Arteries of the Lower Limb: One Vessel That Changes Its Name Five Times

Where the river begins: the abdomen hands the limb over

Every artery in the leg is a descendant of the abdominal aorta, and the family tree branches only twice before the limb begins. At roughly the level of the fourth lumbar vertebra the abdominal aorta ends by dividing into the right and left common iliac arteries. Each common iliac then splits, in front of the sacroiliac joint, into an internal and an external iliac artery. The internal iliac turns down into the pelvis and is the artery of the pelvic viscera, the perineum and — importantly for us — the buttock and the back of the hip: it supplies the superior gluteal artery (which leaves above piriformis with the nerve of the same name), the inferior gluteal artery (below piriformis), and the obturator artery, which passes through the obturator canal to reach the medial thigh. The external iliac artery ignores the pelvis entirely and heads for the limb. On its way it gives just two named branches — the inferior epigastric artery, which climbs the back of the anterior abdominal wall and marks the medial boundary of the deep inguinal ring, and the deep circumflex iliac artery, which runs laterally along the iliac crest. Then it reaches a ligament, passes under it, and stops being the external iliac altogether. Nothing about the vessel changes. Only the name does. This is the whole grammar of the cardiovascular plan in one limb: the tube is continuous, the labels are geographical.

Name change one: the femoral artery

The external iliac artery passes deep to the inguinal ligament and becomes the femoral artery. The exact spot has a name you must know, because it is where you press: the mid-inguinal point, midway between the anterior superior iliac spine (ASIS) and the pubic symphysis. There the artery enters the femoral triangle, wrapped with the femoral vein in a funnel of transversalis fascia called the femoral sheath. Inside the triangle the order from lateral to medial is fixed and famous — NAVEL: Nerve, Artery, Vein, Empty space (the femoral canal), Lymphatics. Two details in that mnemonic are worth more than the mnemonic itself. First, the femoral nerve lies OUTSIDE the femoral sheath, not within it — the sheath contains only the artery, the vein and the canal. Second, the artery is LATERAL to the vein, which is exactly what makes both structures findable: feel the pulse, and the vein is one finger's breadth medial to your fingertip. Every femoral line, every cardiac catheter, every emergency venous access in a shocked patient is placed on that single relationship. The triangle itself, its floor and its contents, is mapped in the femoral triangle, popliteal fossa and gait.

The femoral artery's small branches supply skin; its one enormous branch supplies the whole thigh. Near its origin the femoral artery gives four modest superficial branches that fan out over the groin and lower abdominal wall: the superficial epigastric, the superficial circumflex iliac, and the superficial and deep external pudendal arteries. Then, about four centimetres below the inguinal ligament, it gives the branch that matters: the profunda femoris — the deep artery of the thigh — which is the principal blood supply of the entire thigh. The profunda dives backwards between the adductors and gives, in turn, the lateral circumflex femoral artery (which encircles the femur laterally, supplying the quadriceps and the greater trochanter), the medial circumflex femoral artery (which passes backwards between iliopsoas and pectineus to reach the back of the hip), and four perforating arteries that pierce adductor magnus one after another to supply the posterior compartment and the hamstrings. After giving the profunda, what remains of the femoral artery is often called the superficial femoral artery by clinicians, and it now has almost no job except to keep travelling.

The retrograde supply — and why a hip fracture kills bone

Of all the branches in the limb, one deserves to be learned on its own: the medial circumflex femoral artery. It curls round the back of the femoral neck and gives off a ring of retinacular arteries that run UP the neck, from below the head towards it — that is, retrograde, against the direction you would expect. These little vessels, hugging the bone beneath the reflected fibres of the joint capsule, are essentially the only meaningful blood supply of the head of the femur in an adult. The artery of the ligament of the head (a branch of the obturator) contributes almost nothing after childhood. Now put the anatomy and the ward together. An eighty-year-old woman with osteoporosis slips on a wet floor and fractures the neck of her femur INSIDE the capsule — an intracapsular fracture. The break does not merely separate bone; it tears the retinacular vessels running across it. The head of the femur is left with a shaft of dead-ended supply, and the bone dies: avascular necrosis. That single anatomical fact is why the surgeon's decision changes with the level of the fracture. An intracapsular fracture in an elderly patient is usually replaced (hemiarthroplasty or total hip replacement), because fixing a fragment whose blood supply has been destroyed is fixing a piece of bone that will not live. A fracture below the capsule — intertrochanteric — leaves the retinacular vessels intact and is simply fixed with a screw and plate. The bone itself, its neck angle and its trabecular architecture are covered in the femur and patella, and the capsule that carries these vessels in the hip joint.

Name change two: the adductor hiatus and the popliteal artery

Below the femoral triangle the artery slips into a tunnel: the adductor canal, also called the subsartorial canal or Hunter's canal, a triangular corridor on the medial side of the thigh roofed by sartorius and bounded by vastus medialis and the adductors. It carries the femoral artery, the femoral vein and the saphenous nerve — a protected passage that keeps the vessel deep and safe through the most muscular part of the limb. At the bottom of the canal the artery passes through the adductor hiatus, a gap in the tendon of adductor magnus, and emerges into the back of the knee. Third name: the popliteal artery. Its defining feature is depth. The popliteal artery is the DEEPEST structure in the popliteal fossa, lying directly on the popliteal surface of the femur and the capsule of the knee, with the popliteal vein superficial to it and the tibial nerve most superficial of all. Everything about it clinically follows from that: a popliteal aneurysm is felt as a deep, expansile, pulsatile mass behind the knee rather than an obvious lump; and because the artery is tethered above at the hiatus and below at the fibrous arch of soleus, it cannot move out of harm's way — a posterior knee dislocation or a supracondylar fracture of the femur can shear it, which is why every knee dislocation is treated as a vascular emergency until the foot pulses are confirmed. Along the way the popliteal artery gives the superior, middle and inferior genicular arteries, which weave together over the front of the knee as the genicular anastomosis, a standing collateral network that keeps the leg perfused when the knee is fully flexed and the main vessel is kinked.

Name changes three and four: the leg divides in two

At the lower border of popliteus the single river finally forks, and each branch takes a different compartment. The popliteal artery ends by dividing into the anterior tibial artery and the tibioperoneal (tibiofibular) trunk. The anterior tibial artery does something no other artery in the limb does: it passes ABOVE the upper border of the interosseous membrane to reach the anterior compartment, then descends on the front of that membrane, between tibialis anterior and extensor hallucis longus, with the deep fibular nerve alongside it. At the ankle, midway between the two malleoli, it crosses under the extensor retinaculum and — name change five — becomes the dorsalis pedis artery. You feel it on the dorsum of the foot immediately LATERAL to the tendon of extensor hallucis longus; ask the patient to lift the big toe, the tendon stands up, and the pulse is beside it. The dorsalis pedis gives the arcuate artery to the toes, then dives between the first and second metatarsals as the deep plantar artery to join and complete the plantar arch on the sole. Meanwhile the tibioperoneal trunk divides into the fibular (peroneal) artery and the posterior tibial artery. The fibular artery runs down behind the fibula and is the vessel that supplies the lateral compartment — a compartment with no artery of its own, fed entirely by perforating branches reaching round the bone, a fact worth remembering when reading the muscles of the leg. The posterior tibial artery descends through the deep posterior compartment, and at the ankle passes behind the medial malleolus inside the tarsal tunnel, where it is palpable, before dividing into the medial and lateral plantar arteries. The lateral plantar artery is the larger, and it curves across the sole to form the deep plantar arch, which the deep plantar branch of the dorsalis pedis joins from above — so the two great arteries of the leg, one anterior and one posterior, shake hands under the foot.

The safety nets: cruciate and trochanteric anastomoses

The body does not trust a single pipe. Around the hip it builds two named ring-roads. The trochanteric anastomosis sits in the trochanteric fossa and is formed by the superior and inferior gluteal arteries meeting the two circumflex femoral arteries; it is the main source of the retinacular vessels to the femoral head. The cruciate anastomosis lies lower, at the level of the lesser trochanter, where four vessels meet in a rough cross: the inferior gluteal artery from above, the first perforating artery of the profunda from below, and the medial and lateral circumflex femoral arteries from either side. Together they mean that blood from the internal iliac system can reach the thigh even if the external iliac or femoral artery is blocked. The same principle repeats at the knee (the genicular anastomosis) and around the ankle and foot (the malleolar and plantar arches). It is the reason a patient can occlude the superficial femoral artery almost completely and still walk into clinic with a warm foot — the collaterals have had years to open. Compare this with the arteries of the upper limb, where the scapular anastomosis around the shoulder does exactly the same job for the arm: wherever a joint bends and a vessel could be pinched, evolution has built a bypass.