Lesson Plan - Get It!
How do you control a machine? Usually with knobs and levers and buttons. How does your body tell the prosthetic machine how to move?
When you play an arcade game, you are using messages from your brain to control your hand movements.
This process requires pathways in the body to be active and responsive. So what happens when someone has lost a limb?
In the earlier Related Lessons, found in the right-hand sidebar, you learned that prosthetics tools that can be used by an individual who has experienced limb loss. Early prosthetics did not function effectively, but we know that many people use artificial limbs with great success today.
Before reading on, jot down some quick answers to these questions:
- How do you think artificial limbs interact with the body?
- Why do you think the fit of a prosthetic is important?
- How do prosthetics mimic natural movements?
As you read, think about the answers to these questions.
So, how is the body able to communicate effectively to these artificial limbs? This depends on the type of prosthetic limb and injury. All prosthetics fit together with the patient's body, where the amputation or injury occurred. We call this the "residual limb." It is used to create a mold for the new artificial limb so the limb attaches comfortably. The attachment point on the artificial limb is called the "socket." Think about how a natural joint fits together: your shoulder has a ball and socket, with the ball fitting into the socket; you can model this by placing your left fist in your right hand. This is the same fit used in prosthetics.
These sockets are custom-built for the patient to ensure a good fit. Notice how this Walter Reed Army Medical Center prosthetic technician works on the mold for a prosthetic limb to ensure a proper fit:
Image by Fred W. Baker III for the U.S. Department of Defense is in the public domain.
Your body changes as you grow and age, so multiple molds and sockets must be made during the lifetime of the patient. Today, we can use technologies like laser measurement and three dimensional printing to speed up the process while improving the precision of the design.
Once we have a well-made socket, the prosthetic can be attached to the body. They depend on the skeletal system for support and the muscular system to carry out function. Prosthetic limbs are attached by a suspension system that depends on the type of limb. Some use an elastic sleeve, suction socket, or even straps or a harness. This system depends on how heavy and large the prosthetic is, because more security is needed for larger artificial limbs.
Some artificial limbs are fixed and move based on body movements, but others actually use muscle movements in the body to manipulate objects. Stand up and walk around the room, paying attention to how your legs swing. As you walked, your brain instructed your muscles to move back and forth to keep balance between both legs. A patient with a prosthetic leg must learn how to use gravity and the natural swing of the body in order to walk. This can take a lot of practice, because it is a new way of walking. Most prosthetic patients require physical therapy in order to regain full function of the injured area. Physical therapy focuses on teaching the patient how to complete tasks with the new artificial limb.
As technology has improved, the use of prosthetics has also improved. Today, some patients qualify for myoelectric-controlled limbs that are controlled by muscles and nerves in the body.
- How do you think this technology relies on the nervous system of the body?
Myoelectric control uses the messages sent to the residual limb from the brain along nerves to trigger electrodes, small attachments that are able to send and receive electronic messages. These electrodes communicate with electric motors inside the prosthetic, leading to movement and action. So, when the brain tells the arm to lift, the prosthetic moves as though it were a natural limb. While it does require surgery, it provides the patient with more function in the limb. In cutting-edge prosthetic design, nerves that controlled the muscles in the lost limb are redirected to muscles elsewhere in the body. When those muscles are stimulated through brain activity, the prosthetic moves accordingly.
These new developments have made prosthetic limbs more effective and easier to use. The fit is the most important part of the prosthetic process, because this is where the artificial limb will attach to the body every day. A poorly-fitted limb can cause damage or discomfort to the residual limb. That is why molds are taken to customize the socket. Once the limb is ready, it is attached to the body using suction or a harness, so that the limb is able to move in the desired way. Finally, the patient learns how to manipulate the limb through muscle stimulation, gravity, or the nervous system.
- How do you think prosthetic design could be improved for the comfort of the patient?
In the Got It? section, you will watch an informative video on the development of prosthetic limbs.