Artificial hip replacements are medical devices which have, just like medical drugs, become a central aspect of the healthcare system in the UK and globally. Medical devices assist in preventing, diagnosing, and treating health issues. The range of medical devices in use is wide, ranging from rubber gloves and spatulas to highly complex equipment such as pacemakers and Magnetic Resonance Imaging (MRI) devices.
Artificial replacements of body parts, so-called prostheses, are medical devices that seem fairly simple but their development and use have had considerable impact on healthcare and the lives of patients. For example, the replacement of diseased hip joints with artificial hips has been acknowledged as the outstanding success in orthopaedics in the 20th century (Skinner and Kay 2011).
Indeed, around the world, millions of these artificial joint replacements have been implanted, delivering in most cases pain relief and independence to patients. Artificial hip joints replace the body’s own worn and diseased hip joint.
Metal-on-metal joints: promise and failure
Studying metal-on-metal joints
The aims of engineering research on joint replacements
Metal-on-metal joints: promise and failure
In an effort to bring the benefits of less pain and more mobility to younger patients, metal-on-metal hip replacements have been developed by medical engineers, clinical scientists and prostheses manufacturers. These metal-on-metal hip replacements were considered to be more resistant to wear. They had, until very recently, experienced an increase in use. However, a number of these metal-on-metal designs have failed earlier and in more patients than expected. As the key example, the ASRTM metal-on-metal hip replacements were given to nearly 100,000 patients globally and failure rates of between 1 in 8 and 1in 12 have been reported. These high profile failures have led to a backlash against all metal-on-metal hip prostheses. In order to understand the failures and improve the design and use of such artificial hip joints, Dr Thomas Joyce at Newcastle University has been working together with a team of local orthopaedic surgeons at the University Hospital of North Tees, obtaining and studying a large number of the failed metal-on-metal hip prostheses from this and other clinical centres.
Studying metal-on-metal joints
The articulating surfaces of these metal-on-metal hip prostheses – that is: the surfaces of the two parts of the prosthesis that fit into each other like a fist into a cupped hand and allow for movement – have been studied by Dr Joyce and his team in detail. From research on ‘conventional’ hip prostheses it is known that their life-time is linked to the amount of wear that occurs on the surfaces. Wear means that the surfaces are damaged through use. Strong wear leads to the release of small particles of the core material that the prosthesis is made of – usually Chromium and Cobalt. These particles are set free and enter the surrounding body tissue and the blood stream of people whose hip replacements experience such strong wear. The human body can react quite strongly to any particles that are not its own, leading to inflammation of the surrounding body tissue and to potentially more far-reaching health impact. Wear must hence be minimised as much as possible. The surfaces of tmetal-on-metal hip protheses such as the ASRTM designs and other joint replacements are therefore studied using two key methods, each of which gives valuable insight into where and how such wear occurs, helping to later develop methods on how to minimise it:
All of this information is then compared with other relevant data: the position of the acetabular cup (where and how the cup is located in the pelvis), and the concentrations of metal ions – of Cobalt and Chromium – in the blood.
The aims of engineering research on joint replacements
Overall, the research on metal-on-metal hip protheses undertaken at Newcastle University aims to evaluate and minimise the wear of such hip replacements. Importantly, it also aims to minimise the differences of how the hip replacements affect patients by reducing the sensitivity of patients’ bodies to the prosthesis. This would help increase the life-time of devices. The team around Dr Thomas Joyce not only aims to know why some designs fail, but also why some perform so well – and then want to share this knowledge with everyone concerned with joint replacements – engineers, surgeons, manufacturers, regulators and, perhaps most importantly of all, the millions of current and potential people with hip replacements across the globe.