Week 12 - Bioceramics

Question 1

What are ceramics?

Answer 1

Crystalline materials, usually made up of inorganic salts. Some are metallic salts (Al2O3, MgO, SiO2) and some are ionic salts (CsCl, ZnS)

Question 2

What are some general strengths of ceramics? (3)

Answer 2

Very inert
Mechanically strong
Biocompatible

Question 3

What is the main disadvantage of ceramics?

Answer 3

Brittle (zero failure strain)

Question 4

Where does their strength come from?

Answer 4

Ionic bonding between positive and negatively charged elements in their structure.

Question 5

Why are ceramics particularly well suited for medical applications? (4)

Answer 5

Resistance to chemical attack
Resistance to microbial attack
Resistance to pH changes
Heat resistance

Question 6

What are the three main commercial uses of bioceramics?

Answer 6

Glasswear (glasses, thermometers, fibre optics)
Dental (dentures, porcelain crowns)
Orthopaedic (hip /knee replacements, spinal fusions)

Question 7

Name the three tissue attachment mechanisms for ceramic implants. Rank them in terms of bond strength.

Answer 7

1. Bioactive fixation
2. Biological fixation
3. Morphological fixation

Question 8

In order for biological fixation to occur, how big must the pores be and why?

Answer 8

The pore size must be greater than 100 to 150 microns (to allow for vascularisation - which doesn’t occur in pores smaller than 100 microns)

Question 9

What is the current state of the art method for biological fixation?

Answer 9

Porous titanium coated with HA

Question 10

What is morphological fixation? What materials is it used for?

Answer 10

Attachment via cementing the device into tissue.

Dense, nonporous ceramics (Al2O3)

Question 11

What is biological fixation? What materials is it used for?

Answer 11

Bone ingrowth into pores of device which mechanically attaches bone to device.
Porous inert ceramics (Al2O3)

Question 12

What is alumina made from?

Answer 12

Ore bauxite

Question 13

What is alumina used for? (3)

Answer 13

Ball component of hip implants
Knee femoral component
Dental implants (e.g. bridges & crowns)

Question 14

What are some strengths of alumina? (6)

Answer 14

Immune to oxidation corrosion
Good wear characteristics
Excellent bio-compatibility
Very thin capsule formation
Low coefficient of friction
Very hard (14 GPa)

Question 15

What is zirconia used for? Why?

Answer 15

Used to reinforce alumina. Toughest conventional ceramic

Question 16

Why isn’t silicon nitride widely used?

Answer 16

It is very strong and biocompatible but it doesn’t have widespread long term acceptance - therefore people just use alumina

Question 17

What is porcelain and what is it used for?

Answer 17

Silicate based ceramic made from clay, silica, and feldspar

Dental crowns

Question 18

Why is porcelain used over alumina and zirconia for dental crowns?

Answer 18

Perfect hardness match, zirconia and alumina are too hard and tend to wear down opposing teeth

Question 19

Name two bioresorbable ceramics

Answer 19

Tricalcium phosphate

Calcium sulphate

Question 20

What is the tissue response to bioresorbable ceramics?

Answer 20

Degrades over time and replaced by host tissue

Very thin fibrous capsule

Question 21

What are the main issues with bioresorbable ceramics? (3)

Answer 21

Maintaining strength and stability at the interface during the degradation period
Matching resorption rate to repair rate
Ensuring that the resorbed material is non toxic

Question 22

What are the three fixation methods for bioresorbable ceramics?

Answer 22

Morphological, bioactive, biological (same as bioinert materials)

Question 23

What are the three main uses for bioresorbable ceramics?

Answer 23

1. Tissue engineering scaffolds
2. Bone repairs
3. Drug delivery systems

Question 24

What are the main categories of bioactive ceramics and what are the most common 2 used?

Answer 24

Calcium phosphate ceramics, glasses.

HA and bioglass

Question 25

What is HA?

Answer 25

Hydroxyapatite | A polycrystalline calcium phosphate ceramic similar to bone mineral

Question 26

What is the tissue response for bioactive ceramics?

Answer 26

Formation of direct chemical bond between implant and tissue

Question 27

What are 3 advantages of HA?

Answer 27

Chemically similar to bone material Low elastic modulus (20-120GPa) which means it doesn't lead to stress shielding Bioactive

Question 28

What is the main issue with HA? What is a solution to this?

Answer 28

Brittleness | Using HA as a coating on a tough metal implant

Question 29

Describe the four steps of HA bonding in vivo.

Answer 29

1. Cellular bone matrix from osteoblasts forms, producing a narrow amorphous layer 3-5 microns wide 2. Collagen bundles form between this area and the cells 3. Bone mineral crystals form in amorphous area 4. As site matures, bonding zone shrinks to 50-200nm

Question 30

Why is 45s5 bioglass so sick?

Answer 30

Only material known to form a bioactive bone with soft tissue

Question 31

Which bonds faster to bone in vivo, HA or bioglass 45s5?

Answer 31

bioglass 45s5

Question 32

What is bioglass?

Answer 32

An amorphous hydroxyapatite doped with silicon bone mineralisation catalyst and sodium (to make it biodegradable)

Question 33

What is SBF and what is it used for?

Answer 33

Simulated body fluid Acellular protein free salt solution that mimics the blood serum salt content Used for bioactivity testing

Question 34

What are the 4 stages in bioactive testing?

Answer 34

1. SPF immersion at 37 degrees 2. Osteoblast cell culturing 3. Animal in vivo implantation 4. Human clinical trials

Question 35

What are 7 advantages of bioglass?

Answer 35

``` Can bond to soft tissue & bone Can render polymers bioactive Actively encourages bone growth Non-toxic Non-carcinogenic Non-allergenic Strength of healed bone = strength of healthy bone ```

Question 36

What are 5 disadvantages of bioglass?

Answer 36

``` Excessive bioactivity can lead to too rapid bone growth w poor structure Highly brittle Doesn't bond well to metal Slower bone growth than grafted bone Not very tough ```

Question 37

What are the three main applications of bioglass?

Answer 37

Injectable bone grafts Cosmetics Sensodyne toothpastes

Question 38

What are the 3 key differences between HA and bioglass?

Answer 38

HA: bioactive, stable in the body for years, bonds to bone Bioglass: bioactive & biodegradable, dissolves in weeks to months, bonds to hard and soft tissues

Question 39

How does bioglass work as a sensitive toothpaste?

Answer 39

Very fine bioglass powder plugs the pores in tooth surface

Question 40

Why can't zirconia be used on its own?

Answer 40

In 90s was discovered to be thermodynamically unstable in vivo.