What is a prosthesis?
A prosthesis or a prosthetic limb is an artificial limb used to substitute a missing part of the body. There are cases where a person might have lost a limb in a mishap, an amputation or due to some congenital problem. Prosthetic limbs could help them regain their mobility by fitting a mechanical (or electromechanical) attachment in place of the lost limb. A prosthesis can replace any part of a body. Prosthesis comes under the field of biomechatronics.
How is the science of Prosthesis classified?
A prosthesis can, in theory, be made to replace almost all parts of a body. A subset of a prosthesis is Cosmesis, which includes replacements for body parts that are visible. Cosmetic prosthetics usually don’t add mobility and are used just to restore the original look. With the development of technology, cosmetic prosthetics are designed to look incredibly life-like, complete with veins, fingerprints and even freckles.
Limb Prosthesis includes prosthetics for upper extremities (shoulders, forearms, arms, fingers, etc.) and lower extremities (hips, calves, knees) and is custom built to suit the user’s requirements.
There are four types of prosthetic limbs
Transtibial Prosthesis – Attachment below a natural knee
In this type, the attachment of a prosthetic limb takes place below the knee (BK). The amputation area is such that a new leg can be attached to the knee cap and made functional. Since knee is retained, an amputee is likely to get adjusted with the new limb more readily.
Transfemoral Prosthesis – Attachment with knee, upper and lower leg
This type includes the attachment of the artificial limb above the knee (AK). Transfemoral amputees may need more time in adjusting with the new limb as it takes 80% more energy as that required to walk with a natural pair of legs. It’s hard because of the missing knee. Though to provide more user control, improved designs have inculcated innovative technologies such as carbon fiber (makes it lighter), hydraulics (eases the movement), computer microprocessors (makes it smarter), etc.
Transradial Prosthesis – Forearm (Below the elbow)
This type of prosthesis replaces arm missing below the elbow (BE). Traditionally these used to be attached by a cable around the shoulder of the opposite arm but myoelectric arms are in lately. These have special electrodes that detect tiny electric signals produced in the healthy tendons in residue part to process the type of movement.
Transhumeral Prosthesis – Attachment with elbow, upper and lower arm
This type of prosthesis replaces missing limb above the elbow (AE). The working, in this case, remains quite similar to that of the transfemoral prosthesis.
How does prosthetics work?
With rapidly emerging technologies like 3D printing, faster processors, better sensors and lightweight materials, the world of prosthetics is going through dynamic leaps regarding how prosthetic limbs are designed, created and function. The core underlying principles are still dependent on the following modules.
The prosthetic attachment as a whole consist of four major sub-parts which are responsible for their specific functionality. The body of the limb, the socket, the attachment mechanism, and the control system.
The body of the artificial limb:
A human’s natural pair of legs weighs almost 30-40% of the body’s weight whereas the hands approximate to 10% of it. Hence, taking the weight of natural legs into consideration, the prosthetic should be light weight as it should not tire the user.
For that very reason, the construction of prosthetic, the kind of material to be used shouldn’t be very hard for the user to adjust and lead a daily routine with it. Therefore it is made of materials like carbon fiber covered with a foam or plastic covering to provide a protective interface and soft padding between the natural part and the artificial part.
Socket of the artificial limb
It plays an important role. Because, precise fitting, apart from adding to the comfort factor, is essential for proper functioning of the prosthesis. Think of it as a hand glove that we use in a game of baseball. If it fits in correctly, we’re able to play more efficiently or else we keep fidgeting. In short, a snug fitting helps in governing the voluntary movements in the residual limb.
The connecting part of the prosthesis is called a socket. It is molded around a plaster cast around the residual limb to get a precise measurement. Laser scanning can also be used to get an accurate virtual 3D diagram of the stump. A silicone liner is first attached to the residual limb which gets fixed to the socket. This arrangement ensures a better suction fit.
Sockets are made out of carbon fiber or hard epoxy.
Attachment Mechanism of the artificial limb
A well fit socket has to be assured by a suspension which might be an electric suction, a harness, or a strap. Often the socket itself is a part of the attachment. The terminal device (the actual limb) can be a hand, hook, or a leg. The prosthetic limb is also usually made out of carbon fiber or 3D printing resins (for cheaper alternatives).
Another method that is being researched to provide a more comfortable fit than the stump-socket method is the Direct Bone attachment. (Also known as Osseointegration)
The Artificial Limb Control System
Our brains stimulate the movement of our natural limbs. Nerves running all the way down from the nervous system to parts of the body receive electric signals which are decoded by the brain, resulting in the type of action a person wants to perform.
Similarly, cables that run inside a prosthetic limb receive electric pulses that help the prosthesis to move. An electric motor drives the prosthesis.
The most sophisticated ones are myoelectric prosthesis which uses electrodes to sense muscular impulses passed onto an electronic control system which amplifies the signals and tells a prosthesis’ mechanism to allow the type of movement the user is trying to do. It runs in a similar fashion as that of a natural limb and hence is the most popular one.
What are some breakthrough prosthesis products?
The Bebionic limb is a smart prosthesis that is modeled on the human limb to represent an accurate anatomical representation. Each finger (or digit) of the Bebionic limb is powered by an individual motor allowing the hand to have multiple grips. It’s designed and manufactured in the United Kingdom.
It works by sensing muscle contractions of the residual limb. This electronic data is then used to command individual actuators (motors) in hand to provide the desired movement. Once a user is trained to use it, operations become instinctive and natural. Watch the video below to see it work!
Bebionic uses a programming software named Bebalance (designed by RSL Steeper) which wirelessly programs the unique control settings desired by each user. Moreover, the prosthetist can study a patient’s real-time control signals and optimize the settings of the limb to best suit each pair.
Developed by MIT artificial intelligence researchers Hugh Herr and Ari Wilkenfeld here is another smart prosthetic. This prosthetic substitutes the knee providing possibly the best kind of natural knee functioning. It continually adapts to the user’s gait and external environment. Additionally, there is an auto adjustment feature that changes settings according to the terrain. Easy swinging mechanism and stability mechanism makes it more responsive in every situation for the user. They’ve included kinematic sensors in this knee to ensure stability and safety. Though the sensors need to be programmed by a technician for the first time when attached to the residual leg. The limb is even weatherproof so the user may not have to worry about exposing it to water splashes.
The cost of RHEO knee is around $18,000 though it may go up to $50,000 inclusive of the socket, knee, foot, and labor of a prosthetist to fit the limb.
Hugh Herr, the inventor of the Rheo Knee 3, is an active promoter of technology to make lives easy for the specially abled. Watch his inspiring talk at the TED conference in Vancouver where he speaks about his journey, inspiration, the Rheo, and his personal beliefs. This video is a must watch.
The IRIS hand is a semi-autonomous robotic prosthesis that is competent to grasp objects in possibly the best way any prosthesis can, by determining the grip to grasp a body and controlling the grip.
It uses an advanced vision system to find the most proper grip for grasping an object. It is manufactured using 3D printing with all actuators and sensors contained in the prosthetic hand itself. As can be seen, the vision system used here is similar to how images are captured by our retina and are processed for concluding methods to deal with a task.
What are the technological limitations and disadvantages of a prosthetic limb or prosthesis?
The biggest barrier right now, technologically, is the lack of feedback. If all your senses are stripped from your right hand, then the only way you would be able to understand if you are holding your phone is if you look at your hand. But if you close your eyes, you can’t tell.
Research in this area is ongoing, but even if the technology is developed to be built into commercial products, the costs will increase exponentially. That brings us to another disadvantage.
High-end prosthetics are extremely costly with prices ranging from $15,000 to $90,000. This needs to reduce so that everyone who needs a prosthetic limb can afford it. 3D printing is a solution for this issue as it allows the prosthetist to make design changes with a certain degree of freedom and reduce the number of individual parts in a prosthesis thereby reducing the cost.