I really wouldn't worry too much about having your knee cap removed - I was told it was going to be the worse thing ever as I was only 30, but so far it has done me no harm. I still go to the gym, play footie with my kids, cycle and do a bit (Not much though) rock climbing.
As long as you keep fit mentally and physically then it's no problem.
I was told that by the time I get to 50 I could have a new knee cap, but to be honest after that time I think I will be better off without it.
Nick