3D Printing and the Dream of Affordable Prosthetics
As amazing as the human body is, it’s unfortunately not as amazing as e.g. axolotl bodies are, in the sense that they can regrow entire limbs and more. This has left us humans with the necessity to craft artificial replacement limbs to restore some semblance of the original functionality, at least until regenerative medicine reaches maturity.
Despite this limitation, humans have become very adept at crafting prosthetic limbs, starting with fairly basic prosthetics to fully articulated and beautifully sculpted ones, all the way to modern-day functional prosthetics. Yet as was the case a hundred years ago, today’s prosthetics are anything but cheap. This is mostly due to the customization required as no person’s injury is the same.
When the era of 3D printing arrived earlier this century, it was regularly claimed that this would make cheap, fully custom prosthetics a reality. Unfortunately this hasn’t happened, for a variety of reasons. This raises the question of whether 3D printing can at all play a significant role in making prosthetics more affordable, comfortable or functional.
What’s In A Prosthetic
The requirements for a prosthetic depend on the body part that’s affected, and how much of it has been lost. In the archaeological record we can find examples of prosthetics dating back to around 3000 BCE in Ancient Egypt, in the form of prosthetic toes that likely were mostly cosmetic. When it came to leg prosthetics, these would usually be fashioned out of wood, which makes the archaeological record here understandably somewhat spotty.
While Pliny the Elder made mention of prosthetics like an iron hand for a general, the first physical evidence of a prosthetic for a lost limb are found in the form of items such as the Roman Capua Leg, made out of metal, and a wooden leg found with a skeleton at the Iron Age-era Shengjindian cemetery that was dated to around 300 BCE. These prosthetics were all effectively static, providing the ability to stand, walk and grip items, but truly functional prosthetics didn’t begin to be developed until the 16th century.
These days we have access to significantly more advanced manufacturing methods and materials, 3D scanners, and the ability to measure the electric currents produced by muscles to drive motors in a prosthetic limb, called myoelectric control. This latter control method can be a big improvement over the older method whereby the healthy opposing limb partially controls the body-powered prosthetic via some kind of mechanical system.
All of this means that modern-day prosthetics are significantly more complex than a limb-shaped piece of wood or metal, giving some hint as to why 3D printing may not produce quite the expected savings. Even historically, the design of functional prosthetic limbs involved complex, fragile mechanisms, and regardless of whether a prosthetic leg was just static or not, it would have to include some kind of cushioning that matched the function of the foot and ankle to prevent the impact of each step to be transferred straight into the stump. After all, a biological limb is much more than just some bones that happen to have muscles stuck to them.
Making It Fit
Perhaps the most important part of a prosthetic is the interface with the body. This one element determines the comfort level, especially with leg prostheses, and thus for how long a user can wear it without discomfort or negative health impacts. The big change here has been largely in terms of available materials, with plastics and similar synthetics replacing the wood and leather of yesteryear.
Generally, the first part of fitting a prosthetic limb involves putting on the silicone liner, much like one would put on a sock before putting on a shoe. This liner provides cushioning and creates an interface with the prosthesis. For instance, here is an instruction manual for just such a liner by Össur.
These liners are sized and trimmed to fit the limb, like a custom comfortable sock. After putting on the liner and adding an optional distal end pad, the next step is to put on the socket to which the actual prosthetic limb is attached. The fit between the socket and liner can be done with a locking pin, as pictured on the right, or in the case of a cushion liner by having a tight seal between the liner and socket. Either way, the liner and socket should not be able to move independently from each other when pulled on — this movement is called “pistoning”.
For a below-knee leg prosthesis the remainder of the device below the socket include the pylon and foot, all of which are fairly standard. The parts that are most appealing for 3D printing are this liner and the socket, as they need to be the most customized for an individual patient.
Companies like the US-based Quorum Prosthetics do in fact 3D print these sockets, and they claim that it does reduce labor cost compared to traditional methods, but their use of an expensive commercial 3D printer solution means that the final cost per socket is about the same as using traditional methods, even if the fit may be somewhat better.
This highlights perhaps the most crucial point about using 3D printing for prosthetics: to make it truly cheaper you also have to lean into lower-tech solutions that are accessible to even hobbyists around the world. This is what for example Operation Namaste does, with 3D printed molds for medical grade silicone to create liners, and their self-contained Limbkit system for scanning and printing a socket on the spot in PETG. This socket can be then reinforced with fiberglass and completed with the pylon and foot, creating a custom prosthetic leg in a fraction of the time that it would typically take.
Founder of Operation Namaste, Jeff Erenstone, wrote a 2023 article on the hype and reality with 3D printed prosthetics, as well as how he got started with the topic. Of note is that the low-cost methods that his Operation Namaste brings to low-resource countries in particular are not quite on the same level as a prosthetic you’d get fitted elsewhere, but they bring a solution where previously none existed, at a price point that is bearable.
Merging this world with that of of Western medical systems and insurance companies is definitely a long while off. Additive manufacturing is still being tested and only gradually integrated into Western medical systems. At some level this is quite understandable, as it comes with many asterisks that do not exist in traditional manufacturing methods.
It probably doesn’t bear reminding that having an FDM printed prosthetic snap or fracture is a far cry from having a 3D printed widget do the same. You don’t want your bones to suddenly go and break on you, either, and faulty prosthetics are a welcome source of expensive lawsuits in the West for lawyers.
Making It Work
Beyond liners and sockets there is much more to prosthetic limbs, as alluded to earlier. Myoelectric control in particular is a fairly recent innovation that detects the electrical signals from the activation of skeletal muscles, which are then used to activate specific motor functions of a prosthetic limb, as well as a prosthetic hand.
The use of muscle and nerve activity is the subject of a lot of current research pertaining to prosthetics, not just for motion, but also for feedback. Ideally the same nerves that once controlled the lost limb, hand or finger can be reused again, along with the nerves that used to provide a sense of touch, of temperature and more. Whether this would involve surgical interfacing with said nerves, or some kind of brain-computer interface is still up in the air.
How this research will affect future prosthetics remains to be seen, but it’s quite possible that as artificial limbs become more advanced, so too will the application of additive manufacturing in this field, as the next phase following the introduction of plastics and other synthetic materials.