A cast should hold the fracture still and do nothing else. The two ways in which casting harms the patient are that it is too tight — the swelling comes after the injury, not before — and that an edge or a fold presses on a bony prominence until the skin breaks down. An acute, fresh injury is therefore immobilised in a backslab or in a split cast, never in a closed circumferential cast, and the patient must leave with clear information about the symptoms that should make them seek help immediately.

Indications

An undisplaced or reduced fracture to be treated conservatively.
Immobilisation after reduction of a dislocation.
Immobilisation of a ligament injury, a tendon injury, or after suturing over a joint.
Pain relief and stabilisation for transfer in a suspected fracture.
Clinical suspicion of a scaphoid fracture despite a normal radiograph.

Contraindications

An open fracture or a wound that needs to be inspected — use a windowed splint or external fixation.
Established or impending compartment syndrome.
An unstable fracture requiring operative fixation.
A circumferential cast in a fresh injury, before surgery, or while swelling is progressing.
Take care in impaired sensation (neuropathy, spinal cord injury) — the patient does not feel the pressure sore.

Preparation and equipment

Material	Comment
Stockinette	Innermost against the skin, protects against the edges of the cast
Padding	The wadding that protects the skin and the bony prominences
Plaster bandages and plaster slabs (calcium sulfate)	Mouldable, cheap, heavier, less strong, takes longer to set
Synthetic cast material (fibreglass)	Lighter, stronger, radiolucent, tolerates moisture better, more expensive and stiffer to mould
Elastic bandage	Holds the slab in place; wound on without tension
Tepid water, plastic gloves, plaster scissors, cast saw, protective sheeting	Hot water gives faster setting and more heat production — a risk of burns

Document the distal neurovascular status before casting: sensation, motor function, pulses and capillary refill. Check the skin, and remove rings and jewellery distal to the injury before the swelling develops.

Procedure

Place the joint at the angle the treatment requires before the cast is applied, and hold that position until the cast has set.
Pull on the stockinette so that it extends a few centimetres beyond the planned ends of the cast.
Pad evenly, 2–3 layers with about 50 per cent overlap, and add extra padding over bony prominences, at the ends of the cast and where the edge of the slab will lie. Uneven padding, folds and lumps are the commonest cause of pressure sores.
Dip the plaster in tepid water and squeeze out the excess without wringing.
Put the slab in place and mould with the whole palm, never with the fingertips — fingerprints in the plaster become pressure points against the skin.
Fold the ends of the stockinette back over the edge of the cast and smooth the edges.
Secure with an elastic bandage in even turns without tension.
Check that the slab ends where it should: the metacarpophalangeal joints must be able to flex freely, the thumb must be free unless it is to be immobilised, and the elbow must be able to flex if it is to be free.
Check the distal neurovascular status again once the cast has set, and document it.
Elevate the limb and inform the patient.

Forearm and hand with a white plaster slab on the dorsal aspect from below the elbow to the knuckles, secured with an elastic bandage, with the fingers and thumb free — Figure 1. Dorsal backslab of the forearm and wrist. The plaster slab lies on the dorsal aspect of the forearm and hand and ends just proximal to the metacarpophalangeal joints so that the fingers can flex freely; the thumb is left free. Innermost lies the stockinette, whose edges are folded out over the plaster, and then an even layer of padding extending slightly beyond the edges of the slab. The volar aspect is open and covered only by the elastic bandage, which allows room for swelling — this is why the backslab is the first choice in a fresh injury.

Backslab or circumferential cast

A dorsal backslab or another half-slab for a fresh injury, acute swelling, after reduction and before surgery. The slab allows the volume to increase.
A circumferential cast gives better stability, but only once the swelling has settled, often after about a week. A circumferential cast applied acutely must be split through its entire thickness, including the padding and the stockinette, right down to the skin — a cast split only through the plaster layer does not relieve the pressure.

Common splints and casts

Injury	Immobilisation	Comment
Distal radius fracture, acute	Dorsal backslab, forearm–wrist	Circumferential cast after the swelling phase
Suspected scaphoid fracture	Scaphoid cast including the base of the thumb	Reassessment after 10–14 days
Finger fracture or finger dislocation	Finger splint or buddy taping	Do not immobilise a finger longer than necessary; risk of stiffness
Elbow dislocation after reduction	Dorsal backslab, upper arm–forearm, elbow at 90 degrees	For a short period, then early mobilisation
Ankle fracture, acute	Posterior below-knee backslab, ankle at 90 degrees	Circumferential below-knee cast later
Marked ankle sprain	Splint or orthosis	An orthosis is usually preferable
Metatarsal fracture	Below-knee cast with a heel or a rigid sole	Weight-bearing as directed by the orthopaedic surgeon
Achilles tendon rupture	Below-knee cast in equinus, or an orthosis with heel wedges	The angle is changed in stages

Casting times and weight-bearing regimens vary between regions and units and are governed by the fracture type — follow the local care programme and the orthopaedic surgeon's instructions, not a general rule of thumb.

Compartment syndrome

The feared complication. The pressure in a muscle compartment rises until perfusion ceases, and the muscle necrosis is irreversible within hours.

Pain that is disproportionately severe and increasing despite adequate analgesia is the first and most important sign.
Pain on passive stretching of the muscles in the compartment.
A tense, hard and tender compartment.
Paraesthesiae and numbness distally.
A preserved pulse and normal capillary refill do not exclude compartment syndrome — pulselessness is a late and unreliable sign.

Action when it is suspected: split the cast immediately through all layers down to the skin and spread it open, place the limb at the level of the heart — not higher, since elevation lowers the perfusion pressure, and contact the on-call orthopaedic surgeon urgently. Fasciotomy is the definitive treatment.

Two cross-sections through a limb in a cast, on the left with the cast closed and the vessels compressed, on the right with the cast split on two sides and the vessels open — Figure 2. Cross-section through a limb in a circumferential cast. Innermost are the muscle compartments bounded by fascia, then the subcutaneous fat, the skin, the stockinette, the padding and outermost the plaster shell. On the left a closed cast: the swelling cannot expand, the compartment pressure rises and the small vessels between the muscles are compressed. On the right the cast has been split through its entire thickness on two opposite sides and the halves have been spread apart; the volume increases, the compartment pressure falls and the vessels are open again. The split must go through the padding and the stockinette right down to the skin in order to have any effect.

Complications

Compartment syndrome.
Pressure sores over bony prominences. Common sites: the heel, the malleoli, the head of the fibula, the olecranon, the knuckles, the radial styloid and the base of the thumb.
Pressure on the common peroneal nerve at the head of the fibula causes foot drop — check dorsiflexion of the foot with every below-knee cast.
Circulatory compromise with cyanosis, swelling and coldness distally.
Burns from water that is too hot, or from too many layers setting at the same time.
Joint stiffness and muscle atrophy from prolonged immobilisation.
Venous thromboembolism when the lower limb is immobilised.
Itching, eczema and maceration under the cast, often after it has become wet.

Aftercare and follow-up

The patient must be given both verbal and written information. Seek help immediately if:

The pain increases despite elevation and analgesia.
There is numbness, tingling or weakness distally.
The fingers or toes become blue, white, cold or markedly swollen.
The cast feels too tight, or conversely becomes loose.
There is a discharging patch, a smell or fever.
Something has fallen inside the cast.

Other advice: elevation for the first few days, move the fingers and toes regularly, keep the cast dry, never push anything inside to scratch, and never cut or saw the cast yourself.

A follow-up appointment according to the fracture type — a radiograph after about a week in fractures at risk of displacement, clinical review and a change of cast once the swelling has settled, and reassessment after 10–14 days in a suspected scaphoid fracture.

Common pitfalls

A closed circumferential cast in a fresh injury.
A cast split only through the plaster layer, while the padding and the stockinette still encircle the limb.
Uneven or too thin padding, or padding with folds in it.
Moulding with the fingertips, which creates pressure points.
A slab that ends too distally and blocks the metacarpophalangeal joints — this leads to stiffness.
No documented distal neurovascular status before and after casting.
Elevation above the level of the heart in suspected compartment syndrome, which worsens the perfusion.
The patient goes home without warning information.

Casting and immobilisation