About six or eight years after my tkr, the knee was swollen about the size of a grapefruit.
It had happened slowly, over time, wasn't particularly painful, just a feeling of pressure, and you could push in, and the skin slowly came back.
Went to see the ortho doc, and he said he needed to drain the knee. Slowly, over time, the "plastic" kneecap sheds minute particles of itself into the area around the knee, and the body surrounds them with a coating to protect itself. This forms the "water on the knee effect" we see here...
He poked my knee. The nurse brought in a tray, with a huge syringe on it, fitted with a long 6" horse needle. The surgeon had been my guy for more than a decade at that point, and he knew me. He knew my attitudes and pain tolerances.
Rather than a major production, just wiped on a topical anesthetic, Miss T moved in for a closer look, and he pushed the needle in the side of my knee. When he started his draw, the liquid was pale green, and like syrup. "Yup, that's the stuff!" He said, and grinned. He pushed the needle a little farther each draw... the nurse looked green.
"I will get the wraps," she said, and was gone. He gave another light push, and the skin on the far side tented. "There she is!" He said... and pulled back on the syringe. He drew enough liquid out to nearly fill the body of the syringe, then injected back a small amount. "Lubes the joint." He said.
(The stuff he pulled out and sent for testing, just in case, looked just like the stuff we bought for the kids to play with, called - Slime. Remember it? It came in a small trash can, and stuck to the wall when they threw it.)
As he drained the buildup, we watched the knee deflate, and with only a topical, I could feel the pressure drain off. The procedure really wasn't painful, and didn't have to be repeated. The buildup had begun again, but the revised the replacement, (rebuilt it,) at the 14 year mark.