An orthopedic cast, or simply cast, is a shell, frequently made from plaster or fiberglass, that encases a limb (or, in some cases, large portions of the body) to stabilize and hold anatomical structures—most often a broken bone (or bones), in place until healing is confirmed. It is similar in function to a splint. Plaster bandages consist of a cotton bandage that has been combined with plaster of paris, which hardens after it has been made wet. Plaster of Paris is calcined gypsum (roasted gypsum), ground to a fine powder by milling. When water is added, the more soluble form of calcium sulfate returns to the relatively insoluble form, and heat is produced. 2 (CaSO4·½ H2O) + 3 H2O → 2 (CaSO4.2H2O) + Heat The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; however, the cast is not fully dry for 72 hours. Current bandages of synthetic materials are often used, often knitted fiberglass bandages impregnated with polyurethane, sometimes bandages of thermoplastic. These are lighter and dry much faster than plaster bandages. However, plaster can be more easily moulded to make a snug and therefore more comfortable fit. In addition, plaster is much smoother and does not snag clothing or abrade the skin. Cast types Upper extremity Upper extremity casts encase the arm, wrist, or hand. A long arm cast encases the arm from the hand to about 2 inches below the armpit, leaving fingers and thumbs free. A short arm cast, in contrast, stops just below the elbow. Both varieties may, depending on the injury and the doctor's decision, include one or more fingers or the thumb, in which case it is called a finger spica or thumb spica cast. Lower extremity Lower extremity casts are classified similarly, with a cast encasing both the foot and the leg to the hip being called a long leg cast, while a cast encasing the patient's foot, ankle and lower leg ending below the knee is referred to as a short leg cast. A walking heel may be applied for ambulation. These heels, when properly applied, elevate the toes and offer the wearer the advantage of keeping the toes out of the dirt and moisture of the street. The walking heel provides a small contact patch for the cast and creates a fluid rocking motion during the stride as the cast can pivot