Home Health & Personal Care. How to relieve and stop itching inside a cast. If you or your child has to wear a cast after breaking an arm or a leg, you probably know how itchy it can get inside. Fortunately, there are some ways to prevent the itching (as well as itch!) without having to remove the cast to do it. First, when you need to itch, use a knitting needle to get inside the cast and scratch. The needle is thin enough to slide inside the cast and rub the itchy area. Just be sure the knitting needle isn't too small of a diameter, so you don't have to worry about accidentily poking and breaking the skin. To stop the itching, pour a small amount of baby powder inside the cast at one end. Then use a hair dryer on the cool setting to blow the baby powder inside the length of the cast to provide a non-sticjy barrier between the cast and the skin.