What is first aid for knee injury?

Call for an ambulance is the first thing to do. Make the patient as comfortable as possible. Treat any bleeding that may be present. Don't move the patient unless you absolutely have to. they may have more damage that a broken leg. Don't try to set it. If you suspect the femur is broken, don't move them at all unless their life is in danger where they are. A broken femur can cut the artery in their thigh and they will bleed to death in a very short time.