Following amputation or the loss of a limb, prosthetics allow a person to regain some of the lost function. Many prosthetics help amputees regain a sense of independence and freedom. Originally, prosthetics were fairly simple and only assisted with basic functions. Modern prosthetics are significantly more complex, and thus allow for many more actions similar to the biological limb. In the future, prosthetists hope to create prosthetics that function identically, or even better.
A vast majority of early prosthetics were passive; they were entirely immovable and essentially cosmetic. Most early prosthetics were held in place with leather straps. Tales of the Roman general Marcus Sergius describe his use of an iron hand that could hold his shield in combat. Pathologists have found multiple ancient Egyptian prosthetics consisting of wood and leather. During the middle ages, knights who were missing limbs could use special prosthetics to hold shields, lances, and swords.
Prior to the 20th century, most prosthetic sockets were square and had limited space. This presented various issues, most notably pain and lack of comfort. In the 1980s, John Sabolich created the Contoured Adducted Trochanteric-Controlled Alignment Method or CAT-CAM socket for leg prosthetics. Unlike previous prosthetic sockets, the CAT-CAM sockets had special shapes that could contain muscular tissue more comfortably and reached up to the pelvis. Additionally, they had a bony lock that held the femur in place for certain motions, allowing a more even distribution of body weight and better support.
Most prosthetic attachment methods require a liner that fits over the residual limb. This provides a more comfortable fit and alleviates issues such as sweating. Once the liner is on the limb, there are a few possible methods of attachment:
One of the biggest innovations in the field was the advent of prosthetics moveable using existing muscles. Many upper limb prosthetic users use choose body harnesses that allow them to move their prosthetic elbow, hand, or hook with basic movements. Essentially, a cable connects the prosthetic to a harness that straps along the shoulders and back. Body manipulation pulls the cable and cause certain motions. A shrug of the shoulders may cause the hand to contract, allowing a person to grab and hold objects.
Prosthetists, or individuals who create prosthetics, are constantly trying to address issues that hinder their clients. For example, many people with leg and foot prosthetics experience fatigue, asymmetrical gait, and limited walking speeds. With springs and motors, prosthetists can create prosthetic ankles with variable stiffness levels that address these issues. Other issues research is attempting to solve include stability problems on uneven ground. By using a motor to rotate the ankle and foot, some prosthetics can automatically adjust for irregular terrain.
In most circumstances, complex prosthetics are costly. However, with the rise of 3D printing, some body-powered prosthetics require less than 150 dollars-worth of materials. The printers create each piece individually, and the prosthetics then require assembly. Thanks to flexible rubber tendon cords, the prosthetic becomes entirely body-powered. Popular 3D-printed prosthetics include hands that use wrist movements to close. There are options for 3D-printed prosthetics that attach to body harnesses. Printed prosthetics are great for children, as the prosthetics are highly customizable and lightweight. Additionally, because children grow quickly, they often require constant and expensive adjustments to their new limbs. 3D printers make this as simple as printing another piece.
Though body-powered prosthetics are easy to use, they do lack a certain level of complexity. Newer prosthetics use motorized components for more actions and a higher level of control, but they are much harder for people to use. An electrical sensor sits just over certain muscles in the residual limb. The sensor detects muscle movement and tells the prosthetic to perform an action. This is similar to how biological actions occur. To make these prosthetics more intuitive, researchers are considering attaching electrodes directly to the nerves.
Of the drawbacks of prosthetics, one of the more complex is the lack of feedback. When a biological hand touches an object, the body receives a variety of inputs. The hand can detect texture, temperature, size, and other attributes of the object. Additionally, it can sense how much pressure is required to pick it up. Prosthetics can’t do these things, so researchers use small tricks to simulate these signals. One method that achieves this is using small mechanical parts to squeeze the upper arm as the hand contracts. Another method uses vibrations.
When most people think of prosthetics, they think of complete limb replacements. However, as technology improves, even people without amputations may benefit from prosthetics. Some biomedical engineers are creating exoskeletons for individuals with leg paralysis. Many of these individuals can move slightly, but not enough to support their weight or walk. Researchers connect electrodes to the cortex, spinal cord, peripheral nerves, or nerve endings in muscles. These electrodes can sense the intent of a movement and send a signal to the exoskeleton. The suit then relies on a complex suspension system to move the exoskeleton as the person intends.
The largest flaw of current motorized prosthetics is their inaccuracy. Each prosthetic has a learning curve because the actions to perform a motion differ from the motions that a biological limb would require. As prosthetics advance, researchers hope to accurately receive signals from the brain through electrodes to create seamless and intuitive movements. Researches implanted computer chips into the brain of a 52-year old quadriplegic woman. These chips contain 96 microelectrodes and sit near neurons that control the right arm and hand. If the patient thinks about moving her arm, the chips send signals to a robotic hand. With this incredible procedure, the woman was able to shake hands with others and eat a chocolate bar.
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