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).

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.

• 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), but also crush injury, 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.

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:

Timing and complications

Decompression should be as early as possible (ideally within 6 hours); irreversible muscle necrosis follows prolonged ischaemia. 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.

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.)