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Functional Envelopes (Myoelectric vs Body Powered Arm Prostheses) 



The functional envelope for an upper extremity (arm) prosthesis is the area in which a prosthetic user can operate the prosthesis.  The functional envelope can be limited for various reasons, most commonly either the user's anatomy or innate limitations of the prosthesis.   


A body-powered prosthesis does not rely on an outside power source and is operated via a system of cables, harnesses, and sometimes, manual control.  Exaggerated movements of the body are used to control the prosthesis and these movements are captured by harness systems and cables.  These systems then operate the hand, or terminal, device.  Examples of these exaggerated movements are:

 

·          Bi scapular abduction/adduction

·          Glen humeral flexion to operate terminal device and elbow

·          Shoulder depression to lock and unlock elbow

 

To effectively control this type of prosthesis the user must possess significant strength and control over various body parts, including the shoulders, chest, and residual limb.  There must be sufficient limb length, musculature, and range of motion.   

 

The illustration below demonstrates the typical functional envelope for a 100% body-powered prosthesis.  When wearing a 100% body-powered prosthesis, the functional envelope is limited to directly in front of the user, from waist level to mouth level.  The user can no longer maintain control of the prosthesis when attempting to operate the prosthesis out to the side, down by the feet, or above the head.

 

A myoelectric prosthesis is an electrically controlled prosthesis that uses electrodes mounted within the socket to receive electrical signals from the muscle contraction.  These signals are sent to a motor in the prosthetic elbow and/or wrist.  A myoelectric elbow may then bend or straighten, a wrist can flex, and a hand can open or close.  There are many different types of myoelectric prostheses and there are various manufacturers that produce these devices.  

 

The figure below shows the wide range functional envelope for a myoelectric prosthesis.  The user is able to operate the prosthesis within a wider space, including above the head and below the waist.  Depending on the user and prosthetic design, the prosthesis may also be operated behind the back. 

 

Myoelectric prostheses typically do not involve elaborate harnesses or cables and rely on very subtle muscle movements.  This frees the user to be able to perform more complicated fine motor skills while still operating the prosthesis.

 

If the prosthetic user has experienced severe trauma or damage/atrophy to necessary muscles or joints, the functional envelope may be temporarily or permanently limited regardless of the use of a myoelectric prosthesis.  Limited range of motion in t he shoulder and/or elbow can often be improved with the introduction of physical and occupational therapy.  Regular exercise and stretching of the residual limb can maintain a wider functional envelope for all prosthetic users.   

The ability to have control over the prosthesis in a wide functional envelope can be crucial for certain activities and for certain occupations.  Every effort should be made to maintain the widest functional envelope possible, even with 100% body-powered prosthetic designs.  Designing a low weight prosthesis with a comfortable socket is an easy way for a prosthetist to accomplish this, regardless of the power source.




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