Working at the Diamond Light Source synchrotron facility in the UK, a team from Limerick University found that gamma rays used to sterilise parts of the implant actually make them more stiff and brittle.
Approximately 750,000 orthopaedic implant operations are undertaken in the US each year, in a market estimated to be worth $6.4bn (£4bn). However, research has shown that the failure rates of some orthopaedic materials is around 13 per cent over five years.
‘The polymer bearing material between the two metallic parts wears after a certain length of time and the joint becomes loose, so it has more room to move within the socket and little particles can chip off the polymer, causing pain and infection, that can lead to secondary surgeries,’ said Dr Maurice Collins from the Stokes Institute at Limerick University.
In the majority of artificial joints, ultra-high-molecular-weight polyethylene (UHMWPE) provides a surface against which hard metal or ceramic components connect. Before surgery, these components are sterilised by gamma radiation.
In its experiments, Collins’ team exposed UHMWPE materials with three different doses of gamma rays, then measured the material properties under strain using the Diamond Light Source facilities.
‘We had the material in a tensometer and placed the tensometer within the beam, which consists of electrons that are travelling close to the speed of light,’ said Collins. ‘The electrons hit the materials and diffract off at certain angles, and we measure those angles to determine the material properties.’
The team found that the gamma rays had a profound influence on the polymers’ material properties in a dose-dependent fashion.
‘Polymers are made up of long chains and these gamma rays can chop up the chains into short length, and when they do that it reduces the mechanical properties — they’re not as strong and become more brittle,’ added Collins.
The team will continue to work at the Diamond Light Source facility in an attempt to mitigate this, firstly through optimisation and in future through novel polymer composite materials.
‘Basically what we want to do is optimise the levels of sterilisation, because it will have to be sterilised obviously before it goes into the body, so we need to get a balance between sterilisation dose and reduction in mechanical properties. We can never get rid of it completely.’
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