Cast leg. Plaster cast seen covering the leg, ankle and foot of a woman. A cast is a rigid casing, usually applied to a limb to hold a broken bone or dislocated joint so that it will heal in the correct position. Plaster casts are made using bandages impregnated with plaster of Paris, which hardens after application. Plaster casts are removed using an electric saw that cuts through the cast but does not damage the skin.