Plaster Complications
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Direct answer
A plaster of Paris or fibreglass cast becomes dangerous the moment swelling turns it into a closed compartment. The serious complications are a tight cast causing vascular compromise and compartment syndrome, pressure sores over bony prominences, and nerve palsies — classically foot drop from a common peroneal nerve pressed at the fibular neck by a below-knee plaster. Careful technique, elevation and teaching the warning signs prevent most of them; a painful cast is split to skin without delay.
What you must remember
- Tight cast is the emergency: increasing pain, numbness or tingling and pale or blue digits in the first 24 to 48 hours demand the cast be split immediately, down to and including the padding, and reassessed.
- Compartment syndrome under a cast presents as pain out of proportion and pain on passive stretch; splitting the plaster is the first response and fasciotomy the definitive one.
- Pressure sores: dips in the cast and inadequate padding over the heel, malleoli and ulnar styloid cause burning pain followed by necrosis; a window is cut and dressed, mindful of window oedema — tissue herniating through the opening.
- Nerve palsies: the common peroneal nerve at the fibular neck is the classic victim of a below-knee cast, producing foot drop; never let the limb rest on the cast edge.
- Plaster technique itself: plaster of Paris — calcium sulphate — sets exothermically, so an over-thick cast can burn skin; fibreglass sets faster and is lighter but less mouldable.
- Immobilisation costs: joint stiffness and contracture, muscle wasting, disuse osteopenia and thromboembolism in lower-limb plasters, so uninvolved joints are exercised and prophylaxis considered in high-risk patients.
- Warning signs every patient learns: increasing pain, numbness or pins-and-needles, colour change in the digits, discharge or smell — any means return immediately; digits are checked for being warm, pink, moving and sensate.
Common confusion
The recurring trap is attributing worsening pain to the fracture: pain that escalates after cast application is the cast, not the injury, and needs splitting rather than stronger analgesia. The nerve palsies are then confused by site — the peroneal nerve at the fibular neck under a below-knee cast, not the tibial nerve — and window oedema is mistaken for infection when it is simply tissue bulging through an opening. Urgency also differs: a pressure sore needs windowing and dressing, while compartment syndrome needs the whole cast opened to skin and possible fasciotomy.
Exam-focused takeaway
NEET-PG frames this topic as the day-after-cast vignette: a patient in plaster with escalating pain, numb toes or a drooping foot asks for the next step — split the cast completely, to skin, and reassess. One-liners test the commonest nerve palsy (common peroneal, foot drop), the exothermic setting burn of plaster of Paris and window oedema, while complication-matching stems pair casts with heel and malleolar sores and prolonged immobilisation with stiffness, disuse osteopenia and thromboembolism. Patient-instruction questions test the warm-pink-moving-sensate check of the digits.
Frequently asked questions
What are the danger signs of a tight cast?
Increasing pain, numbness or pins-and-needles, and pale, blue or cold digits — the cast must be split down to skin immediately.
Which nerve is classically compressed by a below-knee plaster?
The common peroneal nerve at the fibular neck, causing foot drop and sensory loss on the dorsum of the foot.
Why can applying a plaster burn the skin?
Plaster of Paris sets by an exothermic hydration reaction, and thick or rapidly setting casts generate enough heat to injure skin.
What is window oedema?
Herniation of swollen soft tissue through an opening cut in a cast, interfering with healing and needing careful window management.
What are the late complications of prolonged casting?
Joint stiffness and contracture, muscle wasting, disuse osteopenia and venous thromboembolism in lower-limb immobilisation.