What Causes Swelling On Top Of Knee With Stiffness Behind The Knee?

knees swollen and stiff, 69 year old male, had polymyalgia rheumatic for 14 months, now knees swollen on top and very stiff behind the knee, limited range of mortion. on 9 ml prednisone at moment, seems very cold weather making it worse. is knee swelling usual with PMR, only happened in recent times