What Causes Swollen Knees When Suffering From Ankylosing Spondylitis?

I suffer from ankylosing spondylitis and currently have a swollen knee i had been given indocap SR but it doesn t seem to be working this time round had also been given nucoxia-mr but that too doesn t help the swelling is not reduced and mobility is restricted. Is there any other med that can be suggested for treatment?