Lower Limb Essentials
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Direct answer
The great saphenous vein begins at the medial end of the dorsal venous arch, runs anterior to the medial malleolus — the classical venous cutdown site — and pierces the fascia lata at the saphenous opening to join the femoral vein about 4 cm below and lateral to the pubic tubercle. Around this vein the FMGE lower limb paper is built: the femoral triangle with its NAVEL contents, the nerve injuries (femoral, obturator, sciatic, common peroneal), the Trendelenburg sign of gluteal weakness, and the drainage of all superficial limb lymph to the inguinal nodes.
What you must remember
- Femoral triangle contents from lateral to medial spell NAVEL: femoral Nerve, Artery, Vein, Empty space (femoral canal) and Lymphatics; the femoral sheath encloses artery, vein and canal but not the nerve.
- Surface marking of the femoral artery is the mid-inguinal point (midway between anterior superior iliac spine and pubic symphysis), while the midpoint of the inguinal ligament marks the deep inguinal ring — a deliberate exam distinction.
- Profunda femoris is the chief artery of the thigh; ligation of the femoral artery proximal to it endangers the limb, distal to it does not, thanks to the cruciate anastomosis. Femoral nerve injury (L2–L4) paralyses quadriceps, abolishes the knee jerk and weakens hip flexion through iliacus; obturator injury weakens adduction.
- The sciatic nerve (L4–S3, the largest nerve) usually divides at the apex of the popliteal fossa; its peroneal component lies lateral and less protected, so partial injuries favour foot drop.
- Positive Trendelenburg sign: standing on the affected leg, the pelvis sags on the opposite side — superior gluteal nerve weakness (gluteus medius and minimus) or hip disease.
- The adductor (Hunter's) canal carries the femoral artery and vein with the saphenous nerve from triangle to popliteal fossa — the seat of catheterisation haematoma and of limb ischaemia after injury.
- Superficial inguinal nodes drain the whole limb, the external genitalia, the anal canal below the pectinate line and the abdominal wall below the umbilicus, their efferents passing to the external iliac nodes.
- The small saphenous vein drains behind the lateral malleolus into the popliteal vein; only the great saphenous is used for cutdown and grafting.
Working through a limb with weak movements
A patient cannot dorsiflex and evert the foot after a fortnight in a tight below-knee plaster. Apply the compartment map rather than a list. Dorsiflexion belongs to the anterior compartment (deep peroneal nerve), eversion to the lateral compartment (superficial peroneal); both nerves are branches of the common peroneal at the neck of the fibula, exactly where the plaster pressed. Inversion through tibialis posterior and plantar flexion are intact, so the tibial nerve has escaped — a common peroneal lesion at the fibular neck, confirmed by sensory loss on the dorsum but not the sole. Reverse the stem: plantar flexion and inversion lost with normal dorsiflexion indicts the tibial nerve in the deep posterior compartment; weak knee extension with an absent knee jerk and anterior thigh anaesthesia moves proximally to the femoral nerve; adduction failure alone means obturator. Pair the lost movement with its compartment, then with the nerve crossing the site of injury.
Where students slip
Two pairs get swapped under pressure. Mid-inguinal point versus midpoint of the inguinal ligament: the femoral artery is marked at the former (spine to symphysis), the deep ring at the latter (spine to tubercle) — the tubercle changes the answer. Great versus small saphenous: only the great saphenous passes in front of the medial malleolus and ends in the femoral vein; a vein behind the lateral malleolus entering the popliteal fossa is the small saphenous. Candidates also mislabel Trendelenburg sign as femoral nerve weakness — it tests the hip abductors (superior gluteal nerve), and the follow-up is usually the waddling gait of bilateral weakness, as in muscular dystrophy or congenital hip dislocation.
Frequently asked questions
Where is venous cutdown performed on the great saphenous vein?
Anterior to the medial malleolus at the ankle, where the vein is constant and superficial.
What are the contents of the femoral triangle from lateral to medial?
Femoral nerve, artery, vein, empty femoral canal and lymphatics — NAVEL — with the nerve outside the femoral sheath.
Which nerve lesion abolishes the knee jerk?
Femoral nerve injury paralysing quadriceps (L2–L4); an L3–L4 root lesion does the same.
What does a positive Trendelenburg sign indicate?
Failure of the hip abductors (gluteus medius and minimus, superior gluteal nerve) or hip pathology, letting the pelvis drop on the unsupported side.
Which surface landmark marks the deep inguinal ring?
The midpoint of the inguinal ligament, just above it — distinct from the mid-inguinal point used for the femoral artery.
Where does the small saphenous vein terminate?
In the popliteal vein, after passing behind the lateral malleolus and up the calf's midline.