I keep reading jokes and text about the metal arm being removable. This bothers me for a variety of reasons. The way that he is able to use the arm in CA: WS is, frankly, amazing. He is able to utilize it as though it were an organic part of his body, and not just in ways that a real armwould work. For example, we see multiple occasions where hydraulics kick in to force the arm forward, but (as far as I can tell - please let me know if you noticed something I did not) there is no preceeding visible command. In fact, the only time that I can remember a gesture that may have been a physical cue was when he was forced to recalibrate following Black Widow’s EMP attack: he sort of spreads his fingers, and even that could simply be to check function rather than an actual command.
So how does the arm work? Obviously he has spacial recognition. This means that there has to be some form of sensation involved. However, it is reasonable to assume that he does not feel pain in that limb like the typical human would; imagine the intense pain he would feel in the following image otherwise?
Speaking of this scene: If the Winter Soldier’s arm could be unattached, it would have at that moment. The amount of force being exerted is more than enough to rip a normal human arm right out of its socket. In fact, this scene is pretty good proof that he’s had a great deal of his skeletal system replaced / reinforced. If they only reinforced the shoulder, the force would rip the entire altered section away from weaker natural bone. Therefore, he’s likely been modified along the entirely of his upper torso (from shoulder to shoulder) as well as down his rib cage. I’m not even going to get into his knee or toes in this scene…
My guess is that he can feel pressure, but nothing as complex or possibly incapacitating as pain/pleasure or hot/cold. Pressure is pretty important in his position: squeezing a trigger, not crushing things when he picks them up, etc.
In order to have any sensation, however, the arm has to be wired into his nervous system. So, somewhere in his shoulder they have connected the arm to not only his muscular and skeletal systems, but also the nervous. At some point Hydra would have had to reconstruct his entire shoulder, not only to deal with the original damage and atatching the new prosthetic, but to fortify the joint and surrounding bone in order to support the extra weight and stand up to the extreme amount of wear and tear.
Furthermore, the Winter Soldier is able to accomplish things with extreme precision. The scene where he catches Steve’s shield shows just how flawlessly he can use his arm. Being able to move that accurately would be impressive for a natural appendage, but considering that this is a prosthetic? Medically we’ve come a long way with modern prosthetics, to the point where the user can flex their fingers and grasp, but to accomplish anything complicated they must keep their eyes on the task at hand (that wasn’t meant as a pun, I swear). And yet, the Winter Soldier manages to snag the shield at exactly the right moment reflexively.
You can see a lot of examples showing how aware the Soldier is of the arm’s position in space. See how he is able to grab the weapon without looking in the following gif:
He doesn’t have to look or even focus, he just snags it as he walks by.
Alright, now let’s talk anatomy. Take a look at the above image to see where he metal meats skin, and then take a look at this link for some nifty muscle references. Look at how many muscles are involved in allowing an arm full range of motion here (hint: it’s a lot). In order for the arm to work so seamlessly, they’d have to attach it to (or replace) the trapezius, the pectorals major, the coracobrachialis, the subclavius, the pectorals minor, and the teres major in the front. In the back it’d be the trapezius, rhombi major, infraspinatus, and the teres major and minor. With as low as the metal bits are positioned, you’re looking at something that’s been locked in at the ribs, clavicle, and the scapula - unless they’ve completely replaced them, of course. Which is quite possible, considering how much abuse the Winter Soldier puts his body through.
There are plenty of other (unanswered) questions concerning the arm, such as how he’s able to move so well without chafing and how the the skin-to-metal boundary works. I have guesses for these, also, but I’ll save those for later. The message for now is: Bucky’s arm is staying right where it is without some major work.
Thanks to @100yearpatriot for the references and your magnificent brain.