The Tech Behind: Prosthetics

Maximum PC Staff

The most concise definition of a prosthetic is any device that replaces a missing body part, and whether you know it or not, prosthetics are a part of our daily lives. From banal applications such as dental crown, to complicated devices which compensate for a life altering injury sustained in an accident or enable an individual who lacks a fully functional body part due to an accident of birth, prosthetics are an awesome technology that the majority of us are fortunate enough to be able to take for granted.

The earliest mention of prosthetic devices date back to the Egyptian New Kingdom era (between 1600 BC and 1100  BC), with archeological evidence such as a wooden toe attached to the foot of a mummified Egyptian corpse proving that ancient man wasn’t any more down with living without all of his physical faculties than we are today. Around the same time that Pompeii was devoured by the eruption of Mount Vesuvius, there was said to be a Roman General bopping around with an iron arm he commissioned from a blacksmith so that he could still hold a shield during combat.

Despite these impressive feats of early ingenuity, prosthetic devices as we understand them today didn’t really start coming into their own until the 14th  and 15th centuries during the Renaissance, when the development of man-made body parts underwent—let’s just call it what it is—a renaissance.  At that time, functional prosthetics were only obtainable by the most affluent of people, and were most often made of wood, steel, copper and iron. The most complex of their devices sought to not only replace a missing appendage, but in some cases, primitively mimic its functionality.

A confirmed example of this technology was used by Götz von Berlichingen: a German mercenary who saw active combat in the mid 1400s. von Berlichingen was said to have had a pair of iron hands that could be articulated into various positions through the tightening or loosening of springs contained within the prosthetics. This made von Berlichingen capable of using a fork, or when the situation warranted, swinging a sword.

In the 1500’s, Ambroise Paré, Royal Surgeon to a number of monarchs of the day, pushed prosthetic technology forward once again when he invented a wooden leg and foot prosthesis the user attached to his body with a leather harness and featured a knee joint that could be unlocked so that the wearer could in effect, kneel. It could be argued that innovations like this and the ones seen in von Berlichingen’s hands were the progenitors of the modern limb prosthetic technologies that millions of people benefit from today—technologies that go beyond providing a specific function—such as holding a shield in battle—instead, offering their users a tool that can be applied to multiple applications every day of their lives.

Over the past 30 years, lower and upper extremity prosthetic technology has been glut with innovation and breakthroughs, thanks in no small part to organizations like The International Society for Prosthetics and Orthotics, which was founded back in 1970 in order for physicians and researchers to share their prosthetics work and findings at conferences, meetings and via the organization’s technical journal.

When you’re talking about modern prosthetics, one of the first things that must be understood, is that playing pound it ’til it fits with an artificial appendage, just isn’t cool. There’s a reason why you can’t just nip out to a pharmacy and pick up an artificial arm or leg: For the fullest range of viability, a prosthetic limb must be matched to its intended user, which isn’t a simple task. Before a fitting can occur, the type of amputation that an individual has undergone must be assessed by a physician or prosthetist so that it can be decided what manner of artificial limb, if any, can be utilized. If a prosthetist finds that a device can be matched to an individual’s upper or lower level amputation, the amputee will be sent for a fitting. No matter what manner of amputation a prosthetic is designed to deal with, the majority of modern prosthetic appliances incorporate a number of elements.

A typical limb prosthetic will be fitted with a custom designed socket, which has been crafted to mate with a specific individual’s residual limb (the part of an arm or leg left after an amputation has occurred). This socket is most often designed by a prosthetist utilizing a Computer Aided Design program to map an amputee’s residual limb. The CAD information is subsequently paired with a Computer Aided Manufacturing process, to build a duplicate of the residual limb that is used to construct a positive segment of a mold from. In order to make the negative segment, a sheet of thermoplastic then heated until it becomes pliable, at which point it is place in a vacuum chamber with the positive mould. Once the chamber has been sealed, the air is removed, creating a—you guessed it—vacuum, that collapses the sheet of thermo plastic down around the positive mold, creating the mold's negative. The two halves of the mold are then used to make a socket for the limb. As an amputated limb may shrink or otherwise change its shape over time, it may be necessary for an amputee to have several sockets made for them as time passes.

Even with the socket being built thanks to CAD/CAM software and hardware, a socket may still prove uncomfortable for an amputee to wear. To mitigate this, many people opt to wear a suspension system that consists of Velcro or leather strapping and a liner—often made of silicone or urethane—which mates with the prosthesis’ socket through the use of a series of pins. Under this, a tailored sheath is worn to sheath the residual limb where it interfaces with the prosthetic. Wearing this sheath, often referred to as a “stump sock”, helps to prevent chaffing and inflammation due to bacterial infections caused by perspiration. To ensure the maximum benefit is received by a an amputee wearing a stump sock, the sheaths are often made using moisture-wicking materials such as Dupont’s CoolMax fabric and can also incorporate an X-Static weave which utilizes pure silver, a materiel that retards bacterial growth… and werewolves. While the use of a liner and stump sock are often the de rigueur, some devices, such those worn by many above the knee amputees, may utilize what is called a standard suction socket, disallows for additional appliances to make it more comfortable.

A custom-fitted socket isn’t any good to anyone without something connected to it. Moving outwards from the socket, most modern prosthetics also include an internal skeleton, often referred to as a pylon. Traditionally, the pylon was made of steel. Fortunately, steel has given way to lighter, high-strength materials such as titanium, aluminum and most recently, carbon fiber. For aesthetic purposes, the pylon is often covered in polypropylene foam, which has been sculpted and colored to match the skin color and shape of the amputee’s sound limb whenever possible. Surprisingly, despite the availability of any number of modern, light-weight materials some components, such as the foot on an artificial leg often still contain an internal wooden core with a urethane foam and rubber exterior, as the combination of materials provide for an excellent balance of shock absorption and durability. However, times are changing for fake feet as well, with many amputees favoring carbon fibre over traditional dead tree materials, as the use of the synthetic further reduces the weight of the device.

And now for the cool stuff.

While socket and pylon prosthetics have allow their users the look and a limited amount of additional mobility or dexterity that comes from their application, they were no where near close to being a true replacement for the absence of a whole leg or a hand. Other than the materials utilized, not much could be said to have changed in the design of artificial limbs since the opening of the 20th century. At that time, an amputee by the name of D.W. Dorrance invented a prosthetic arm with a split hook for a hand, designed to allow the wearer to open and close the device’s hand by tightening or loosening a set of heavy elastic bands running across his back to his other shoulder. While this basic design has been getting the job done for pedestrian assignments such as holding a tube of toothpaste or a drinking cup for over a century, it’s not exactly a high-performance solution, even once the technology to augment the design with assistive motors and electric switches became available.

Enter myoelectric devices. “Myoelectric”, you ask? Yeah, myoelectric. Unlike mechanically controlled prosthetics, an artificial limb that utilizes myoelectrics is controlled by voluntary muscle contractions in the wearer’s residual limb. The electrical activity caused by the user’s muscle activity is captured by sensors in contact with the flesh of an amputee’s residual limb. By taking the time to train the muscles in a residual limb, a skilled owner of a myoelectric-controlled limb could effect the rotation of their mechanical wrist, open or close their prosthetic’s hand or move an artificial elbow with nothing more than a few minute muscle twitches.

But the science doesn’t end there: In the 47 years since the first crude myoelectric arm was built by the former USSR’s Central Prosthetics Research Institute, scientists have been hella busy refining and improving upon the basic soviet design; refining the arm over the years as breakthroughs in material science, microprocessors and other technologies became available for inclusion in their research. One of the coolest breakthroughs in recent years comes to us thanks to funding provided by the U.S. Army Research Office and the Defense Advanced Research Project Agency (DARPA) to Deka, a company founded by the man who gave us the Segway, Dean Kamen. The fruit of Deka’s labor is the DEKA, or “Luke” (named after Star Wars’ Luke Skywalker) Arm, a device that’s as awesome as anyone on a Segway is snicker-worthy.

With breakthroughs as innovative as this, just think of where the state of the art for prosthetic technology will lay twenty years down the road. Mind-controlled artificial limbs? Implanted micro-processor aided gray-matter interfaces? What will pass for cutting-edge in the field of prosthetics two decades from now or even tomorrow for that matter is anyone's guess. We'll say this however: The future of physically enabling technologies has never looked brighter, or cooler than it does today.

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