What are the functions of bone and outline its importance.
- Mechanical properties –> provides protection and support for vital organs and it provides support and site of muscle attachment.
- Biological –> site of generation of white and blood cells and store of calcium and other ions.
• There are many serious pathologies of bone – aging and its related osteoporosis is major issue in the developed world and is of great clinical and economical importance.
- Millions suffer from osteoporosis/bone fractures every year and it has a huge burden on quality of life.
Why is it difficult to mimic bone?
Bone has a highly complex structure:
- crystals of apatite that are dispersed in collagen matrix
- it’s a porous structure that allows access for blood vessels
- conveys flexibility while maintaining stiffness and low density. It is also very light.
- -> These rare properties are very difficult to replicate in synthetic material – which have a much simpler structure.
What are the current materials used in bone implants?
i. Metals – usually light metals that are strong and compatible in biological environment. Titanium, Ti-alloys, Co-alloys.
ii. Ceramics – they are very resistant to wear and corrosion compared to metals and polymers, but are very brittle. Alumina, zirconia, TCP and synthetic HA.
iii. Polymers – they have problems with strength. PMMA, LDPE.
NB: less than 20 out of 1.5 mil candidates have been approved for use.
List the problems associated with synthetic implants.
- Mismatch in properties between tissue and synthetic material leading to stress shielding.
- Wear of implant due to friction
- Limited implant-tissue interaction preventing fixation.
Describe the issue of implant stress shielding.
Implant stress shielding –> caused by a mismatch in properties.
o Because the metal is much stiffer than bone (it has a higher Young’s modulus - which is a measure of stiffness), it will take most of weight load when you are moving.
(the implant ‘shields’ the bone from applied stress)
– The bone will now take less load than normal (70-80% less) and since these pressures are necessary for the function of normal bone, this will cause the bone to resorb around the implant.
– This will eventually lead to implant failure -> it’s one of biggest causes for implant failure.
Describe the issue of implant wear.
o When moving, friction between the two surfaces can cause the liberation of wear particles.
- These particles will surround implant and will trigger an inflammatory response.
- The inflammation will cause the osteolysis (degradation)of the implant triggering failure.
- Mainly seen in joint replacements.
What is a possible solution to implant wear?
Ceramic implants as they have better resistance to wear and resistance to chemicals.
(However they have low toughness).
Describe the features of zirconia ceramics in the use of implants, including its three temperature phases and its issues.
Zirconia –> A crystalline oxide of zirconium, first introduced in 1985.
- Zirconia has three temperature phases and the expansion caused when there’s change in phase will cause stress, leading to formation of cracks.
- By blending with other oxides it can be stabilised at the lower phase tegragonal at room temperature. E.g. by adding yttrium oxide.
- If stress is applied to tegragonal phase, it will change to monoclinic phase causing volume expansion and will close existing cracks. This makes zirconia implants very tough.
- -> Aging of zirconia: A major problem!!
- in a water environment, the water will diffuse into grains causing the grains to become larger and change to monoclinic from tegragonal and stress is applied causing cracks to be formed.
- The transformation will transfer to neighbouring grains leading to failure.
- This led to mass recall of thousands of zirconia implants in 2001 – the mechanical integrity of zirconia is extremely dependent on quality control.
How can we overcome the issue of zirconia and water wear?
A solution is to use nanocomposites, consisting of aluminium oxide which will separate the zirconia grains from each other making it more resistant to wear from water.
What is the process of implant fixation? What are the three methods of fixation.
Process of fixing the implant inside the body. There are three main strategies:-
- Biological fixation
- Cement (metacrylate) fixation
- Coatings
Describe the process of biological fixation.
– Involves the process of making pores in the implant’s surface so that bone will grow into pores and fix it there.
–The large pores can compromise the strength and mechanical integrity of the implant - therefore might be better in younger and more active patients.
Describe the process of cement fixation.
Process of using cement to fix the implant on bone – reduces rate of loosening and absorb forces on implant to make sure it stays here for years.
Method of mechanical fixation to make a customised fit – however it can lead to bone damage and the cement will deteriorate over the years.
- Metacrylate is a polymer which is most commonly used in cement fixation – it is inert and biocomptabible.
- -They have some problems – it is not resorbable (which is good for stabilisation but prevents the natural bone healing), it is not very strong, there is lack of direct contact to the bone, applying heat is dangerous. - Calcium phosphate bone cements
- - made up of calcium phosphate powder in aqueous solutions to make paste which is applied on bone, it will become rigid and set – takes days.
- - A space has to be made for paste to be applied. Once it precipitate, an acid-base reaction occurs to cream hydroxyapatite, making it very biocompatible.
- - It is brittle so it’s used in dental and facial implants.
- - two types exist: apatite cements (setting product is apatite) and brushite (setting product is brushite).
Bone cements are not only used on implants:- also used in malignant neoplasia where there’s extensive bone loss, joint replacement, osteoporosis and osteomyelitis.
Explain fixation by using coatings. What are its associated problems?
Process of coating implants with material to fix it to the bone
- A variety of surface coatings are used to enhance implants by encouraging bone ingrowth and providing enhanced fixation.
- The most common example is plasma spray – thermal coating process. The HA powder is used with a flame, that will melt the powder. The small droplets are then pushed onto the bone and coating it.
Problems –>
- It is difficult to control the solidification of the HA into bone, so some highly soluble calcium phosphate phases will be there which can deposit and have a harmful effect on mechanical integrity.
- Other problems include weak bonding between implant and coating
- stress due to mismatch between thermal stress in implant and coating.
- It also reduces fatigue endurance of the implant - was found to reduce it by 40%, attributed to the bead blasting used in the plasma spraying that promoted crack formation.
–The growth factor TGF-beta is currently in development for surface coating.
Why are scaffolds so important in bone regeneration (tissue engineering)?
- Scaffolds are needed that can act as temporary templates for bone regeneration and actively stimulate vascularized bone growth so that bone grafting is no longer necessary.
- To achieve this, the scaffold must have a suitable interconnected pore network and be made of an osteogenic material.
What are the properties of the ideal scaffold?
- Biocompatible, non-toxic
- Good cell adhesion
- High porosity – allowing seeding of cells and vessels.
- Bioresorbable
- Appropriate mechanical properties.
- Promote formation of bone tissue - osteogenic.
Why is bioactive glass preferred for scaffold?
Bioactive glass is an ideal material because it rapidly bonds to bone and degrades over time, releasing soluble silica and calcium ions that are thought to stimulate osteoprogenitor cells.
What are the different processes for scaffold fabrication?
- Freeze-casting
- Solid free form fabrication
- Robocasting
