I'm not sure if it affects hedgies, but an edema is, very basically, an accumulation of fluid in the tissues, and the limbs are commonly affected by it.
Has the vet determined whether there's fluid in his legs and if so, has he tested the fluid? If he hasn't done anything other than a physical observation of Sonic, personally I'd push for a fluid test or biopsy. But perhaps Nancy or Kalandra can weigh in on that.