Can't get it on because of his knee

So, the hubby and I would like to have sex more, but there's one problem: his knee. He has knee issues- cartilage gone; bone rubbing on bone. The slightest move or tap can cause his knee to pop. Surgery is scheduled for March-knee replacement. It's hard to have sex because we don't know what will make his knee go out. We change positions like me on top or reverse cowgirl so I'm not putting pressure on him, but that doesn't work either. Any ideas on what we can try? Any advice helps.

Thanks