<s>[INST] I have a slight soft painless lump behind my right knee,  ultrasound shows nothing.  Now I am getting a painless warm sensation in that same leg. I do have lower back soreness,  could this warm sensation be due to sciatica or could this lump be something the ultra sound is not showing like a blood clot? [/INST] Hi. Thanks for your query.*Blood clot is very well shown in ultrasonography. This is probably fat the Monologist can miss easily. Morent Baker cyst is easily picked up and is never warm. Painless warm sensation can be due to sciatica as you have rightly suspected. Get a clinical examination as this can not miss the swellings like lipoma. If required and if you are sure about the soft painless lump behind the right knee, you can get the diagnosis confirmed by an MRI. </s>