Leg compartments

Four compartments

The tibia, fibula, interosseous membrane and tight deep fascia (with two intermuscular septa) divide the leg into four closed compartments.

• Inversion uses tibialis posterior (tibial nerve) and tibialis anterior (deep peroneal); eversion uses peroneus longus and brevis (superficial peroneal) with peroneus tertius.
• A patient with a fibular fracture who cannot evert but has no foot drop has damaged the peroneus longus and brevis or the superficial peroneal nerve, not the common peroneal nerve.

Common peroneal (fibular) nerve

• From the sciatic nerve at the apex of the popliteal fossa, it follows biceps femoris and winds round the neck of the fibula, where it lies on bone just under the skin. It is the commonest nerve injured in the lower limb: fibular neck fracture, a direct blow, a tight plaster or splint, knee surgery, prolonged crossing of the legs or squatting.
• It divides in peroneus longus into:

• The deep peroneal nerve runs down with the anterior tibial artery. At the ankle it lies between the EHL and EDL tendons, beside the dorsalis pedis artery, deep to the fascia. In an ankle block only the tibial and deep peroneal nerves lie deep to fascia; the superficial peroneal, sural and saphenous are subcutaneous.
• Common peroneal palsy: foot drop with a high-stepping gait, weak eversion and dorsiflexion, numb lateral leg and dorsum of foot. Inversion and plantarflexion are preserved (tibial nerve), and the ankle jerk is normal.

Tibial nerve

• The larger terminal branch of the sciatic; runs superficially in the popliteal fossa, then under the soleal arch with the posterior tibial artery, and behind the medial malleolus through the tarsal tunnel, where it divides into the medial and lateral plantar nerves.
• Injury in the popliteal fossa (for example a stab behind the knee): cannot stand on tiptoe or curl the toes, weak inversion, lost ankle jerk, numb sole; dorsiflexion and eversion normal. Calf muscles waste. Toe clawing appears later from loss of the small foot muscles. Injury lower down, behind the medial malleolus, spares the calf and affects only the sole.
• The sural nerve is formed from tibial and common peroneal cutaneous branches. It runs with the small saphenous vein behind the lateral malleolus to the lateral side of the foot. It is the nerve usually taken for nerve biopsy and grafting and can be injured in small saphenous vein surgery.

Foot drop: where is the lesion?

• *Mycobacterium leprae* grows best at about 30°C, so it attacks cool, superficial nerves: common peroneal at the fibular neck, ulnar at the elbow, great auricular in the neck. Nerve reactions need early steroids, and foot drop needs a splint to prevent contracture.

Acute compartment syndrome

• Mechanism: pressure rises inside a closed fascial compartment (bleeding, oedema, reperfusion). Once compartment pressure exceeds capillary and venous pressure, capillary flow fails and muscle and nerve become ischaemic, while arterial pressure is still high enough to keep distal pulses present. Waiting for a pulseless pale limb means waiting for dead muscle.
• Causes: tibial fracture (commonest, including after fixation), crush injury, reperfusion after vascular repair, tight casts and dressings, burns.
• Early sign: pain out of proportion to the injury, worst on passive stretch of the muscles in the compartment (passive toe flexion stretches the anterior compartment).
• Later: paraesthesia (numb first web space from the deep peroneal nerve in anterior compartment syndrome), weakness, then paralysis.
• Diagnosis is clinical; pressure measurement helps: a delta pressure (diastolic pressure minus compartment pressure) under 30 mmHg supports it.
• Treatment: split casts and dressings to the skin, keep the limb at heart level (not elevated), and perform urgent fasciotomy of all four compartments.

Crush injury and rhabdomyolysis

After prolonged crushing (for example under rubble), reperfused muscle releases myoglobin, potassium and phosphate.

• Dark brown urine, dipstick positive for blood but no red cells on microscopy (myoglobin reacts with the haem test), very high creatine kinase, falling urine output.
• Myoglobin injures and blocks the renal tubules (direct toxicity, cast formation in acidic urine and vasoconstriction), causing acute kidney injury.
• Hyperkalaemia can cause early cardiac arrest, so monitor potassium closely; give generous intravenous fluids.

Chronic exertional compartment syndrome

Young runners or recruits get tight, aching pain over the front of both legs after a predictable distance or time, sometimes with numbness in the first web space, settling within about 30 minutes of rest. Examination at rest, X-rays and bone scans are normal and pulses are good. Compartment pressures measured after exercise confirm it. Unlike the acute form, it is not an emergency. Differentials: tibial stress fracture (focal bony tenderness, positive bone scan) and medial tibial stress syndrome.

Vessels and tibial fractures

• Anterior tibial artery passes forwards above the interosseous membrane and becomes the dorsalis pedis at the ankle, felt lateral to the EHL tendon. Posterior tibial artery gives the peroneal artery and is felt behind the medial malleolus.
• The tibia is subcutaneous medially, so open fractures are common; the distal third has a poor blood supply and heals slowly. A toddler's fracture is a spiral tibial fracture after a twist.
• Varicose veins in an otherwise healthy adult result from incompetent valves in the superficial veins (see the vessels chapter).

Remember Pain on passive stretch with pulses present is compartment syndrome until proved otherwise.

Exam trap Common peroneal palsy spares inversion; an L5 root lesion does not.

How FCPS asks about this

• Nerve injured with fibular neck fracture or crossed legs: common peroneal; foot drop with weak eversion, inversion preserved.
• Loss of eversion without foot drop: peroneus longus and brevis or the superficial peroneal nerve.
• Stab behind the knee, cannot tiptoe, absent ankle jerk, numb sole: tibial nerve in the popliteal fossa.
• Deep peroneal nerve at the ankle: between EHL and EDL tendons; supplies skin of the first web space.
• Nerve with the small saphenous vein used for biopsy: sural nerve.
• Severe pain on passive toe movement with strong pulses: compartment syndrome; capillary flow fails before arterial flow.
• Dark urine, no red cells, high CK after crush: myoglobin injuring and blocking the tubules; watch potassium.