Do you mean Septic Arthritis?
A friends foal had this quite a few years ago. I don't know if treatment is still the same nowadays,but back then, he had to go on a very long course of antibiotics, and then the effected joints were catheterised and flushed, and then some sort of antibiotic was administered into the joint. Can't remember the names of the antibiotics, but your vet will surely know.
What has your vet said?
The treatment on my friends foal, worked better than hoped, he's not 100%, but he's not as bad as they can be.
I hope your foal is ok.