I’ve written on this blog before about how advances in our understanding and application of genetic engineering and stem cell technology is raising the realistic possibility of growing replacement human organs. What I haven’t really covered is replacing limbs. This is a somewhat different proposition: if we grew human organs in a dish (so to speak), we’d transplant them into the people that needed them. These kinds of transplant operations now have a substantial surgical history and practice behind them, so it wouldn’t require the development of new techniques. Replacing limbs, however, does not: people have instead relied on artificial prostheses. These are a staple of science fiction too: from Luke Skywalker to the 6 million dollar man to Robocop, prosthetics and – at the extreme end – full human “cyborgs” are everywhere. Replacing limbs with actual biological limbs, however, well…the first thing that springs to mind is Frankenstein, which is unfortunate. There are a few scifi societies where regrowing replacement limbs is the norm (notably lain M Banks’ Culture) but scifi seems to think prostheses are the future, as they are our present. But are they?

Despite the dominance of prostheses, there have been some notable firsts recently in transplanting that most sophisticated human appendage, the hand. Last month, there was remarkable success reported in a child who received a double hand transplant. This boy has gained a quite astonishing use out of his new hands, being able to now use scissors and play baseball, for example, but that’s come with extensive physiotherapy and psychological counselling. It’s also the case that he wouldn’t have been considered for the operation so young because of the need to take lifelong immunosuppressants, but for the fact that he’d previously received a donor kidney from his mother and was taking them anyway. Without these drugs, the immune system would attack the donor tissue and destroy it, leading to graft rejection.
Why are hand transplants a relatively new thing? And why aren’t we trying foot or leg transplants as well? Well, although replacing limbs and organs feels like it should be the same sort of problem, in many ways it’s not. Organs such as kidneys don’t need to move; a leg and particularly a hand needs to be able to perform a wide variety of movements. Legs also need to be able to bear weight (which is one reason why leg transplants aren’t done: the pressure on a surgically attached leg is too damaging). A kidney, however, whilst it is never going to need to move, is an incredibly complex biological tissue which performs highly technical filtration; similarly a liver is a biochemical wizard, performing all sorts of metabolic reactions. Making synthetic versions of those organs is so technically challenging it would be far easier to grow biological ones from stem cells, which I suspect is what will happen first. This would also potentially overcome the problem of having to take immunosuppressant drugs to prevent transplant rejection, if the organs can be grown from the patient’s own cells.
Are human hand transplants the future? Well, I would say probably not at the moment, because prosthetic technology has come along so far. Gone are the days when someone just wears a wooden leg, or a plastic arm with a hook on the end. For example, another recent story highlighted a new type of knee prosthetic that is cheap, durable and will allow the user to walk and work on uneven ground. This will be critical in helping many young amputees, particularly in developing countries, find employment rather than living in poverty.
Making an artificial human hand that can mimic even a fraction of the many complex movements a natural hand can make, however, is always going to be difficult. Our incredible manual dexterity sets us apart from pretty much every other species on the planet. But there have been huge advances in this field too: the development of new materials, microprocessors, bionics and 3D printing, to say nothing of more sophisticated software and user interfaces, is transforming what an artificial hand is capable of doing.
I was really struck by just how advanced robotic hands had become when I went to the
Science Museum’s Robots exhibition. There was a robotic hand there made by Shadow that could perform the full range of human hand movements (just about). It was eerily beautiful to watch. Here’s a video of Shadow’s dexterous hand doing a job I know well, pipetting (starts about 50 seconds in). This isn’t just to look clever: it could be really useful to get a robot to do that sort of job where dangerous pathogens are concerned.
Of course, that’s a robotic hand, not a prosthetic, but these too have developed. Here’s a video of what is currently the most advanced bionic hand prosthetic, the bebionic. It looks amazing, doesn’t it? Very science fiction, as the user herself notes with delight.
Luke Skywalker and his prosthetic hand…compared to the bebionic
There are prosthetics that are more lifelike in appearance, but they’re far less functional. So, if it’s this versus a transplant, what would you choose? My personal inclination is to say the prosthetic, but then I’m not an amputee, so I’ve no idea what I would really feel like in such circumstances. There would also be a number of critical medical considerations, such as the level at which the limb had been amputated, whether it’s a single or double amputation, the age of the patient, etc.
It’s also important to remember that it’s not just about movement, but also about sensation, appearance and the psychological impact of something that isn’t your arm stuck where your arm used to be. Remember that sensation too isn’t just about being able to feel things, but sensory feedback – to feel how much pressure to put on an object to hold it successfully, for example. Over the past 25 years, about a fifth of prosthetic upper limb users have actually stopped using them (1). Most prostheses available make it very difficult to perform essential everyday tasks such as personal grooming and hygiene. If you’ve only lost one hand, a prosthetic may be a better option: if you’ve lost two, or if you also have issues e.g. with blindness (common for example amongst soldiers who have lost limbs in explosions) then the superior sensory feedback and movement of a transplant may be a better option. It’s notable that a recent study comparing outcomes of prosthetic versus transplanted hands found little functional difference, but significantly improved psychosocial outcomes for transplanted hands. These patients had no other impairment such as blindness, however (2).
So what’s for the future? I still think prosthetics will edge out over transplants when it comes to hands, at least for now. There is ongoing research into providing tactile feedback for prosthetic devices, and more sophisticated pattern-recognition control algorithms. Whilst there remains a need to take immunosuppressant drugs, transplants are not likely to become the standard of care except in particular cases, such as bilateral amputations and cases where there is also visual impairment. It’s also the case that, unless limbs can one day be grown, the availability of donors will always be limiting. One can even see cases in the not too distant future where a human might have a replacement pancreas grown from their own stem cells, and an artificial prosthetic limb, and that this would seem perfectly normal.
(1). Salminger et al, 2016. Hand transplantation versus hand prosthetics: Pros and cons. Curr Surg Rep. 2016; 4: 8.  doi: 10.1007/s40137-016-0128-3. Featured image taken from here (creative commons).
(2). Salminger, et al, 2016. Functional and Psychosocial Outcomes of Hand Transplantation Compared with Prosthetic Fitting in Below-Elbow Amputees: A Multicenter Cohort Study
PLoS One. 2016; 11(9): e0162507. doi: 10.1371/journal.pone.0162507

2 thoughts on “The future of artificial limbs

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