I have used Flexi-Freeze Cold Therapy Wraps for years. When I recently fell and injured my knee badly, I immediately went for my Flexi-Freeze wrap. The wrap holds quite well on the kneecap without slipping. I like that you can use it just cold from the refrigerator or you can freeze it completely or (as I do) just freeze it until it is slushy. Holds the cold quite well but having the two ice pillows is really handy -- while using one, the other can be getting cooled.
A great item to have on hand. For swollen ankles especially