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
Describe the working time
This is the difference between the dough and setting time which is in the range of 5-8 minutes
Describe the graph of polymerization times of PMMA bone cements (Temperature vs. Time)
Temperature vs. time
Dough time: horizontal, 2-3 minutes long
Setting time: increasing, 8-10 minutes, specifically at 0.5Tmax
Working time: (Setting time) - (Dough time)
Factors affecting polymerization times
Rapid mixing: this can accelerate dough time which increases the porosity and weakens the cement
Increase in temp: can reduce the dough setting time by about 5% per degree C
High humidity: accelerates setting
What are the characteristics of PMMA bone cements? (i.e. factors affecting properties of bone cements)
Exothermic reaction
Shrinkage due to curing
Describe the exothermic reaction present in PMMA bone cements
The reaction of the polymerization is highly exothermic and temperatures in excess of 80 degrees can be reached which can cause the thermal bone death during THR
Describe the shrinkage due to curing of PMMA bone cements
The liquid solid conversion leads to a reduction of volume of up to 21% due to shrinkage. You can calculate the shrinkage using an equation.
[STUDY THE CALCULATIONS]
What is shrinkage of PMMA bone cements dependent on?
- The number of C=C in the monomer relative to the molar mass
- Amount of pre-polymerized filler and volume of material
How to reduce the shrinkage of PMMA bone cements?
Prepolymerized PMMA particles are mixed with the monomer and the shrinkage is reduced to about 3%
What are the factors that affect the adhesion of PMMA bone cements?
- Presence of blood (must be dry)
- New bone cements will not adhere to old bone cements (old bone cement must be fully removed)
- Adhesion promoter (e.g., silane coupling agent) is used to help the adhesion between the metal and bone cement
