Upper Limb Prosthetics

Externally Powered:

An externally powered prosthesis has components that are moved by motors and powered by batteries.  The system is controlled by a microprocessor that uses signals from the body to tell the prosthesis what to do.  Signals are generated one of two ways: body movement (see input devices below) or, more commonly, electrical signals generated by the wearer's muscles. 

An externally-powered prosthesis will consist of:

1. Socket or Interface
2. Suspension system
3. Input device
(see the section on controlling the prosthesis below)
4. Microprocessor
5. Battery
6. Terminal Device (TD)

Other possible components are:
 7. Wrist Rotator
 8. Elbow
 9. Harness (usually for additional suspension in above elbow prostheses)

 

 

 

 

 Externally powered components:



PARTIAL HAND OPTIONS


 

Prodigits by Touch Bionics           

 
Next-gen myo fingers by Vincent Systems 


TERMINAL DEVICES

In a traditional powered hand, there is an internal mechanism and a thick outer glove made of silicone or rubber-like material.  Although from the outside it is the shape of a normal hand, the internal mechanism only has three fingers:  the thumb, index and middle.  The ring and pinky fingers are empty inside the glove, and grasp is generated only from the first 3 fingers.  The hand opens and closes only one way, and all the fingers move together to form a pinching motion.  

Examples:
 

Ottobock's system electric hands:
(internal structure and outer glove)
Motion Control's myoelectric hand:
Ottobock's Transcarpal hand: 
A low profile design for people with
long residual limbs and not much room
for components.


Hook shapes have some benefits over hands (see "Which is Best" page) and so there are externally powered hook-like devices available, too.

Motion Control's ETD (Electronic Terminal Device):
Ottobock's Greifer 


Newer designs of externally powered hands have individually moving fingers and are called "articulated hands."  However, individual fingers can't be controlled separately; e.g. you can't actively point your index finger.  What happens is that the fingers will start moving together and stop individually when they meet resistance.  For example, to point the index finger, you would start from a fully open position.  Next, you would tell the hand to close while using your other hand to hold the index finger in place.  The other four fingers would curl around to form a fist, and the index finger would remain straight.  Even though this seems awkward, it is still an improvement over the limited, jaw-like, pinching motion of traditional externally powered hands.   When grabbing an object, articulated fingers will wrap around it.  All fingers will make contact with the object and hold it more securely, as a human hand does.

Touch Bionic's iLimb
RSL Steeper's bebionic hand bebionic
Ottobock's Michelangelo hand



Not yet available:

Orthocare Innovations Contineo hand


WRIST ROTATORS

These components provide rotation at the wrist (palm up/palm down).  Currently, only two manufacturer's offer wrist rotators.
Examples:

 

Ottobock's wrist rotator
Motion Control's MC rotator and ProWrist Controller. 

 

 

Other wrists are wired for use with myo-hands and provide passive or manually positioned flexion and extension (wrist up/wrist down).  They can be used as the only wrist component or they can be paired with rotators to provide powered rotation.

Examples:

Ottobock's MyoWrist 2Act
Ottobock's MyoWrist Transcarpal 
A "Low Profile" wrist for people with
especially long residual limbs. 
Motion Control's Multi-flex Wrist


ELBOWS and ELBOW/FOREARM UNITS

The most commonly used myoelectric elbows are actually elbow/forearm units.  The elbow down to the wrist is one piece and pre-made, already containing the battery and the wiring necessary to be connected to a myo TD.   These units simplify things for the prosthetist and the resulting prosthesis is usually more cosmetic than if the forearm is made separately.

Examples:

 

 

 

Motion Control's Utah arm
Ottobock's Dynamic Arm

 

 

HYBRID ELBOWS

These elbows are body-powered elbows wired for use with a Myo-TD.

Example:  Ottobock's Ergo Arm Hybrid Plus:


How Externally Powered Prostheses Work:



The brain tells muscles to move with electrical signals sent through nerves.  When a signal is received, it is multiplied as it spreads throughout the muscle and can be detected with special equipment placed on the skin.   When your arm is amputated, muscles may not have a joint to move anymore, but they are still there and can still receive signals from your brain.  For example, your brain can still tell your bicep to flex your elbow, and the muscle will still generate an electrical signal the same way it does normally. It will even still tense up, but it doesn't have an elbow to move anymore.  Sometimes the signal is still strong enough that it can be detected and used to control the operation of a prosthetic elbow or Terminal Device (TD). The signals are picked up via electrodes embedded in the prosthetic socket and transmitted to a microprocessor, where the signal will be interpreted and used to operate the prosthesis.  Essentially, we are using the signal that used to move your arm to move the prosthesis instead.  There are other ways of controlling an externally powered prosthesis as well, and I will talk about them a little later. 

Terms and Concepts

Contraction
of a muscle, for simplicity sake, is when a person activates the muscle or tenses it up.  For example, your biceps are contracting when you bend (flex) your elbow.  Your triceps are contracting when you straighten (extend) it.  It is the contraction of the muscle that generates the signal we pick up with electrodes.

myo-site is a place on the residual limb where we can pick up the electrical signal generated by a muscle contraction.  An electrode is placed there to pick up the signal.  We usually like to use opposing muscles to avoid them being active at the same time, causing the prosthesis to become confused about what you want it to do.  The larger and stronger the muscle, the better the signal.

 

 

The microprocessor is the "brains" of an externally powered prosthesis.  It takes the signals and uses them to control the prosthesis.  This is done with a computer program, or control scheme, that dictates which signal will trigger what  function  and under what conditions. A function is an action of a prosthetic component, such as "hand open" or "hand close."

 

 

 

 

 

Controlling an Externally Powered Prosthesis



The control scheme may be simple: signal present = hand open, no signal = hand close. Or the system may be more complex, allowing the speed or strength of the component to be controlled directly by the strength or speed of the muscle contraction.  For example, strong contraction = fast close or strong grip of a hand, or slow contraction = slow closing of hand with less force.  Some of these systems are  more intuitive than others, although that seems to be a subjective opinion.  Control schemes are chosen based on the number of good quality muscle signals/myo-sites available and the number of devices and functions you wish to control.  Generally, the higher the amputation, the harder it is to find and isolate good quality signals.  At the same time, there are usually more components you want to control!

There are two main categories of control schemes:

 

 

Digital control can be thought of as a simple on/off, like a light switch.  Signal present = on, absent = off

Proportional control is a strategy of controlling an externally powered prosthesis in which the strength or speed of the signal controls the strength or speed of the prosthetic component (in proportion to the signal).

Controlling multiple functions:
If you are using an above-elbow prosthesis with an externally powered hand, wrist, and elbow, there are 6 different functions to control:  1) open the hand, 2) close the hand, 3) rotate the wrist to the right, 4) rotate the wrist to the left,  5) bend (flex)  the elbow, and 6) straighten (extend) the elbow.   Therefore, the prosthesis needs 6 different cues for it to know what it should do.  People rarely have that many myo-sites, so we have to ‘double up’ functions for each myo-site.   We do this in one of two ways:
1) By having the signal from 1 myosite site control different functions depending on whether or not it is fast or slow, hard or soft.
2) By using another cue to tell the prosthesis to switch between two functions.   Sometimes the cue is a switch (see below), or it can be a co-contraction of two muscles at the same time. 

 

 


Input Devices



Input devices are methods of generating a signal to operate an externally powered device.  (They are the in-put to the microprocessor--the brain of the prosthesis.)  Electrodes pick up an electrical signal generated by your muscles.  Force transducers, Linear Potentiometers, Touch Pads, and Switches translate the body's actual movement (not muscle activity) into an electrical signal that can be used similarly to that from the electrodes. 

ELECTRODES

Electrodes are the most commonly used input device.  They detect the electrical activity of muscles and are usually found in conjunction with other elements to filter out ambient electrical activity and magnify the signal they detect.  These things together are referred to as the "Electrode assembly", athough we usually still call the whole thing an electrode.  The electrical signal they detect is called an Electromyographic signal, or EMG.

Electrodes must be placed close to the muscle whose activity they are supposed to detect.  Usually they are placed over the biggest/main part of the muscle (called the muscle belly).    Placement is very precise, and being off, even just a few millimeters, can make a big difference in the quality and strength of the signal the electrode sees.  Intermittent contact or sliding of the electrode over the skin produces "noise" that can confuse the microprocessor, so they must be held firmly in place.

 

Electrodes can be adjusted for sensitivity, a parameter that we refer to as the Gain.  A higher gain means it is more sensitive.  We can’t increase the gain infinitely because we are amplifying the ambient noise as well, just like turning the radio up really loud will make the static in the background get louder too.  There is an optimal range of strength of signal, and gains are adjusted so that the signal getting to the microprocessor is in that ideal range.     

 

 

 

 

 

 
FORCE TRANSDUCER

A force transducer measures force.  It is attached to a harness and uses a cable or strap to produce tension on the device.   This tension is measured and translated into a signal the way an EMG signal is picked up by an electrode.  You can pull really hard on it, or just a little.  The variable force is converted into a variable signal that can operate a prosthetic component in proportion to the strength of the force.  It is a type of proportional strain gauge. 
  

LINEAR POTENTIOMETER
 

A linear potentiometer measures linear displacement.  You pull on one end of it and it stretches.   This action is accomplished by attaching the linear potentiometer to a harness, like a control cable is attached to a body-powered prosthesis. The systems are similar in that body motion = prosthetic motion,  except that with the linear potentiometer the amount you need to move is much less.   The distance that you pull it out is measured and translated into a signal that looks similar to that of an electrode.  The signal is used in the same way, too:  amplified and used to operate the prosthesis. 
Linear potentiometers can provide proportional control like an electrode:  smaller movement causes slower movement of the prosthesis, and larger movement causes faster movement.
Usually the linear potentiometer is placed across the back of the shoulder, so that the act of hunching or rounding your shoulders (called protraction) pulls on the device and triggers closing of the hand or bending of the elbow.
These are helpful in cases where your ability to generate large movements is decreased for whatever reason,  such as higher level amputation or other injuries. 

TOUCH PADS/FORCE SENSING RESISTORS (FSR'S)
 

FSR's measure a compression force, or how hard something pushes on it.  They are usually used in case of shoulder-disarticulation and the shoulder is used to press on the touch pad to activate the FSR.  Touch pads can provide proportional control, where the amount of pressure applied determines the speed of the prosthesis. 

SWITCHES


Switches are used in digital control schemes and only provide a simple on/off signal.  Movement of the switch causes function of the component the way a linear potentiometer does.  However, the response is not proportional to the signal as it is with electrodes, linear potentiometers, or FSRs.  Switches only produce an on/off signal, and the component will only move at one speed or with a constant amount of force. A switch is attached to a harness, and pushing or pulling on the switch causes function of a component, such as opening or closing of a hand.  A switch can also be used as a way to alternate between activating or controlling components, such between the elbow and the hand.  
Switches can be be a "pull" design (shown) or a "push" design.
A “dual switch” is one in which a gentle pull or a hard pull have two different functions and can be used to control two different devices.

 

 

 

 

 



For more information and examples of commercially available myo technology, visit the manufacturer's websites:

Motion Control: 
www.utaharm.com

Liberating Technologies: 
http://www.liberatingtech.com/products.asp


OttoBock:  http://www.ottobockus.com/cps/rde/xchg/ob_us_en/hs.xsl/6874.html  

Touch Bionics:  www.touchbionics.com

RSL Steeper:
   http://www.steeperusa.com