How Prosthetic Limbs Work

Ancient literature contains references to prosthetic limbs in stories and poems, but some of the earliest historical accounts of prosthetic limb use were recorded in Greek and Roman times. For instance, there's the historical account of of Marcus Sergius, a Roman general who lost his right hand while battling in the second Punic War. Famously, he had a replacement hand fashioned out of iron for the purpose of holding his shield and was able to return to battle and continue fighting.

In the year 2000, researchers in Cairo, Egypt, unearthed what they believe to be the oldest documented artificial body part -- a prosthetic toe made of wood and leather. The device, found attached to the nearly 3,000-year-old mumified remains of an Egyptian noblewoman, is a good representation of how little prosthetic limbs have changed throughout history. With the exception of very recent times, prosthetic devices have been constructed of basic materials, such as wood and metal, and held to the body with leather attachments.

To show how little prosthetic limbs have advanced through most of history, consider the artificial hands and legs of the Dark Ages -- nearly 2,000 years later. Armored knights of this era often relied on iron prosthetic limbs, usually crafted by the same metalworker who made their armor. These bulky limbs were admittedly not very functional and were actually used more for the purpose of hiding the lost limb, which was considered at the time to be an embarrassing deformity.

Most famously attributed to seafaring pirates, peglegs with wooden cores and metal hands shaped into hooks have actually been the prosthetic standard throughout much of history. While Hollywood has exaggerated their use of hooks and peglegs, pirates did sometimes rely on these types of prostheses. The required materials for these devices could be scavenged from a common pirate ship; however, a trained doctor would have been rare. Instead, the ship's cook typically performed amputation surgeries, albeit with poor success rates.

In the early part of the 16th century, French military doctor Ambroise Paré, also famous for his work with amputation techniques, contributed some of the first major advances in prosthetics seen for many years. Paré invented a hinged mechanical hand as well as prosthetic legs that featured advances such as locking knees and specialized attachment harnesses. Around 1690, a Dutch surgeon, Pieter Verduyn, later developed a lower leg prosthesis with specialized hinges and a leather cuff for improved attachment to the body. Amazingly, many of the advances contributed by these two doctors are still common features of modern day prosthetic devices.

With the advent of gaseous anesthesia in the 1840s, doctors could perform longer, more meticulous amputation surgeries, allowing them to operate on the limb stump in such a way as to prepare it for interfacing with a prosthesis. Advances in sterile, germ free surgeries also improved the success rate of amputation procedures, increasing the need for prosthetic limbs.

As artificial limbs became more common, advances in areas such as joint technology and suction-based attachment methods continued to advance the field of prosthetics. Notably, in 1812, a prosthetic arm was developed that could be controlled by the opposite shoulder with connecting straps -- somewhat similar to how brakes are controlled on a bike.

The National Academy of Sciences, an American governmental agency, established the Artificial Limb Program in 1945. The program was created in response to the influx of World War II veteran amputees and for the purpose of advancing scientific progress in artificial limb development. Since this time, advances in areas such as materials, computer design methods and surgical techniques have helped prosthetic limbs to become increasingly lifelike and functional.

How do modern prosthetic limbs compare to those of historical times? One major difference is the presence of newer materials, such as advanced plastics and carbon-fiber composites. These materials can make a prosthetic limb lighter, stronger and more realistic. Electronic technologies make today's advanced prosthetics more controllable, even capable of automatically adapting their function during certain tasks, such as gripping or walking.

While new materials and technologies have certainly modernized prosthetics over the past century, the basic components of prosthetic limbs remain the same. Let's go over some of these.

The pylon is the internal frame or skeleton of the prosthetic limb.  The pylon must provide structural support and has traditionally been formed of metal rods. In more recent times, lighter carbon-fiber composites have been used to form the pylons. The pylons are sometimes enclosed by a cover, typically made from a foam-like material. The cover can be shaped and colored to match the recipient's skin tone to give the prosthetic limb a more lifelike appearance.

The socket is the portion of the prosthetic device that interfaces with the patient's limb stump or residual limb. Because the socket transmits forces from the prosthetic limb to the patient's body, it must be meticulously fitted to the residual limb to ensure that it doesn't cause irritation or damage to the skin or underlying tissues. A soft liner is typically situated within the interior of the socket, and a patient might also wear a layer of one or more prosthetic socks to achieve a more snug fit.

The suspension system is what keeps the prosthetic limb attached to the body. The suspension mechanism can come in several different forms. For example, in the case of a harness system, straps, belts or sleeves are used to attach the prosthetic device. For some types of amputations, the prosthetic is able to stay attached just by fitting around the shape of the residual limb. One of the most common types of suspension mechanisms relies on suction. In this scenario, the prosthetic limb fits snugly onto the residual limb, and an airtight seal keeps it in place.

Though most prosthetic limbs have these basic components in some form, each device is unique and designed for a specific type and level of amputation. Whether an amputation is above or below major joints, like the elbow or knee, makes a big difference in what type of prosthetic limb is required. For example, a transfemoral amputation -- an amputation above the knee -- requires a prosthetic device with an artificial knee, while a transtibial amputation -- an amputation below the knee -- allows the patient to retain the use of his or her own knee.

So now we know the components that make up a prosthetic device, but how are prosthetic limbs made, anyway?

Because each patient and his or her amputation are unique, each prosthetic limb must be custom fitted and then built. This is the task of a prosthetist, who specializes in the fabrication and fitting of prosthetic limbs. Because prosthetists work to interface artificial devices with the human body, they need a wide range of skills in areas such as engineering, anatomy and physiology.

The design and fabrication process consists of several different steps and begins with a precise measurement process later used to design the prosthetic limb. If possible, a prosthetist begins taking measurements before the patient's limb is even amputated, so that the fabrication process can get started. For example, detailed measurements of the patient's body are taken to help correctly size the prosthetic limb. The prosthetist and doctor also meet before the surgery to discuss details of the operation.

Several weeks after the amputation surgery, once the wound has had a chance to heal and the swelling has gone down, a plaster mold is taken of the residual limb. This mold then serves as a template for making a duplicate of the residual limb. The duplicate of the residual limb is then used to test the fit of the prosthetic limb as it's being built. Newer technologies allow computerized digital measurements to be taken as well. Careful attention is also paid to the structure of the patient's residual limb, including the location of any muscles, tendons and bones. The health of the patient and condition of the skin are other factors taken into account when designing the prosthesis.

Physical therapy after an amputation and prosthetic device fitting is extremely important. Learning to walk with a prosthesis can be an especially difficult undertaking, requiring several months of rehabilitation and training. Therapy might also focus on using the prosthetic device to perform important everyday activities. For a leg prostheses, the prosthetist carefully monitors the walking gait of the patient and makes adjustments as necessary.

The prosthetist pays especially close attention to the interface between the patient's residual limb and the prosthetic socket. After an amputation, a patient's residual limb will typically shrink over the course of several months as swelling diminishes and muscles begin to atrophy, or shrink from lack of use. It's possible that new sockets may need to be fitted to accommodate the reduction in size. Layers of sock-like dressings can also be varied to accommodate for the changing size of the residual limb. A prosthetist must work especially closely with children, to make sure that their prosthetic limbs are resized or replaced as necessary to keep up with their natural growth.

A patient will continue to visit the prosthetist throughout his or her life, since residual limbs can always change shape and prosthetic devices eventually break down. In fact, according to the National Limb Loss Information Center, an average prosthetic device has a lifespan of only three years.

Read on to find out how a patient is able to control a prosthetic limb.

Different types of prosthetic limbs are designed with different goals in mind. Often these goals depend on the site of the amputation and the needs of the patient.

For example, a cosmetic prosthetic limb, called a cosmesis, is designed with appearance in mind rather than controllability. Advanced plastics and pigments uniquely matched to the patient's own skin tone allow a modern day cosmesis to take on an amazingly life-like appearance. Even details such as freckles, hair and fingerprints can be included, bringing the cosmesis to the point where it's nearly indistinguishable from the original missing arm or leg.

Other prosthetic limbs are designed with usability and function as a central purpose. As an example, a common controllable prosthetic hand might consist of a pincer-like split hook that can be opened or closed to grip objects or perform other types of tasks. This type of prosthetic device can be covered with a glove-like covering to make it appear more like a natural hand. Functional prosthetic limbs can actually be controlled in a variety of ways.

Body-powered prosthetic limbs are controlled by cables connecting them to elsewhere on the body. For example, a prosthetic arm can be controlled through a cable attached with a strap or harness to the opposite, healthy shoulder. The working shoulder is then moved in certain ways to control the prosthetic device -- similar to how you might use a hand lever on your bike to control the brakes.

Externally powered prosthetic limbs are powered with motors and can be controlled by the patient in several ways. The switch control method allows a patient to move his or her prosthetic device by toggling switches or buttons. The patient toggles the switches using the opposite shoulder, or he or she might be able to use remaining muscles in the residual limb to push the switches. Because a prosthetic hand or arm can perform a wide variety of motions, different sequences of switch toggling might be required to perform desired tasks.

A more advanced way to control a prosthetic limb is by listening to muscles remaining in the residual limb that the patient can still contract. Because muscles generate small electrical signals when they contract, electrodes placed on the surface of the skin can measure muscle movements. Although no buttons are physically pressed by the muscles in this case, their contractions are detected by the electrodes and then used to control the prosthetic limb -- in a way similar to the switch control method that was just described. Prosthetic limbs that function in this way are called myoelectric.

When a prosthetic arm has several joints, such as a transhumeral, or above-elbow, prosthesis, each joint might need to be controlled by the same switch or muscle. To accomplish this, sequential control methods allow one joint be positioned at a time. For example the patient might first use a switch or muscle contraction to signal for the prosthetic limb to bend the elbow joint, then signal for the prosthetic hand to close in order to grip an object.

Advanced lower extremity prostheses are equipped with a variety of mechanisms that help them to move naturally as a patient walks or runs. A prosthetic knee is particularly difficult to engineer, as it must constantly adjust to allow for normal walking, standing and sitting. Advanced artificial legs have a computer-controlled knee that automatically adapts to adjust to the patient's walking style.

Unfortunately, the price of prosthetic limbs tends to be very high. This is especially true of the prosthetic limbs containing electronic components. In fact, myoelectric prostheses and prostheses equipped with computer-controlled knees can cost many tens of thousands of dollars.

So, do you think these prosthetics are the most advanced on the market? Well, researchers and scientists have taken prosthetics to the next level. Read the next page to find out how.

One of the most cutting-edge technologies used to control prosthetic limbs is called targeted muscle reinnervation (TMR) and was developed by Dr. Todd Kuiken at the Rehabilitation Institute of Chicago. To understand TMR, you need to know some basic physiology. Your brain controls the muscles in your limbs by sending electrical commands down the spinal cord and then through peripheral nerves to the muscles. Now imagine what would happen to this information pathway if you had a limb amputated. The peripheral nerves would still carry electrical motor command signals generated in the brain, but the signals would meet a dead end at the site of amputation and never reach the amputated muscles.

In the surgical procedure required for TMR, these amputated nerves are redirected to control a substitute healthy muscle elsewhere in the body. For example, the surgeon might attach the same nerves that once controlled a patient's arm to a portion of the patient's chest muscles. After this procedure, when the patient attempts to move his or her amputated arm, the control signals traveling through the original arm nerve will now cause a portion of chest muscles to contract instead. This is valuable, because the electrical activity of these chest muscles can be sensed with electrodes and used to provide control signals to a prosthetic limb. The end result is that just by thinking of moving the amputated arm, a patient causes the prosthetic arm to move instead.

If electrodes can sense the electricity caused by muscle contractions, why can't they just go to the source of the information and measure the electrical signals carried in the nerves, or even the brain? The answer is that they can, but recording from the brain and nerves is more challenging for several reasons. For example, electrical signals in the brain and nerves are very small and hard to access. The field of neural interfacing is dedicated to developing ways to listen and communicate with the brain and nerves.

As an example of neural interfacing technology, scientists can implant micro-scale electrodes in the brain to listen in on brain activity. When the patient mentally tries to move his or her amputated limb, the microelectrodes can intercept motor command signals generated in the brain, and these signals can then be used to control a prosthetic device. One exciting implementation of this technology comes from lab of Dr. Miguel Nicolelis lab at Duke University. Remarkable video footage documents the ability of monkeys implanted with microelectrodes to use their thoughts to control a prosthetic arm to feed themselves snacks.

Future advances in neural interfacing will allow artificial devices to more effectively stimulate the nerves or brain in order to restore a sense of touch and allow patients to feel their artificial limb. This capability will go a long way in closing the gap between prosthetic limbs and the natural limbs they're designed to replace.

These types of technological innovations are just some of the examples that show how the field of prosthetics is constantly advancing. While the challenges are great, remarkable progress has been made over the past few decades, and dedicated researchers around the globe are working each day to make prosthetic limbs as close as possible to the real thing.

Follow the links on the next page for lots more information about prosthetic limbs and related topics.