Upper Limb Prosthetics

Body-Powered Prostheses

Body-powered prostheses work by using cables to link the movement of the body to the prosthesis and to control it.  Moving the body in a certain way will pull on the cable and cause it to open, close, or bend. 

A body-powered prosthesis will consist of a:

1. Socket or interface
(see Sockets and Interfaces page)
2. Suspension system
(see Sockets and Interfaces page)
3. Harness 
4. Control cable
5. Wrist unit
6. Terminal Device (TD - hook or hand)
7. And possibly:
          - a triceps cuff (below-elbow)
          - hinges (below elbow) 
          - elbow (above elbow) 
          - shoulder (shoulder disarticulation or higher)

Body-powered Components



TERMINAL DEVICES (TD's)

Body powered terminal devices may be hooks or mechanical hands.  They can be voluntary opening (remain closed until pull on the cable opens them. Relaxing closes the TD around an object with a grip force determined by a pre-set resistance)  or voluntary closing (remain open until pulling on the cable closes it with a grip force proportional to the amount of force the person puts on the cable).

Hooks come in various shapes and sizes.  They can be made of aluminum, steel, or titanium and can be rubber lined for better gripping.  The amount of force a voluntary opening hook can hold with (called grip force) is determined by the number of rubber bands holding the hook closed.
 

 

Body-Powered hook (5x hook, Hosmer)                       










TDs may also be hands.  They are more cosmetic, but they are also more bulky and grip force tends to be less.

  Body-Powered mechanical hand  (OttoBock)              

TDs take on other forms, too.  These are sometimes called prehensors because they resemble neither hook nor hand. (UCLA CAPP TD and TRS Grip prehensor)








Others are made for specific activities, like holding a baseball bat or driving.  See Adaptive page.

WRIST UNITS

Wrist units connect the TD to the prosthesis and restore some of the function of the anatomical wrist.

Friction Wrists allow passive rotation and positioning by the other hand. (Hosmer friction wrist)


Locking Wrists can lock in a variety of positions to prevent rotation when grasping or lifting
. (Hosmer) 



Quick Disconnect Wrists allow swapping of TDs.  Some models lock in various positions as well. (Hosmer) 


Flexion units provide wrist flexion or bending.  Using this feature usually requires pressing a button to allow bending and to release it from a locked/bent position. (Hosmer wrist flexion unit)

And some units have a combination of features.
(TAD's N-Abler + Hosmer 4 function wrist)

ELBOWS

Body-powered elbows are controlled by the same cable that operates the TD with an additional attachment or button that operates the elbow lock.  When the elbow is unlocked, pulling on the cable will flex (bend) the elbow.  When the elbow is locked or fully flexed, pulling on the cable will operate the TD.
(Hosmer)
   

OttoBock's Ergo Arm is an Elbow-forearm unit that just requires a TD
Some models of the Ergo Arm are wired to accommodate a myoelectric TD, and in some models the elbow lock is operated electronically as well. 


Controlling a Body-powered Prosthesis


These prostheses are operated by a harness and cable that is connected to the Terminal Device (TD).
  
The socket may be suspended (kept on) with a harness (called a Figure 8 harness).









In order for the harness-cable system to work properly, the cable (a) has to be anchored on the back of the arm between the elbow and the shoulder.  It is attached to a "triceps cuff" (b) that is held in place by the harness.  The triceps cuff is attached to the socket below the elbow with hinges.  Hinges (c) can be flexible (basically more straps) or rigid (providing more structure and support or for a very short residual limb).

The prosthesis may also be self-suspended by using a special design of socket.  A self-suspended body-powered device still need a harness for control of the TD (called a Figure 9 harness).  But it is simpler, less cumbersome, and doesn't have a triceps cuff or hinges. 













To see an old school (and rather dry, but informative) video on below-elbow harnessing and controls:
Part 1:
http://www.youtube.com/watch?v=0_tkXDXrbUE&feature=related
Part 2: http://www.youtube.com/watch?v=UR3tv-tLTeI&feature=related


Above-elbow harnesses are more complicated versions of the figure 8 harness, with additional straps to help hold it on (d) and to operate the elbow lock (e).  Alternative design of above elbow harness may be viewed here: pic.













To see a similar video on above-elbow harnessing: 
Part 2 (starts at 2:00 in)  http://www.youtube.com/watch?v=UR3tv-tLTeI&feature=related
Part 3:  http://www.youtube.com/watch?v=lHDik5MPk6M