My knees aren't the best in the world but my left knee is the worse of the two. As my ortho said it looks like a knee of 90 year old
and I'm 24. I'm thinking about have my knee repaired which will be a partial replacement and take me out of TKD for about a year with healing and rehab. 
My knees have been getting stronger, unless I have surgery I'll only be at 80 percent and with surgery I can get to about 95%. I haven't had a chance to sit down and talk with my instructor about it. It's not that I'm scared that she would support my decision or welcome me back after I was cleared, just haven't had time to talk.
I can do a 10 min mile which is impressive for a 300 plus pound guy. and I can do almost of the kicks expect for the tornado kick going clockwise because I can't jump with that much weight on my knee.
I hate the idea of surgery, it's not the idea of getting opened up and rehab that scares but what I have to sacrifice to do it. I don't want to stop TKD training but I do want to be able to get down on the ground and play with my nieces and nephews and even my own kids someday.
I've had 3 surgeries on my left knee since 97, I've been needing more since 2002 but have been putting it off. Since doing that, it will take to surgeries to repair everything, one is a prep with removing of a lot of scar tissue and the other would be the acutual repair job. I would have to in the hospital for 10 days and wouldn't be able to walk for 5 months, then the rest would be added on crutches and then a cane. The biggest issue that wil be is the fact that I can't have too much persription pain killers, I was addicted once and don't want to relapse.
I'm thinking about putting it off until I at least get married (Nov. 2009) because right now I live a lone in a second floor apartment and would have noone to help me. Maybe I'm scared of just not being able to come back, I've had so much taken away because of my knee injury and i don't want to loose anymore.