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I am sure you have seen or heard of artificial hand or artificial leg especially in Paralympic games. This artificial replacement of missing body parts such as teeth, legs, arms etc. is regarded as prosthetics.
Marine Corp trial practice with prosthetic legs
source: wikimedia commons, public domain
Losing a body part can be a painful experience with devastating psychological consequences. Fortunately, Prosthetic devices (prostheses) have really bridged this gap by restoring appearance and lost functionality for many people who have suffered from amputation or missing one body part or the other as a result of trauma, accidents, diseases or natural birth defects. It is a big leap in medicine and in fact, it has been considered as one of the greatest inventions in human history. But the obvious questions now are: how are these devices interfaced with human body? Is there any adverse effect associated with the use of prostheses? How do they work? Do read on to find out.
Fast Facts about Prosthetics
Below are some key points about prosthetics. More details can be found in the main article
The latest advancement in prostheses are the myoelectric prosthetic limbs which give amputees control over their artificial hands or legs through the muscle signal in the remaining part of that missing hand or leg.
Prosthetic devices are custom made and can last 2-5 years depending on activity level, growth and age. There is a need for yearly replacement when it is used on children due to their body growth.
Modern prostheses are commonly made from tough yet lightweight plastic materials (polyethylene, polyurethane, and polypropylene), carbon fiber and metals like titanium and aluminum.
The residual body part needs to heal completely before prosthesis can be used.
Direct bone attachment, stump-socket and the suspension mechanism are the common methods used for attaching prosthetic devices to the human body
THE PROSTHETICS STORY
Man has been improvising since time immemorial. The use of prosthesis dates back to the 15th century but at that time most of the devices were just for cosmetic purposes. Functional prostheses is believed to have started by a man named Ambroise Pare who introduced the socket mechanism which is still in use till date.
Nowadays, prostheses are made from lightweight materials instead of the traditional heavy metals and woods. Technological advancements such as biometrics, bionic implants, 3D printing etc. have also contributed greatly to the manufacture and use of these devices.
The recent improvements in prostheses include: myoelectric prostheses, i-limbs which enable the wearers change hand grips and carry certain weights, the Power Foot and many more.
Now that you have a little penetration into what prosthetics entail, let us talk about the types of prosthesis and their working principles.
Types of Prostheses
- Cosmetic prosthesis: If you are familiar with term cosmetic products used for make-ups especially by the female folks, you should have an idea of what I am about to say. Yeah, you guessed right. The situation here is quite the same. As the name implies, cosmetic prostheses are passive prosthetic devices mainly used to restore wholeness and appearance.
Interestingly, modern cosmetic prosthetic limbs are equipped with features such as veins, hairs, freckles and even tattoos. They are made to match the skin tone of the wearer. It is also worthy to note that some of these passive prostheses still offer some basic functions such as pushing and light grasping.
- Myoelectric prosthesis: this is one of the most exciting developments in prosthetics. It combines appearance with functionality.
myoelectric is the term for electric properties of muscles.
Is there anything electric in the human body? Yes and you can read more about the human body as an electrical system in this post by @pangoli
Myoelectric prosthesis involves the use of electrical signals generated from the muscle in the residual body part to control the artificial limbs. This device at its most basic consists of some electrodes, an amplifier circuit and a relay system. The electrodes are used to detect the Electromyographic (EMG) signals (electrical signals generated by muscle activity) from the residual limb. These signals are then amplified and conveyed by the amplifier circuit to the relay system which controls the electric motors that drive the robotic hand or leg. These components are battery-powered and are concealed within the artificial part.
wikimedia creative commons: Myoelectric control of ankle exoskeleton
One of the major breakthroughs in myoelectric-controlled prostheses is the robotic hand called bebionic3 capable of lifting weight up to 40kg. It works very fast, easy to use and has eight functional grip patterns.
- Body powered Prosthesis: This is mostly used by upper limb amputees. Other parts of the body (e.g. chest, elbow or shoulder) are used to move the artificial part through a system of cables. In the case of upper limb amputees, the cable is usually attached to the shoulder harness to move the prosthetic device by protracting the shoulders. For instance, if you tied a stick to your elbow such that the strap passes through your back and over your shoulders, you can manoeuver the stick by simply moving your shoulders
A major challenge of this arrangement is that it does not allow simultaneous movement of joints and ineffective when the level of amputation is high
How are these devices interfaced with the human body? Let’s find out
The Attachment Mechanisms
- Stump-socket and suspension method: let me start by defining what a stump is. A stump refers to the remaining part of the limb after amputation. A socket in this case is an opening/case that encloses the residual limb. It is the connecting point of the stump to the prosthetic device. It is mostly made from rubber, plastic or light metallic materials.
This method simply involves fastening socket to the residual limbs through straps, sleeves or belts. A silicone liner or co polymer material is usually worn on the stump to reduce the friction and heat between the socket and the skin.
- Osseointegration: This method is commonly used by dentists for prosthetic teeth replacement. Unlike the stump-socket method, this is a permanent attachment mechanism achieved through surgical procedures. It involves the direct attachment of titanium implant to the bone in the stump. The implant is usually allowed to integrate with the body system for a period of 6-7 months after which a connecting element (abutment) is attached to it. The prosthetic device to be used is then screwed to the abutment when any pain is no longer felt in the stump
let’s talk about the challenges
- Irritation, infection and discomfort: Many patients have complained about stump inflammations, irritations and harness discomfort, particularly with the use of body-powered prostheses. The irritations and inflammations are mostly caused by the heat generated from constant rubbing of the limbs against the socket especially when it is metal. The surface of the stump of some amputee may be very soft and unable to withstand the heat. Also, as the limb grows over the years, there is likelihood of discomfort within the socket. Even without any growth, you will agree with me that it is not an easy thing to fasten something to one’s body for hours, let alone days.
Ossoeintegration obviously eliminates the adverse effect inherent in the use of stump-socket method but it sure leaves its own trail. The implant used sometimes cause different skin infections.
- Cost: Prosthetic devices are generally considered very expensive to purchase and maintain. For instance, myoelectric controlled prostheses are made up of several components that are battery powered and therefore need periodic replacement and maintenance.
Future possibilities
Like most tech devices, work is still ongoing to improve the current prosthetic devices. Despite the recent advancement in prosthetic limb, one common feedback from its users is that they feel as though they are wearing a tool and would not know when somebody touches their artificial part without them looking. As a result of this, many neuroscientists and biomedical engineers have developed prototypes of prosthetic limbs with a sense of touch. Wow, right? Yeah.
One of such prototypes is the near-natural bionic hand developed by the Defense Advanced Research Projects Agency [DARPA] in 2015. In their demonstration, the prosthetic hand was wired directly to the sensory cortex (the section of the brain that identifies sensation) of a 28 years old paralyzed volunteer. With his eyes closed, the volunteer was able to say the exact finger touched within seconds. The researchers concluded that this was just alpha testing and a better version will be fully developed within few years.
Similarly, a company known as Rewalk recently brought to the market, a wearable robotic suit prototype called Restore that can help people with paralysis to stand up and walk.
The Restore transmits power to key joints of the legs with cable technologies powered with software
We cannot explore it all. Let me draw the curtain here.
CONCLUSION
The truth is that missing body parts can never be fully replaced but anyone missing one part or the other would agree that this seeming half bread is better than none. Prosthetic devices have really saved a lot of people from the crushing weight of amputation.Despite some of the challenges highlighted above, many amputees would still prefer to get one. The prosthetic revolution has begun and one can say with certitude that the coming years will be brighter.
In this article, we have seen the different types of prosthetic devices and the methods of attaching them to the human body
See you next time.
References and Further readings
Prosthetics
Myoelectric Prosthetics 101
What is osseointegration
Harnessing thought to defeat paralysis
Bionic hand can feed physical sensations directly to the brain
Jonathan T., Kerstin H., Lars H. & Rickard B. (2010). Osseointegrated Titanium implants for limb Prostheses Attachments: Infectious Complications. Clinical Orthopedics and Related Research. 468(10): 2781-2788
Elaine A.,Biddiss & Tom T.C. (2007). Upper limb prosthesis use and abandonment: A survey of the last 25 years. Prosthetics and Orthotics International. 31(3): 236-257
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