I get that on my right knee cap. It feels very tender to touch on the bottom edge, less so across the front, not on the top.
It all goes back to my quad muscle. Hot spots are between the IT band/vastus lateralis and the front quad muscle, rectus femoris.
http://www.britannica.com/science/quadriceps-femoris-muscle
Get your thumb and dig in there between the front of the quad and the side. You will find some sore spots on the rect muscle. It is attached to the hip under some other band-like muscles then runs down your leg to attach to the patella. Get a golf ball or your thumb and find the hot spots and press like buggery, rubbing them as well. You want them to relax and let off the tension to you patella.
You may also find some hot spots under the knee at the top of your calf muscle.
One physio or ART appointment won't do it. You need to do it every day, not just on leg day either.
I don't know the full story of your condition but it most probably does not need anything invasive, just some work on your quad. Make sure you stretch your quads out well every day, once they are warmed up from a walk or run or leg work.
Check this out: