Types of Artificial Limbs

Types of Artificial Limbs

 

There are four main types of artificial limbs. This includes the following types of prostheses:

TRANSTIBIAL.

TRANSFEMORAL.

TRANSRADICAL.

TRANSHUMERAL.

The type of prosthesis depends on what part of the limb is missing.

 

 

Transtibial Prosthesis

Transtibial Prosthesis is an artificial limb that replaces a leg missing below the knee. Transtibial Amputees are usually able to regain normal movement more readily than someone with a Transfemoral Amputation, due in large part to retaining the knee, which allows for easier movement. In the prosthetic industry a Transtibial Prosthetic Leg is often referred to as “BK” or Below the Knee Prosthesis.

The majority of prosthetic devices are for Below the Knee Amputees an intimate socket fit will provide improved comfort and gait patterns. Prosthetic devices commonly use silicone, urethane or elastomeric gels fit directed to the residual limb and hold the prosthetic device with or without pin locks. Elevated vacuum socket use is also on the rise and the intimate fit provides better blood flow to the residue limb for greater limb health for the amputee.

 

 

Transfemoral Prosthesis

Transfemoral Prosthesis is an artificial limb that replaces a leg missing Above the Knee. Transfemoral Amputees can have a very difficult time regaining normal movement. In general, a Transfemoral Amputee must use approximately 80% more energy to walk than a person with two whole legs. This is due to the complexities in movement associated with the knee. In newer and more improved designs, after employing hydraulics, carbon fiber, mechanical linkages, motors, computer microprocessors, and innovative combinations of these technologies to give more control to the user. In the prosthetic industry a Trans-femoral Prosthetic Leg is often referred to as “AK” or Above the Knee Prosthesis.

 

 

Transradial Prosthesis

Transradial Prosthesis is an Artificial Limb that replaces an Arm Missing Below The Elbow. Two main types of prosthetics are available. Cable Operated limbs work by attaching a harness and cable around the opposite shoulder of the damaged arm. The other form of prosthetics available is Myoelectric Arms. These work by sensing, via electrodes, when the muscles in the upper arm moves, causing an artificial hand to open or close. In the prosthetic industry a Trans-Radial Prosthetic Arm is often referred to as a “BE” or below elbow prosthesis.

 

 

Transhumeral Prosthesis

Transhumeral Prosthesis is an artificial limb that replaces an arm missing Above the Elbow. Transhumeral Amputees experience some of the same problems as Transfemoral Amputees, due to the similar complexities associated with the movement of the elbow. This makes mimicking the correct motion with an artificial limb very difficult. In the prosthetic industry Transhumeral Prosthesis is often referred to as “AE” or Above the Elbow Prothesis.

In recent years there have been significant advancements in artificial limbs. New plastics and other materials, such as Carbon Fiber (Carbon-fiber-reinforced polymer or carbon-fiber-reinforced plastic CFRP or CRP or often simply carbon fiber, is an extremely strong and light fiber reinforced which contains carbon fiber. The polymer is most often epoxy, but other polymers, such polyester, vinyl ester or nylon, are sometimes used. The composite may contain other fibers, such as Kevlar, aluminum, or glass fiber, as well as carbon fiber) have allowed artificial limbs to be stronger and lighter, limiting the amount of extra energy necessary to operate the limb. This is especially important for Transfemoral Amputees. Additional materials have allowed artificial limbs to look much more realistic, which is important to Transradial and Transhumeral amputees because they are more likely to have the artificial limb exposed.

In addition to new materials, the use of electronics has become very common in artificial limbs. Myoelectric Limbs, which control the limbs by converting muscle movements to electrical signals, have become much more common than cable operated limbs. Myoelectric Signals are picked up by electrodes, the signal gets integrated and once it exceeds a certain threshold, the prosthetic limb control signal is triggered which is why inherently, all myoelectric controls lag. Conversely, cable control is immediate and physical, and through that offers a certain degree of direct force feedback that myoelectric control does not. Computers are also used extensively in the manufacturing of limbs. Computer Aided Design and Computer Aided are often used to assist in the design and manufacture of artificial limbs.

Most modern artificial limbs are attached to the stump of the amputee by belts and cuffs or by suction. The stump either directly fits into a socket on the prosthetic, or—more commonly today—a liner is used that then is fixed to the socket either by vacuum (suction sockets) or a pin lock. Liners are soft and by that, they can create a far better suction fit than hard sockets. Silicone liners can be obtained in standard sizes, mostly with a circular (round) cross section, but for any other stump shape, custom liners can be made. The socket is custom made to fit the residual limb and to distribute the forces of the artificial limb across the area of the stump (rather than just one small spot), which helps reduce wear on the stump. The custom socket is created by taking a plaster cast of the stump or, more commonly today, of the liner worn over the stump, and then making a mold from the plaster cast. Newer methods include laser guided measuring which can be input directly to a computer allowing for a more sophisticated design.

One problem with the stump and socket attachment is that a bad fit will reduce the area of contact between the stump and socket or liner, and increase pockets between stump skin and socket or liner. Pressure then is higher, which can be painful. Air pockets can allow sweat to accumulate that can soften the skin. Ultimately, this is a frequent cause for itchy skin rashes. Further down the road, it can cause breakdown of the skin.

Artificial limbs are typically manufactured using the following steps:

- Measurement of the stump.

- Measurement of the body to determine the size required for the artificial limb.

- Fitting of a silicone liner.

- Creation of a model of the liner worn over the stump.

- Formation of thermoplastic sheet around the model – This is then used to test the fit of the prosthetic.

- Formation of permanent socket.

- Formation of plastic parts of the artificial limb – Different methods are used, including vacuum forming and injection molding.

Creation of metal parts of the artificial limb using die-casting.

Assembly of entire limb.

 

 

Body_Powered_Arms

Current body powered arms contain sockets that are built from hard epoxy or carbon fiber. Wrist units are either screw-on connectors featuring the UNF 1/2-20 thread (USA) or quick release connector, of which there are different models. Terminal devices contain a range of hooks, hands or other devices. Hands require a large activation force, which is often uncomfortable. Hooks require a much lower force. Current high tech allows body powered arms to weigh around half to only a third of the weight that a myoelectric arm has.

 

Actor Owen Wilson gripping the myoelectric prosthetic arm of a United States Marine

 

 

Myoelectric

Myoelectric Prosthesis uses electromyography (Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles).

Signals or potentials from voluntarily contracted muscles within a person’s residual limb on the surface of the skin to control the movements of the prosthesis, such as elbow flexion/extension, wrist supination/pronation (rotation) or hand opening/closing of the fingers. Prosthesis of this type utilizes the residual neuro-muscular system of the human body to control the functions of an electric powered prosthetic hand, wrist or elbow. This is as opposed to an electric switch prosthesis, which requires straps and/or cables actuated by body movements to actuate or operate switches that control the movements of prosthesis or one that is totally mechanical. It is not clear whether those few prostheses that provide feedback signals to those muscles are also myoelectric in nature. It has a self suspending socket with pick up electrodes placed over flexors and extensors for the movement of flexion and extension respectively.

The first commercial myoelectric arm was developed in 1964 by the Central Prosthetic Research Institute of the USSR, and distributed by the Hangar Limb Factory of the UK.

 

 

Robotic Limbs

Advancements in the processors used in myoelectric arms has allowed for artificial limbs to make gains in fine tuned control of the prosthetic.

Some artificial limb that has taken advantage of these more advanced processors, as an arm that allows movement in five axes and allows the arm to be programmed for a more customized feel.

Recently the I-Limb Hand, invented in Edinburgh, Scotland, by David Gow has become the first commercially available hand prosthesis with five individually powered digits. The hand also possesses a manually rotatable thumb, which is operated passively by the user and allows the hand to grip in precision, power and key grip modes. Raymond Edwards, Limbless Association Acting CEO, was the first amputee to be fitted with the I-LIMB by the National Health Service in the UK. The hand, manufactured by “Touch Bionic” of Scotland (a Livingston company), went on sale on 18 July 2007 in Britain.

Robotic leg exists too: the Argo Medical Technologies ReWalk is an example or a recent robotic leg, targeted to replace the wheelchair.

Targeted muscle re-innervation (TMR) is a technique in which motor nerves which previously controlled muscles on an amputated limb are surgically rerouted such that they re-innervate a small region of a large, intact muscle, such as the pectorals major. As a result, when a patient thinks about moving the thumb of his missing hand, a small area of muscle on his chest will contract instead. By placing sensors over the re-innervated muscle, these contractions can be made to control movement of an appropriate part of the robotic prosthesis.

An emerging variant of this technique is called targeted sensory re-innervation (TSR). This procedure is similar to TMR, except that sensory nerves are surgically rerouted to skin on the chest, rather than motor nerves rerouted to muscle. The patient then feels any sensory stimulus on that area of the chest, such as pressure or temperature, as if it were occurring on the area of the amputated limb, which the nerve originally innervated. In the future, artificial limbs could be built with sensors on fingertips or other important areas. When a stimulus, such as pressure or temperature, activated these sensors, an electrical signal would be sent to an actuator, which would produce a similar stimulus on the “rewired” area of chest skin. The user would then feel that stimulus as if it were occurring on an appropriate part of the artificial limb.

Recently, robotic limbs have improved in their ability to take signals from the human brain and translate those signals into motion in the artificial limb.  DARPA, Defense Advanced Research Projects Agency is working to make even more advancements in this area. Their desire is to create an artificial limb that ties directly into the nervous system.

 

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