I also have bad knees and after falling last winter on the ice and suffering a severe ankle sprain and a fractured fibula I figured it was time for a wading staff. I have been careful when wading, usually take my time and always wear studded wading boots but there have been a few falls and a couple of scary moments.
I bought this wading staff, liked the idea of being able to clip the retractor to my wading belt, figured I could get to a good spot and cast away without worrying about the staff.
http://www.waterworks-lamson.com/staff.html
Probably the younger guys are wondering what all the fuss is about, but once you get into the mid to late 50's things can get a little wobbly. I'm hoping that with all the precautions I can keep fishing for another 20 years.