Compartments of the Leg

The leg is divided into four osteofascial compartments by the tibia, fibula, the interosseous membrane and the anterior and posterior intermuscular septa. Each is a relatively inelastic box containing a functional muscle group with its motor nerve and (with one exception) a named artery — and it is precisely this inelasticity that makes the leg the classic site of acute compartment syndrome. The exam value of the anatomy is therefore inseparable from the emergency it underlies: you learn the contents so you can recognise which function and sensation fail first, confirm that every box has been opened at fasciotomy, and protect the structures that lie at the decompression incisions. Hold three layers together — the contents of each compartment, the clinical syndrome (pain on passive stretch, the differential-pressure threshold), and the operation (a two-incision four-compartment release). This page is the anatomy and the decision thresholds; the syndrome itself is developed in compartment syndrome of the leg and compartment syndrome, its end-stage in Volkmann's ischaemic contracture, and the exercise-related variant — a different disease with a different threshold — in chronic exertional compartment syndrome. The two nerves that give the compartments their clinical signature have their own pages: the common peroneal nerve and the tibial nerve.

The single highest-yield table in this topic — know the muscles, nerve, artery and action of each compartment cold, because the examiner will ask you to recite them and then link each to a clinical deficit.

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• Anterolateral incision (lateral to the tibial crest, centred over the anterior intermuscular septum): decompresses the anterior and lateral compartments. Identify the septum and the superficial peroneal nerve in the distal lateral compartment.
• Posteromedial incision (about 1-2 cm posterior to the medial tibial border): decompresses the superficial and deep posterior compartments - protect the saphenous vein and nerve, and ensure the deep posterior compartment (tibialis posterior) is genuinely released.
• A single-incision perifibular (Matsen) technique can release all four compartments through one long lateral incision over the fibula, but it is more demanding, gives poorer access to the deep posterior compartment, and risks the superficial peroneal nerve — the two-incision technique is preferred for completeness and is the safer answer in a viva.
• Whichever technique, incisions must be generous (typically the length of the leg segment) — short skin incisions cause skin-level constriction and an incomplete release.
• Wounds are left open and managed with delayed closure or skin grafting.

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Acute compartment syndrome (ACS) results from raised pressure within a closed osteofascial compartment that exceeds capillary perfusion pressure, producing ischaemia, muscle and nerve necrosis, and ultimately an unsalvageable limb if not decompressed. It is the clinical reason the compartment anatomy matters.

Causes: most commonly a tibial diaphyseal fracture (the single commonest cause — see tibial shaft fractures), but also crush injury (crush syndrome), reperfusion after vascular injury/ischaemia, a tight cast or dressing, burns with circumferential eschar, high-energy soft-tissue injury, bleeding (anticoagulation/coagulopathy), and extravasation. It can occur with open fractures — an open wound does NOT decompress all compartments and does not exclude ACS.

How often, and in whom

Quoted incidences after a tibial diaphyseal fracture differ several-fold, and the reason is the diagnostic threshold, not the biology — so quote a figure with its denominator and its definition rather than a single number. McQueen & Court-Brown's 1996 prospective series of 116 consecutive fractures with 24-hour anterior-compartment monitoring recorded ACS in 3 patients (2.6%) (PMID 8898137). The same unit's later 13-year cohort of 1,388 tibial diaphyseal fractures, diagnosed on clinical signs, pressure monitoring or both, recorded ACS in 160 patients (11.5%) (PMID 25882967) — a broader ascertainment in a unit with a low threshold to monitor and decompress.

What that larger cohort adds is the risk profile, and it is not the one most candidates offer: youth is the single strongest independent predictor (p < 0.001), with the highest prevalence between 12–19 and 20–29 years. Age, male sex, blue-collar occupation, sporting injury, fracture classification and treatment with an intramedullary nail were all predictive on initial analysis, but after adjusting for age only occupation and implant type remained significant — and implant type lost significance once stratified by Tscherne soft-tissue grade, which argues the nail was a marker of the injury rather than a cause. The translation for the ward round: the young patient with a tibial shaft fracture is the one to monitor, and a benign-looking soft-tissue envelope is not reassurance. The same principle drives the regional variants — see region-specific compartment syndrome, foot and forearm compartment syndrome.

Diagnosis is clinical

The earliest and most reliable findings are pain out of proportion to the injury and pain on passive stretch of the muscles in the affected compartment. A tense, swollen compartment supports the diagnosis. The classic "5 P's" are LATE and unreliable — by the time they appear the limb is often already lost.

Intracompartmental pressure measurement

Where the patient is obtunded, anaesthetised, a child, or the picture is equivocal, measure intracompartmental pressure (within about 5 cm of the fracture, all compartments). Act on the differential (delta) pressure rather than the absolute value:

How accurate is the monitor you are being asked to trust? The page above tells you to measure; this is the evidence that the measurement is worth acting on, and it is a fair viva follow-up. In 850 monitored tibial diaphyseal fractures from the Edinburgh unit, continuous intracompartmental pressure monitoring had an estimated sensitivity of 94%, specificity 98%, positive predictive value 93% and negative predictive value 99% (PMID 23595064); 152 patients (17.9%) underwent fasciotomy, of whom 141 had confirmed ACS, 6 did not, and 5 more had ACS at operation despite a normal differential-pressure reading. Set that against the clinical findings' 13–19% sensitivity and the case for monitoring the at-risk limb makes itself. Note the honest limitation in those five false negatives: a normal ΔP does not license you to stop examining the patient — the monitor and the serial examination are complementary, not alternatives.

Timing and complications

Decompression should be as early as possible; irreversible muscle necrosis follows prolonged ischaemia. The familiar "within 6 hours" (and the companion teaching that myonecrosis begins by 6–8 hours) is a conventional benchmark carried through the textbooks rather than a validated threshold derived from a human tibial series — treat it as an urgency instruction, not a deadline you have until. Two things are firmer and more useful in a viva: the clock that matters runs from the onset of the compartment syndrome, not from the injury (so a late-presenting ACS may be hours old on a day-old fracture), and what demonstrably shortens time to decompression is acting on a sustained ΔP below 30 mmHg rather than waiting for the clinical picture to complete itself. Complications of missed or delayed ACS include Volkmann's ischaemic contracture (fibrotic, contracted, functionless muscle), rhabdomyolysis with acute kidney injury and hyperkalaemia, infection, amputation, and death. A very late presentation (established necrosis, typically beyond 36–48 hours) is one situation where fasciotomy may be withheld, because opening dead muscle risks fatal infection — a difficult, senior decision. Missed ACS is one of the commonest sources of successful orthopaedic litigation, so document serial examinations and act decisively.

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Global Practice Picture

Leg compartment anatomy is universal core knowledge for trauma and the basis of fasciotomy. The internationally consistent teaching: know the four compartments and their contents, diagnose acute compartment syndrome clinically (pain out of proportion, pain on passive stretch) with pressure measurement when equivocal, and decompress all four compartments emergently - most reliably via the two-incision technique. (The full neurovascular contents are tabulated in the Neurovascular Contents by Compartment section above.)