Femoral canal and femoral sheath anatomy

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Femoral Sheath and Femoral Canal - Anatomy

Femoral Sheath

The femoral sheath is a funnel-shaped sleeve of fascia that surrounds the upper ~4 cm (about 1.5 inches) of the femoral vessels as they pass beneath the inguinal ligament into the anterior thigh.

Formation

The sheath has two walls:

Anterior wall - downward prolongation of the transversalis fascia of the anterior abdominal wall
Posterior wall - downward prolongation of the iliac (iliopsoas) fascia

At the inguinal ligament level, the iliopsoas fascia forms the iliopectineal arch, which divides the space inferior to the inguinal ligament into two regions:

Lacuna vasorum (medial) - for the femoral vessels, enclosed within the sheath
Lacuna musculorum (lateral) - for the iliopsoas muscle, femoral nerve, and lateral femoral cutaneous nerve

The femoral nerve lies lateral to the femoral sheath and is NOT enclosed within it.

The Three Compartments

Two anteroposterior septa divide the sheath into three compartments (lateral to medial):

Compartment	Contents
Lateral (arterial)	Femoral artery + femoral branch of genitofemoral nerve
Intermediate (venous)	Femoral vein
Medial (lymphatic)	Lymphatic vessels + deep inguinal lymph node = Femoral canal

A useful mnemonic - from lateral to medial: NAVEL (Nerve, Artery, Vein, Empty space [canal], Lacunar ligament)

Femoral Canal

The femoral canal is the smallest and most medial compartment of the femoral sheath. It is cone-shaped (wide above, narrow below).

Length: approximately 1.25 cm (about 0.5 inches)
Shape: conical, wider proximally
Contents: loose areolar connective tissue, lymphatic vessels, and the Cloquet node (a large deep inguinal lymph node at its upper end)

Femoral Ring (Superior Opening)

The femoral ring is the upper (proximal) opening of the femoral canal - its inlet from the abdominal side. It is covered by a plug of extraperitoneal fat (the femoral septum).

Boundaries of the femoral ring:

Border	Structure
Anterior	Inguinal ligament (+ iliopubic tract)
Posterior	Pectineal (Cooper's) ligament + superior pubic ramus
Medial	Lacunar (Gimbernat's) ligament - the curved medial edge of the inguinal ligament
Lateral	Femoral vein (periadventitial connective tissue septum)

Note: Mulholland and Greenfield's Surgery clarifies that the medial border is more precisely the aponeurosis of transversus abdominis (not the lacunar ligament, which inserts more medially). This is an important surgical distinction.

Saphenous Opening (Inferior Opening)

Inferiorly, the canal opens at the saphenous opening (fossa ovalis) in the deep fascia of the thigh, where the great saphenous vein pierces the fascia to enter the femoral vein.

Clinical Significance

Femoral Hernia

The femoral ring is a natural weak point in the abdominal wall. Increased intra-abdominal pressure can force peritoneum (and sometimes bowel or omentum) through the ring into the canal, creating a femoral hernia.

Key clinical points:

More common in females because the pelvis is wider, leaving a larger femoral ring
The hernia exits below and lateral to the pubic tubercle (distinguishing it from inguinal hernias, which exit above and medial to the pubic tubercle)
The rigid boundaries of the ring (especially the sharp lacunar ligament medially) make femoral hernias prone to incarceration and strangulation
The canal is relatively rigid and difficult to obliterate with sutures from below, so a prosthetic plug is often used after reducing the hernia

Anomalous Obturator Artery (Corona Mortis)

An accessory obturator artery (a branch of the inferior epigastric artery replacing the normal obturator origin from the internal iliac) passes close to the femoral ring in up to 30% of individuals. It can be injured during femoral hernia repair attempts to enlarge the ring - earning the name "corona mortis" (crown of death) due to the risk of severe hemorrhage.

Vascular Access

The femoral artery and vein are accessible just inferior to the inguinal ligament at the midpoint between the ASIS and pubic symphysis. This is a common access point for:

Cardiac catheterization / coronary angiography
Peripheral vascular interventions
Central venous access (femoral vein)

Summary Relationships

Inguinal ligament
        |
   [Femoral Sheath - funnel of fascia]
   ┌──────────────────────────────┐
   │ Lateral  │ Middle  │ Medial  │
   │ Femoral  │ Femoral │ Femoral │
   │  artery  │  vein   │  canal  │
   └──────────────────────────────┘
              ↓ (narrows distally)
       Saphenous opening

Sources:

Gray's Anatomy for Students, p. 676
Mulholland and Greenfield's Surgery: Scientific Principles and Practice, 7e, p. 3652
Fischer's Mastery of Surgery, 8e, p. 6051-6052

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