Plaster & Backslab Application

Backslab vs full cast

• Backslab (non-circumferential) is the default in acute ED immobilisation. An incompletely encircling slab, held with a bandage, allows for the swelling that follows fresh fracture or soft-tissue injury without becoming a tourniquet.
• A full circumferential cast applied acutely to a fresh, swelling-prone injury risks compartment syndrome as the limb swells against unyielding plaster — reserve full casts for definitive, non-acute, already-settled injuries, per local/orthopaedic protocol.
• Suspected or evolving compartment syndrome is a contraindication to any circumferential immobilisation — split down to skin and escalate urgently instead.

Common ED slabs

Equipment

Below-elbow backslab — positioning

Materials & padding technique

1. Position and support the limb

   Hold the joint in the required position throughout — an assistant supporting the limb makes correct moulding much easier.

2. Apply padding, distal to proximal

   Wrap orthopaedic wool smoothly with roughly 50% overlap between turns; extend it 2–3 cm beyond the planned edges of the slab so no plaster contacts skin directly.

3. Extra padding over bony prominences

   Add extra layers over the olecranon, ulnar and radial styloids, malleoli, tibial crest, patella, fibular head and heel — these are the classic pressure-sore sites under a cast.

4. Measure and prepare the slab

   Measure the slab material against the uninjured side or the limb itself before wetting; POP slabs are typically 8–10 layers thick for adequate strength.

5. Wet and apply

   Dip in tepid (not hot) water — hot water accelerates the exothermic setting reaction and risks thermal skin injury, especially through thick, freshly-applied plaster. Smooth the slab onto the limb over the padding.

6. Secure with a bandage

   Hold with an open-weave or crepe bandage rather than a second layer of plaster, preserving room for swelling.

Moulding & joint positioning

• Three-point moulding — apply gentle counter-pressure at three points along the slab (not fingertip indentation, which creates a pressure point) to hold the fracture in the reduced position while the plaster sets, mirroring the natural curve of the limb.
• Position the joint as required by the specific injury — e.g. wrist neutral/slight flexion for a reduced distal radius, ankle at 90° for an ankle injury, thumb in a scaphoid slab.
• Hold the position without moving until the plaster has fully set — moving too early loses the correction.

Post-application checks & written advice

• Check capillary refill, colour, sensation, movement and pain immediately after application and before discharge.
• Give clear written safety-netting advice: elevate the limb, keep fingers/toes moving, and return immediately if there is increasing pain not controlled by analgesia, tingling/numbness, pale or blue fingers/toes, or the cast feels tight.
• Explicitly warn about compartment syndrome — pain out of proportion to the injury, or pain on passive stretch of the digits, needs urgent reassessment; this is the single most important safety-netting message for any new cast.
• Document the type of slab, position achieved, padding used, neurovascular checks, and advice given.

Cast complications

• Pressure sores — inadequate padding over bony prominences, or a cast that has been moulded with fingertips rather than a flat palm.
• Thermal injury — from hot-water dipping or a thick freshly-applied cast generating exothermic heat against the skin, particularly if the limb rests on an insulating surface (e.g. a pillow) while setting.
• Joint stiffness — from prolonged or unnecessarily extensive immobilisation; immobilise only the joints that need it, for only as long as needed.
• Compartment syndrome — from a cast that is too tight, or one applied circumferentially too early in a swelling-prone injury.
• Nerve palsy from prolonged localised pressure (e.g. peroneal nerve at the fibular head in a below-knee cast).

References