biological)

Question 14

Dense tissue ceramic biomaterial interactions

Answer 14

Non-porous inert materials
Attach to tissue through morphological fixation
Stable interface is crucial, otherwise implant will loosen and fail

Question 15

Porous tissue ceramic biomaterial interactions

Answer 15

Biological fixation:
With pores > 100 micrometers
bone ingrowth occurs, anchoring bone to material
Good mechanical stability, but lower mech stress and brittleness in implant

Question 16

General manufacturing steps of oxide ceramics

Answer 16

1. Starting material in form of fine powder
2. Green body formed (has shape of final component but lower strength)
3. Sintering to increase strength and density. Usually Hot isostatic pressing is used
4. Grinding and surface polishing

Question 17

What is the main source of alumina? What is the structure of alumina at 1200C?

Answer 17

Main source is bauxite and corundum
HCP alumina formed about 1200 celsius
Used in high purity with small amount of MgO to help sintering
Average grain size of 4 micrometer to exhibit wear resistance due to low surface roughness

Question 18

Which compound is added to help sintering of alumina? What is the average grain size?

Answer 18

Alumina used in high purity with small amount of MgO to help sintering
Average grain size of 4 micrometer to exhibit wear resistance due to low surface roughness

Question 19

What material is used as the femoral head in THR? What properties must this material have?

Answer 19

Alumina has been used for 40 years
Must have high degree of sphericity due to load bearing and wearing surfaces
Long term friction of alumina joint decreases wear properties over time

Question 20

Rank the wear rates of different acetabular cup and femoral head combinations?

Answer 20

1. Metal/Metal (Co-Cr) 0.0003 mm/year
2. Ceramic/Ceramic (alumina-alumina) 0.003 mm/year
3. Ceramic/Polymer (alumina-UHMWPE) 0.008 mm/year
4. Metal/Polymer 0.07-0.19 mm/year