I think pain is typically a good message that some adjustment is needed, and if a lower cadence keeps you pain free, then that's what you do. I'm two years post-op on a knee that was really put through the wringer (including cracking the patella in half). To be honest, cycling is the one thing that doesn't tend to give me problems UNLESS I slack with weight training (strong muscles around the knee compensate for the loss of my medial ligament; others were repaired or replaced with cadaver, yummy). That said, I can't use a spin bike at the gym b/c the fit is never right, and then I pay for it in pain later. However, it hurts BAD if I try and mash for too long, which has been a problem for me doing hills (I have to learn to stand more). If I keep my cadence upwards of 82+, I can go all day 100 mi+ no problem. However, 30 minutes at 70 rpm and I'm hurting the next day. So, I don't think your problem is spinning per se, but the extra pressure on your knee doing hill work. For me, body positioning when climbing is key. I try and think of my leg like a piston - if I'm sitting, I slide my butt back and try and make an imaginary straight line forward and when standing I stand forward and my leg makes a straight line going back somewhat towards the rear wheel. If I'm sloppy, the knee let's me know the next day and then some with a deep ache. Of course, at 30 my body is still in the range where I think I got fairly good injury recovery, considering. However, I didn't actually feel that way until the last year when I committed to dragging myself to the gym 2-3x per week (no less) for weight training . Up until then, I still had a noticeable limp.

And, might I add, knee pain SUCKS!