Pain Out of Proportion After a Tibial Fracture
A 22-year-old motorcyclist is brought to a rural hospital six hours after a fall, with a closed, isolated tibial shaft fracture that has been immobilised in a full above-knee plaster backslab. He is in severe pain (9/10) that has not eased after 10 mg IV morphine, and the pain is worse when the toes are passively dorsiflexed. The anterior compartment is tense and shiny; sensation over the first web space is reduced compared with the other foot; the dorsalis pedis pulse is palpable and capillary refill is normal. Blood pressure is 110/70 mmHg. A single compartment pressure reading is 34 mmHg. Creatinine kinase is 1,200 U/L. The hospital has no orthopaedic surgeon on site; the on-call surgeon is four hours away by road, and the theatre team can be assembled in 45 minutes.
1. Name the compartment involved and the structures at risk within it, and explain why a palpable dorsalis pedis pulse does not exclude compartment syndrome.
Read the worked answer
The anterior (tibial) compartment of the leg contains tibialis anterior, extensor hallucis longus, extensor digitorum longus and peroneus tertius, together with the anterior tibial artery and the deep peroneal (fibular) nerve. The deep peroneal nerve supplies the dorsal first web space - the reduced sensation there is the earliest objective sign of nerve ischaemia and is more dependable than a pain score.
A palpable dorsalis pedis pulse does not exclude compartment syndrome. Capillary perfusion fails at pressures far below systolic pressure, so the arterial pulse persists until the compartment pressure approaches systolic pressure - by which time muscle is already necrotic. Pulses are a late and unreliable sign, and their presence must never be used to delay fasciotomy. The same applies to capillary refill and to the absence of pallor.
The reliable early signs are pain out of proportion to the injury, pain on passive stretch of the compartment's muscles (dorsiflexion stretches the anterior group), a tense compartment, and a progressive sensory deficit. A tight backslab can also cause pressure at the knee and ankle, so the whole limb must be examined.
2. Calculate the delta pressure and interpret the 34 mmHg reading. Would you split the cast and observe, or take him to theatre? Justify your answer, including the pitfalls of compartment pressure measurement.
Read the worked answer
Delta pressure = diastolic pressure − compartment pressure = 70 − 34 = 36 mmHg. Conventional thresholds are an absolute compartment pressure above 30–40 mmHg, or a delta pressure below 30 mmHg. On the numbers alone, 36 mmHg sits just above the delta threshold, which would permit observation with serial examination.
But the decision is clinical, not numerical. This patient has pain out of proportion, pain on passive stretch, a tense compartment and an objective sensory deficit - that is established compartment syndrome, and the pressure reading merely supports it. Take him to theatre for fasciotomy of all four compartments (anterior and lateral through one incision, superficial and deep posterior through another), with fixation of the fracture at the same sitting.
Pitfalls of the measurement: a single reading rather than a trend; a needle sited more than 5 cm from the fracture or inside a haematoma giving a falsely high or low value; and hypotension, which lowers the delta and hides the syndrome - if his systolic pressure fell, the same 34 mmHg would be far more dangerous.
Waiting for a pulseless limb means waiting for an amputation. Had the pressure been 34 mmHg with a soft compartment and no sensory deficit, splitting the cast and hourly reassessment would be reasonable.
3. List what you must do before and during transfer to theatre, and what you must communicate to the family and the receiving surgeon.
Read the worked answer
Immediately:
- Bivalve and remove the backslab and any circumferential dressing now; keep the limb at heart level, not elevated, because elevation lowers perfusion pressure.
- Oxygen, IV crystalloid, adequate analgesia; keep systolic pressure normal and avoid hypotension.
- Group and save, coagulation screen, urea and electrolytes, repeat CK. A CK of 1,200 U/L is an early warning for rhabdomyolysis: maintain a good urine output, monitor urine colour, and consider urine alkalinisation if CK climbs steeply.
- IV antibiotics and tetanus prophylaxis if there is any breach of skin; mark and document the dorsalis pedis pulse and the sensory level before transfer.
- Consent for fasciotomy with possible delayed primary closure or skin grafting, keep him nil by mouth, and book theatre for 45 minutes' time.
Communicate to the family: this is limb-threatening; fasciotomy leaves a large open wound that may need further surgery; some muscle may already be dead and full recovery of sensation is not guaranteed.
Communicate to the receiving surgeon: the history and time of injury, the examination findings with timings, the pressure reading and the diastolic pressure it was measured against, the CK, and the treatment given. Send photographs and speak by telephone before transfer.
Document with times exactly what you found, what you did and whom you spoke to - this is a classic medico-legal scenario.