Is the joint pain constant or on movement? Does it react to heat/cold?

IANAD (I Am Not A Doctor) but I'd check a variety of inflammation markers (the more the better) with special attention paid to possible autoimmune issues. I assume the usual suspects (arthritis, gout) were ruled out?

If I were you I'd try more doctors (third, fourth, fifth, etc.) hoping to hit one of three: (1) someone highly competent; (2) someone lucky to order the right test; (3) someone with determination to figure it out.