I had ACL surgery about 1.5 years ago (which is nothing compared to what you are dealing with).  Although I'm not a physical therapist, I can tell you what I've been told by a variety of people after asking similar questions. 

Cycling is supposed to be pretty easy on your knees as aerobic exercise goes, so you are lucky in that respect.  It is imperitive, however, that as you begin to train, you keep your cadence above 60 RPM's.  As a general rule, keeping it above 60 places the least amount of strain on your knees.  Once you drop below that level, all bets are off.  This probably means you should avoiding climbing.  Also try to integrate some cross training in the form of swimming into your routine.  This way, you can still build muscle and aerobic threshold, but won't be do a weight-bearing exercise.  If you don't swim, you can still do workouts in the water that will be easier on your knee like jogging or walking in 2-3 feet of water.

Good luck--I admire you for sticking with it!

Stephaie

First off I have been a fairly avid cyclist for over 20 years, riding 1 to 2 or 3 centuries per year. I ride with a regular group on week ends and would ride at lease 100 miles a week. In 1999 both my knees had degenerated to the point that total knee replacements were no longer an option. And according to the doctors my cycling actually kept me from having to have them replaced sooner. Sorry for the long version. So in December of 1999 I had both of them done at once, fortunately I was able to keep my knee caps. after about 2 weeks into rehab I was allowed on a stationary bike to peddel backwards. At 3 weeks I started light weight training, streching flex excersizes and peddling forward. In the 4th week I increased the level from week 3. This routine continued thru the 7th week where I was actully putting some power to the peddels and spining at about 90 rpm. In week 8 I went and rode about 15 mile on my road bike. In May of 2000 I rode a 50 mile charity event.

Now not any of this was with out some degree of pain especially in the begining.

So do I think you pushing it? Not really unless you are incurring a lot of pain and it continues for days afterwards. Remember the more you use you body the quicker it will recover, let your knees be your guide.

PS I was 55 when I had them replaced.

I thought I'd kick in some thoughts regarding the cycling after knee replacement.

I'm 53 (now) but Oct 04 had partial inner replacement.  I was not a cycler then but the doctor said that his most successful recoveries were cyclist.  Equiped with that bit of information I was in.  I started riding in Feb of 05 and haven't stopped since.  I do 3 big rides a year and try to go out 3-4 times a week based upon my travel schedule. 

Key for this program is building a solid foundation of schedule and program.  Be committed to consistency and the rest will come.  As someone else said, your body will tell you if your pushing to hard, but my philosophy is that a little pain is good.  I know I'm not going to hurt the joint, titanium and nylon is strong stuff, so it is a function of rebuilding muscles and endurance which is where consistency comes in.  Strange thing is that I have more discomfort when I'm not riding - go figure.  Hope this helps, good luck.

B