I've suffered with knee problems since my mid teens, but despite various trips to hospital and an op, no-1 has ever suggested any stretches for it. I tend to pull my knee up to my chest, and to gluts. Ialso walk, then gentle jog, up and down the stairs a few times to warm them up. You can also try straight leg raises, but inclined to the side, to warm up the inner and outer patella muscles. That involves lying flat on you back, raise the leg about6 inches up, but turn your leg slightly to the side. You can turn your body slightly instead if you find that easier, its how I tend to do it.
As always though, worth asking your Dr, incase it is something they can answer!