Week 12 - Bioceramics Flashcards
What are ceramics?
Crystalline materials, usually made up of inorganic salts. Some are metallic salts (Al2O3, MgO, SiO2) and some are ionic salts (CsCl, ZnS)
What are some general strengths of ceramics? (3)
Very inert
Mechanically strong
Biocompatible
What is the main disadvantage of ceramics?
Brittle (zero failure strain)
Where does their strength come from?
Ionic bonding between positive and negatively charged elements in their structure.
Why are ceramics particularly well suited for medical applications? (4)
Resistance to chemical attack
Resistance to microbial attack
Resistance to pH changes
Heat resistance
What are the three main commercial uses of bioceramics?
Glasswear (glasses, thermometers, fibre optics)
Dental (dentures, porcelain crowns)
Orthopaedic (hip /knee replacements, spinal fusions)
Name the three tissue attachment mechanisms for ceramic implants. Rank them in terms of bond strength.
- Bioactive fixation
- Biological fixation
- Morphological fixation
In order for biological fixation to occur, how big must the pores be and why?
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)
What is the current state of the art method for biological fixation?
Porous titanium coated with HA
What is morphological fixation? What materials is it used for?
Attachment via cementing the device into tissue.
Dense, nonporous ceramics (Al2O3)
What is biological fixation? What materials is it used for?
Bone ingrowth into pores of device which mechanically attaches bone to device.
Porous inert ceramics (Al2O3)
What is alumina made from?
Ore bauxite
What is alumina used for? (3)
Ball component of hip implants
Knee femoral component
Dental implants (e.g. bridges & crowns)
What are some strengths of alumina? (6)
Immune to oxidation corrosion Good wear characteristics Excellent bio-compatibility Very thin capsule formation Low coefficient of friction Very hard (14 GPa)
What is zirconia used for? Why?
Used to reinforce alumina. Toughest conventional ceramic
Why isn’t silicon nitride widely used?
It is very strong and biocompatible but it doesn’t have widespread long term acceptance - therefore people just use alumina
What is porcelain and what is it used for?
Silicate based ceramic made from clay, silica, and feldspar
Dental crowns
Why is porcelain used over alumina and zirconia for dental crowns?
Perfect hardness match, zirconia and alumina are too hard and tend to wear down opposing teeth
Name two bioresorbable ceramics
Tricalcium phosphate
Calcium sulphate
What is the tissue response to bioresorbable ceramics?
Degrades over time and replaced by host tissue
Very thin fibrous capsule
What are the main issues with bioresorbable ceramics? (3)
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
What are the three fixation methods for bioresorbable ceramics?
Morphological, bioactive, biological (same as bioinert materials)
What are the three main uses for bioresorbable ceramics?
- Tissue engineering scaffolds
- Bone repairs
- Drug delivery systems
What are the main categories of bioactive ceramics and what are the most common 2 used?
Calcium phosphate ceramics, glasses.
HA and bioglass
What is HA?
Hydroxyapatite
A polycrystalline calcium phosphate ceramic similar to bone mineral
What is the tissue response for bioactive ceramics?
Formation of direct chemical bond between implant and tissue
What are 3 advantages of HA?
Chemically similar to bone material
Low elastic modulus (20-120GPa) which means it doesn’t lead to stress shielding
Bioactive
What is the main issue with HA? What is a solution to this?
Brittleness
Using HA as a coating on a tough metal implant
Describe the four steps of HA bonding in vivo.
- Cellular bone matrix from osteoblasts forms, producing a narrow amorphous layer 3-5 microns wide
- Collagen bundles form between this area and the cells
- Bone mineral crystals form in amorphous area
- As site matures, bonding zone shrinks to 50-200nm
Why is 45s5 bioglass so sick?
Only material known to form a bioactive bone with soft tissue
Which bonds faster to bone in vivo, HA or bioglass 45s5?
bioglass 45s5
What is bioglass?
An amorphous hydroxyapatite doped with silicon bone mineralisation catalyst and sodium (to make it biodegradable)
What is SBF and what is it used for?
Simulated body fluid
Acellular protein free salt solution that mimics the blood serum salt content
Used for bioactivity testing
What are the 4 stages in bioactive testing?
- SPF immersion at 37 degrees
- Osteoblast cell culturing
- Animal in vivo implantation
- Human clinical trials
What are 7 advantages of bioglass?
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
What are 5 disadvantages of bioglass?
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
What are the three main applications of bioglass?
Injectable bone grafts
Cosmetics
Sensodyne toothpastes
What are the 3 key differences between HA and bioglass?
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
How does bioglass work as a sensitive toothpaste?
Very fine bioglass powder plugs the pores in tooth surface
Why can’t zirconia be used on its own?
In 90s was discovered to be thermodynamically unstable in vivo.
