If you ever get a wild hair, Turn Key, you might want to check out a Townie. My father-in-law (Justin's dad) has a
Townie, and a problematic knee and back, and he rides it daily with good results. It's an especially good bike for folks with back problems.
Bicycling is one of the most knee-friendly activities one can do. I know, since I've had ACL reconstruction, partial meniscus (cartilage) removal and a meniscus repair on my left knee. I didn't get back into cycling (was active growing up, for exercise & transportation,) until *after* my knee surgery in 2005.