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I understand your concern.
Thanks for the detailed description of your problem. I encounter similar issues very frequently in my practice. I do not consider 38 degrees as a fever. According to the literature, it is common to have this body temperature after surgery. In order to reduce my anxiety, I get a few blood tests which include complete blood count (CBC), erythrocyte sediment rate (ESR), and C-reactive protein (CRP). These are also deranged after surgery but I get them every alternate day three times and I get to know the trend. If total leukocyte count (TLC), or CRP has a falling trend, then I am worried. Now in your case, did you have any recent blood tests after surgery? Could you please send me a picture of your knee? Also, send me your X-rays. Do you have any complaints of a burning sensation while micturition? I can help you better after getting a picture of the area.
I hope this information will help you. Please revert in case of more queries.