Most orthopedic casts are made from one of two materials: plaster of Paris or fiberglass. Plaster casts use strips of cotton gauze coated in calcium sulfate hemihydrate, a white powder that hardens when mixed with water. Fiberglass casts use a knitted fabric impregnated with a polyurethane resin that also sets after being dipped in water. Both types are wrapped over a soft inner lining to protect your skin.
Plaster of Paris Casts
Plaster of Paris has been the standard casting material for well over a century. The active ingredient is calcium sulfate hemihydrate, a fine white powder made by heating gypsum (a naturally occurring mineral) until most of its water content evaporates. When the plaster-coated bandage strips are dipped in water, the calcium sulfate absorbs moisture and recrystallizes back into gypsum. This chemical reaction generates heat, which is why a fresh plaster cast feels warm on your skin as it sets.
A plaster cast typically takes 24 to 72 hours to fully harden, though it feels firm within minutes. During that initial curing window, the cast can still be dented or cracked if you put weight on it or press against a hard surface. Plaster molds closely to the shape of your limb, which makes it easier for a clinician to get a precise, custom fit. That moldability is one reason plaster is still preferred for certain fractures, especially those that need an exact contour around joints or curves.
The drawbacks are practical. Plaster is heavy, which can be tiring if you’re wearing a full-leg or full-arm cast for weeks. It also falls apart when it gets wet. A soaked plaster cast softens, loses its structural integrity, and can irritate the skin underneath as trapped moisture breeds bacteria. You’ll need to wrap it in plastic before showering.
Fiberglass Casts
Fiberglass has largely replaced plaster as the go-to casting material in many clinics. The casting tape is a woven fiberglass fabric coated with polyurethane resin. Like plaster, it’s activated by dipping in water, but it cures faster and produces a harder shell in less time.
Fiberglass is lighter and more durable than plaster, which makes a noticeable difference in comfort over a six- to eight-week healing period. It’s also more porous, allowing better airflow to the skin. Fiberglass doesn’t disintegrate when it gets wet the way plaster does, though the cotton padding underneath can still trap moisture against your skin. That’s where waterproof liners come in.
What Goes Underneath the Shell
No matter the outer material, a cast has layers. The first layer against your skin is a stockinette, a thin tubular fabric sleeve. Over that goes cast padding, usually made of cotton, polyester, or synthetic fiber. This padding protects bony areas like your ankle bones or wrist from pressure sores and cushions the skin from the rigid shell.
For people who want to swim or shower without covering their cast, waterproof liners made from expanded polytetrafluoroethylene (the same material as Gore-Tex) can replace the standard cotton padding. These liners let water pass through and drain out rather than trapping it against the skin. Research on infants in hip casts has shown that Gore-Tex waterproof liners reduce skin irritation and make daily care significantly easier. Waterproof liners are typically paired with fiberglass rather than plaster, since plaster itself can’t handle water exposure.
Splints vs. Full Casts
Splints, sometimes called half-casts, use many of the same materials but are applied differently. A splint is a slab of plaster or fiberglass placed along one side of the limb and held in place with an elastic bandage. It doesn’t wrap all the way around, which leaves room for swelling. Splints are common as a first step after an injury, before a full cast is applied once the swelling goes down.
Custom-molded splints used in hand therapy and rehabilitation are often made from low-temperature thermoplastics, sheets of plastic that soften in warm water (around 150°F to 170°F) and can be shaped directly on your hand or wrist. Once they cool, they hold their shape firmly but can be reheated and reshaped if the fit needs adjusting. These are lighter and more adjustable than plaster or fiberglass splints, making them practical for conditions that require gradual repositioning over time.
3D-Printed Casts
A newer option gaining traction is the 3D-printed cast, a rigid lattice structure custom-designed from a scan of your limb. These casts are typically printed from thermoplastic filaments like PLA (polylactic acid) or PETG (a type of polyester). The open lattice design makes them breathable, lightweight, and waterproof, solving several comfort problems at once. They also allow access to the skin for wound care or itching.
The tradeoff is structural. Common 3D-printing plastics don’t match the strength of fiberglass or plaster composites, and some clinical studies have reported minor breakage or deformation under load. Availability is another limitation. 3D-printed casts require a limb scan, custom design software, and printing time, so they’re not yet standard in most emergency departments. They’re more commonly used in follow-up care for stable fractures where the initial swelling has resolved and the injury geometry is straightforward.
How Cast Material Affects X-Rays
One reason your doctor may choose one material over another is how well it shows up (or doesn’t) on X-rays. Fiberglass is more radiolucent than plaster, meaning X-rays pass through it more easily. That makes it simpler to check how a fracture is healing without removing the cast. Plaster can obscure finer details on imaging, which sometimes means the cast needs to come off for a clear follow-up X-ray and then be replaced.