I suggest a CAT scan and a bone scan.
X-rays are useless except for diagnosing fractures or some other diseases.
The cat scan and bone scan showed my doctor that the plate was loose at the femur.

After two manipulations from my prior surgeon, a CPM that was not implemented until 2 1/2 months, and Dynasplint I was very relieved to find someone that was willing to tell me if there was actually something wrong and it’s not my lack of exercising.
You have to be pushy because they’re not going to offer to you.
Maybe a very good doctor would but most of them will not.

I'm writing this with sympathy and total understanding of the pain you are dealing with. I lived with severe chronic pain for years and understand how you are feeling. Lack of sleep from pain, then brief fitful sleeps and then getting up in the morning to face it all over again, just like the movie "Groundhog Day" will break your will to live. To entertain myself while lying in bed I plotted the best way to commit suicide. I was so close to following through. Fortunately because of another discussion site on Mayo unrelated to knee replacement a blogger shared an experimental solution others had tried with some success. A miracle, it worked for me.

My point is dear auxiliary it is not possible to live with 24/7 pain and not start to loose your mind. If I were you I would travel the country,
and see multiple specialists; you may have already done this. Never count on one or two specialists to diagnose your issue. I am speaking from experience. If they still can't determine the cause I personally would have the amputation. I would have to adjust my life to accommodate being in a wheelchair. Yet I know, without pain, I would find activates I could find happiness in and appreciate the gift of life. The pleasure of being able to sleep soundly at night is a blessing.

Believe me I am not suggesting amputation lightly. I already know what is in my future, probably a wheelchair. I follow this site even though I have NOT had a knee replacement but because I am always seeking knowledge because as of today no implant will not cause an allergic reaction in me leading to rejection. I may end up in a wheelchair because I do not qualify for a knee replacement. I have had extensive allergic testing which documents my allergies to metals and among a host of other allergies I am allergic to 5 different metals and my ortho told me even titanium, which I didn't test allergic to has traces of other metals which I am allergic to. There is a ceramic coated knee implant which we considered but now this company has a class action lawsuit against them because the bone cement, which cements the implant to the bone is place is not adhering to the implant. I do have hope that before I need a wheelchair they will have figured out this issues.