Last Updated on March 16, 2025
A plaster cast, slab, or splint is applied to broken bones of the upper or lower extremities. Plaster can be applied for a temporary period while waiting for definitive treatment or it could be definitive treatment in itself.
The basic aim of a plaster cast application is the immobilization of the fracture fragments and joints.
A plaster cast could be used in the following situations
- A temporary measure of pain relief and reduction of swelling so that definitive treatment could be executed.
- For immobilization of joints in case of ligament injury or joint swelling due to disease.
- A splint for undisplaced fractures.
- A definitive treatment where plaster is applied to maintain the reduction of the fragments.
- External splint to aid with the internal fixation of fractures, osteotomies, and other bone surgeries where internal fixation alone would not be able to withstand loads.
- External splint for blocking movements in case of nerve, tendon, or vessel repair.
Difference Between Plaster Slab and Plaster Cast
A plaster cast is a circumferential application of plaster bandages. The cast is stronger being circumferential than slabs, controls the reduction of fragments better, and withstands mechanical stresses better. Most of the closed reductions of the fractures require a cast application.
The slab is a support created from plaster material which is applied on one aspect of the limb and secured with cotton bandages. Sometimes, two slabs may be applied, for example, one anterior and the second posterior, for better strength and control.
Slabs are applied in cases where there is swelling or a poor skin condition that needs to be observed like blisters, after internal fixation to aid the fixation, while waiting for definitive treatment. Many undisplaced fractures are treated by slab application.
Different Materials Used in Plaster Cast
Plaster of Paris
Traditionally, plaster of Paris casts are used. That is why they called plaster casts. But the name plaster cast is interchangeably used for casts from other materials. Y
Plaster of Paris, in its raw state, is termed gypsum hydrated calcium sulfate with impurities. The surgical form is pure anhydrous calcium sulfate.
Adding water allows for a return to the original crystalline state of full hydration and is an exothermic reaction.
Plaster of Paris is extremely safe and does not produce allergies. It is infinitely adaptable to the part being splinted and can be applied speedily without gloves. It is cheaper in comparison with more modern materials.
Although the setting takes only a few minutes, drying may take many hours – roughly 36 hours for an arm cast, 48-60 hours for a leg cast, and up to 7 days for a hip spica, especially if the atmosphere is moist and cool.
The optimum strength is achieved when it is completely dry. Movement of the plaster, while it is setting, will cause gross weakening.
The cast strength depends upon the thickness of the plaster and the shape of the cast
Fiberglass
A fiberglass cast is a plaster cast made from fiberglass material which is a lighter, synthetic alternative plaster of Paris. Fiberglass is also called glass-reinforced plastic[GRP] or glass fiber-reinforced plastic [GFRP]. It is made of a plastic matrix reinforced by fine fibers of glass.
The fiberglass cast is also popularly called the synthetic cast. Traditionally, the orthopedic cast has been made of plaster of Paris and is called a plaster cast. However, it is common to use a plaster cast for both types.
Compared to the traditional plaster cast, it is light in weight and more durable. It is three times stronger but is only one-third in weight.
Casts made of this material require less maintenance.
The plaster of Paris casts is white. Fiberglass casts come in many colors and some doctors even let the patient choose the color.
Fiberglass cast is less yielding to molding and to hold fracture fragments, molding is required. In contrast, the plaster of Paris provides excellent molding.
Fiberglass sets quickly, therefore experience is required to time the application well. It is hygroscopic [absorbs water] and setting time decreases with humidity and warmth.
How to Apply Plaster Cast?
Extent
Plaster cast application aims to block a joint above and below the fracture. This is done as a movement of either joint can cause movement in fracture fragments.
Different types of casts based on the extent of the plaster are
- Long Arm Plaster Cast: From distal palmar crease anteriorly and to covers the lower two-thirds of the arm. Aims to immobilize the forearm and elbow
- Short Arm Plaster Cast: From the distal palmar crease to the proximal third of the forearm. Aims to immobilize the wrist and distal forearm.
- Long Leg Plaster Cast: From the base of the toes to the middle of the thigh. A cylinder cast has a similar upper extent but ends at the ankle.
- Short Leg Plaster Cast: a cast that extends from the base of the toes to the below knee
- Body Casts: Body casts are those casts that also include the trunk. The examples are shoulder Spica, hand ip spica [used in case of femur fracture in very young children and in cases of congenital hip dislocations.
Padding – Paddeded vs Unpadded Plaster Casts
Though few people had advocated this in the past, a plaster cast or slab is not applied without the padding.
Soft resilient material such as wool felt and special orthopedic padding is placed on the skin before plater application.
Padding serves to
- prevent sores
- increase comfort
- act as a spacer to aid removal.
An unpadded cast has the danger of swelling which is less with padded casts as the padding buffers the space needed for the swollen tissue.
The cast is also difficult to split and bivalve.
Removal of the cast is also difficult and uncomfortable for the patient.
Plaster of Paris Cast Application- The Procedure
First of all, stockinette is applied over the area to be plastered and covered by the soft cotton padding of the required width firmly over the area.
For securing, each turn is overlapped by one-third in order to secure layers.
The patient is settled in a comfortable position with clothing protected. The equipment is readied [water in bowl, bandages]. The water used should be neither very cold or very hot. If a bandage is immersed in cold water the initial plaster setting will be delayed and thus “working time” lengthened. However, if a very rapid set is required soaking the bandage in warm water will accelerate the rate of reaction.
The affected part should be covered with suitable padding and the desired position secured and held correctly. Bandages of the correct size are immersed in water, one at a time, and held there until bubbling stops. The bandage is removed by holding it at the ends. The ends are gently squeezed towards the center and then pulled back to shape.
The bandage is unrolled around the limb in an even manner. Minimum tension should be exercised and this should be directed towards the center of the bandage-not at the edges.
Molding of the bandages to the contours of the limb should be done by constant smoothing with the palms of the wet hands.
When the required thickness has been obtained, the extremities of the plaster of Paris cast may require trimming to ensure that a free range of movement is possible at joints that are not immobilized. The completed wet cast is handled carefully and supported correctly to protect it from damage.
The cast does not fully dry out until 36 to 72 hours after application.
When a dry plaster cast is tapped with the knuckles, it gives a crisp, clear sound, but the damp cast gives a dull sound.
Slab Application
Slabs can be prepared by unrolling plaster bandages to a required length or by withdrawing the required length of layers from a plaster dispenser. This may be done with either wet or dry bandages which are folded out on a smooth surface. The average thickness of slabs for strengthening is 12-16 layers. The thickness might be adjusted from patient to patient.
When using bandages care must be taken to see extends for the same distance. Short ends should be discarded because they cannot be held securely when the slab is immersed in water. The slab can be prepared in any width, depending on the needs of the situation. Sometimes a slab is used for initial immobilization and is completed later. Slabs must be smoothed carefully on a flat surface after they have been soaked.
Plaster of Paris casts become supportive in three to five minutes, depending on the water temperature and the thickness of the cast.
Care of Plaster Cast
Following instructions for care of plaster cast should be given to every patient who has got a cast applied.
Do Not Interfere
The patient should be instructed not to wet, cut, heat, or otherwise interfere with the plaster.
Report At Once If-
All the casts in case of acute fractures should be recalled the next day to note any discomfort, bluishness, swelling, and numbness of the digits.
- If the plaster cast cracks and becomes loose or otherwise uncomfortable.
- If there is any pain that is not relieving or worsening.
- If there is any discharge or high-grade fever
- If the fingers or toes become numb or difficult to move.
- If the fingers or toes become swollen or blue.
- If there is a burning sensation at any point throughout the cast extent. It usually occurs due to plaster digging in the skin
Elevate As Instructed
The plaster cast may feel tight for some time after application. This can usually be relieved by lying down and elevating the arm or leg on one or more pillows and by constantly moving those joints of the arm and leg that are not covered by plaster.
For the first twenty-four hours keep the limb raised as much as possible. In the case of the upper limb, a sling should be used to elevate the limb. For lower limb injuries, the limb is placed on a cushion or pillow.
Move The Digits
The patient should be instructed to move the fingers or toes of the affected limb. This encourages the return of the blood due to the pumping of blood by muscles and also in the reduction of swelling.
Avoid Pressure on Wet Cast
Plaster of Paris casts become supportive in three to five minutes, depending on the water temperature and the thickness of the cast. The cast does not fully dry out until 36 to 72 hours after application.
When a dry cast is tapped with the knuckles, it gives a crisp, clear sound, but the damp cast gives a dull sound.
Till the cast dries, it has not reached its full strength and might break with stress. It is important in weight-bearing casts of walking casts and the patient should be instructed accordingly.
The patient must be advised not to rest the wet cast on a firm surface such as an unpadded chair. This will dent the cast and produce a ridge on the internal surface which may cause soreness. Unsuspected hazards, such as the ridge on the arm of a chair, can cause a dent in still-damp plaster.
Sitting in sunlight or where the air is circulating adequately would hasten the drying time.
Fiberglass cast on the other hand dry in a few hours.
Cast Protection
In the case of walking casts, a reinforcing sole or heel of wood or rubber should be added after the cast has dried.
Forearm casts are particularly prone to getting wet at the lower edge which may fray. Small fragments may be detached and slip between the skin and plaster causing irritation.
The patient should be instructed to be careful.
Wetting does not affect the fiberglass cast per se, but even in these cases, water may percolate to the padding where it would affect the skin.
Assistive devices Persons with Casts
Ambulation with casts may require assistive devices. Most of the assistive devices are required for lower limb or trunk casts. These are
- Crutches
- Walkers
- wheelchairs
Other Dos and Don’ts
- Check for cracks or breaks in the cast.
- Pad the rough edges to protect the skin from scratches.
- Do not scratch the skin under the cast by inserting objects inside the cast. Instead, blow cool air from the dryer to relieve the itch. Never blow warm or hot air into the cast.
- Do not put powders or lotion inside the cast.
- Elevate the cast to decrease swelling and move the fingers or toes to promote circulation.
- Do pull support or lift the person by cast or bar on the cast.
- Special care of hip spica-
- Use a diaper or sanitary napkin around the genital area to prevent leakage or splashing of urine.
- Place toilet paper inside the bedpan to prevent urine from splashing onto the cast or bed.
- Keep the genital area as clean and dry as possible to prevent skin irritation.
