Aerokitty raises an interesting point in asking if OP notified UA of the need for an aisle seat on one side of the plane. How exactly does one get it into UA's computer system that the person has selected not just an aisle seat but an aisle seat on a particular side of the plane because of an injury on one side of their body. I'm guessing all one can do it book the preference and if it is switched see if someone will switch with you?