Go to this link for a comprehensive look at knee problems. It is a very good NHS information site.
This exercise is called the clam or Oyster because the movement is like opening a shell. There are lots of knee exercises but this is our favourite because it helps so many different knee problems.
Repeat this movement x10 slowly, x10 quickly and x10 in 1cm steps,
Keep going through this until you are fatigued and the muscle on the outside of your hip is really tired. Repeat to fatigue 3 times.
This exercise should strengthen the muscles that control the knee without hurting the knee at all.
The exercise should be done at least twice a day for at least 3 weeks and then if the problem is resolved keep going once or twice a week.
When this is too easy try these variations.
Variations. In this position separate your ankles!
2: lift your leg straight to the perpendicular.
3: as in 2: but now Rotate your toes to the ceiling