As far as the last part...stairs. I personally would not rely on others, but imo, I don't see how a knee replacement would be any different than a full leg cast as far as getting around and up & down stairs without using the "bad" leg...btdt. I could haul azz on crutches, and though it's been a long time ago, I don't recall any issues with stairs, and I lived in a 3rd floor apt at the time. Maybe someone else can ease your concerns about the surgery....'cause I can't
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Best of luck Scott!