Prosthetics need to be able to hold on to the body.
If you're giving your amputee something similar to 99.99% of modern prosthetics, this will be done through a socket. This is a ridged cup made perfectly for the amputee that holds the prosthetic onto the body. Older prosthetics (mostly anything before the 90's) made the prosthetic socket intentionally tight in spots, which is what held it in place. Some people with sensitive skin still use this style of prosthetic today but they've mainly fallen out of use in favour for suction sockets.
Suction sockets create a vacuum seal that holds the prosthetic in place. These can work in two ways, either just by forcing excess air out of the socket and creating the seal that way, or for some legs, sucking that excess air out and into an "ankle" mechanism to offer some extra suspension and padding in the step.
Some prosthetics will also use additional measures as well as suction, such as pin-locks, where the amputee wears a sock with a screw at the bottom that clips into a mechanism at the bottom of the socket, or a prosthetic with movable panels that can be tightened via cables running through the socket.
I've used all of these except the pin lock socket, and they all have one thing in common: The sockets need as much space as possible. For prosthetics using suction in particular, this is to spread out the amount of force being applied to the leg. If all the suction is being applied to the very end of the stump, it's going to get sore and could even damage the skin. If that same amount of suction is applied to a much wider area, it's going to feel less intense. Likewise, older prosthetics needed as much space to work with as possible too, as applying tight pressure to a small area as opposed to a larger surface to keep the tension isn't good for your skin or muscles in that spot.
For this reason, the sockets will take up all of the space available without limiting movement, meaning they will go all the way up to the next major joint. An amputee who lost their hand through the wrist will have a socket that goes all the way to their elbow. An amputee who lost their leg through or above the knee will have a socket that goes all the way to their hip.
Sometimes, if an amputation is particularly close to a major joint and there isn't a lot of space left between the stump and the next major joint, prosthetists will opt to immobilise the closest joint and take the socket all the way up to the next major joint. This was something I've actually discussed with my prosthetist in the past. My left leg is amputated below the knee, but I only have a few centimetres of space below the knee. That leg occasionally needs revisions, meaning they take the very tip off of the stump to help correct issues with weird bone growth, scarring, infections etc, but if I get another revision, my leg will be too short to comfortably wear a socket, so my knee will need to be immobilised and my leg will become, functionally, an above knee amputation, despite still having the joint. This is rare, but it happens on occasion, showing that sometimes that need for space trumps even the use of a still functional joint. It's really that important.
I wanted to bring this up because I see a lot of people draw sockets on their amputee's prosthetics, but they're much too tiny to be comfortable!
I did mention most prosthetics use a socket, but not all do. Some old prosthetics did not have sockets and were held in place using other methods.
This is a "prosthetic" my prosthetist found in his company's back room. He's not sure when it was made, but together we came up with an estimate of it being made around the 70's for a through-hip amputee (meaning someone who's whole leg was amputated with no stump at all)
[ID: an image of someone holding what appears to be some kind of crutch. The crutch has a cushioned pad with a tall handle extending from the side. It appears as though you are supposed to rest your hip on the pad, and use the handle to hold it in place/move it. /End ID] - Photo by me
It's designed so that the user would rest their hip on the cushion and use the handle to hold it in place and move it in time with their walk. This kind of mobility aid isn't often used any more (me nor my prosthetist have even seen one out in the world), and seems to have faded in use during the 80's as sockets were invented that could better hold onto the hip and pelvis for through-hip amputees and the use of wheelchairs for amputees became less stigmatised.
There's also a new type of prosthetic has been developed called the Osseointegration prosthetic, which also doesn't use a socket. These are very rare as they are incredibly expensive and still very risky, but these prosthetics bypass the socket and implant the prosthetic directly into the body through a rod planted inside one's stump bone. This rod has a clip at the end of the stump, so the external part of the prosthetic can be removed as needed (and replaced). The reason they are risky though is that they are EXTREMELY prone to infection. I only know one person who had this implanted successfully, but he has to be very careful to keep his leg clean or else it will get infected (and it frequently does, he's constantly on antibiotics). Everyone else I know who got it had to get it removed.
With time these implants will get safer, but we are a very, very long way off from that right now.
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