My knees "crack" when i bend down or get up off the floor. it feels like they're about to dislocate. sometimes i can't walk. do you know what's wrong?

Patellofemoral pain: This is usually pain from your knee cap or patellofemoral pain. This could be due to softening or fraying of the cartilage behind your knee cap or because the knee cap doesn't track well in its groove. You should be evaluated by a physician and usually physical therapy can improve your symptoms.

Patellar instability: It sounds like you have patellofemoral instability. This is when your knee caps tend to track abnormally creating popping and clicking. Sometimes this can be painful or the kneecap can even dislocate. There are some anatomical differences that may predispose you to this problem therefore i recommend an evaluation by a specialist to determine what you can do to minimize these symptoms.

Patella femoral : If your pain is in front/under your knee cap, then most likely patella femoral pain syndrome/chondromalacia patella. Best managed with exercise to strengthen your quads to improve tracking, and avoiding deep knee bending activities. Also oral or topical nsaids can be of benefit, as well as a brace. Injection of cortisone or hyaluronic acid also very popular. If all else fails, then scope.