Dislocating knee cap"s !!!?

I am 14 and for about a year and a half I have had dislocating knee caps. I have been taught how to put them in myself at physio, however it's very painful, I have other joint and bone problems aswell and am under going tests to see if it may be artharitas, cancer, musculardisk malformation and scoliosis of the mestra. I am having physio for the lee cap which I have had twice before however so far hasn't worked they said if it doesn't stop in the next 4 weeks after intense physsio I will have to have the oporation to replace my kneecap cartalige for it Has been warn thin from dislocating so often. Because I learnt how to replace it I don't have to go to hospital like others however I cannot do certain sport. Does anyone no any good ways to try and jeep the kneecaps in. I have tried a brace and a metal implanted brace(had to have op for this one) but neither have worked. Any ideas to strengthen the knee will be appreciated thankyou :)