By: 27 June 2018
MeDe researchers are putting next generation knee implants through their paces

Dr Louise Jennings, of MeDe Innovation, and Ian Revie, of Invibio, discuss how the techniques and equipment designed by MeDe researchers are being used, not only to evaluate existing devices, but also to support the development of new designs using novel materials – paving the way for new types of implant that are better tailored to individual patients

Sophisticated simulation methods developed by researchers at MeDe Innovation are improving our ability to predict how well replacement knee joints will perform. Working with commercial partner, Simulation Solutions, MeDe Innovation has developed the world’s largest independent joint simulation facility. Using a combined computational and experimental approach, it is possible to more accurately predict the effects of a wide range of different activities on the wear and performance of the knee replacements.

Knee replacement surgery is a routine option for patients suffering from pain caused by arthritis or injury – but although more than 90,000 of these operations are performed in the UK each year, and 2.5 million globally, many do not meet the rising expectations of patients.

Patient-reported outcomes show up to 30 per cent of patients are unhappy with their replacement knees one year after their operation and, depending on the region, between 5 and 12 per cent of knee implants will need revision surgery after 10 years – mostly among younger, more active patients who had surgery in their 50s and who want to be able to maintain an active lifestyle.

Despite the prevalence of knee surgery, there has been little change in materials and techniques over the past 20 years – the bearing couple of cobalt chrome and UHMWPE, with its high wear resistance and biocompatibility, is still the gold standard in arthroplastic surgery. But as patients are beginning to have more of a voice in what constitutes successful surgery, new technologies are starting to emerge, meeting their need for implants that are both comfortable and long-lasting.

These new materials require increasingly sophisticated methods to test performance, especially the effects of wear as the implant has to be able to stand up to a huge range of different movements and loads. This testing is exactly what the joint simulation facility, developed by the EPSRC Centre for Innovative Manufacturing in Medical Devices (MeDe Innovation) is designed to do.


Pre-clinical testing innovation

Based at the University of Leeds, our researchers at MeDe Innovation are developing enhanced simulation equipment and experimental and computational methods to investigate the effect of different kinematics on the wear and performance of knee prostheses with greater rigour and accuracy than has previously been possible.

Working in partnership with experimental simulation equipment specialists, Simulation Solutions, we have designed and developed one of the world’s largest facilities for pre-clinical simulation of total joint replacements.

The facility combines dynamic computational simulations with electromechanical joint simulators. For knee replacements, this means that, for the first time, computer models can be created that predict the effect of a wide range of kinematic conditions on contact stress and cross shear on wear in knee prostheses. These can be validated with experimental simulations and measurements, which extend far beyond current international standards, reflecting a wider range of clinical conditions.

Because of its expertise in this field, MeDe Innovation’s researchers have been able to lead on the drafting of new international standards for the pre-clinical testing of joint replacements. The electro-mechanical knee simulators co-designed with Simulation Solutions are now being manufactured and sold across the UK, Europe and Asia.


Developing the materials of the future

MeDe Innovation isn’t just developing technologies that can be sold by commercial companies. Its research expertise is increasingly being used by med tech companies wanting to work in partnership to design and test promising new products at our facility.

Over the past five years, our researchers have been working with medical technology developers Invibio, to develop and test replacement knee products using the synthetic Super Engineering polymer, PEEK-OPTIMA™ (polyether ether ketone).

Over the past 20 years, Invibio has led the introduction and growth of PEEK in a variety of surgical procedures including trauma plates, suture anchors, and most commonly in spinal surgery, where more than nine million devices have been produced using Invibio’s PEEK-OPTIMA™. Invibio is now leading the highly challenging development of PEEK in replacement knee implants.

The product Invibio has designed is based on the Maxx Freedom knee system and is composed of a femur component made from PEEK instead of cobalt chrome, while the tibia and patella are made from ultra-high molecular weight polyethylene (UHMWPE). Using MeDe Innovation’s simulation facility, Invibio has been able to conduct rigorous wear testing of the implants, ahead of clinical trials, which are due to start later this year.

Why is PEEK-OPTIMA such a promising material for knee implants? Aside from significant manufacturing advantages – it takes just a few minutes to make each component – the material behaves much more like natural bone than metal implants could be expected to. In particular, it has a similar stiffness and weight to bone – metal implants can weigh around ten times more than the bone that has been removed. It also has similar thermal conductivity to bone, which is likely to mean it will feel more comfortable for patients. All of this means simply that the patient will notice the implant less and be less likely to experience discomfort.

There’s another advantage to PEEK as well: it’s possible to use X-ray and MRI to look behind the implant to diagnose infections or other complications which can’t be seen behind standard metal implants.


Putting PEEK through its paces

These are good reasons to pursue the regulatory approval required to start using PEEK knees in the clinic. With support from MeDe Innovation, Invibio has already embarked on a rigorous programme, including testing PEEK against other materials solutions, combining it with components made from cobalt chrome and ceramic to arrive at the most effective combination of PEEK and UHMWPE. These PEEK/UHMWPE devices have been studied extensively through our full range of kinematic tests.

Because PEEK is temperature sensitive, we’ve taken this even further, testing components at both body temperature and room temperature.

Results so far have shown that current PEEK implant designs are at least as good a solution as cobalt chrome, showing equivalent wear rates.

There are still questions about the performance of this material that we need to understand before it can be launched as a commercial product. We need to investigate the effects of wear debris and how debris might be reduced. We also need to address the interesting question of why there is more friction between the two components than there is in metal devices – but equivalent wear. Alongside this, a clinical investigation will need to be satisfactorily completed before the implants can be CE marked.


A PEEK into the future

The ease and speed with which these components can be manufactured, as well as their improved performance, make it highly likely that we will see traditional cobalt chrome implants replaced in the future. The emergence of personalised treatment options for patients, where implants are more tailored to specific patient groups will mean patients can expect better – and longer – outcomes from surgery. Implants that can be manufactured within minutes and customised for best fit will be key to achieving this progress.


Louise Jennings, pictured, is a MeDe Innovation Researcher, and Associate Professor of Medical Engineering in the School of Mechanical Engineering, University of Leeds.

Ian Revie is Business Manager – Knees, Invibio Ltd.