Upper Limb Prosthetics

Sockets and Interfaces


The socket is the part of the prosthesis that encases the residual limb and to which other components are attached.  Because newer designs are being made of less and less material in innovative shapes, some people prefer to use the term "interface."
The socket is the foundation of the prosthesis.  It must fit well to provide comfort and function.  The prosthesis should feel and move like an extension of your body.  It has to be fairly snug; otherwise, when you move your limb, it will just move inside the prosthesis and not move the prosthesis itself.  You expend energy without a full response from the prosthesis, and it's very inefficient.  It also results in rubbing of the skin and can create soreness and skin problems.  If you are using an externally powered prosthesis and it is too loose, the electrodes won't have good contact with the skin and it won't work well or at all.

Socket designs are continually evolving.  For an article on the evolution of socket design,
click here

Examples of current, commonly used socket designs:
(Please excuse the line drawings. I'm trying to avoid copyright issues.)


 

Below elbow /
Transradial    

        
Above Elbow /
Transhumeral
 
     


Shoulder Disarticulation    
  








Other Socket features:

 - 3/4 modification to transradial sockets

 - Use of a flexible inner liner to increase comfort 

(test socket with flexible inner liner and 3/4 modification)
 


There is also a new design for upper and lower limb absences of all levels called the
High Fidelity Interface.
It was originally developed as a part of the DARPA's Revolutionizing Prosthetics program, but it has been making its way into the public sector.  For an article about this design, click here.

Suspension systems


Suspension is the term we use to describe how we hold the prosthesis on the body.
This can be accomplished in a number of ways:

SUCTION
Suction suspension relies on negative pressure like a suction cup, except the suction cup is the prosthetic socket on your limb.  A one-way air valve is incorporated into the socket to allow air to come out but not go back in, thus maintaining the suction.  The valve has a release button to allow air in and allow you to take off the prosthesis.  With this type of design, fit of the socket is even more important, because unwanted leakage of air might cause you to slip out of the prosthesis.
This intimate of a fit usually requires a special method for putting it on.
You may "push in," often using a special lotion that facilitates sliding into the socket but dries to a powder so that you don't just slip out again.  Or you may "pull in" using a donning sleeve. 
click for diagram
Video of patient putting on iLimb with donning sleeve:
http://www.youtube.com/watch?v=I81Wfh7F9gM&feature=grec_index

Shown is a flexible inner liner that has been removed from the outer frame.  Electrodes are embedded in the plastic and there is a black valve at end (The black part would be removed and donning sock pulled through that hole when putting on the prosthesis.)


SELF-SUSPENDING SOCKET
A self-suspending prosthesis stays on the limb due to its shape and design.  This is usually accomplished by "grabbing" areas of the residual limb, like just above the elbow.  The upper arm bone (humerus) widens near the bottom.  Those wider, knobby sides are called the epicondyles of the humerus.  If you have a socket that fits tightly above these, it will not come off easily.  This is called "supracondylar" suspension.


GEL AND SILICONE LINERS
Ossur's Iceross Upper-ex liner
Liners are made of silicone or gel with an outer layer of fabric and a pin that screws into the end of it.  This pin will engage in a lock inside the socket.  You roll the liner on and then push into the socket until the pin engages in the lock.  To take the prosthesis off, you push a button to disengage the lock and pull out your limb.  This offers a secure suspension and can increase comfort by providing a soft squishy layer between your limb and the hard socket.  Electrodes can also be attached and incorporated into the liner.   However, some people's skin reacts to the material of the liners, they require washing, and they can get kind of hot and sweaty.  (Ossur's Iceross Upper-ex liner)


HARNESSING
Harnessing is used in body-powered and above-elbow externally-powered prostheses and is described on the
Body-Powered page.


For an article with a table describing the pros and cons of each suspension system,
click here.