Neurovasculature of the Lower Limbs

Knowledge of lower limb nerves and vessels allows accurate localisation of neuropathies, interpretation of pulse deficits and ischaemia, and safe procedures such as injections, arterial access and fasciotomy.

Arterial supply

• The external iliac artery becomes the common femoral artery at the inguinal ligament, at the mid-inguinal point (midway between the anterior superior iliac spine and pubic symphysis).
• The common femoral artery divides into the profunda femoris, which supplies the thigh, and the superficial femoral artery, which runs in the adductor canal and passes through the adductor hiatus to become the popliteal artery.
• The popliteal artery gives genicular branches and divides into the anterior tibial artery (continuing as the dorsalis pedis) and the tibioperoneal trunk (dividing into the posterior tibial and peroneal arteries).
• The internal iliac artery supplies the buttock through the superior and inferior gluteal arteries, and the medial thigh through the obturator artery.
• Palpable pulses are the femoral, popliteal, dorsalis pedis (lateral to extensor hallucis longus) and posterior tibial (behind the medial malleolus).

Venous drainage

• Deep veins accompany the arteries and carry about 90% of blood. The muscle pump of the calf drives venous return against gravity.
• The great saphenous vein runs from anterior to the medial malleolus up the medial leg and thigh to the saphenofemoral junction, about 3 to 4 cm below and lateral to the pubic tubercle.
• The small saphenous vein ascends from behind the lateral malleolus and drains into the popliteal vein.
• Perforating veins (Cockett, Boyd, Hunter) connect the systems. Their valves direct flow inward.

Nerve supply

• Lumbar plexus (L1 to L4): femoral nerve (L2 to L4), obturator nerve (L2 to L4), lateral femoral cutaneous nerve (L2 to L3), genitofemoral, ilioinguinal and iliohypogastric nerves.
• Sacral plexus (L4 to S4): sciatic nerve (L4 to S3), superior gluteal nerve (L4 to S1), inferior gluteal nerve (L5 to S2), pudendal nerve (S2 to S4), and posterior cutaneous nerve of the thigh (S1 to S3).
• The sciatic nerve exits the pelvis below piriformis (in a minority it pierces or passes above it), divides into tibial and common peroneal nerves usually at the popliteal fossa apex, and sends the tibial nerve behind the medial malleolus through the tarsal tunnel.
• The common peroneal nerve winds around the fibular neck and divides into superficial (evertors, dorsolateral foot sensation) and deep (dorsiflexors, first web space sensation) branches.

Segmental correlations

• Myotomes: L2 hip flexion, L3 knee extension, L4 ankle dorsiflexion, L5 great toe extension and hip abduction, S1 plantarflexion and eversion.
• Reflexes: knee jerk (L3 to L4), ankle jerk (S1).
• Dermatomes: L4 medial leg, L5 dorsum of foot and lateral leg, S1 lateral foot and sole.

Fascial compartments

• Thigh: anterior (femoral nerve), medial (obturator nerve) and posterior (sciatic nerve).
• Leg: anterior (deep peroneal nerve, anterior tibial vessels), lateral (superficial peroneal nerve), superficial posterior and deep posterior (tibial nerve, posterior tibial and peroneal vessels).

Neuropathies

• Femoral: quadriceps weakness with giving way, absent knee jerk, and anteromedial thigh and medial leg numbness (saphenous). Causes include iliacus or retroperitoneal haematoma in anticoagulated patients, pelvic surgery retractors, lithotomy position, and diabetic amyotrophy.
• Obturator: weak thigh adduction, medial thigh numbness and a circumducting gait, after pelvic surgery, obturator hernia, or childbirth.
• Lateral femoral cutaneous (meralgia paraesthetica): burning numbness and pain over the anterolateral thigh with no weakness, from tight belts, obesity, pregnancy and diabetes.
• Sciatic: weak hamstrings, foot drop and sensory loss below the knee (sparing the medial leg), after posterior hip dislocation, arthroplasty, pelvic fracture and gluteal injections. The peroneal division is usually more severely affected.
• Piriformis syndrome: buttock pain radiating down the leg, worse on sitting, without a root deficit. It is a diagnosis of exclusion.
• Common peroneal: foot drop with sensory loss on the dorsum of the foot and anterolateral leg, from fibular neck compression (leg crossing, casts, weight loss, prolonged squatting, knee dislocation).
• Tibial and tarsal tunnel: plantar numbness, burning and weak toe flexion, with a positive Tinel sign behind the medial malleolus.
• Superior gluteal: abductor weakness and Trendelenburg gait.

Differentiation of L5 radiculopathy from peroneal neuropathy: L5 radiculopathy weakens inversion (tibialis posterior) and hip abduction and may preserve the sensation loss to the lateral leg, whereas peroneal palsy spares inversion and hip abduction.

Vascular syndromes

• Acute limb ischaemia: pain, pallor, pulselessness, paraesthesia, paralysis and poikilothermia.
• Chronic occlusive disease: buttock and thigh claudication points to aortoiliac disease, calf claudication to femoropopliteal disease.
• Popliteal artery injury: associated with knee dislocation (up to a third of cases), supracondylar fractures and tibial plateau fractures.
• Popliteal artery entrapment syndrome: calf claudication in young active patients from compression by the medial head of gastrocnemius.
• Femoral artery pseudoaneurysm or arteriovenous fistula: after arterial puncture, with a pulsatile mass and bruit.
• Compartment syndrome: disproportionate pain, pain on passive stretch, tense swollen compartment, with paraesthesia and later weakness. Pulses may be preserved.
• Chronic venous disease and DVT: varicosities, oedema, skin changes, and venous ulceration.

Clinical assessment

• Palpate and compare femoral, popliteal and pedal pulses. Examine for bruits, capillary refill, skin temperature, hair and nail changes.
• Test motor power by myotome, sensation by dermatome and peripheral nerve territory, reflexes, and gait (steppage gait, Trendelenburg sign).
• Tinel sign over the fibular neck, tarsal tunnel and inguinal ligament.

Vascular testing

• Ankle brachial index: below 0.9 diagnoses peripheral arterial disease, 0.4 or below indicates severe disease, and above 1.3 suggests non-compressible calcified vessels, when toe pressure and toe brachial index (abnormal below 0.7) are used.
• Duplex ultrasound for stenosis, occlusion, aneurysm, pseudoaneurysm and venous reflux.
• CT angiography or MR angiography for planning intervention. Catheter angiography is used for diagnosis combined with treatment.
• In knee dislocation, an ABI below 0.9 or any hard sign of vascular injury needs CT angiography. Normal pulses do not exclude intimal injury.
• Stress testing (post-exercise ABI) for suspected popliteal entrapment, with dynamic MRI or duplex during plantarflexion.

Compartment pressure measurement

• Delta pressure (diastolic pressure minus compartment pressure) below 30 mmHg supports the diagnosis. Absolute compartment pressure above 30 mmHg is suspicious.
• Diagnosis remains primarily clinical, and measurement helps in obtunded patients.

Neurological testing

• Nerve conduction studies and EMG at 3 weeks or more after injury to localise and grade axonal injury. Compare lumbar paraspinal muscle involvement to distinguish radiculopathy from plexopathy.
• High resolution ultrasound or MR neurography for nerve continuity, entrapment, tumour and cyst.
• MRI of the lumbosacral spine for radiculopathy, and pelvis for plexopathy or retroperitoneal haematoma.
• Blood tests: HbA1c, B12, TSH, and CBC and coagulation profile when haematoma is suspected.

Acute limb ischaemia

• Immediate unfractionated heparin 5000 units IV bolus (80 units/kg), then infusion at 18 units/kg/h aiming for aPTT 1.5 to 2.5 times control, analgesia and oxygen.
• Urgent revascularisation by catheter-directed thrombolysis, thrombectomy or embolectomy, or bypass according to limb viability. Threatened limbs need revascularisation within 6 hours.
• Monitor for reperfusion injury, hyperkalaemia, myoglobinuria and compartment syndrome. Perform fasciotomy when indicated.

Compartment syndrome

• Remove constricting dressings, keep the limb at heart level, and perform urgent four-compartment fasciotomy of the leg through medial and lateral incisions.
• Treat rhabdomyolysis with IV crystalloid to maintain urine output at 200 to 300 mL/h.

Knee dislocation

• Immediate reduction and splinting. Vascular injury is repaired within 6 to 8 hours using reversed saphenous vein graft, with shunting if delayed. Ligament reconstruction is staged later. Peroneal nerve injury is observed for 3 to 4 months, then explored.

Peripheral arterial disease and venous disease

• Antiplatelet therapy (aspirin 75 to 100 mg daily or clopidogrel 75 mg daily), high-intensity statin (atorvastatin 40 to 80 mg), supervised exercise, smoking cessation and blood pressure control. Low-dose rivaroxaban 2.5 mg twice daily with aspirin can be added in stable disease.
• Revascularisation (endovascular or bypass) for disabling claudication refractory to therapy and for critical limb ischaemia.
• Chronic venous disease: compression therapy, endovenous ablation of incompetent saphenous trunks, and sclerotherapy for tributaries.

Nerve disorders

• Meralgia paraesthetica: remove constrictive clothing and belts, lose weight, and use gabapentin 300 mg at night titrated to 1800 mg/day or amitriptyline 10 to 50 mg at night. Inject methylprednisolone 40 mg with 5 mL of lidocaine 1% near the nerve medial to the anterior superior iliac spine. Neurolysis is reserved for refractory cases.
• Common peroneal palsy: remove external compression, use an ankle foot orthosis, and perform physiotherapy. Decompress the fibular neck when progressive, or explore after trauma if no recovery by 3 to 4 months. Tibialis posterior tendon transfer for permanent foot drop.
• Femoral neuropathy from retroperitoneal or iliacus haematoma: reverse anticoagulation, with surgical evacuation for progressive deficit. Diabetic amyotrophy is managed with glycaemic control, analgesia and physiotherapy.
• Sciatic nerve injury: explore after penetrating trauma, treat hip dislocation urgently by reduction within 6 hours, and consider exploration for deficit after hip arthroplasty.
• Tarsal tunnel syndrome: orthoses, NSAIDs, steroid injection (triamcinolone 10 to 20 mg), and surgical release for persistent symptoms.
• Piriformis syndrome: stretching and physiotherapy, injection of local anaesthetic with methylprednisolone 40 mg under ultrasound guidance, and botulinum toxin for refractory cases.
• Popliteal artery entrapment: surgical release of the gastrocnemius head, with bypass if the artery is damaged.
• Neuropathic pain: gabapentin, pregabalin 150 to 600 mg/day, duloxetine 60 mg daily and amitriptyline.

Prevention of iatrogenic injury

• Intramuscular injections into the ventrogluteal site or the upper outer quadrant of the buttock, or anterolateral thigh.
• Padding of the fibular head and avoidance of prolonged hip flexion or abduction in lithotomy positioning.
• Ultrasound-guided femoral arterial puncture below the inguinal ligament and over the femoral head, to reduce pseudoaneurysm and retroperitoneal haemorrhage.
• Tourniquet times kept below 2 hours.