Energy harvesting knee implants
Jim Newman looks at a new prototype designed to help monitor knee loads – and has some concerns.
Knee loading is of critical importance to a knee surgeon. For most of us, balancing the loads (that is, the forces passing through our knees) is something we do naturally. Whenever you shift your stance, perhaps because you’re picking up something heavy or you’ve been stood in one position for too long, you’re managing the load passing through your knees. When you have a knee replacement, the knee surgeon has to position the implant so that it can continue to manage loads just as it did when the knee was healthy. That’s not a simple exercise. Get the alignment wrong, and you can introduce a new set of unnatural stress forces to the knee and other joints which can cause a whole new set of problems.
Over the past few years, we’ve seen a number of technological advances designed to help knee surgeons achieve perfect alignment first time. A couple of years ago, we explored a custom knee replacement tool that takes a scan of the limb – not just the knee but the hip and ankle too. This is important, because alignment isn’t just about what the knee is doing; it is achieved through achieving the right balance across every joint in the leg.
The scan provides precise dimensions of the knee joint and illustrates how weight is distributed along the leg and through knee. It then forms the basis for a 3D printed model of the knee, which fits exactly on the patient’s knee and acts as the perfect cutting guide to ensure alignment.
But what if you could have a continual assessment of the load forces going through a patient’s knee? Rather than having to wait for a patient to complain that they are experiencing knee pain, their implant could monitor the load and give advance warning of a problem. It sounds like an exciting prospect, but there are a couple of issues…
The first issue is that when you ask an implant to collect and feed back data from within the body, you have to power the implant in some way. That issue has potentially been resolved by a new prototype which uses the energy of the load itself to power the implant monitoring it.
It’s called triboelectric harvesting. The energy is collected via a tiny harvester which sits between the resurfaced bone and polyethylene bearing, which gives the implant enough power to sense the load within the knee several times a day.
It’s clever stuff, but one which leads to the second big challenge.
One of the most crucial aspects of knee implant design is the small bearing which allows the joint to flex. Years of research and millions of pounds/dollars of funding have gone into perfecting it. Right now, we know the implants we use will often last 25 years.
By introducing another element, there’s a risk that you compromise that longevity. Now, this is a prototype. We don’t know that it increases wear. We don’t know it has any adverse effect whatsoever, but the introduction of any new element is likely to have some effect.
And that’s the challenge as technology like this moves forward. Do you give a patient an implant which you know is likely to last 25 years? Or do you introduce an undoubtedly clever piece of kit that may provide useful data, but which may also increase the wear rate, so the knee doesn’t last the distance?
Right time for load balancing
The time for load balancing has to be during the initial operation. That’s when the knee surgeon has the opportunity to definitively resolve the issue and eliminate the requirement for time consuming further redo surgery. I’m all for technology which can help you measure and achieve dynamic load stability in real time at the point of replacement. I’m less convinced about getting the data once the op is over – because doing something about it becomes a far more complex prospect.
If you would like to know more about knee replacement surgery, contact Jim Newman now.