Touching Lives - June 2005
Hip replacement - the next generation
A new hip can mean a new lease of life for someone suffering from degeneration of the hip joint. Most operations are carried out on people with osteoarthritis, a condition that affects the ‘shock absorbing’ qualities of the cartilage that lines the joint.
On average, replacement hip joints last between ten and 15 years, and an ageing population means that this surgery is likely to become more common in the future. There are two types of hip replacement, the most common uses a bone cement to hold the implant in place, to give a good ‘fit and fix’. But over time this can start to degrade, causing a loosening of the joint and the need for revision surgery.
This means that many people who have a hip replacement have to undergo some sort of revision procedure, usually because the cemented replacements in the hip become loosened over time. Constant movement and loading of the hip joint causes the cement to degrade. Once loosened, the joint deteriorates because of the abnormal stresses placed upon both the implant and the bone.
A study funded by Action Medical Research has been looking at why these problems can arise, and what might be done to monitor the degradation of the cement used to fix the metal stem of the replacement hip joint within the bone.
Dr Nader Saffari, Reader in Ultrasonics at the Department of Mechanical Engineering, University College London, together with Dr Martin Browne of the Bioengineering Sciences Research Group, University of Southampton, have been leading the recent study. The team has developed ways to assess the mechanisms that cause damage to the cement and to the interface between the cement and the implant. The hope is that further development of these techniques will enable the designs of ‘next generation’ implants to be modified to reduce the incidence of loosening and the resultant revision surgery.
This would be great news for patients — who would avoid follow-on surgery — and it would save the NHS millions of pounds every year.
Dr Saffari said: “We have employed a technique for monitoring defect formation and growth during laboratory loading tests called Acoustic Emission (AE) where you listen to the sound produced by a defect as it grows through the cement layer or along the cement-stem interface. At the same time, we have also looked inside the specimen using ultrasound and visualised the defects actually growing in the sample. This approach allows us to understand the different phases of the initiation and growth of defects in existing implants or new implant designs by continuously monitoring the degradation of the sample. ^In this way, we can identify implant designs that are likely to perform poorly^ and assess new cement formulations for strength and crack growth resistance.”
The new techniques could ultimately lead to the development of longer lasting implants, and these would be better suited to younger patients who want to get maximum wear from their new joints without the need for further surgery.