Given the tech level of that arm, there's no way it was built in the 1950s; I've always assumed the scar tissue is from multiple bouts of surgery/stress/injury as the arm is damaged (because look at how he uses it - it must take a heck of a lot of damage!) and repaired and replaced.
And as others have noted, the attachment point (i.e. shoulder) is the place where the most strain would happen, because it's an interface/transition; the arm is presumably anchored underneath the muscle at some point and somehow attached to it, which would create a natural place where stress could inflict damage. Ergo, that's where most of the repairs would be needed after missions, AND where they'd have to poke around when fitting a new one.
If Steve and Bucky can still be killed, then that logically means their enhanced healing has limits, and everyone seems to take it as fact that they can die, so. Yeah. I'd think repeated stress & stress injury + surgical damage could overwhelm the non-scarring properties of Bucky's healing factor, when done in a continuous cycle!
Cryo couldn't be helping, either; even if his frozen tissue doesn't take the kind of damage (i.e. fatal) that a regular person's would, it's still going to be elevating the stress levels on that tissue during the freezing process. Not to mention that a cryogenically frozen body has stopped normal processes, so even if the damage is minor, no healing is happening! So Hydra would need to be sure that a certain level of healing has happened before refreezing him, and check again when he comes out. They don't allow a lot of time out of cryo on either end of missions, so you're potentially looking at a dozen minor stresses to the same area as he's injured, frozen, thawed, injured, frozen, thawed...
My headcanon says that if the Winter Soldier ever came out of cryo permanently and stopped taking missions, a lot of the scar tissue there would eventually repair itself and fade away - but as things stand it's never been allowed to. Hydra doesn't care about minor aesthetic issues or even pain/injury that doesn't impede function; it's far more important to protect the asset's programming by getting him refrozen quickly than to make sure his shoulder is at 100% rather than 98% functionality.
Add to this: Bucky presumably lost the arm falling in the Alps, and we don't know how long it took for Hydra to find him there. Some extreme damage could have happened then, as the arm was wrenched/crushed away and he lay in freezing temps exposed to the elements, and the trauma extreme enough to overcome his enhanced healing and leave scarring. So he may well have had scar tissue there well before any prosthesis was introduced, which consequently was never given time to fade/heal completely because of what came after.
Plus, we don't know exactly how his version of the serum works; it's entirely possible that he received a lot more of it after his second capture, and at the time of the fall his healing wasn't as enhanced as it became later. (For maximum angst points... it's entirely possible that the serum wouldn't have removed old scars in and of itself, as the body would consider them healed, but Hydra didn't want their asset to have identifying marks from his old life, so they reinjured those sites in order to make the enhanced healing kick in.)
Re: Physical Damage
And as others have noted, the attachment point (i.e. shoulder) is the place where the most strain would happen, because it's an interface/transition; the arm is presumably anchored underneath the muscle at some point and somehow attached to it, which would create a natural place where stress could inflict damage. Ergo, that's where most of the repairs would be needed after missions, AND where they'd have to poke around when fitting a new one.
If Steve and Bucky can still be killed, then that logically means their enhanced healing has limits, and everyone seems to take it as fact that they can die, so. Yeah. I'd think repeated stress & stress injury + surgical damage could overwhelm the non-scarring properties of Bucky's healing factor, when done in a continuous cycle!
Cryo couldn't be helping, either; even if his frozen tissue doesn't take the kind of damage (i.e. fatal) that a regular person's would, it's still going to be elevating the stress levels on that tissue during the freezing process. Not to mention that a cryogenically frozen body has stopped normal processes, so even if the damage is minor, no healing is happening! So Hydra would need to be sure that a certain level of healing has happened before refreezing him, and check again when he comes out. They don't allow a lot of time out of cryo on either end of missions, so you're potentially looking at a dozen minor stresses to the same area as he's injured, frozen, thawed, injured, frozen, thawed...
My headcanon says that if the Winter Soldier ever came out of cryo permanently and stopped taking missions, a lot of the scar tissue there would eventually repair itself and fade away - but as things stand it's never been allowed to. Hydra doesn't care about minor aesthetic issues or even pain/injury that doesn't impede function; it's far more important to protect the asset's programming by getting him refrozen quickly than to make sure his shoulder is at 100% rather than 98% functionality.
Add to this: Bucky presumably lost the arm falling in the Alps, and we don't know how long it took for Hydra to find him there. Some extreme damage could have happened then, as the arm was wrenched/crushed away and he lay in freezing temps exposed to the elements, and the trauma extreme enough to overcome his enhanced healing and leave scarring. So he may well have had scar tissue there well before any prosthesis was introduced, which consequently was never given time to fade/heal completely because of what came after.
Plus, we don't know exactly how his version of the serum works; it's entirely possible that he received a lot more of it after his second capture, and at the time of the fall his healing wasn't as enhanced as it became later. (For maximum angst points... it's entirely possible that the serum wouldn't have removed old scars in and of itself, as the body would consider them healed, but Hydra didn't want their asset to have identifying marks from his old life, so they reinjured those sites in order to make the enhanced healing kick in.)