MRCS Part B (OSCE) - Online Resources

"The lower leg is divided into four compartments. "

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The compartments of the lower leg are the:

• Anterior (extensor) compartment.
• Lateral (peroneal) compartment.
• Superficial posterior (flexor) compartment.
• Deep posterior (flexor) compartment.

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The muscles of the anterior compartment are the:

• Tibialis anterior.
• Extensor digitorum longus.
• Extensor hallucis longus.
• Peroneous tertius.

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"The anterior tibial artery, a branch of the popliteal artery, supplies the structures in the anterior compartment of the lower leg. "

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• The anterior tibial artery is one of the main branches arising from the popliteal artery.
• It runs in the anterior compartment of the leg between the extensor hallucis longus muscle and the tibialis anterior muscle close to the interosseous membrane.
• At the ankle it can be palpated in the mid-point between the two malleoli in the ankle, just lateral to the tendon of extensor hallucis longus.
• The deep peroneal nerve accompanies the anterior tibial artery throughout its course in the anterior compartment and lies just lateral to the artery at the ankle.
• The anterior tibial artery runs beneath the extensor retinaculum where it becomes the dorsalis pedis artery at the distal edge of the retinaculum.
• The dorsalis pedis artery can be palpated between the bases of the first and second metatarsals of the foot.

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"The deep peroneal (fibular) nerve, one of the two terminal branches of the common peroneal nerve, innervates the anterior compartment. "

The anterior compartment muscles help in dorsiflexion of the ankle and extension of the toes. In addition, the tibialis anterior also contributes to inversion of the foot and the peroneus tertius helps in eversion of the foot at the subtalar joint.

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The muscles of the lateral (peroneal) compartment are:

• Peroneus longus.
• Peroneus brevis.

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"The muscles of the lateral compartment are supplied superiorly by perforating branches of the anterior tibial artery and inferiorly by perforating branches of the peroneal artery. The peroneal artery itself lies within the deep posterior compartment and not within the peroneal compartment. "

"The superficial peroneal (fibular) nerve, a branch of the common peroneal nerve, innervates the lateral compartment. "

Peroneus longus and brevis plantar-flex at the ankle joint and evert the foot at the subtalar joint.

The posterior compartment is the largest of the compartments. The ‘calf’ muscles in the posterior compartment are divided into superficial and deep groups by the transverse intermuscular septum:
Superficial group:
 Soleus.
 Gastrocnemius.
 Plantaris.

Deep group
 Tibialis posterior.
 Flexor hallucis longus.
 Flexor digitorum longus.

"The posterior tibial artery and the tibial nerve supply both the superficial and deep group of muscles and run just deep to the transverse intermuscular septum. "

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• Compartment syndrome, a surgical emergency, is defined as raised pressure within the osseo-fascial compartment of sufficient magnitude that can result in myo-neural necrosis.
• The intermuscular septae of the limbs are strong and resistant to stretch. Therefore, any rise in the interstitial fluid pressure within the fascial compartments can compress the vessels and nerves, resulting in microvascular injury.
• Prompt diagnosis is essential to prevent irreversible damage to the limb and significant morbidity.

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The key clinical feature of compartment syndrome in the conscious patient is severe pain out of proportion to the injury, which fails to improve in the expected clinical time course, and is aggravated by passive muscle stretch. Sensory loss within the distribution of the nerves traversing the involve compartments can also be a useful early sign. Compartment syndrome does not usually result in the loss of peripheral pulses (this is a late sign).

 

The 6 P’s are also often use to characterise the symptoms of compartment syndrome:

• Pain (severe and constant often disproportionate to the injury).
• Paraesthesia (due to nerve compression).
• Pallor.
• Paralysis.
• Pulselessness (absent dorsalis pedis pulse) due to arterial compression.
• Perishingly cold (poikilothermia) due to reduced perfusion.

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• Trauma.
• Fractures.
• Haemorrhage.
• Burns.
• Tight POP cast.
• Toxins (such as snake venom).

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The management of compartment syndrome is by immediate surgical decompression of the fascial compartments (a ‘fasciotomy’) to relieve the pressure in the compartments. The wound is left open and closed at a later stage.

Note: Intracompartmental pressures can be measured using specific devices. A difference of 30mmHg or less between the diastolic blood pressure and the intracompartmental pressure (known as the differential pressure) is a recommended threshold for fasciotomy. This condition is however a clinical diagnosis and if there is suspicion of compartment syndrome, there should be a low threshold for operative intervention rather than relying on measurements.

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