perhaps you are looking at the wrong place most likely it is a inflammation of the hip area ,possibly caused via a latent viral infection ,I would put the bird on noninflammatory drugs or you could choose to feed the bird ginger ,capsicum and chilli's ,I would also give Bananas these have an inbuilt protease inhibitor that will stop a virus (or at least slow ) from replicating it's DNA , I would also have a Google for Viral Tenosynovitis ,I doubt if this is the cause but perhaps further links may help ....If this problems develops further within your flock I would increase Manganese in their diets and back this up with B group vitamins , B group vitamins are particularly help full in calming stressed and in pain birds as well as ligament and bone growth problems especially those associated with legs