An orthopedic cast is a hard shell used to encase parts of a body after a broken bone is set to hold the body parts in relative immobility until it can be confirmed the bone has properly healed. Traditionally, casts were made by using a bandage made of a mixture of cotton gauze and Plaster of Paris that becomes pliable when immersed in water, can be moulded around the limb or other body part to be immobilized, and then hardens to near full strength in as little as 45 minutes. Although modern alternatives exist, such as fiberglass bandages mixed with polyurethane, or thermoplastic, plaster bandages are still preferred as they can be more closely molded to the patient's skin and can be more comfortable as a result.
Although a splint can serve the same function, a cast is preferred when the bone is large and is subject to larger outside forces.
Casts do have limitations. They interfere with the normal loss of skin cells and can cause considerable dermatitis. They are also quite heavy, particularly in comparison to the weight of a child. Plaster casts must also be kept dry. Modern fiberglass and plastic casts allow the patient to bathe.