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So I have a steampunk setting with some magic and I have a young, 20 year old female character who is supposed to have a full prosthetic arm. It might be assumed that the answer to this question could also be relevant for the integration of sci-fi prosthetics.

Such prosthetics are commonly attached to the body with osseo-integrated implants, which secures metal directly to bone, and the issue of skin adherence to an artificial surface has been solved with an artificial bone/antler/ivory like material. A certain degree of control and feedback is achieved through magical means, but it is not relevant to this question.

However it seems problematic to me that in this case, the entire upper arm bone is gone, and therefore also much of the ligaments and muscles that would normally attach the arm to the body. Also, potential hard points, like the collar bone and the scapula are mobile, so I'm not entirely sure how you might mount it. Would artificial ligaments be needed? Actual medical knowledge would be greatly appreciated.

How would you mount the prosthetic permanently to the body in a load bearing way?

To be clear, I am looking for a permanently attached and bio-integrated prosthetic as opposed to a traditional harness.

(Edit - I was hesitant to use flexible materials in the implanted parts because I wanted the parts directly fused with the body to be extremely long lasting and hard wearing. I was worried that given most materials' lack of self healing ability, that you would want most parts of the prosthetic outside the body, where parts can be easily replaced.)

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  • $\begingroup$ I was halfway through an answer and realized I didn't have all the information I need. Is the metal an orthetic (support of the arm) or prostetic (complete replacement of the arm), or is it inbetween where a part is an orthetic which is in the shoulder bit and then the prostetic is basically attached to that orthetic part? What part of the arm constitutes a "full" prostetic? There are arguments to be made that it's just the upper and lower arm but you can also argue that it's everything from the ribs, shoulder-blades, collar bone, spine and arm and even variants inbetween. $\endgroup$
    – Demigan
    Dec 15, 2020 at 23:17
  • $\begingroup$ @Demigan - I am talking about the complete replacement of the arm up to the shoulder. The ribcage is intact as well as the collarbone and shoulder blade, and some surrounding muscle. I suppose you could say the "socket" part of the arm connection is intact, but the "ball" part and the rest of the arm beyond that is completely gone. What I want is a permanently attached and integrated prosthetic, rather than something that uses a cup-like harness over the area as a mounting point. $\endgroup$
    – Redbud201
    Dec 15, 2020 at 23:24
  • $\begingroup$ Was the prosthetic replacement planned at the time the native arm came off, or did the arm come off because it was sick and now at a later date the prosthesis is going on. If it is all in one surgery you could use some of the native muscular support of the arm for the prosthesis. This would be tricky if you are doing the prosthesis some time later. $\endgroup$
    – Willk
    Dec 15, 2020 at 23:43
  • $\begingroup$ @Willik - Her arm was lost in an unexpected accident, but got surgery fairly soon. The doctors doing the surgery would have probably planned to allow for a prosthetic to be mounted in the future, as such prosthetics, while expensive are not uncommon. $\endgroup$
    – Redbud201
    Dec 15, 2020 at 23:49
  • $\begingroup$ Is all the muscle that attaches to the humerus also missing? most of those muscles are on the torso not the arm. $\endgroup$
    – John
    Dec 16, 2020 at 6:12

2 Answers 2

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The humerus is not solidly attached. It is attached by ligaments and muscles.

If it were solidly attached it would not be so mobile! The head of the humerus is in a pretty shallow joint that allows the many positions of the arm. That joint does not bear much weight. The weight of the arm and weight lifted by the arm are borne by muscles which attach at a variety of places on the trunk and connect to the humerus.

muscles https://www.pinterest.com/pin/624663410806133670/

Look at all the muscles that connect to the head of the humerus! The latissimus dorsi in particular is a colossal muscle and attaches along the whole upper spine. The pectoralis major is also very big and solidly moored to the chest.

In people who get really weak (usually hemiparetic after a stroke) the shoulder will dislocate itself simply from the weight of the arm. The muscles are what is doing the work.

Ideally you have this new arm planned and in the operating room when the real one comes off. You can then detach the muscles supporting the humerus with the idea of immediately reattaching them - probably with the same "skin adherence to an artificial surface has been solved" tech to stick ligaments to prosthesis.

You will attach and support the new arm with the same muscles and same tendon attachments that supported the old arm.

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  • $\begingroup$ This is a big part of what I was unsure about, so I'm glad you cleared that up for me. I'll have to look at how muscles and ligaments attach to bone. $\endgroup$
    – Redbud201
    Dec 15, 2020 at 23:56
  • $\begingroup$ If part of the upper arm did remain, how much of it would be necessary for full motion? It seems to me that based on the ligament/muscle attachment points, that you'd need approximately 1/3, or maybe up to 1/2 of the upper arm remaining? The deltoid muscle in particular, attaches rather far down from the shoulder. $\endgroup$
    – Redbud201
    Dec 16, 2020 at 0:11
  • $\begingroup$ Alternatively, if using the tendons and shoulder muscles was not possible, would it be feasible to mount a socket over the scapula's natural "socket" and mount the arm from that, assuming that the new mechanical joint WOULD be load bearing without tendons? Or would that possibly cause too much arm strain? $\endgroup$
    – Redbud201
    Dec 16, 2020 at 0:43
  • $\begingroup$ I worry the scapula and its own attachments are not as robust as what anchors the whole upper arm to the sternum and spine. As regards some but not all of the natural attachments - if people lose some, they lose some mobility but the arm is still useful. $\endgroup$
    – Willk
    Dec 16, 2020 at 3:23
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    $\begingroup$ @Redbud201 artificial shoulder joints exist and they rely on the muscle to hold the bone in place there is no way to make a self contained shoulder joint that fits in the human body. (Nasa actually offers a quite large reward for anyone that can design one that can fit in the same space as human shoulder). The human arm is notable for being almost completely supported by only musculature, it is the free floating articulation that allows the huge range of motion the human shoulder has. $\endgroup$
    – John
    Dec 16, 2020 at 6:21
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Body suit:

A good prosthesis should be comfortable to wear, functional, and still be able to use leverage. Yours is a powered device, but I'm guessing you'll have a problem making it do super-amazing things because it doesn't attach well.

Implanted prosthetics are mounted into bone, typically, which does give increased ability to wear for long periods and somewhat better function, but usually relies on existing joints. Unfortunately, the bones tend to break and take damage. And steampunk is less cybernetic and more mechanical. So let talk armored breastplates. We want your character to be able to do some kick-ass stuff.

I'm going to suggest something that looks a bit like a breast plate. It would conform to the upper body (somewhat restricting motion) but secure around the other arm and possibly around the abdomen as well (especially if they have a buff 6-pack of abs). This might also be a little harder to breathe in, unfortunately, but knights in armor did okay in worse.

The prosthetic would be more like a body suit then, and applying force would transfer and pull against a much broader area of the body. Since we're going magical, I might make the suit actual enhanced armor around the other arm and then the two limbs can act as a pair and increase your flexibility to do "cool" things. This also allows you to distribute the weight more evenly across the shoulders. You still won't be able to lift cars (pesky backs, legs) but I do have visions of your hero holding open a massive closing gate with arm strength.

enter image description here Or for something a little more scaled-down... enter image description here

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  • $\begingroup$ It's a good idea, to use a more distributed-load mounting setup for bracing against intense forces, but it's not quite what I was looking for. I was thinking something more lightweight and functional for everyday use, as she doesn't need to perform any fantastic feats of strength herself, (though if she did, your suggestion might be a good idea), since she has a big robot friend for muscle. $\endgroup$
    – Redbud201
    Dec 15, 2020 at 23:40
  • $\begingroup$ Too bad, but kick-ass is a lot of fun. I still think the mounting is better with an across the shoulder suit. Make it lighter and it can be more of a day-to-day thing. Plus if it's removable, it can be swapped out/upgraded for that fun action scene... $\endgroup$
    – DWKraus
    Dec 15, 2020 at 23:47
  • $\begingroup$ And the same design could work for a complete second set of arms mounted off the shoulder. Then she'll have three arms. Dr. Steinman says symmetry is the enemy of the goddess. $\endgroup$
    – DWKraus
    Dec 15, 2020 at 23:51
  • $\begingroup$ There will be enough robots, power armor and flying ships in the story to satiate any aficionado. ;) She, however, is more of and engineer and pilot rather than a fighter. $\endgroup$
    – Redbud201
    Dec 16, 2020 at 0:22

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