Anatomy, Abdomen and Pelvis: Femoral Sheath
The femoral sheath is a fascial tube encapsulating the key vascular structures passing through the retro-inguinal space, a critical transition point between the abdomen and the anterior thigh compartment. It is an important anatomical landmark for understanding the structures in the femoral triangle within which it lies and has clinical importance as the site of femoral hernias.[1]
The inguinal ligament spans the gap between the anterior superior iliac spine and the pubic tubercle. Beneath it, the retro-inguinal space is created and is divided by the iliopectineal arch into a muscular compartment and a vascular compartment. The femoral sheath lines the vascular compartment of the retro-inguinal space, which contains the femoral artery, femoral vein, and deep inguinal lymph nodes.[2]
The femoral sheath has a conical shape and is of varying length, typically 3 to 4 cm, before it blends with the adventitia of the femoral vessels. Its anterior aspect is a continuation of the transversalis fascia, whereas the posterior aspect is formed from a continuation of the iliopsoas fascia.[3]
Anteriorly, the femoral sheath is covered by the fascia lata of the thigh, and posteriorly it is related to the underlying iliacus and pectineus muscles. The femoral sheath is bordered medially by the lacunar ligament, and on its lateral side lies the femoral nerve.
The femoral sheath is divided into three compartments by vertical septa of connective tissue that separate it into distinct functional areas:
The function of the femoral canal (medial compartment of the femoral sheath) is to allow expansion of the femoral vein into this space following increased venous return.[5] The superior opening of the femoral canal under the inguinal ligament is termed the femoral ring. The borders of the femoral ring are:
The ‘mouth’ of the femoral ring is covered transversely by a sheet of peritoneal fatty tissue termed the femoral septum; this area is weak and abdominal viscera can traverse through it into the femoral canal, causing a femoral hernia.[2][4]
The general function of the femoral sheath is to allow the femoral vascular structures to glide underneath the inguinal ligament during hip flexion and extension. It provides a smooth fascial layer to facilitate this.
The femoral sheath is a fascial structure formed anteriorly by a continuation of the transversalis fascia and posteriorly by a continuation of the iliopsoas fascia. Both of these fascia layers are part of the thoracolumbar fascia, which is key to maintaining posture and the transmission of force between the scapular and pelvic girdles.[6] The embryological origin of the fascial layers of the body is intimately associated with the development of their corresponding muscular structures. Somitogenesis begins on the twentieth day of the embryo, forming somites from the segmentation of the paraxial mesoderm.[7] The dermomyotome is one of the constituent parts of the resulting somite, alongside the sclerotome. Further differentiation of the dermomyotome forms the dermis of the skin, as well as the epaxial and the hypaxial muscle compartments, the latter of which include the ventrolateral muscles of the abdominal wall.[8]
The femoral artery travels in the lateral compartment of the femoral sheath. It is a continuation of the external iliac artery, commencing as it passes under the inguinal ligament.[9] Distally, the femoral artery passes into the adductor canal, becoming the popliteal artery as it passes through the adductor hiatus. It gives off several important branches, none of which arise within the femoral sheath itself.
The femoral vein occupies the intermediate compartment of the femoral sheath. It is a continuation of the popliteal vein and enters the femoral sheath before becoming the external iliac vein as it passes under the inguinal ligament.
The femoral canal contains deep inguinal lymph nodes and associated lymphatic vessels.[10] The superior-most lymph node is termed the node of Cloquet.[11] These lymph nodes drain the glans clitoris in females, and the glans penis and the distal part of the spongy urethra in males.
The femoral branch of the genitofemoral nerve (from L1-L2) is given off as it lies on psoas major. It then runs inferiorly under the inguinal ligament, accompanying the femoral artery in the lateral compartment of the femoral sheath, before piercing it anteriorly to innervate the skin over the femoral triangle.[12][13]
The femoral nerve proper (from L2-L4) travels in the muscular compartment of the retro-inguinal space, lateral to the vascular compartment lined by the femoral sheath.[14] Thus, although an important part of the femoral triangle anatomy, it does not form part of the femoral sheath.
Behind the femoral sheath are the pectineus muscle (posteromedial) and the iliopsoas muscle (posterolateral).[2] The origin of the pectineus muscle is the pectineal line of the pubis, and its distal insertion point is the pectineal line of the femur, which is a continuation from the inferior part of the lesser trochanter. The iliopsoas muscle is a congruence of two posterior abdominal wall muscles; the iliacus, which arises from the iliac fossa, and the psoas major, which arises from the transverse processes of lumbar vertebrae L1-L5. Together, as the iliopsoas muscle, it inserts at the lesser trochanter of the femur. There is no muscular covering anteriorly over the femoral sheath; instead, it is covered by the fascia lata and cribriform fascia, subcutaneous tissue, and skin.
The medial part of the femoral sheath, termed the femoral canal, is the site through which abdominal viscera can protrude, resulting in a femoral hernia. Femoral hernia repair can be performed with mesh (hernioplasty) or primary suture closure (herniorrhaphy). Modern practice internationally has favored mesh repair, either via an open approach or laparoscopically, in the elective setting for both male and female groin hernia repair.[15][1] However, controversy remains as to the best management for femoral hernias.[16][17][18][19] This is because they are far less common than inguinal hernias, and there is a lack of long-term outcome data from the various repair techniques.[20]
The three classic open approaches to repair of femoral hernia are the infra-inguinal (Lockwood) approach, the trans-inguinal (Lotheissen) approach, and the high (McEvedy) approach, with each offering advantages in certain situations. The Lockwood approach, for example, involves a parallel incision approximately 1cm below the inguinal ligament and is suitable for elective repair as there is little opportunity to allow for bowel resection if required in emergency surgery[21]. Contemporary surgical practice has moved towards the laparoscopic approach for elective repair of femoral hernia, however, with classic open approaches increasingly being superseded. Indeed, the laparoscopic approach is increasingly recognized also as an option for emergency repair.[22]
The two laparoscopic surgical techniques for groin hernias are the transabdominal preperitoneal (TAPP) and the totally intraperitoneal (TEP) approach.[23] In the former, the peritoneal cavity is entered, and an incision is made pre-peritoneally to enable repair of the herniated site, whereas, in TEP, the peritoneum remains intact, and repair is performed without entering the cavity[24]. Current evidence involving a meta-analysis of 15 randomized-control trials suggests the recurrence rate and long-term outcomes of the two techniques are comparable.[25]
Femoral Hernia
Femoral hernias are the least common kind of groin hernia, accounting for between 20% and 31% of groin repairs in women and only 1% of groin repairs in men.[26] There is a strong preponderance towards women, with one study identifying 88% of femoral hernias as presenting in women, and the average age of presentation is typically in the 6th to 7th decade.[27] However, they may be underdiagnosed, particularly in patients with a high body-mass index (BMI); one study reported that 37% of women undergoing endoscopic inguinal hernia repair were found to have a synchronous undiagnosed femoral hernia.[28]
The typical clinical presentation of femoral hernia is a swelling inferior to the inguinal ligament and inferior and lateral to the pubic tubercle.[1] The risk of strangulation in femoral hernia is higher than for inguinal hernia.[26] Incarcerated bowel within a strangulated femoral hernia may present with severe pain and an acute bowel obstruction requiring emergent surgical intervention, and this is associated with worse outcomes than in patients managed electively.[29][30]
Femoral Vascular Access
The femoral vein is a relatively safe option for gaining central venous access when more proximal locations are less easily accessible. Ultrasound guidance helps reduce vascular bleeding by 60% when used for femoral vein cannulation.[31][32] The femoral artery is also a commonly used site of arterial access, both for obtaining blood in emergency situations and diagnostic procedures, such as digital subtraction angiography in neurosurgery.[33][34]
Node of Cloquet
The node of Cloquet, which lies within the medial compartment of the femoral sheath (otherwise known as the femoral canal), is the junctional point between the external iliac lymph chain and the deep inguinal nodes.[35] As such, it is the sentinel node for the iliac and obturator lymphatic system. Studies investigating the predictive value of metastatic deposits in Cloquet's node for deep pelvic malignancy have shown a positive predictive value of 70% and a negative predictive value of 84%.[11] This means that the decision on whether to perform deep pelvic lymphadenectomy can potentially be made based on a sentinel node biopsy of the node of Cloquet.[11] A further study found the pathological status of Cloquet's node to be superior to the radiological diagnosis when determining the appropriate extent of lymph node dissection that should be undertaken in patients with lower limb melanoma and palpable inguinal lymphadenopathy.[36]
Vascular Lacuna, Femoral Nerve, Ilioinguinal nerve, Femoral sheath, Femoral artery, Femoral vein and ring
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
Genitofemoral Nerve. The genitofemoral nerve arises from the lumbar plexus and supplies sensation to the thigh and genitalia.
Illustration by E Gregory
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