Swelling in knee after injury is very common. It is due to internal bleeding in the knee. In most cases there is no need to take out this blood. In some cases the knee may be too tight due to swelling. Only then the blood must be taken out. After aspirating the blood from the knee it is very important to cover the knee with cotton and crepe bandage to stop internal bleeding, else the knee will swell with blood again. I usually don't recommend repeated aspiration as it can cause infection.