Life as an amputee can be difficult – memories of what you used to be able to do are all around and experienced on a daily basis. The current prosthetics available, using microprocessors and motors and bluetooth and carbon fibre, are far better than anything we’ve had before and allow amputees to carry out elements of daily life without hindrance. However, there is a long way to go before they even come close to biological limbs. I’ve picked out four of the technologies currently under development that I find most exciting to, hopefully, give an insight into where the prosthetics industry and amputee treatment are heading.
1. 3D Printing
3D printing is all the rage these days. At the Detroit Motor Show in January, a company called Local Motors unveiled their latest model of fully functioning 3D printed car and was featured on the BBC. The Open Hand Project is a crowd-funded start up project that utilised 3D printing to create a functioning robotic hand for use by arm amputees. Whilst not offering function above and beyond what is already on the market, the technologies involved allow a price tag around 100 times cheaper than some of the more advanced available options. What’s so exciting about 3D printing within prosthetics is the cost factor. Expensive manufacturing processes can be avoided as well as centralised production – designs can be emailed and produced locally, drastically reducing the logistical burden. Spare parts are equally accessible. The open-source nature of the Open Hand Project allows for customisations and modifications depending on user requirements. Components aren’t the only part of prosthetics that can benefit from 3D printing processes. This article from The Manufacturer talks about the University of Southampton’s research into 3D shape measurements and patient specific biomechanical analysis. Whilst this research is intended for rehabilitation analysis, it carries exciting implications for creating a patient specific, medically appropriate 3D printed socket. The Media Lab at MIT are also developing sockets that are not only easier to fabricate, but beneficial to the long term comfort and performance of the prosthetic limb. The VIPr socket uses 3D stump measurement and subsequent biomechanical analysis to create a ‘smart socket’ that allows both stiff and pliable areas of the socket depending on required contact pressure according to the user’s internal stump structure. Engineers on the Innovation and Design Engineering course at the Royal College of Art and the Imperial College, London, have used bone structure algorithms in conjunction with novel materials selection and 3D printing to create the Endura socket, designed with a high strength to weight ratio for maximum comfort and performance for high level athletes.
The flexibility and affordability of manufacture allow for customised designs to be easily produced without shipping, tooling and labour costs. 3D measurement techniques will lead to medically appropriate 3D printed sockets and innovations in 3D carbon fibre printing offer the high strength and durability required for the loads experienced during walking or running. These technologies will drastically reduce the time, cost and labour burden for prosthetic components. These reductions will allow the more amputees access to higher technology components as the cost is reduced and much shorter lead times for the fitting and delivery of sockets which will lead to a marked improvement in quality of life.
The scope of possibilities for 3D printing are already making steps into the medical world and go beyond the manufacture of traditional prosthetic components. Patient Specific Implants (PSIs) are already used in orthopaedic implants such as knee replacements and resurfacing. A CT scan is conducted and the 3D data is sent to the product manufacturer and a CAD code is created to machine a one-off PSI. As can be imagined, this often has large cost implications and a much longer product lead time compared to off-the-shelf products. Advances in printing using techniques grouped under the umbrella term ‘Additive Manufacturing‘ have allowed for the creation of products constructed from metal or ceramic powders. These techniques, used in the medical field, have received a large amount of press when a 3D printed jaw was manufactured from titanium powder and implanted into an 83 year old woman’s face in the Netherlands in 2011.
A very similar technique has recently been used in Israel to recreate the lower half of a patient’s face after he was injured in a rocket attack in Syria last year. The technology goes beyond jaws and bones. Ears, kidneys, blood vessels and skin grafts have all also been 3D printed, conveniently described in this short article from Popular Science. This ability to manufacture ‘replacement’ body parts is going to increase in quality and capacity, expanding the potential for stump lengthening and joint and limb recreation, described in more detail in the Limb Regeneration section.
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There’s been a remarkable amount of interest in the press recently about a medical technique called osseointegration, with respect to its application to prosthetic interfaces. This is a process by which an implant is firmly anchored into the bone structure and the technique is most commonly used in dental surgery, but is also used as an anchoring method in joint replacement implants and more recently as an innovative technique for directly attaching a prosthetic limb to the skeleton without the requirement for a socket. The resurgence of public interest in the technique in the UK is largely down to the use of an Australian technique on a British soldier. A quick Google search of the key words osseointegration and soldier brings up a whole raft of press pieces from the last few months. But why? What’s so exciting about this development?
Well, first off, you need to consider the components of the traditional prosthetic leg. As an above knee amputee you would have the liner, the socket, the prosthetic knee, the shank, then the ankle and then the foot. Those are the 6 main components that make up the majority of all prosthetic legs for that level of amputation. Some users prefer sockets that don’t require liners, some knee joints have the shank built in and a lot of feet incorporate a built in ankle unit of some description, so those are the key variations from user to user. Despite all the recent advances in prosthetic technology like the X3 knee from Otto Bock, the BeBionic hand from RSL Steeper or the BiOM foot and ankle, any amputee will tell you that if the socket doesn’t fit right, then it doesn’t matter what technology you’ve got underneath.
The socket is typically a hard plastic or composite carbon fibre shell fabricated from a plaster cast of the user’s stump. The socket is designed to give structural support to the user, secure the prosthesis to the body, and transfer energy from the body to the prosthesis for onward transmission to the ground in order to create forward motion (in the case of leg prostheses). This method of socket construction, whilst giving good support and structural strength, can often lead to severe discomfort for the user. The rigid outer shell allows little room for soft tissue volume fluctuations, such as happens in higher temperatures or after heavier exercise, and can often lead to muscle pain, sores or rashes for the user. Extended periods of sitting can also be uncomfortable, with the rim of the socket digging into the soft tissue of the thigh. Osseointegration applied to prosthetics aims to tackle these issues.
The procedure is usually carried out in two stages with an extended period of adjustment and rehabilitation following the second stage of implantation. The stump is opened at the distal end and a hole is bored into the femur. A titanium rod is press-fit into the bore and the skin is closed around the end of the rod. The rod is normally coated in a mesh-like structure (the Australian technique uses Spongiosa Metal) which allows for bone to heal into and around the titanium rod, fixing it in place biologically – hence osseo (bone) integration. This initial healing period takes around 6-8 weeks. After, an opening is created through the skin to the rod and a secondary fixing is attached to the end of the rod. This secondary fixing allows for the attachment of a prosthetic limb and usually incorporates a safety mechanism that will release the prosthetic device under a specified torque loading to prevent extreme stresses from being passed to the skeleton. Once the secondary fixing is in place weight bearing exercises can begin, sometimes as early as a few days after the procedure. From there it is a progressive transition to full weight bearing and walking.
What is so exciting about osseointegration is that it has the potential to take away a large proportion of the discomforts associated with wearing a prosthetic. The hard shell of the socket can be incredibly uncomfortable after extended periods of time in the same position – standing, sitting and even lying down can all become painful because of the way it presses into your soft tissue. That’s before considerations have been made to walking or exercising in general – then comes sweating and muscle expansion and can cause blisters or sores. By taking away the necessity for the hard shell, osseointegration can leave the amputee free to concentrate on life with a much improved comfort level.
It does, however, come with significant risk. The opening (or stoma) presents an uninterrupted pathway into the bone which creates a significant risk of infection. A silicon covering over the external entrance to the stoma reduces the risk, and proper hygiene can reduce it further. Proper administration of antibiotics will effectively clear most infections but deeper bone infections and inflammation can lead to osteolysis – the resoprtion of bone material – that reduces the structural integrity of the implant fixation and will usually require implant removal. However, clinical studies being conducted by Stanmore Implants in the UK are using deer antlers as a model in order to create a soft tissue seal that removes this risk (read the science here).
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Try this article about osseointegration in animals or this one from InMotion magazine from the Amputee Coalition or this one from the American Academy of Orthotists and Prosthetists or see this news clip from Forces TV
3. Neural Interface Prosthetics
The surge in popularity of the Paralympics, and with particular regard to the impressive performances from amputee athletes such as Jonny Peacock, Richard Whitehead and Marie-Amelie Le Fur have shown just what can be achieved with modern materials. Academically, an increased availability to prosthetics based research funding due to battlefield casualties in Iraq and Afghanistan has paved the way for technological advancement after technological advancement. One of the key areas that scientists and engineers are particularly interested in is the ability to link the microprocessor of the machine to the human brain – neural interface prosthetics. Currently, all microprocessor controlled (MPC) prosthetic knee joints that are on the market have no control system link to the user. Instead, sensors measure force vectors, knee angle, leg speed and leg position to feed information into a mathematical algorithm to dictates the knee response. The response is normally limited to speed of knee collapse (yield) and speed of leg swing, but in active knees such as the Ossur Power Knee it can also dictate motor torque and speed. Whilst the user can learn to use body position and leg movements to manipulate the response of the knee, the lack of integrated control system means movements are rarely sub concious (with the exception of perhaps level ground walking, after time). Using a form of neural interface offers the user a real time, natural control system. One of the forerunning techniques for mind-controlled prosthetics uses a technique called targeted muscle reinnervation (TMR). This process has primarily been trialled for use with arm amputees with amputation at shoulder level and involves reassigning nerve pathways that once controlled the amputated body part to a muscle that has little residual function due to the amputation. For shoulder level amputees, the nerves are normally reassigned to different pectoral muscles. The pectoral muscles are denervated (nerves cut away) and are reinnervated (replaced / reactivated) with those nerves that once controlled the arm and the hand. After a period of healing between 3 to 6 months, the nerve pathways will have begun innervating the pectoral muscles and when the patient thinks about moving their arm or hand, a contraction will occur in the reinnervated pectoral muscle. A sensor array placed over the chest picks up the electromyography (EMG) signals and microprocessors are used to filter and translate the signals into movement of the prosthetic arm. See this Journal of the American Medical Association article for more science.
Although the majority of the primary research and development into the use of TMR has been for use with arm amputees, the Rehabilitation Institute of Chicago (RIC) have developed a prosthetic leg that utilises TMR (in conjunction with some of the features normally found in MPC knee joints) to create the first prosthetic leg with a neural interface. This RIC press release gives an overview of the technology and this one highlights the global press that the research received.
However, like most emerging technologies, the techniques involved aren’t yet trouble free. The surgical and financial costs of TMR are extreme and hence why the technique tends to be restricted to shoulder amputees. The surface measurement of EMG signals is subject to interference from both external sources and other muscle groups, often restricting output to either ‘on’ or ‘off’ – rather than, as an example, flex the elbow at this speed with this force to this position. Implanting the electrodes into the muscle removes surface interference but can still be subject to other muscle group EMG interference, albeit at a reduced level. This technique allows for refined measurement and interpretation of data and hence a larger degree of control but comes at added surgical cost and the requirement for powering sub-surface signal measurement.
The US Department of Defense (DoD) has it’s own in-house research agency called DARPA (Defense Advanced Research Projects Agency) – and it’s awesome. Within their Biological Technologies Office sits two programs which should be of keen interest to amputees – Revolutionizing Prosthetics and RE-NET (Reliable Neural-Interface Technology). The Revolutionizing Prosthetics programme was started in 2006 as a result of high numbers of amputees returning from modern conflicts in Afghanistan and Iraq. It’s aim was to push the performance of upper limb prosthetics in line with the technologies available for lower limb amputees – an entirely different engineering challenge. However, after the creation of two advanced upper limb prosthetics, it was realised that the capabilities of the modern prosthetic would never be fully realised without an intuitive, high capacity user interface, hence the creation of the RE-NET programme. It’s ultimate aim is to develop viable technologies that allow intuitive, flexible and reliable neural control of prosthetic limbs. Incidentally, its Reliable Central Nervous System Interfaces programme (a sub-programme of RE-NET) has aims that are also applicable to repairing or bypassing damaged areas of the spinal cord to improve, increase or restore motor control function to spinal cord injury sufferers.
That’s why neural-interface prosthetics are so exciting. The human central nervous system is highly complex and naturally tuned to perform everyday limb functions. Tapping into this pre-programmed control system and using it to sub-consciously and intuitively control prosthetic limbs will help replicate lost function and improve user quality of life.
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4. Limb Regeneration
Limb Regeneration remains the holy grail of prosthetic technology – why bother with external prostheses if we can regrow a natural limb? Although it may sound like the stuff of science fiction, parts of the technology at least, are not that far off. Advances in the ability to map genetics have allowed scientists to map how the cells of a newt respond to limb amputation, and have been able to identify the processes that take place that allow the amphibian to fully regrow the limb over a period of around 30 – 90 days. This animation from BioInteractive is a good, basic, overview of what happens and this article from EuroStemCell goes into a bit more of the biology.
Although we are far from succeeding in unlocking individual cell response, we are closer than ever at identifying the complex chemical processes that occur during limb regeneration in these animals. This detailed biography from Matter about a Russian scientist called Michael Levin, working at Tuft’s University in Medford, USA, details a number of breakthroughs that may just hold the key for unlocking limb regeneration in species other than newts or salamanders. I would highly recommend reading the whole article.
Within a much smaller timeframe, existing practices in regenerative medicine, such as bone and cartilage and regeneration, will become much more mainstream. By using these techniques in conjunction with existing orthopaedic strategies, such as total knee replacement and bone repair will allow for the recreation of joints, represented graphically in this BBC Health clip. The recreation of joints and the increase in functionality that is implied is a huge step forward in prosthetic technology. For example, currently there are no realistic products on the market that recreate the true functionality of the knee joint for a through- or above-the-knee amputee. In this cohort of amputees, the quadriceps and hamstrings groups of muscles are reduced to stabilising functions and acting as part of the hip flexors / extensors rather than force generation at the knee. Using replacement and regeneration technologies that already exist to replicate not only the motion (kinematics – movement without consideration of force) of the knee joint, as approximated in conventional prosthetics, but also the ability to apply force at a chosen velocity for a chosen duration of time will re-open a whole avenue of functionality to the amputee – stair ascent and descent, cycling, acceleration and deceleration, perhaps even jumping. The implied quality of life benefits of such a technology are not only vast but timely and realistic. The use of 3D printing to create bone augmentation scaffolds will allow stump length to be increased – improving the mechanical lever arm and potentially allowing for more comfortable socket fit. Taking this one step further, replacement joints can be manufactured using similar processes and fitted to the end of the stump and reconnected to the different muscle groups, reinstating and recreating joint function.
Further into the future, improvements in bone and cartilage technologies will allow for the complete biological regeneration of joints such as the knee and elbow, regaining function lost at that level of amputation without artificial materials. As the stem cell technology improves and the ability to reprogramme cell transformation is perfected, complete limb regeneration may just become a reality.
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Each one of these technologies is exciting in its own right. 3D printing offers convenience, cost saving and speed of manufacture, not to mention customisation and enhanced design opportunities. 3D printed implants hint at a whole new level of surgical options for the amputee. Osseointegration looks to improve the comfort and feel of prosthetics to drastically improve the quality of life. Neural Interface technology looks to improve control systems so that prosthetics become more intuitive, more natural and offer an increase in functionality. Limb regeneration aims to negate the whole problem. Whilst some of them are available now and some will be in the not too distant future, others, like limb regeneration, have a much, much longer time-frame before they could even be considered a possibility. However, each small stage of progression – printing prosthetic limbs and joints, attaching a prosthesis directly to the skeleton, controlling a bionic arm and recreating the functionality of a lost joint – all point towards an incredibly hopeful future for amputees.