It's been two years since having both knees replaced. I'm 69 y/o now and in the gym every day. I do a lot of cardio, mostly spinning, some rowing. I'm grateful, to say the least.

I've seen people post here about their TKR experiences, and they certainly vary. Post-surgery fatigue though is new to me. At 8 weeks it could just be emotional exhaustion from the physical therapy and rehab. It can take up to one year for the knee to fully heal. That may be causing some fatigue.

As for pain in the lower part of the kneecap - it could be incorrect prostheses size but that would show on an Xray, and the surgeon should always correct that during surgery. I don't think that's it.

Have you been doing exercises on a leg extension machine? On this machine, the legs start at a 90 degree angle and the ankles are underneath two pads. The knees are then extended to work the quadriceps.

My surgeon made it very clear - DO NOT USE A LEG EXTENSION MACHINE, EVER! The reason is that the patella tracks unnaturally when the knee his bent and unbent. If you're doing this, I'd stop.

And the leg extension exercise has no functional use, well unless you walk backwards a lot. A better exercise is simply to extend your legs from a seated position, lock the knee and hold for 30 seconds, give or take. I still do this and squat exercises to strengthen my quads.

Hope you find an answer, and all the best to you. Joe