While fiberglass material is newer, many casts used today are still made from plaster. Plaster casts are most often used when a fracture reduction (repositioning of the bone) is performed. The reason plaster is used after repositioning the bone is that plaster can be well molded to the patient, and therefore it can support the bone more precisely. When a bone was out of position, and is manipulated back into position, plaster may be used to help hold the bone in the proper position.
The problem with plaster is that it is heavy and must remain dry. Plaster casts are a burden for the patient because of their bulky and heavy material. Furthermore, water will distort the cast shape and can cause problems for healing should the cast get wet.
Fiberglass casts are usually fitted when the bone is not out of position, or if the healing process has already started. Fiberglass casts are lighter weight, longer wearing, and more breathable than plaster. The fiberglass casts are sturdier than the plaster and require less maintenance.
Both plaster and fiberglass casts are wrapped over a few layers of cotton that serve to protect the skin. Keeping this cotton clean and dry will be of utmost importance for your comfort. There is a special type of padding material that can be used under fiberglass casts to allow the cast to get wet. Ask your doctor if you are interested in a "waterproof" cast.