Well into my second year after total knee replacement (TKR) surgery on my left knee, I'm doing well, except for grinding, popping, and a bit of pain coming from under the knee cap area at times. I've found that there is a term for this sort of noise--it's called patellar crepitus.
According to an article I found on the Orthosupersite, "crepitus is caused by the same conditions responsible for patellar clunk: frank catching of a synovial nodule in the intercondylar box during knee extension." The way I think about it is that there is some bad bits floating around under my kneecap that catch and cause some grinding and occasional pain.
I plan to talk to my surgeon about it next month when I see him for a checkup and x-ray. The crepitus first became noticable early last summer, about seven months after the surgery. It's been getting steadily worse, even though my knee feels stronger than ever. The crepitus doesn't keep me from cycling hard, but I've quit doing squats in the gym, because when I tried doing squats with free weights last fall that seemed to make the crepitus worse. On the bike, it's most pronounced when I first start riding, and also when going uphill seated but putting out a lot of power under lower RPMs. On flats at cruising speed, it's not an issue. When I walk it's not an issue, but if I do a deep knee bend I experience some crepitus.
I'll do some more posts about this condition soon. My basic outlook, however, is that I'm still happy I had the surgery. I was hoping my knee would be less noisy post-TKR, but overall, my knee condition and ability to do off-the-bike activities like standing and walking is so dramatically improved versus my pre-TKR condition that I'm OK with a bit of crepitus. For sure, I'll do another "crepitus" post after talking to my surgeon. Maybe there is something that can be done about it, such as special taping or bracing.