upper-limb terminal device

On an arm prosthesis, the part at the very end that does the actual gripping is called the terminal device — it is the prosthetic equivalent of a hand. It is the working tip of the whole device, the part that holds a cup, turns a key, or grips a hammer, and its design is the single biggest decision in an upper-limb prosthesis: do you want something that looks like a hand, or something that works like a tool?

Terminal devices come in two broad families. A hook is a simple split-hook clamp, two curved fingers that open and close; it looks unlike a hand but it is light, rugged, lets the user see exactly what is being gripped, and excels at picking up small objects and doing hands-on work. A prosthetic hand looks human and is socially comfortable, but a hand shape is bulkier, heavier, and often blocks the user's view of the object. Either kind can be powered two ways: body-powered, where a cable running from a shoulder harness opens or closes the device as the user shrugs or moves the shoulder; or externally powered (myoelectric), driven by motors. Many experienced users own more than one — a hook for the workshop, a lifelike hand for a dinner party.

The plain reality is that no terminal device, however clever, comes close to a real hand, mainly because it cannot feel. A natural hand constantly senses how hard it is gripping and adjusts; a prosthetic device grips blind, so the user must watch it to avoid crushing an egg or dropping a glass. This is why a simple body-powered hook, which is light and gives a little tension feedback through the cable, is preferred by many people for real work over a heavier, fancier hand — function and reliability often beat appearance.