I Have Had A Fatty Feeling Type Lump On The

I have had a fatty feeling type lump on the back of my knee for many years. Over the past few years it has grown. I had an mri and the doctor told me that he did not see anything! It is very visable!!! Now I get a tingly feeling, sort of numb burning feeling in my calf! What concerns should I have and what type of doctor should I be consulting with? The doctor that I saw was an orthopedic surgeon!