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 holding a functional muscle group with its motor nerve and, with one exception, a named artery, and that inelasticity is what makes the leg the classic site of acute compartment syndrome.

Why the anatomy is examined. The contents tell you which function and which patch of sensation fail first, let you confirm at fasciotomy that every box has been opened (you can see tibialis posterior in the deep posterior compartment), and name the structures to protect 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).

Where the rest lives. 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.

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The walls. The four boxes are bounded not just by the tibia and fibula but by named fascial structures, and the examiner will make you draw them:

• The deep (crural) fascia forms the outer envelope
• The interosseous membrane separates the anterior compartment from the deep posterior
• The anterior intermuscular septum (fibula to deep fascia) separates the anterior from the lateral compartment
• The posterior intermuscular septum separates the lateral from the superficial posterior compartment
• The transverse intermuscular septum separates the superficial posterior (gastroc-soleus) from the deep posterior compartment

Name the septa, not just "intermuscular septa": each compartment is a fascial box with a specific wall between it and its neighbour. Drawing it correctly also explains the incisions. The anterolateral fasciotomy is placed over the anterior intermuscular septum so that one skin incision reaches both the anterior and lateral boxes, and the posteromedial incision lies behind the medial tibia to reach the superficial and, through the transverse septum, the deep posterior boxes.

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Anterior. Tibialis anterior, extensor hallucis longus, extensor digitorum longus and peroneus tertius, supplied by the deep peroneal (fibular) nerve and the anterior tibial artery, dorsiflex the ankle and extend the toes. The compartment is bordered by the tibia, fibula, interosseous membrane and anterior intermuscular septum, and it is the most common compartment to develop acute compartment syndrome.

Lateral. Peroneus (fibularis) longus and brevis, supplied by the superficial peroneal (fibular) nerve, evert the foot and assist plantarflexion. This is the one compartment with no major named artery; it is supplied by perforating branches.

Superficial posterior. Gastrocnemius, soleus and plantaris, the "calf", plantarflex the ankle. The muscles are supplied by the tibial nerve; the sural nerve in this compartment is cutaneous.

Deep posterior. Tibialis posterior, flexor digitorum longus, flexor hallucis longus and popliteus, with the tibial nerve and the posterior tibial and peroneal (fibular) arteries, invert the foot, flex the toes and assist plantarflexion. It is the compartment most often inadequately decompressed at fasciotomy.

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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The operation. The standard surgical decompression of the leg is the two-incision technique, releasing all four compartments. Failure to release every compartment is the commonest reason a fasciotomy fails.

• Anterolateral incision, lateral to the tibial crest and 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.

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The single-incision alternative. A 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.

The skin. 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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The "fifth compartment". A recurring anatomical reason the deep posterior compartment is the one most often left under-decompressed is that tibialis posterior frequently lies in its own separate fascial subcompartment within the deep posterior space; some describe it as the fifth compartment of the leg. Opening the deep posterior fascia and seeing FDL and FHL is therefore not enough. The tibialis posterior sheath must be specifically identified and released along the back of the tibia, or it can remain tight and ischaemic despite an apparently complete fasciotomy.

The drill at the posteromedial incision. After releasing the superficial posterior (gastroc-soleus) and the main deep posterior fascia, deliberately detach soleus from the posteromedial tibia, follow the neurovascular bundle, and confirm the tibialis posterior fascia itself is opened.

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. The single commonest is a tibial diaphyseal fracture (see tibial shaft fractures). The others:

• Crush injury (crush syndrome)
• Reperfusion after vascular injury or ischaemia
• A tight cast or dressing
• Burns with circumferential eschar
• High-energy soft-tissue injury
• Bleeding (anticoagulation or coagulopathy)
• 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

Quote the denominator. 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.

Who is at risk. The larger cohort adds the risk profile, and it is not the one most candidates offer: youth is the single strongest independent predictor (p less than 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.

On 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 signs. 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.

Weak alone, strong together. "Clinical diagnosis" does not mean one sign is enough. In Ulmer's systematic review of the four eligible prospective studies, the sensitivity of individual clinical findings was only 13-19% and their positive predictive value 11-15%, while specificity and negative predictive value were each 97-98% (PMID 12352566). The probability of ACS rises steeply with the number of findings present, approximately 25% with one finding and 93% with three, so the signs are cumulative and the examination that matters is the serial one that documents a rising count rather than a single snapshot.

Can a normal examination exclude it? Taken as Ulmer found them, the findings are more useful by their absence in excluding ACS than by their presence in confirming it. That point is genuinely contested: Long et al. (PMID 30685220, cited below) conclude that history and examination are unreliable to rule out the diagnosis, and Ulmer himself cautions that with so few eligible studies the predictive value "has yet to be defined." The safe reconciliation, and the one McQueen's group act on, is that a single normal examination in an at-risk limb is not an exclusion, which is precisely why the at-risk patient gets monitored rather than merely re-examined.

Intracompartmental pressure measurement

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

The McQueen rule. ΔP = diastolic blood pressure − measured compartment pressure. Decompress when the differential pressure drops below 30 mmHg; the original recommendation is a ΔP falling under 30 mmHg, and the widely taught "≤ 30" is the same rule rounded. Using the differential pressure, which accounts for the patient's perfusion pressure, rather than an absolute pressure threshold avoids both missed cases and unnecessary fasciotomies.

Why the differential and not the absolute. In McQueen & Court-Brown's prospective series of 116 tibial diaphyseal fractures, a ΔP threshold of 30 mmHg missed no cases, whereas an absolute threshold of 30 mmHg would have sent 50 of the 116 patients (43%) to fasciotomy and an absolute threshold of 40 mmHg still 27 patients (23%), against a true ACS rate of 2.6%. That is the argument for the differential in one line: the absolute pressure would have operated on roughly one patient in two who did not need it.

Add the duration. A threshold with no time attached is half the rule. The Edinburgh recommendation is decompression on a differential pressure below 30 mmHg sustained for more than 2 hours, read from continuous monitoring (PMID 26814506). A single transient dip below 30 on a one-off needle reading is not the trigger; a sustained fall is. Getting this right cuts both ways: it stops the isolated low reading from generating an unnecessary fasciotomy, and, because the criterion is met before signs declare themselves, monitoring on this basis shortens time to definitive treatment compared with waiting for clinical symptoms and signs to develop.

How accurate is the monitor. A fair viva follow-up, and the evidence that the measurement is worth acting on. 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. The honest limitation is in those five false negatives: a normal ΔP does not license you to stop examining the patient, because the monitor and the serial examination are complementary, not alternatives.

Timing and complications

The clock. 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, so 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 (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.

What a missed ACS costs. The complications of missed or delayed decompression are:

• Volkmann's ischaemic contracture: fibrotic, contracted, functionless muscle
• Rhabdomyolysis with acute kidney injury and hyperkalaemia
• Infection
• Amputation
• Death

When fasciotomy may be withheld. A very late presentation with 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.

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.)