My main character has a metal arm (think the Winter Soldier? Or maybe Adam Jensen?) that is permanently attached to his body. His body is going through the growth stages similar to puberty and I know he'd have to get a new arm. However, I'm not sure how to go about showing that the arm no longer 'fits' him and that it is damaging his body because, unfortunately, bionic arms of that ilk don't exist in real life. Any ideas, guys?
I'm not trying to be facetious here, but why can't you just simply say he's outgrowing his prosthetic arm? A prosthetic arm is a prosthetic arm. I'm not sure how the fact the one in your story doesn't actually exist makes any difference.
I also think it's something you could downplay, just going with general descriptions of the problem and associated discomfort, but If you want something more specific than Jud's suggestion, I think you need to imagine up more specific details on exactly what the arm is like and how it fits onto the organic part of his body. To what does the arm attach? I'd imagine that the attachment point would be the most painful/uncomfortable part, so you could focus on that. It might be a good idea to learn the anatomy of the region if you're not familiar. E.g. if the prosthesis replaces his whole arm, perhaps it articulates into the glenoid cavity of his shoulder blade just like the head of his humerus (the top arm-bone) would have. If his glenoid cavity has grown, then I'd imagine the prosthesis might become loose and rattle around a bit: maybe he has more trouble using it, maybe there are some jarring pains, maybe that puts a strain on the ligaments that stabilise the artificial joint (are those ligaments bionic or organic?), maybe the wobbly prosthesis damages the cartilage that lines the glenoid cavity and he's at risk of osteoarthritis in the joint... That's the sort of thought process I'd follow. Happy to help if you get stuck with yours. Good luck!
I'm not sure it might help, but if you have a gaming worthy computer or a console I fully recommend that you play Deus Ex: Human Revolution for inspiration with your story, if you've never heard of it, it's a near future RPG that deals with the theme of human augmentation(robotic arms, legs, enhancements, etc) Here's a picture of it: Spoiler https://ginguelona.files.wordpress.com/2011/03/deus-ex-human-revolution-02.jpg
