Walking down stairs is, and has always been, one of the most difficult tasks with this.
Your knee has to bend a lot and you use the muscles that have been decimated by surgery.
So I am extremely happy that I can go down stairs with ease these days.
The popping/cracking in my knee has migrated. This is apparently normal, and makes sense, as things reorganise. The swelling's going down and things are shifting around. Scar tissues tearing away and generally there's a lot going on in there.
The popping that used to happen when I straightened my leg came and went a long time ago. One day, I had it pop when I was sitting with my leg straight and shifted my weight a bit... that was weird.
These days it's when I'm walking down stairs.
It's great when it goes.
Before it lets go, I can feel a little resistance under my kneecap... sorta like an air bubble.. and my knee doesn't flex totally right. Once it pops, my knee moves normally. I've taken to walking up and down the tallest stairs in town like it's a treadmill ;)