The femoral triangle and femoral canal: boundaries, contents and femoral hernia

Boundaries

The floor is a gutter: iliopsoas and pectineus slope towards each other, so the femoral vessels lie in a groove between them. The great saphenous vein pierces the cribriform fascia at the saphenous opening, a few centimetres below and lateral to the pubic tubercle, to join the femoral vein at the saphenofemoral junction.

Contents and the femoral sheath

From lateral to medial the contents are the femoral nerve, the femoral artery, the femoral vein and the femoral canal (often remembered as NAVY, with the Y for the empty space). The femoral sheath is a funnel-shaped extension of the abdominal fascia: transversalis fascia in front and iliac fascia behind. It extends about 3–4 cm below the inguinal ligament and has three compartments.

• Lateral compartment: the femoral artery and the femoral branch of the genitofemoral nerve.
• Intermediate compartment: the femoral vein.
• Medial compartment (femoral canal): fat, lymphatics and a lymph node (of Cloquet), in a space that lets the vein expand.
• Outside the sheath: the femoral nerve (L2–L4), which lies in the groove between iliacus and psoas and divides into anterior and posterior divisions soon after entering the thigh. The saphenous nerve continues with the artery into the adductor canal.

Superficial epigastric artery

Runs up towards the umbilicus in the superficial fascia.

Superficial circumflex iliac artery

Runs laterally towards the anterior superior iliac spine; the axial vessel of the groin flap.

Superficial and deep external pudendal arteries

Run medially to the external genitalia.

Profunda femoris artery

Arises from the lateral side of the femoral artery about 3.5 cm below the inguinal ligament. Its branches are the medial and lateral circumflex femoral arteries and four perforating arteries.

Descending genicular artery

Arises in the adductor canal and joins the anastomosis around the knee.

Surface anatomy that is easy to confuse

For vascular access, the femoral vein lies medial to the pulse and the femoral nerve lateral to it. Puncturing the artery over the femoral head lets it be compressed afterwards: too high risks retroperitoneal haemorrhage, and too low risks the bifurcation, a pseudoaneurysm or an arteriovenous fistula.

The femoral canal and femoral hernia

The femoral canal is the medial compartment of the sheath, about 1–2 cm long. Its upper opening, the femoral ring, has four boundaries:

Three of the four boundaries are unyielding, so a femoral hernia has a narrow neck and a high risk of incarceration, strangulation and Richter’s hernia, in which only part of the bowel wall is trapped. The hernia passes down the canal, emerges through the saphenous opening and may then turn upwards over the inguinal ligament. Femoral hernias are commoner in women than in men, but even in women inguinal hernias are more common than femoral ones.

• Position. A femoral hernia lies below and lateral to the pubic tubercle; an inguinal hernia lies above and medial to it.
• Differential diagnosis of a groin lump. Inguinal or femoral hernia, saphena varix (a cough impulse and thrill that disappear on lying flat), lymph node, femoral artery aneurysm, psoas abscess, ectopic testis, lipoma and a hydrocele of the canal of Nuck.
• Repair. Femoral hernias are repaired promptly because of the risk of strangulation. The HerniaSurge guidelines favour a laparo-endoscopic approach for groin hernias in women, partly because it shows and treats an occult femoral hernia. Open approaches are low (infra-inguinal, Lockwood), high (extraperitoneal, McEvedy; useful when bowel may need resection) and trans-inguinal (Lotheissen).

The adductor canal

The adductor (subsartorial, or Hunter’s) canal runs from the apex of the femoral triangle to the adductor hiatus in adductor magnus. Vastus medialis lies anterolaterally, adductor longus and adductor magnus posteriorly, and sartorius over the roof. It contains the femoral artery and vein, the saphenous nerve and the nerve to vastus medialis. At the hiatus the femoral vessels become the popliteal vessels. An adductor canal block anaesthetises the saphenous nerve for knee surgery while sparing most of quadriceps.

Clinical correlations

• Femoral nerve injury, from a psoas haematoma in an anticoagulated patient or a retractor in pelvic surgery, weakens knee extension, abolishes the knee jerk and numbs the anteromedial thigh and, through the saphenous nerve, the medial leg.
• Lymphatic drainage. The superficial inguinal nodes drain the lower limb, the perineum, the anal canal below the dentate line, the lower abdominal wall below the umbilicus and the external genitalia. The testis drains to the para-aortic nodes.
• The lateral cutaneous nerve of the thigh passes under or through the inguinal ligament near the anterior superior iliac spine; compression causes meralgia paraesthetica.
• The inferior epigastric artery arises from the external iliac artery just above the inguinal ligament and runs up behind rectus abdominis. It is the usual source of a rectus sheath haematoma, the subject of the second free question below.