Throughout the human body there are numerous openings that allow structures to pass from one area to another. They are referred to by different names, such as foramina, canals, or hiatus. The anterior abdominal wall – which extends in a craniocaudal fashion from the xiphisternum and adjacent lower borders of the lower eight ribs to the inguinal ligaments and the pubis – has naturally occurring paired canals in the lower regions known as the inguinal canals. More specifically, the inguinal canals are found in the inguinal region, which extend from the lower abdomen into the medial part of the proximal thighs on each side.
The oblique tunnels serve as a conduit that allows passage of the male gonads from their intra-abdominal point of origin to the final destination in the scrotal sac. The boundaries of this hiatus are formed by the muscles of the anterior abdominal wall in the left and right iliac fossa. The size of the canals increase with age and are significantly more developed in males than in females. This article will discuss the embryology and anatomy of the inguinal canals. Furthermore, disorders of the inguinal canal and how they are treated will also be addressed.
- Embryology of the Inguinal Canal
- Anatomy of the Inguinal Canal
- Contents of the Inguinal Canal
- Clinical Application: Inguinal Hernia
- Related Atlas Images
Embryology of the Inguinal Canal
The sole purpose of the inguinal canal is to provide a conduit that facilitates gonadal descent. In males, this results in the testes leaving the lumbar region of the posterior abdominal wall to enter the scrotal sac. In females, however, this means movement of the ovaries from the same area, to the pelvic region. The presence of the inguinal canal in both males and females are indicative of the stage of sexual development before the differentiation of sexes. Since the formation of the canal is more pronounced males, it will be discussed as the prototype for inguinal canal development.
Initially, around the eighth week of gestation, the testis shares a urogenital mesenteric attachment with the mesonephros (primitive kidney) to the posterior abdominal wall. The bilateral formation of the ligamentous gubernaculum occurs shortly after the degeneration of the mesonephros (the mesentery is left attached to the gonads). These mesenchymal derivatives have proximal attachment to the caudal pole of each ipsilateral testis. Distally, each ligament takes an oblique course through the developing anterior abdominal wall before inserting on the internal surface of the ipsilateral labioscrotal swellings. Gonadal descent is also facilitated by testicular enlargement and loss of the paramesonephric ducts (under the influence of Müllerian inhibiting substance [MIS]). The latter event allows the testis to migrate across the abdomen from the posterior abdominal wall to the anterior abdominal wall. In addition to these processes, an increase in intra abdominal pressure (as a result of gastrointestinal organogenesis), along with foetal testosterone also promote the descent of the glands into the scrotum.
However, the key feature that supports formation of the inguinal canal is the development of the processus vaginalis (during the 12th gestational week) in the region ventral to the gubernaculum. The flask-shaped processus vaginalis is an evaginated portion of peritoneum that bulges through the anterior abdominal wall. As the processus vaginalis migrates along the path created by the gubernaculum, it takes the layers of the anterior abdominal wall with it; resulting in the formation of the inguinal canal. The deep and superficial inguinal rings correspond to the openings created by the processus vaginalis through the transversalis fascia and external oblique aponeurosis, respectively. Under normal circumstances, the testicular descent begins around the 26th gestational week and may conclude over the course of three days. In other instances, it spans the course of six weeks and by the 32nd gestational week both testes are present in the scrotal sac. A small percentage of infants are born with undescended testes (cryptorchidism). This is likely to resolve by the 3rd month of extrauterine life.
In genotypic females, although the gonads never leave the pelvic cavity, there decent is also preceded by the formation of a relatively smaller processus vaginalis. Similarly, this promotes the formation of the inguinal canal in females as well. The processus vaginalis often fuses long before birth.
Anatomy of the Inguinal Canal
Landmarks of the Inguinal Canal
The inguinal canal is an oblique intramuscular slit that may range from 3 – 5 cm long in an adult. It originates superolaterally at the deep inguinal ring. This opening occurs at the medial half of the inguinal ligament of Poupart at the midpoint between the anterior superior iliac spine (ASIS) and the pubic tubercle; also known as the midpoint of the inguinal ligament. Please note that this is different from the mid inguinal point, which is the midpoint between the ASIS and pubic symphysis. However, the deep inguinal ring can also be found 1.25 cm above the mid inguinal point.
The canal then terminates at the superficial inguinal ring, which can be found about 1 cm superolateral to the pubic tubercle. Also, the superficial inguinal ring is bordered medially by the inferolateral border of rectus abdominis, laterally by the inferior epigastric vessels and inferiorly by the medial third of the inguinal ligament. These three borders are often referred to as Hesselbach's (inguinal) triangle and serve as an important landmark for the superficial ring.
Borders of the Inguinal Canal
The various layers of tissue of the lower abdominal wall play an important role in the formation of the inguinal canal. The canal has four borders – a roof and a floor, and anterior and posterior walls. It should be noted that the deep ring and thinnest part of the postural wall (i.e. the lateral aspect) is supported by the thickest part of the anterior wall. Similarly, the reflected inguinal ligament and conjoint tendon provides additional support to the posterior wall immediately behind the superficial inguinal ring.
A simple mnemonic to recall the borders of the inguinal canal is MALT:
- The roof is formed by muscles (internal oblique and transversus abdominis).
- The anterior wall is derived from aponeuroses (internal and external oblique aponeuroses).
- The floor is formed by ligaments (inguinal and lacunar ligaments).
- The posterior wall is formed by the conjoint tendon and transversalis fascia.
Deep Inguinal Ring
The initial opening of the inguinal canal is located at the midpoint of the inguinal ligament, and is referred to as the deep (lateral) inguinal ring. It is an oval defect in the transversalis fascia, which is widest along the vertical axis of the opening. The defect is significantly larger in males than in females, as it is meant to accommodate the passage of the testes into the scrotal sac.
The deep ring is lateral to the inferior epigastric vessels as well as the interfoveolar ligament (fibrous band extending from the lower margin of the transversalis fascia to the superior pubic ramus) when they are present. The deep ring may also function as a valve whenever it is made taught by contractions of the internal oblique muscle in response to raised intra abdominal pressure.
Anterior Wall of the Inguinal Canal
Beneath the skin and Scarpa’s fascia, sandwiched between two layers of Camper’s fascia, are the anterior abdominal wall muscles. The most superficial anterior abdominal muscle plays a crucial role in the formation of the anterior wall of the inguinal canal. The external oblique muscle has numerous points of origin along the lower eight ribs and their associated costal cartilages.
While some of the caudal fibers have attachments to parts of the iliac crest, some fibres continue caudally as the anterior aponeurosis. This fibromuscular structure, coupled with fibers of the internal oblique muscle form the anterior wall of the inguinal canal. The medial third of the anterior wall is formed by the patent superficial inguinal ring.
Inferior Wall (Floor) of the Inguinal Canal
The free lower border of the external oblique aponeurosis rolls inwards to form the inguinal ligament. Extending from the anterior superior iliac spine to the pubic tubercle, this ligament has an antero-inferior convexity toward its lower limit. It commences as a tubular, oblique structure at the lateral extent.
As it travels medially, the ligament then begins to widen as it approaches the pubis. The abdominal surface of the ligament contains a groove, which forms the floor of the inguinal canal. The fibres that do not insert at the pubic tubercle diverge to form the lacunar ligament (posterolaterally) and to join the linea alba (superomedially). The lacunar ligament also contributes to the floor of the canal.
Superior Wall (Roof) of the Inguinal Canal
Deep to the thick, bulky external oblique muscle is the internal oblique muscle. Unlike its superficial counterpart, it originates from the groove in the lateral two thirds of the inguinal ligament as well as the iliac crest. It has superior attachments to the lower border of the lower four ribs and their associated cartilages. Additionally, other fibres also join the anterior aponeurosis to attach to the costal cartilages of the 7th to 9th ribs.
However, it is the set of fibers that arise from the inguinal ligament that are needed to form the roof of the inguinal ligament. These fibers travel superiorly, and then arch inferomedially as it crosses the contents of the canal. The roof is further supported by fibers of the transversus abdominis muscle and the transversalis fascia.
Posterior Wall of the Inguinal Canal
Like the internal oblique muscle, the transversus abdominis muscle also (albeit, partly) arises from the lateral third of the inguinal canal and curves inferomedially to join the internal oblique aponeurosis as both structures insert at the pectineal line and pubic crest as the conjoint tendon. This robust tendon provides additional support to the medial third of the posterior abdominal wall. The lateral part of the posterior wall is also supported by the in-rolled part of the inguinal ligament, alongside the transversalis fascia.
Superficial Inguinal Ring
The medial opening of the inguinal canal occurs within Hesselbach's triangle and is known as the superficial inguinal ring. It is a ‘V’ shaped defect in the external oblique aponeurosis, whose apex is parallel with the deep aponeurotic fibres. The side walls of the superficial ring are formed from the aponeurotic crura of the external oblique. The lateral crus is thicker than the medial one, as the former is reinforced by the inguinal ligament.
The medial crus inserts at the pubic crest, while the lateral crus attaches to the pubic tubercle. There is also grooving of the lateral crus (not observed in females) that provides an atraumatic area for the spermatic cord to lay. The base of the triangular opening falls along the pubic crest. While there is variability in the size of the ring among individuals, it is also markedly larger in males than in females in order to accommodate testicular descent.
Contents of the Inguinal Canal
The contents of the inguinal canal vary dramatically between males and females; as there are significantly less structures that traverse the female inguinal canal. The ilioinguinal (arising from T12 and L1) and genital branch of the genitofemoral nerves (arising from L2) are the only structures that are found in both male and female inguinal canals. Note that the ilioinguinal nerve does not pass through the deep inguinal ring. Instead, it gains access to the canal by piercing its roof midway along the length of the canal.
Recall that the ilioinguinal nerve provides general somatic afferent innervation to the anterior perineum and medial part of the proximal thigh. Additionally, it innervates the mons pubis and labia majora in females, as well as the base of the penile shaft and anterior scrotal sac in males. The genital branch of genitofemoral nerve participates in the general somatic efferent supply to the cremasteric muscle (in males) and general somatic afferent to the labia and scrotum.
Inguinal Canal in Females
The round ligament of the uterus (ligamentum teres uteri) passes through the inguinal canal to insert on the posterior surface of the labia majora. It is a derivative of the embryonic gubernaculum that originates at the uterotubal junction (also called the uterine horn) and functions as a uterine anchor that keeps the uterus in the anteverted position.
Inguinal Canal in Males
In the simplest form, the male inguinal canal contains only the spermatic cord in addition to the same nerves found in the female inguinal canal. However, the spermatic cord contains several layers of fascia, and houses several arteries, nerves, and other reproductive structures. These structures can be readily recalled using the mnemonic “FANO x 3”, which translates to:
Fascia – these are continuations of the muscular and fascial layers that were traversed by the processus vaginalis during development.
- The deepest of the three fascial layers is the internal spermatic fascia. It is a continuation of the transversus abdominis fascia that was pulled through the canal during development.
- The cremasteric muscle and fascia form the middle musculofascial layer of the spermatic cord. Both are continuations of the internal oblique muscle and fascia, respectively.
- Finally, the external spermatic fascia is the outermost layer, which developed from the external oblique aponeurosis.
Arteries – the testes maintain the arterial supply it acquired during early development. These vessels migrate caudally with the gonad during its descent.
- The testicular artery is responsible for supply the testes. Also historically known as the internal spermatic arteries, these vessels are direct branches of the aorta.
- The cremasteric artery (formerly known as the external spermatic artery) supplies the cremaster as well as other fascial layers of the spermatic cord. The artery arises from the inferior epigastric artery and traverses the inguinal canal. It also shares anastomoses with the testicular artery.
- The third artery in the inguinal canal is the artery of the vas (ductus) deferens. The vas deferens is responsible for transporting sperm from the testes to the urethra. The arteries that supply it arise indirectly from the internal iliac artery via either the superior or inferior vesical arteries.
Nerves – in addition to the general somatic afferent and efferent nerves mentioned earlier, there are also autonomic nerves that pass through the canal to supply the gonads and associated structures.
- Ilioinguinal nerves
- Genital branch of genitofemoral nerve
Other – miscellaneous structures found in the cord include:
- The vas deferens.
- The pampiniform plexus of veins - These vessels encircle the arterial structures and participate in a countercurrent heat exchange process that ensures that the blood being carried to the testes is at ambient temperature.
- Testicular lymphatics will drain lymph from the testes to the para-aortic lymph nodes.
Clinical Application: Inguinal Hernia
A hernia is the abnormal protrusion of viscera out of the walls of its containing cavity as a result of a weakness in these walls. There are numerous potential points of herniation throughout the body. Below is a laundry list of possible herniation sites and types of hernias that may occur. It is by no means an exhaustive recall of all possible hernias:
Intracranial hernias as a result of raised intracranial pressure:
- Tonsillar herniation
Intrathoracic hernias following traumatic or congenital injuries to the diaphragm:
- Traumatic herniation
Ventral abdominal wall hernias:
Posterior abdominal wall hernias occur through weakened points in the upper and lower lumbar triangles. However, these are extremely rare.
- Grynfeltt-Lesshaft (Superior Lumbar Hernia)
- Petit (Inferior Lumbar Hernia)
- Obturator (rare)
There are several etiological factors that influence the occurrence of a hernia. The following concepts can be applied to hernias throughout the body, but will be discussed with respect to abdominal wall and groin hernias. Essentially, hernias can be considered as disorders of collagen characterized by an imbalance of the types of collagen present in the tissue or an inherent problem with collagen synthesis.
Some hernias occur through areas of innate weakness – as is the case with posterior abdominal wall hernias. In other instances, the weakness exists at a hiatus that is normally used for structures to enter and exit the primary cavity. This is the case for inguinal hernias, as the deep and superficial rings are innate points of weakness. During development, if there is failure of a structure to form completely, then there is a risk of herniation through that structure (e.g. diaphragmatic hernias). Both sharp and blunt force trauma can produce areas of weakness through which viscera may herniate (e.g. acquired diaphragmatic hernia, incisional hernia). Aging is associated with progressive, widespread tissue degeneration that results in increased laxity of the ligaments. The hormonal changes associated with pregnancy also promote ligament laxity to facilitate growth and subsequent delivery of the foetus. In the absence of weakening of the abdominal wall, raised intra abdominal pressure is unlikely to produce a hernia. However, in cases where there are inherent defects (i.e. the inguinal region) the positive pressure within the abdominal cavity may cause the abdominal viscera to herniate.
Inguinal hernias are among the most common forms of hernias that require surgical intervention. They are more commonly encountered in males than in females. These hernias may be congenital (in which case the hernia is associated with a patent processus vaginalis) or acquired (related to an imbalance of type I and type III collagen). Smoking, weight training and lifting heavy objects have also been identified as risk factors for developing hernias. The hernia usually begins as a saccular outpouching that contains extraperitoneal tissue. Over time, as the hernia gets larger, positive pressure from the intra abdominal cavity forces peritoneum into the sac. Eventually, abdominal viscus may enter the hernia sac as well.
Types of Inguinal Hernias
Inguinal hernias that protrude through the deep inguinal ring and traverse the canal are known as indirect hernias. In these cases, the swelling is noted at the midpoint of the inguinal ligament. If the sac emerges through the superficial ring, then this is a direct inguinal hernia, with the contents emerging within Hesselbach's triangle. In either case, the contents of the hernia may pass all the way through the inguinal canal and enter the scrotal sac. This inguinoscrotal swelling should be adequately examined to determine if this is primarily a testicular or scrotal disorder, or a large inguinal hernia.
On rare occasions, the herniation can occur through both the deep and superficial rings at the same time. These defects are referred to as Pantaloon hernias. The astute clinician should also examine the contralateral groin as there is a possibility that the patient may have a bilateral inguinal hernia.
Symptoms of an Inguinal Hernia
Patients often present with a spectrum of symptoms depending on the complexity of their inguinal hernia. These hernias may be occult swellings that are difficult to elucidate on clinical examination, but are typically associated with immense pain. Other patients with reducible hernias present with complaint of a mass that is absent in the morning, but becomes more obvious throughout the course of the day. The same mass is worsened with standing, bearing down, or lifting heavy weights. The patient may even be able to reduce (push in) the hernia themselves.
As time progresses where the hernia is left untreated, the same patient may note that the mass is no longer reducible. Incarceration of a hernia is not uncommon as the hernia sac is likely to be filled with additional visceral content and is now too large to re-enter the abdominal cavity through the inguinal opening. These hernias require urgent attention as they are at increased risk of causing bowel obstruction (more commonly small bowel). Another detrimental concern of incarcerated hernias is the likelihood of them progressing to strangulation if left unattended. A strangulated hernia contains non-viable bowel that will become necrotic (and possibly infected) and is likely to perforate if left unattended. The pain associated with strangulation is severe, and the patient may have features of septic shock.
Examination of a Hernia
On conducting the groin examination, it is important for the patient to be positioned in the supine position. The patient should be exposed from the nipple line to the mid-thigh region, as it is important for the clinician to observe the genitalia during this examination. Inspect the abdomen and groin area for any obvious swellings. Make note of any surgical scars and their association with the swellings (i.e. incisional hernias). Asking the patient to raise their head off the bed while supine can also reveal any divarication of the rectus abdominis. If a swelling is obvious, note if there is any bruising or cellulitis to the overlying skin as this suggests incarceration with venous engorgement and impending strangulation.
At the foot of the bed, ask the patient to cover their mouth and cough. The raised intra abdominal pressure will exaggerate the hernia. If the swelling does not extend into the scrotal sac, then attempt to reduce the hernia. If it is adequately reduced, then using one finger, demonstrate that the swelling can be controlled by asking the patient to cough again with the examining finger over the reduced swelling. If the patient coughs and the selling does not recur, then it is controlled.
If the swelling extends into the scrotal sac, then proceed with examination of the scrotum. Palpate the testes to determine their presence and orientation within the respective hemi-scrotum. Attempt to palpate the spermatic cord, which is more readily done if the mass is due to a testicular hydrocele (fluid within the scrotum) as opposed to an inguinoscrotal hernia (abdominal viscera within the scrotum). Additionally, hydroceles will transilluminate but a hernia will not. If it is more likely that the mass is due to a hernia, then attempt to reduce it and demonstrate that it can be controlled (as described above).