Casts are custom-made. They must fit the shape of your injured limb correctly to provide the best support. Casts can be made of plaster or fiberglass — a plastic that can be shaped.
Splints or half-casts can also be custom-made, especially if an exact fit is necessary. Other times, a ready-made splint will be used. These off-the-shelf splints are made in a variety of shapes and sizes and are much easier and faster to use. Some have Velcro straps which make the splints easy to put on, take off, and adjust.
Fiberglass or plaster materials form the hard, supportive layer in splints and casts.
Fiberglass is lighter in weight and stronger than plaster. In addition, x-rays can "see-through" fiberglass better than plaster. This is important because your doctor will probably schedule additional x-rays after your splint or cast has been applied. X-rays can show whether the bones are healing well or have moved out of place.
Plaster is less expensive than fiberglass and shapes better than fiberglass for some uses.
Both fiberglass and plaster splints and casts use padding, usually cotton, as a protective layer next to the skin. Both materials come in strips or rolls which are dipped in water and applied over the padding covering the injured area. In some cases, special waterproof padding and cast material may be used. Your doctor will let you know if your cast is made and padded with these waterproof materials.
The splint or cast must fit the shape of the injured arm or leg correctly to provide the best possible support. Generally, the splint or cast also covers the joint above and below the broken bone.
In many cases, a splint is applied to a fresh injury first. As swelling subsides, a full cast may replace the splint. If a cast is initially applied to your injury, it may be “valved” (cut) to allow for swelling, then repaired at your first follow-up appointment.
Sometimes, it may be necessary to replace a cast as the swelling goes down and the cast gets "too big." As a fracture heals, the cast may be replaced by a splint to make it easier to perform physical therapy exercises.