If the plane is not booked solid, it is quite reasonable to request and receive an empty seat between you and your wife so 3 seats across, you may even lay your legs on her part of the way.

Another idea is 'to look the part' i.e. have a walking stick and/or crutches and/or ask for wheel chair service etc. I don't think you are cheating you have a real medical problem. BTW u would lose any chance of emergency exit