Orthopedic Plaster Casts – Material, Application and Care

A plaster cast, slab or splint is applied in broken bones of upper or lower extremity. Plaster can be applied for a temporary period while waiting for definitive treatment or it could be definitive treatment in itself.

Materials used in Orthopedic Casts

Plaster of Paris takes its name from Paris, France, where it was first widely used chemically, surgically and in construction works.

Its first documented medical use dates back to 1852 when A. Mathyson, a Dutch Army Surgeon, rubbed powdered plaster into cotton bandages to form splints.

Plaster of Paris, in its raw state, is termed gypsum hydrated calcium sulphate with impurities. The surgical form is pure anhydrous calcium sulphate.

The essential chemical step is the heating of gypsum to 120 degree Celsius (250 degree F).

Adding water allows for a return to the original crystalline state of full hydration. Twenty percent of added water is incorporated into the hydrated crystal lattice but the other 80 percent of water eventually evaporates.

The absorption of water while setting gives out heat (an exothermic reaction).

Plaster of Paris extremely safe and does not produce allergies. It is infinitely adaptable to the part being splinted and can be applied speedily without gloves. It cheaper in comparison with more modern materials.

However, plaster of Paris is slow to dry, to gain full strength and is seriously weakened if it becomes wet again. It is very heavy when wet but becomes much lighter when dry.

It is partially radio-opaque, obscuring bone detail on radiographs.

[Read more on fiberglass cast]

Mechanical Characteristics

Low temperatures and sugar solutions retard setting of plaster of Paris while high temperatures and salt or borax solutions accelerate it.

The setting time is three times longer at 5 degree Celsius (40 degree F) than at 50 degree Celsius (125 degree F).

Although 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.

Movement of the plaster, while it is setting, will cause gross weakening.

The optimum strength is achieved when it is completely dry.

Plaster of Paris casts are frequently used to put casts in fracture patients. Although Plaster of Paris only depends on one simple chemical reaction for its setting, it is possible to vary the features of the process according to different requirements.

Temperature

If a bandage is immersed in cold water the initial set 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.

Strength

The cast strength depends upon the thickness of plaster and the shape of the cast which follows the contours of the affected limb. However, excess plaster will also increase weight and bulk and heat product. Therefore these different factors must be weighed against each other.

Because plaster of Paris is infinitely mouldable in the wet state it can be set around cast brace hinges and walking heels with ease.

Alternatives to Plaster of Paris

Fiberglass is a commonly used material for casts and external splints. These are light, durable and waterproof but require protective packaging and are difficult to apply without wearing gloves.

They are more radiolucent than plaster.

A number of preformed plastic components are available as an alternative to plaster. They are made to fit different sizes of limbs and to allow movement at joints.

What Are Uses of Plaster Cast Application

The basic aim of a plaster application is immobilization of the fracture fragments and joints.

A plaster could be

• 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, 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 the 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.

Principles of Plaster Cast Application

Material

Traditionally plaster of Paris casts is used. You must have seen those white colored casts, those are made of plaster of Paris or POP.

Another material which is commonly used now is fiberglass cast. It is a special kind of plastic which when comes in contact with water turns hard. It is available in many colors.

Extent

Rule of thumb for plaster applications block a joint above and below the fracture. This is done as movement of either joint can cause movement in fracture fragments.

For immobilizing the joint, a plaster should be adequate enough to counter the forces of the muscles that move the joint.

Majorly, a plaster, depending on its extent, could be

Above Elbow

An above elbow plaster cast or slab is applied from knuckles of hand [distal palmar crease anteriorly] and covers lower two thirds of arm

Below Elbow

While distal extent is same as above, proximally the plaster ends below elbow crease.

Above Knee

Distal extent is up to metatarsophalngeal joints and proximally it covers lower two-thirds of the thigh.

Below Knee

Distal extent is same, proximal extent ends below the knee.

There are various other types of casts used in case of a number of fractures where the extent and position is defined by the type of plaster applied.

Plaster slabs or cast may have same extent but different joint positions depending upon conditions.

Plaster Slabs and Plaster Casts

A plaster cast is a circumferential application of plaster bandages. Cast is stronger by virtue of being circumferential than slabs, control  the reduction of fragments better and withstand mechanical stresses better. Most of the closed reductions of the fractures require cast application.

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 second posterior, for better strength and control.

Slabs are applied in case where there is swelling , 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.

Padding

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 orthopaedic padding is placed on the skin before plater application.

Padding serves to

• prevent sores
• increase comfort
• act as a spacer to aid removal.

Traditional casts are made of plaster of Paris material. Other materials like fiberglass casts are also applied now. Fiberglass  is a type of plastic that can be shaped.

Fiberglass casts are lighter and more durable than traditional plaster casts. There is better air circulation in the cast and xrays also have better penetration making it easier for the clinician to  evaluate the union of fracture

How to Apply Plaster

For details of the procedure, please click on the relevant link.

Plaster of Paris Cast Application

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 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 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 which 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 of Paris cast is tapped with the knuckles, it gives a crisp, clear sound, but the damp cast gives a dull sound.

Video of Plaster of Paris Cast Application

A video describing application of plaster of Paris cast is shown below.

Slabs

Read more about slabs and casts

A slab is prepared When the required length and width have been decided, the slab is lightly folded from each end to the centre. It is immersed in water immediately removed, then carefully and quickly smoothed on the flat surface. The layers must be pressed together and the bubbles excluded. If this is not done, the layers become brittle when dry and can separate.

Plaster of Paris may be used for making plaster slabs or removable splints. These fit the part exactly and are light to wear and easier to apply than many of the traditional splints.

Any complete plaster of Paris cast can be converted into a removable splint by bivalving it.

If a cast has been bivalved to facilitate daily dressing of a wound, and immobilization has to be continued, the anterior and posterior sections are both preserved and are held in place by a firm bandage.

Plaster slabs are applied as follows

Plaster Slab Application

Slabs can be prepared by unrolling plaster bandage 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. This 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.  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.

Video of Plaster of Paris Slab

What is Unpadded Cast?

The complete, unpadded, skin-tight cast is not used often. It had been advocated in the past by some authors because it produces a lightweight, comfortable and perfectly fitting cast.

The advocates recommended this type of cast only in situations where swelling is unlikely to develop.

This cast has many drawbacks and potential risks. The danger of swelling always is there in an injury and if there is no padding the cast constricts the swelling leading to a vicious circle of swelling.

The danger is less with padded casts as the padding buffers the space needed for the swollen tissue. But in the case of unpadded cast, the swollen tissue encounters the cast material which does not yield.

This, in turn, leads to pressure on the swollen tissue leading to the constricting effect. Because of this constriction, the flow of venous blood stops leading to further swelling.

If the patient neglects the swelling, it might lead to ischemia. Limbs have been lost due to plaster swellings.

The cast is difficult to split and bivalve.

Removal of the cast is also difficult and uncomfortable for the patient.

An electric cutter must never be used in this situation.

This occurs because the cutter is at risk of striking the skin after the plaster because there is nothing between. The unpadded cast should be cut with hand instruments that do not use power.

Some people apply this cast on stockinette to protect the skin from the heat that exothermic reaction of plaster setting would generate.

The advantage of the unpadded cast is a very accurate molding of the plaster to the contours of the limb and thus better grip of the plaster.

But stockinette can be considered as padding and the cast is still called unpadded.

Care of Plaster Cast

Plaster cast application is not the end of the treatment. Care of plaster cast is very necessary. All patients need to be given instructions about how to take care of plaster cast and preferably in writing.

Following instructions for care of plaster cast should be given to every patient who has got a cast applied.

Do Not Interfere

Plaster cast extent and thickness is decided by the injury to be treated, body habitus and type of the cast.

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 next day to note any discomfort, bluishness, swelling and numbness of the digits.

• If the plaster cracks, 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 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 case of 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 pumping of blood by muscles and also in the reduction of swelling.

Drying Time of 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.

At home, 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 in where the air is circulating adequately would hasten the drying time.

Cast Protection

In case of walking casts, a  reinforcing sole or heel of wood or rubber should be added after the cast has dried.

Canvas or plastic coverings may be used for short periods to protect the cast from water. These must be removed regularly to prevent the plaster from becoming damp as sweat fails to evaporate.

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

General Instructions For Patients

For Care of Plaster Cast

• Keep the cast clean and dry.
• 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 one can use a hairdryer placed on a cool setting to blow air under the cast and cool down the hot, itchy skin. Never blow warm or hot air into the cast.
• Do not put powders or lotion inside the cast.
• Prevent small toys or objects from being put inside the cast.
• Elevate the cast above the level of the heart 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.
• Spica casts need special care especially using bedpan or urinal. Tips to keep body casts clean and dry and prevent skin irritation around the genital area include the following:
  • 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.