Polymer Composites 1 Flashcards
Non-Cemented Press Fit Fixations in Total Hip Replacements
- Metal or calcium phosphate coatings
- In the long term, this may create bone ingrowth within the porous coating and biological or bioactive fixation
- Slow process
- More controllable for revision surgery
Cemented Fixation
- A methacrylate monomer is injected as a liquid that undergoes polymerization to a polymethyl methacrylate bone cement
- A patient is able to exert load on the implant
What is the main application of polymeric bone cements discussed in class?
Total Hip Replacements
- PMMA bone cement is used to fix the stem into the femur
What were PMMA based bone cements originally intended to do?
- Be a space filler between a metal femoral stem and bone in order to distribute the stress and prevent stress concentrations
- Reduce pain by fixing the prosthesis relative to the bone
How are PMMA based bone cements prepared by physician?
- PMMA is the major ingredient in bone cements for orthopedic implants and joint replacements
- A viscous paste is mixed by the surgeon and is used as a packing material between the metal and femoral prosthesis and the internal bone cavity
Describe the implant-cement-tissue interface
Metallic implant has a high elastic modulus, as the distance increases from the metallic implant the elastic modulus decreases. The lowest modulus is reached at the bone cement. The elastic modulus begins increasing again at an even further distance, as the tissue region.
How are PMMA based bone cements supplied?
Supplied in a sterilized two-part kit with a 2:1 ratio of powder (g) and liquid (mL)
What is used as a radiopaque agent in PMMA based bone cements?
Barium sulphate (BaSO4)
- It can be seen in an x-ray
- It is added in at 10 wt%
Describe the components of PMMA based bone cements
- Powder particles: 30um spheres of PMMA
- Radical source: Benzoyl peroxide
- Liquid component: 97% methyl methacrylate monomer (MMA) with DMPT
What extra components can be added to PMMA bone cements?
- Inhibitors: to stop premature curing or polymerization
- Antibiotics: Gentamicin (5-10 wt%)
Through which process does curing occur?
Free radical chain polymerization where the monomer polymerizes around the powder component
Describe the polymerization mechanism
- Initiation (creation of a radical and an active center)
- Propagation (addition of monomers to active center)
- Termination (the inactivation of two active centers)
(STUDY THE CHEMICAL DIAGRAMS)
What are the three polymerization times?
- Dough time
- Setting time
- Working time
Describe the dough time
Occurs: at 25 degrees and 65% humidity
Time: 2-3 minutes, this is the typical time for the mix not to stick to the surgeon’s glove
After mixing, it is loaded into a syringe or a pressure gun
Describe the setting time
Occurs: when the surface temperature of the dough reaches 0.5 of Tmax
Time: 8-10 mins
- The increase of temperature is due to the conversion of chemical to thermal energy (exothermic reaction).
- The surface temperature increases (which may reach 80 degrees) as the mass of the cement increases
- In vivo, the mass of the femoral stem absorbs the heat
- The thickness of the cement between the implant and the bone is typically 5mm