Sockets and Stumps and Science
A PROSTHETIC MEANS ‘an artificial part supplied to remedy a deficiency’. But always artificial, and not, in the words of the design historian, with much chance of ever matching the performance of the real thing. The problem is the stump-and-socket model, the impermanence, the changeability, the inefficiency of the interface, the pain. One solution seems to be to take changeability out of the equation by replacing the limb permanently.
There are two models of permanent limb replacement. The first is transplantation: where a donated limb is surgically attached to a stump. This is already happening with hands and arms. The surgery is straightforward – no techniques that aren’t already in use – but nothing else about the procedure is. Prior to surgery the immune system is suppressed, and suppressed hard, and then it is suppressed by heavy medication loads for as long as the transplant remains attached. There’s no forgetting it’s a transplant for the patient, because they have to look for signs of rejection every day, painstakingly. Rejection starts as a rash or a change in skin tone and can appear anywhere on the new limb. In the early stages it doesn’t hurt, but every little thing has to be reported to the transplant surgeon. Limb transplant can only be done if the patient can manage the heavy immune-suppressant regime (and not everyone can).
Transplants are not just about soft tissue and bone. They also reconnect nerves to brain, and although it takes a while, some sensation does eventually return as the nerves and brain rewire themselves. This process seems to be about length, so the longer the limb being transplanted, the longer it will take to register sensation. Sensation is needed not just so the person can touch and feel again, but also so that their own body can protect them – so pain can be registered. Ideally this means the restoration of nociceptor pain, the really useful form of pain that tells the brain that damage and danger are imminent and to react. With hand and arm transplants some function can be returned (but it takes intensive, specialist rehab) to the point at which patients can write, feed and clean themselves, and when they speak, their hands have begun to be expressive again, vague but discernible motions of body language. None of this works for legs. Return of sensation isn’t really relevant; limb transplants aren’t load-bearing – they can’t take weight – so there is no possibility of ever leaving the wheelchair behind.
So if we are a very long way from leg transplants, what are the other options? Something permanently attached, that gets away from the stump-and-socket model but which can take weight and doesn’t need the sledgehammer immune-suppressant regime. There is another technique called osseo-integration, where metal prosthetics are attached directly and permanently on to the bones in the stump. It’s an integration technique because the idea is that the metal prosthetic will integrate with the bone to create a whole – a new join, not something detachable or changeable. No more socket: just a stump with a metal clip for a prosthetic sticking out of the end.
As with limb transplants, none of the components of osseo-integration is new, just the way they are being used. We know that bones can grow new pieces of themselves, which is how fractures heal if they are set right. We also know that bone cells will regrow on a surface not made of bone and that the resultant integrated area can take weight and generate movement. Osseo-integration is what makes knee and hip and dental implants work, and the vertebral screws that pin together smashed bones. So osseo-integration at amputation stumps builds on all these things that we know already work.
Unlike limb transplantation, osseo-integration is being done on amputees today. The surgery involves attaching a bespoke metal implant, measured to match the existing or former limb, to a neatly trimmed bone end. This is the thing that will replace the socket. This implant is designed to be transcutaneous, which means that the other end of the implant sticks out of the stump, so a plastic surgeon has to close up the soft tissue of the stump carefully around the sides of the implant end. There is a coupling at the end of the implant to which prosthetics can then be attached. The coupling is designed to break, like a ski binding, if it comes under pressure.
After surgery the patient goes back to rehab, where they and their new implant learn how to take loads again. Specialist physios help them develop the bone density, millimetre by millimetre, so that eventually they will be able to put their weight down on their new legs through their new implant. Bone grows and becomes denser at the interface as its owner moves and places loads on it – just as runners have better bone density than people who don’t move around so much. Loads equal density equals a good level of osseo-integration. The first time patients put weight on their big legs at Headley it’s all about balance. With osseo-integration it’s about load and developing bone density – growing something back that had been taken away. That’s where this system has it over the stump-and-socket replacement model. Soft tissue, like skin and muscles worked over bone remnants, isn’t supposed to take load and responds inefficiently when it has to. Bone ends in stumps don’t increase their density, because the bone has nothing to engage or interface with. Implants promote regrowth – not much, but significant.
Integrated implants mean that prosthetics are attached to the skeleton again, not a socket which is controlled by muscles that weren’t ever supposed to do that. The patient’s brain says Leg Lift and Go Upstairs, and it does. The prosthetic leg clips on to the implant in twenty seconds, without the palaver of socks and linings and stumps being manoeuvred into sockets. Integrated implants work well for amputees with knees. Above the knee, and things are not so clear. Above the knee osseo-integrated implants remind us that this is a very new technique and one where we do not yet know the long-term outcomes, because there haven’t yet been any long-term outcomes to know. No one has had one of these for more than five years. We know a few things: that patients can’t run with integrated implants, that the implants can still break and that managing tissue around the bit of implant that sticks out through the skin is difficult. Apart from our gums and nail beds, human skin is designed to be a closed system, with no direct access for micro-organisms. If the transcutaneous section of the implant gets infected, it can spread, and ultimately the whole system can fail. Very few of the patients who end up back in their chairs back in their surgeons’ consulting rooms are suitable candidates for osseo-integration. But that doesn’t change the fact that sockets are the worst of their problems every day of the week, every week of the year. If someone tells an amputee there’s a system where the socket can be replaced, they don’t hear anything else after that.
So another way is to make the socket better. Make it smarter. Make it tell the patient when things are starting to go wrong: temperature, moisture, weight slowly shifting to the wrong place, red marks that last for longer than ten minutes, muscle strain, deep tissue damage, ulcers. Or just simple things, like they aren’t walking quite as well or efficiently as they might. Too much energy being expended for not enough effort. Choose who you listen to, and one day they will be able to listen to their socket because it’s going to ring them on their phone and tell them how it’s feeling. Optimum stump health – there’s an app for that.
How to make a socket smart? Take existing sensor technology and the idea of wearables and put them together – sensors inside a socket. One model currently being researched is a paper-thin liner that goes inside the socket to take measurements. Hold out your hand, fingers spread, and then take your other hand and make a fist. Put it in the palm of your flat hand and then wrap your fingers around your fist. As you move your fist about, your fingertips sense every muscle and tendon as it works. The smart socket is the same principle. It’s shaped like a cobweb made of flat, plasticised film, with a suite of sensors on every point. It fits down inside the socket, between the exterior and the elastic sock lining that goes over the stump.
When the stump is inserted in the socket and the patient stands up and moves, the sensors transmit data via Bluetooth to processors stuck on the back of the outside of the socket casing. There’s lots of spare room all over the leg and the technology is light, so no extra weight.1 If everything works well (and the batteries for transmitters charge properly, which is the main problem at the moment), this gives sockets the potential to be their own little data-processing centres. And the sensor data that spins out from their web is pure gold, almost infinitely useful.
Most importantly of all, and whatever the eventual model will be, smart sockets tell their wearer when something is going wrong: too hot, wrong loading, sweat, swelling, deep tissue dysfunction, balance wrong, symmetry off. It’s an early warning system, so adjustments can be made quickly – sit down, but only for a few minutes, use a wipe to clean an over-heated, sweaty stump, fix the padding, get out in front of the problem and shut it down. Nearly as important as telling the wearer what is wrong, the smart socket also tells when things are going right. Balance, weight loading, symmetry, gait. Press a button and the wearer and their physio can see a graph that plots their progress now against progress they made last month, so they can see the accumulation of things going right, and keep doing them.
The really smart thing about the idea of a smart socket is that it gives everyone involved in the injury a shared language, for things that go wrong and for things that go right. Smart socket data could be used by prosthetists, physios or Paralympic coaches for repairs, maintenance, improvements, refinements. Shared languages, as the pain consultant will testify, are essential. It takes the guesswork out if the patient can watch their own biofeedback in real time, with their physio, seeing what effect new exercises or step patterns have, right there on a screen. A shared language means that whenever they get a new physio or move to a new NHS catchment area, they hand over a data stick, and they never quite have to start from scratch. A smart socket makes a smarter patient – expert in their own condition, with stats to prove it. This is where Mark Ormrod’s personal research programme and the technical wizardry of university research departments meet.
This could be a significant step forward, if it works out as it should. Smart sockets will suit the global amputee population because, although they may not have 3D printers (currently the solution to everything, if you believe the gospel according to TED), they do have smartphones. Smart socket sensor arrays, whatever their model, will be small. Fit the sensor in the socket, clip on the hardwear, download the app and off it goes. Upload the data and study it, upload the data to the Cloud, contribute to a new kind of global analytics. What works all over the world, for everyone taking the same journey on whatever kind of road is laid out before them. A new meaning for the global amputee population, becoming part of their own solution. Hurry up, I say to the quietly brilliant PhD student whose work this is, whose hands shake because we are all watching him when he puts the batteries in Dave Henson’s processor on the day he first tests it at the running track at Battersea Park. Hurry up, not just because it’s a cold day and it’s getting dark, but because this is so important.
If we can have a smarter socket, how about a smarter stump? This is what Dave Henson is working on, now he is back from Rio. He wants to build on the initial insights from back before he was injured, when physios and surgeons in the UK told surgeons at Bastion that the through-knee amputation was the best, leaving as much leg to work with as possible. His through-knee stump is evidence of that. He has a complete femur kneecap and an entire calf muscle that a plastic surgeon at Bastion carefully connected up to some surviving tendons, bundled up and tucked into the soft tissue around Dave’s remaining bone, just in case. This is all material that could be usefully recycled into some kind of knee – and any kind of knee is better than no knee at all. So, using maths, and scans and the kind of engineering models that are second nature to him, Dave is finding ways to optimise the surgical anatomy of the stump. He is aiming to give surgeons a formal anatomical model to guide them when they are presented with a leg blasted into fragments, random avulsive injuries, seemingly chaotic. They will be able to compare their patient with the Henson model and handle bones, muscles, tendons, ligaments on the operating table in a way that later on will be useful to the patient and their physio and their prosthetist. They will make their decisions about all the elements in the stump they are about to create, not just in case or just for now or for the time in the prosthetic workshop, but for a whole lifetime. The Henson model will take into account likely gait patterns, functionality, pain. It will project how the new stump will affect bone density in areas, such as hips, that will have to take more strain than they were originally intended for, so how to avoid creating areas that are likely to develop osteoarthritis and other conditions that cause fractures. The model won’t just be for surgeons; it will also guide engineers and designers in the development of better technology. That’s what optimised stump surgical anatomy is; if there has to be a stump, then make it the best possible, most useful, least painful stump there can be.
There are other ways to make the stump be all it can be. Make the skin on the stump better, more resilient, able to take the loads thumping down on it with every step. We do already have this kind of skin, on the palms of our hands but mainly on the heels of our feet. Here it is ten times thicker than anywhere else on the body, and it’s compliant, not brittle, it doesn’t break under strain of the big heel bone (the calcaneus) in the base of your foot thumping down on it with every step you take. When your shoes don’t fit properly, you get blisters on your ankle bones, on your toes, everywhere else, but not your heel, because here the skin has adapted to take the pressure.2 There’s a gene (HOXA13, if you’re interested) that determines protein levels in the epidermis, and that’s what makes your heels tough as old boots. If there could be heel skin on a stump where the residual bone comes thumping down with every step, then all those problems that come from ordinary skin not being great under strain might be resolved.3
It would mean reprogramming the skin at the genetic level, but this is another thing we already do for something else (hair transplants: move a hair follicle and it goes on making hair wherever it gets put because the skin around it changes into becoming hair-follicle-supporting skin). So cells that know they are heel skin cells could be transplanted into skin on other parts of the body (provided that body has heels left), which then start to operate like a heel and start to toughen up. We are only just starting to do this in the lab, but when it’s ready, one day, HOXA13 can go in a cream, and the amputee can use their smart socket data to see exactly where they should put it on their stump. All of this research is at an early stage; when I wrote this, the scientists who came up with it were just starting to fill out the forms for funding. That’s how science works. Someone has a really, really smart idea that could change the world, and then they fill out forms for funding.
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Postscript
Both projects got funding.