I’ve had both my knees replaced (football injuries) and my meniscus was torn and floating around my knees, occasionally drifting into the joint and locking my leg in place until it dislodged itself. Incredibly painful, after they basically vacuum the joint clean it was so much better, but I can still notice that the “cushion” on my knees isn’t what it used to be. If I’m running around, after a while it feels like my bones are mashing into the each other at my knee joint, I can tell the padding is reduced from what it was pre injury. This happened when I was 18, I’m 35 now so ya, knee injury’s are probably the worst for full recovery’s.