Cast For a Broke Bone: What Is It Made Of?

Casts, whether for a broken arm, leg, or other part of the body, are used to support and protect injured bones and give them a chance to heal. Casts are typically made of plaster or fiberglass, each of which have pros and cons.

While casts can be uncomfortable and hard to deal with, they are an effective and efficient way to treat a broken bone (fracture).

This article explains the differences between plaster and fiberglass casts, as well as what a soft cast is and when one is used.

Most casts used today are fiberglass. They are usually fitted when the bone is not out of position, or when the healing process has already started.

Like plaster casts, fiberglass casts are wrapped over a few layers of cotton that protect the skin. Keeping the cotton clean and dry is very important for your comfort.

Compared to plaster casts, fiberglass casts:

While fiberglass material is newer, many casts used today are still made from plaster. Plaster casts are most often used when repositioning the bone (fracture reduction) is done. This is because plaster can be molded to the individual patient and support the bone better.

When a bone is out of position and put back, plaster casts can be used to help hold the bone in the right place.

The problem with plaster is that it is heavy and must stay dry. Water will ruin the cast's shape and can cause problems with healing.

Plaster casts can also be hard to deal with because they're bulky and heavy.

A soft cast is otherwise known as a splint or temporary cast. This is used when more rigid immobilization is not needed or in the early stages after a fracture has happened.

For example, a person who goes to the ER and is diagnosed with a broken bone usually goes home wearing a split because it allows more room for swelling than a hard cast.

It's important not to cast a broken bone right away because if the limb is constricted, it can lead to a serious complication called compartment syndrome. This condition happens because too much pressure has built up in the body—for example, after a fracture if swelling is happening in a space that's confined by a cast.

While compartment syndrome can cause severe pain, it can be hard to tell the pain from normal fracture pain. Most healthcare providers do not want to take the risk and will use a splint to leave enough room for swelling.

Once the swelling has gone down, a person can be fitted for a cast. If they need surgery to fix the bone, they will have to wait until after the procedure to transition from a splint to a cast.

Casts for broken bones are commonly made of plaster or fiberglass. While plaster is supportive, it is bulky and heavy. Fiberglass is often preferred because it's lighter and can get wet.

A soft cast or a splint might be used at first—especially right after an injury where there is a lot of swelling. Splints are also made of various materials (like foam or plastic), but are not as inflexible as a cast. Once the swelling goes down, you can get fitted for a cast.

American Academy of Orthopedic Surgeons. Care of casts and splints.

American Academy of Ortopaedic Surgeons. Care of casts and splints.

American Academy of Orthopaedic Surgeons. Compartment syndrome.

By Jonathan Cluett, MDJonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.