Femoral Sheath and Inguinal Canal Anatomy

The inguinofemoral region is a relatively complex one that is significant for clinicians across various fields. Recognition of normal and pathologic variation within this region is important to many surgical disciplines, including general, vascular, plastic, oncologic, gynecologic, and urologic surgery. Medical intensivists, cardiologists, general physicians, and pediatricians also require a working understanding of the femoral sheath and inguinal canal. Inguinofemoral anatomy has been well described in detail;[1]  the focus of this article is on anatomic considerations relevant to the clinical diagnosis and care of patients, with particular emphasis on procedural and urgent medicine.

In both pediatric and adult patients, the inguinal region represents the confluence of the fasciomuscular component of the lower abdomen with the adjacent lower extremities. Contained and traveling therein are neurovascular structures that are crucial to the function and neurovascular integrity of the lower limbs, as well as the reproductive function in males. For a better understanding of the clinical significance of the inguinal canal and the adjacent femoral sheath, this region may be conceptually broken down into osseous, myotendinous, neurovascular, lymphatic, and genitourinary/spermatic cord components.

Osseous components

Any structure, anatomic or architectural, requires a foundation or scaffold. In the case of the inguinofemoral region, this essential framework is provided by portions of the bony pelvis, specifically the anterior iliac spine (AIS) of the ilium and the pubic tubercle of the pubis. Both of these structures can be readily palpated in most patients (with the exception of the morbidly obese).

For traumatologists, intensivists, vascular surgeons, interventional radiologists, and cardiologists, the AIS and the pubic tubercle provide a relatively constant set of landmarks by which to gauge the course of the femoral artery or vein when central vascular access is required. An imaginary oblique line is drawn between the AIS and pubic tubercle. This line is then subdivided into 3 equal components. The femoral artery should lie at the junction of the middle segment and the most medial segment.

The femoral vein, in turn, should lie 1-2 cm medial to the artery. Even when the vessel is camouflaged by tissue in an obese patient or when there is no palpable femoral pulse in an underresuscitated trauma patient, the femoral vein may be successfully located in 90% of cases with this technique.[2]

Myotendinous components

Likewise, the inguinal ligament, which spans the AIS and the pubic tubercle, can be appreciated in most patients. This interposing ligament represents the inward-scrolling, aponeurotic, inferiormost external oblique muscle that forms the floor of the inguinal canal[1] and the superior border of the femoral triangle[3] and marks the transition between the external iliac and femoral vessels.[4]

The inguinal (or Poupart) ligament is a key element in the repair of most inguinofemoral hernias.[5] When femoral central vascular access is desired, the inguinal ligament may serve as an important landmark in adequately perfused nonobese patients. In such patients, the femoral pulse may be appreciated just below the inguinal crease.[6] During surgical cases requiring more proximal exposure of the femoral vessels, this ligament can be divided to facilitate adequate vascular exposure.[7, 8] However, failure to stay below the inguinal ligament during attempted percutaneous cannulation of the femoral vessels can lead to serious and even lethal consequences in both adult and pediatric patients (see the image below) and observance of the inguinal ligament in percutaneous femoral vessel cannulation.

In addition to the inguinal ligament, the inguinal canal is composed of the aponeurosis of the external oblique muscle (anterior wall), the transversalis fascia (posterior wall), and fibers of the internal oblique muscle and the transversus abdominis (roof).[1] Along its 4-6 cm inferior medial course, it transmits the spermatic cord in males and the round ligament in females. In addition, the ilioinguinal nerve and lymphovascular components pass through the canal in both sexes.[1]

The entry point of the inguinal canal formed by a ballooning of the transversalis fascia that is known as the internal inguinal ring. Through this ring pass the vas deferens and the gonadal vessels. The internal ring is situated lateral to the inferior epigastric arteries in a position that is understood and well respected by open and laparoscopic hernia surgeons.

The exit point of the inguinal canal, known as the external inguinal ring, is formed by aponeurotic fibers of the external oblique muscle and is situated superolateral to the pubic tubercle. It represents the initial access point for entry into the canal during open inguinal hernia surgery; like the internal ring, it transmits the spermatic cord in males and the round ligament in females.[1] The external ring can be appreciated in nonobese male patients by invaginating the skin superolateral to the pubic tubercle with the index finger of the examiner. Should an inguinal hernia be present, a cough impulse can be appreciated with this technique.[1, 9]

Neurovascular components

Of all the structures within the inguinofemoral region, perhaps few are more relevant to daily practice of surgical, intensive, and interventional medicine than the vascular components of the femoral sheath.

The well-described NAVEL mnemonic (see the image below) has been used to help medical students and surgical trainees remember the relationship of the femoral sheath structures in a lateral-to-medial direction: femoral Nerve, femoral Artery, femoral Vein, Empty space, and Lymphatics.[10] Although this mnemonic is not necessarily anatomically complete,[4] it is relevant for daily clinical practice in terms of properly orienting the crucial neurovascular structures of the femoral region.

The femoral sheath itself is an extension of transversalis and iliopsoas fascia that encloses the proximal parts of the femoral vessels 3-4 cm inferior to the inguinal ligament. This composition accounts for how retroperitoneal hematomas may develop in instances of femoral vascular injury (iatrogenic or other). The sheath is subdivided into 3 compartments: lateral (femoral artery), intermediate (femoral vein), and medial (femoral canal, site of femoral hernias). Notably, the femoral sheath does not enclose the femoral nerve (L2-4),[11, 12] which provides motor innervation to the quadriceps and sensory input from the medial thigh. These relationships bear particular significance for those attempting safe and timely femoral vascular access.

Other named nerves of the region include the lateral femorocutaneous nerve, the ilioinguinal nerve, the iliohypogastric nerve, and the genitofemoral nerve. The lateral femorocutaneous nerve (L2-3) typically courses deep to the inguinal ligament en route to providing sensation to the lateral portion of the thigh. It is particularly susceptible to injury during laparoscopic herniorrhaphy, which consequently leads to the development of a condition known as meralgia paresthetica. The ilioinguinal nerve (L1 ± T12) courses along the spermatic cord within the inguinal canal and should be preserved during surgical dissection because it provides innervation to the thigh, scrotum, and labium majus. The iliohypogastric nerve (L1 ± T12) courses superior to the ilioinguinal nerve and provides sensation to the skin above the pubis.

The genitofemoral nerve (L2-3) splits into femoral and genital components, which provide sensory innervation to the medial thigh and motor innervation to the cremasteric muscle, respectively.[11]

Lymphatic components

Immediately adjacent to the structures of the femoral sheath are the inguinofemoral lymph nodes. These nodes may serve as harbingers of occult systemic disease of an infectious or oncologic nature and are situated superficial and deep to the deep fascia of the thigh.[13] The superficial nodes receive lymph from the external genitalia (including the inferior vagina), the inferior anal canal, the adjacent abdominal wall, and the umbilicus. The deep nodes, of which there are generally only 1 to 3, are located medial to the femoral vein and receive lymphatic return from the lower extremities.[13]

When adenopathy exists, it is useful to recall the lymphatic basins that empty into the inguinofemoral region and to examine the lower extremities as well as the area inferior to the umbilicus.

Although interruption of the lymphatic channels emptying into the inguinofemoral nodes may be less dramatic than injury to the adjacent femoral vessels, the long-term results of injudicious or deliberate oncologic dissection may lead to development of lymphedema with its attendant, sometimes irreversible, morbidity (see the image below).

Genitourinary/spermatic cord components

The previously described inguinal canal serves as a conduit for the spermatic cord. Contained within the cord are a number of clinically relevant structures.[1] The vas deferens conveys sperm from the epididymis to the ejaculatory duct, whereas the testicular artery and pampiniform plexus are vital to testicular vascular integrity. Testicular lymphatics may inadvertently serve as a route of metastasis in cases of testicular cancer, and the genital branch of the genitofemoral nerve (L2-3) provides innervation to the cremasteric fibers of the internal oblique muscle. Observance of these structures is of obvious importance to the inguinal hernia surgeon.

The relationship of the structures within the femoral triangle is well described. The NAVEL mnemonic (femoral N erve, femoral A rtery, femoral V ein, E mpty space, and L ymphatics) has been used to help medical students and surgical trainees remember the relationship of these structures in a lateral-to-medial direction (see the image below).[10] Although confirmation of this relationship is borne out in day-to-day clinical practice, an anomalous relationship in which the artery is located medial to the vein has been described.[14]

Another, perhaps more common, anatomic variant with which surgeons who operate in the femoroinguinal region should be familiar is the aberrant obturator artery. This variation represents an "aberrant" anastomosis connecting the inferior epigastric artery to the otherwise "normal" obturator artery. Some studies suggest this variant may in fact be present in 60-90% of individuals. Also referred to as the artery of death by some texts, the aberrant obturator artery may represent a source of copious bleeding when injured and unrecognized. Injury is best prevented by careful dissection in the region of the obturator canal and a wary mindfulness of the existence of this variation.[4, 15]

Vascular anomalies are not the only clinically relevant anomalies of the inguinofemoral region. The femoral nerve, typically described as bifurcating below the inguinal ligament,[16] has also been found to branch into anterior and posterior divisions above the inguinal ligament.[17] The lateral femoral cutaneous nerve has also been described as more variable in its course than was previously thought.[18] This variability is of particular significance for laparoscopic inguinal hernia surgeons.

The inguinofemoral region is also affected by variation in urologic anatomy. Pediatricians, family physicians, surgeons, and urologists may encounter hydroceles. During development and descent through the inguinal canal, the testicles are enveloped in a dual layer of peritoneum, which becomes the tunica vaginalis. The communication between the peritoneal space and the tunica vaginalis is known as the processus vaginalis. Although this structure normally obliterates, failure to do so results in a hydrocele.

Communicating hydroceles represent a persistent processus vaginalis through which peritoneal fluid can track and accumulate around the testicles. Should the processus obliterate, thereby trapping fluid around the testicle, it is referred to as a noncommunicating hydrocele.[19] In female patients, a persistent processus vaginalis that traverses the inguinal canal (a so-called canal of Nuck) may facilitate later inguinal hernia development.[1] These conditions may have pathologic significance.

Numerous important pathologic conditions manifest in the inguinofemoral region. For the purposes of clinical classification, it is helpful to divide these into infectious, oncologic, vascular/hypercoagulable, mechanical/connective tissue, urologic, and iatrogenic disorders.

Infectious disorders

Infectious inguinofemoral lymphadenopathy is an important condition to recognize clinically; it may point to or be associated with a primary abscess or infection that refers lymphatically to the groin. Infections involving the external genitalia, rectum, lower abdomen and back, and upper third of the thigh typically drain to the inguinal nodal basin, whereas a nidus of infection in the lower extremity may refer to the femoral nodes situated beneath the inguinal ligament.

Although perhaps less common in industrialized nations than in the tropical and undeveloped world, parasitic or sexually transmitted infectious lymphadenopathy of the inguinofemoral nodes can be dramatic in clinical presentation. Examples of such disorders include filariasis and lymphogranuloma venereum (LGV).

Worldwide, filariasis is the most common cause of lymphedema.[20] Easily recognizable lower extremity lymphedema follows infestation of lymph nodes by the parasite Wuchereria bancrofti, which has a predilection for the inguinal lymph nodes.[21]

LGV is representative of sexually transmitted diseases that may manifest in the lymphatics of the groin. Chlamydia trachomatis is markedly lymphotrophic, and regional lymph nodes are quickly invaded, causing an acute purulent lymphadenitis (bubo formation) 4-8 weeks after coitus. The inguinal bubo, which is the most common site (see the image below), must be differentiated from swollen nodes of syphilis, chancroid, pyogenic lesions of the lower limbs, and even bubonic plague.[22]

In those less accustomed to such manifestations, confusion may arise. The author has been consulted in the emergency department to evaluate a patient with a tender inguinal mass suspected of being an incarcerated inguinal hernia. The young man had developed an inguinal bubo consistent with LGV and needed a course of doxycycline rather than an operation, something that became apparent after history and physical examination. In cases where such confusion exists, plain films, ultrasonography, or computed tomography (CT) may help to delineate the relationship of the mass to intraperitoneal contents and surrounding structures.[23, 24]

Oncologic disorders

Melanoma may involve the inguinofemoral region with metastatic disease of the inguinal nodes. Other malignancies, including lymphoma, retroperitoneal sarcoma, nonmelanoma metastatic disease, and testicular cancer, may also present as inguinofemoral adenopathy or may even be mistaken for an inguinal hernia.[23] Non-Hodgkin lymphoma may manifest as well, with adenopathy both above and below the inguinal ligament.[25]

Vascular/hypercoagulable disorders

Just as the vessels of the femoral sheath provide a reliable point of access into the central circulation, so they also provide an important window into systemic or localized vascular disorders. Absence of a femoral pulse in its typically appreciable position below the inguinal ligament signals severe atherosclerotic disease that may warrant surgical or catheter-based intervention in a setting of claudication or rest pain. Acute loss of the femoral pulse with attendant severe lower extremity pain points to an embolic event, particularly in a patient with atrial fibrillation and or valvular disease.[26]

Perhaps the most clinically significant manifestation of femoral venous disease is deep venous thrombosis (DVT). Femoral DVT may be catheter-related related or may develop in association with an inherited or acquired hypercoagulable state. Venous congestion and pain point to this diagnosis, which if suspected may be accurately assessed with duplex ultrasonography in the emergency department setting.[27]

Mechanical/connective tissue disorders

Hernias representing pathologic laxity and degeneration of the connective tissues of the inguinofemoral region are of unarguable clinical significance. More than half a million hernia operations are performed in the United States each year.[28] In terms of lifetime risk, up to 27% of men and 3% of women develop an inguinal hernia.[29] Many excellent texts are available detailing all aspects of inguinofemoral hernia diagnosis and management. There remain, however, some salient points worth emphasizing here.

Regarding diagnosis, physical examination remains the best way to determine the presence or absence of an inguinofemoral hernia.[23] Although distinguishing direct from indirect hernias is difficult, femoral hernias are classically appreciable as a bulge below the inguinal ligament and medial to the femoral vessels.[23, 30, 31] In thin patients, soft, asymptomatic protrusions inferior to the inguinal ligament and medial to the femoral vessels may be noted on examination. Such a bulge may appear to reflect a femoral hernia, when in fact it is a prominent femoral fat pad that disappears when the patient assumes a supine position. In cases of ambiguity, ultrasonography, CT, or magnetic resonance imaging (MRI) may be useful.[23]

Urologic disorders

Communicating and noncommunicating hydroceles represent delayed or failed closure of the processus vaginalis. Their pathologic significance stems from their potential either to become symptomatically tense and thereby impair testicular circulation or, in the case of communicating hydroceles, to provide a ready conduit for visceral herniation into the scrotum or labium majus (ie, an indirect inguinal hernia).[19] Such hydroceles, or those that persist beyond the first year of life, rarely resolve and typically warrant surgical intervention.[19]

Although not part of the inguinal canal or femoral sheath per se, testicular torsion is a urologic emergency with which emergency physicians, family physicians, general surgeons, and urologists should be familiar. Minevich and McQuiston note that the condition is perhaps better termed spermatic cord torsion, in that it is torsion of the spermatic cord that leads to strangulation of the gonadal vessels and subsequent testicular loss.[32] Indeed, proper recognition of this condition with immediate surgical attention, as opposed to waiting for unnecessary confirmatory studies, can lead to a more than 90% salvage rate, provided that surgery is performed within 4-6 hours of the onset of pain.[19]

Iatrogenic disorders

Failure to correctly discern the vascular components of the femoral sheath and surrounding structures can lead to uncommon but disastrous procedural consequences, including bowel and bladder perforation.[33, 34, 35, 36, 37] Delayed complications of unintended femoral vessel puncture may also arise and include development of arteriovenous fistulas and pseudoaneurysms. Although ultrasonography may be beneficial in securing femoral vascular access, its use does not necessarily preclude morbidity in patients with complex histories.[33]

Neural structures, including all those listed previously (see Neurovascular components) are susceptible to injury in both open and laparoscopic hernia repairs.

Surgical disruption of the inguinofemoral lymphatics after inguinal node dissection has been associated with a 30% incidence of lower extremity lymphedema,[38] although this rate does not seem to be affected by whether the surgeon takes a deep or a superficial approach to groin dissection.[39] The morbidity of lower extremity lymphedema may be substantial (see the image below).

Femoral access in trauma resuscitation

Despite the emphasis on central access in trauma resuscitation, peripheral access is preferable to femoral (or subclavian) access on the basis of Poiseuille's law.[40] For example, a 14-gauge, 5-cm peripheral catheter delivers flow at twice the rate of a 16-gauge, 20-cm central catheter.[41] Central access via the femoral approach can be used in cases in which peripheral access is not achievable, preferably through unaffected skin in the case of burn patients.[6, 40, 42] Femoral venous and arterial access can also be safely and reliably achieved even in pediatric burn patients.[43, 44]

The femoral vein is generally considered the easiest of the large veins to cannulate. Risks attendant on femoral vein cannulation include venous thrombosis (10%), femoral artery puncture (5%), and diminished hip flexion.[6] Popular belief to the contrary, no evidence exists that the infection rate with femoral catheter use is greater than that with internal jugular vein catheter use.[45, 33]

Observance of inguinal ligament during percutaneous femoral vessel cannulation

As earlier mentioned, failure to stay below the inguinal ligament during attempted percutaneous cannulation of the femoral vessels can lead to serious and even lethal consequences.[46, 47, 48] The author has consulted on a patient in hemodynamic collapse who was labeled with a diagnosis of an acute surgical abdomen. Review of the patient's history demonstrated percutaneous angiography completed via the right groin on the evening before consultation. Ecchymosis and swelling within the groin were consistent with iatrogenic retroperitoneal hematoma, and the patient was taken urgently to the operating room. There, an actively bleeding suprainguinal iliac injury was noted and repaired after evacuation of a large amount of retroperitoneal clot.

Such misadventures may prove especially disastrous in anticoagulated patients, given the propensity for bleeding into the relatively uncontained retroperitoneal space.[47, 48] This is also true of pediatric patients, given the relatively short distance between the femoral sheath structures and the peritoneal space.[49] Iatrogenic injuries such as this may potentially be avoided through use of bedside ultrasonography (see below), real-time fluoroscopy,[50] and observation of the previously described bony landmarks, including the anterior iliac spine (AIS) and the pubic tubercle.

Nonemergent femoral access: ultrasonography versus anatomic landmarks

Various studies have demonstrated higher success rates for first cannulation attempts, fewer arterial punctures and hematomas, fewer sticks necessary to achieve adequate access, and equal timeliness of insertion and efficacy with the use of ultrasonographic guidance for nonemergent femoral venous cannulation.[51, 52, 53] These findings are consistent in both adult and pediatric populations.[54]

On the basis of the current literature and the author's own experience,[15, 55] ultrasonographic survey of the femoral vascular anatomy is particularly beneficial for helping to delineate the patency and or existence of anticipated vessels under circumstances in which patients bear scars indicative of previous inguinofemoral surgery (vascular or otherwise). The author has been consulted to assist with difficult femoral access on occasions in which the previous clinician was seeking access through a chronically thrombosed vessel or one that had been surgically altered by way of an iliofemoral graft. Regardless of surgical or medical training background, an astute clinician will be closely attuned to the potential for significant anatomic alteration in the previously operated groin.