Bioactive Ceramics 2 Flashcards
What is the Ca/P of synthetic hydroxyapatite
1.67
What is one way to process calcium phosphates?
using compaction (die or hot isostatic pressing) and solid state sintering
Comment on the effects of temperature and water on the phases of CaPs during production
- with water present, HA is formed and it stable up at 1360C
- at higher temperatures and no water, a-TCp and TTCP are present
- unhydrated calcium phosphates react with water at 37C to form HA on their surfaces
High temperature processing of hydroxyapatite
Solid-state reactions for HA synthesis: stoichiometric mixing of reagents
- calcium components are mixed, formed and then sintered above 950C
What are the disadvantages of high temperature processing of HA?
- difficult to estimate the time required to produce a homogeneous end product
- thus, tendency to produce materials which are inhomogeneous at the final processing stage
Low temperature processing of hdyroxyapatite
Aqueous precipitation produced by either a reaction between
- calcium hydroxide and phosphoric acid, or
- a reaction of calcium salt and an alkaline phosphate.
- the reactants are mixed in the presence of water (controlled T, P and pH of 10), and stirs for 12h to precipitate, followed by drying and filtering
Disadvantages of low temp processing of HA
- Precipitated HA (pHA) is less crystalline and perfect than sintered HA (not necessarily a disadvantage, just fact)
- more difficult to process consistent pHA due to potential difficulties in controlling pH, temperature, and Ca/P ratio
Benefits of low temp processing of HA
- can be used to generate ion substituted HA, which affects solubility and generates more bioactive, ion releasing HA
- results in pHA close to that in vivo
- gives a wide variety of apatite sizes and morphologies
What are the two types of substitutions that can be made to create carbonated apatites (incorporated carbonate ions)
1) Carbonate for hydroxyl group
2) Carbonate for phosphate group
What is the rationale for creating silicon substituted HA?
- enhanced bioactivity of silica based bioglasses
- silicon is active in bone formation
- potential of increasing the strength of ceramics
- can be made at low temperature while in an aqueous environment
Clinical applications of CaPs
Powders, small unloaded implants, dental implants with reinforcing metal posts, low loaded porous implants, bioactive phase in a polymer matrix composite, coatings on metal implants, bone cements
Benefits of HA coatings
- improve fixation of orthopedic devices, affecting bone ingrowth and combing biologic and bioactive fixation types
- rapid increase in interfacial bond strength when using HA coatings rather than porous metals
what are the factors that affect HA coating properties?
- thickness of coating (50-100um)
- adhesion strength (5-65 MPa)
- biodegradation which is affected by chemical purity, porosity and crystallinity of HA
What is the sequence of interfacial reactions involved in forming a bond between tissue and HA?
1) differentiated osteoblasts form a cellular bone matrix at the surface producing a narrow amorphous band between 3-5um wide
2) bundles of collagen between the bone matrix and the cells are found with some bone mineral crystals
3) As the site matures, the bonding zone shrinks to a depth of 0.05-0.2um
4) the result is normal bone attached to the implant
What are the two types of CPC setting reactions?
1) an acid-based reaction where a relatively acidic CaP reacts with a relatively basic CaP to give a neutral CaP
2) The initial and final CaP have the same Ca/P molar ratio
