Overview of Inguinal Canal
The canal serves as a pathway by which structures can pass from the abdominal wall to the external genitalia. It is of clinical importance as a potential weakness in the abdominal wall, and thus a common site of herniation.
Development of the Inguinal Canal
During development, the tissue that will become gonads (either testes or ovaries) establish in the posterior abdominal wall, and descend through the abdominal cavity and a fibrous cord of tissue called the gubernaculum attaches the inferior portion of the gonad to the future scrotum or labia, and guides them during their descent.
It is the pathway by which the testes (in an individual with an XY karyotype) leave the abdominal cavity and enter the scrotum. so, In the embryological stage, the canal is flanked by an out-pocketing of the peritoneum (processus vaginalis) and the abdominal musculature.
The processus vaginalis normally degenerates, but a failure to do so can cause an indirect hernia, a hydrocele, or interfere with the descent of the testes. The gubernaculum (once it has shortened in the process of the descent of the testes) becomes a small scrotal ligament, tethering the testes to the scrotum and limiting their movement.
Individuals with an XX karyotype also have a gubernaculum, which attaches the ovaries to the uterus and future labia majora. Because the ovaries are attached to the uterus by the gubernaculum, they are prevented from descending as far as the testes, instead moving into the pelvic cavity. The gubernaculum then becomes two structures in the adult:
- The ovarian ligament and
- Round ligament of uterus
Boundaries of Inguinal Canal
The inguinal canal is bordered by anterior, posterior, superior (roof) and inferior (floor) walls. It has two openings – the superficial and deep rings.
- Anterior wall – aponeurosis of the external oblique, reinforced by the internal oblique muscle laterally.
- Posterior wall – transversalis fascia.
- Roof – transversalis fascia, internal oblique, and transversus abdominis.
- Floor – inguinal ligament (a ‘rolled up’ portion of the external oblique aponeurosis), thickened medially by the lacunar ligament.
During periods of increased intra-abdominal pressure, the abdominal viscera are pushed into the posterior wall of the inguinal canal. To prevent herniation of viscera into the canal, the muscles of the anterior and posterior wall contract, and ‘clampdown’ on the canal.
Rings of Inguinal Canal
The two openings are known as rings.
The deep (internal) ring is found above the midpoint of the inguinal ligament. which is lateral to the epigastric vessels. The ring is created by the transversal fascia, which invaginates to form a covering of the contents of the canal.
The superficial (external) ring marks the end of the canal and lies just superior to the pubic tubercle.
It is a triangle-shaped opening, formed by the evagination of the external oblique, which forms another covering of the inguinal canal contents.
This opening contains intercrural fibers, which run perpendicular to the aponeurosis of the external oblique and prevent the ring from widening.
Contents of Inguinal Canal
In biological males, the spermatic cord passes through the inguinal canal, to supply and drain the testes. In biological females, the much smaller round ligament of the uterus traverses through the canal on its way to the labia majora.
The walls of the inguinal canal are usually collapsed around their contents, preventing other structures from potentially entering the canal and becoming stuck.
There are two known nerves that pass within the structures of the inguinal canal. These nerves are the ilioinguinal and the genitofemoral nerves. A third nerve, iliohypogastric nerve, supply sensation to the skin above the genitalia does not pass through the inguinal canal. It pierces the transversus abdominis then the external oblique in the inguinal area.
The ilioinguinal nerve is a branch of L1. It passes through the deep inguinal orifice along with the cord structures. It provides sensation to the anterior perineum and medial and upper thigh. In males, it also provides sensation to the anterior scrotal area. In women, the nerve provides sensation to the labia majora and mons pubis.
The genitofemoral nerve is derived from the L1-L2 spinal nerve roots.
It divided above the inguinal canal to the genital branch that passes through the deep inguinal ring with the cord structures, and the femoral branch that passes below the inguinal canal.
It provides a motor function to the cremasteric muscle and sensory innervation to the scrotum (genital branch) and the upper thigh (femoral branch) in males, and labia in females.
A hernia is defined as the protrusion of an organ or fascia through the wall of a cavity that normally contains it. Hernias involving the inguinal canal can be divided into two main categories:
- Indirect – where the peritoneal sac enters the inguinal canal through the deep inguinal ring.
- Direct – where the peritoneal sac enters the inguinal canal though the posterior wall of the inguinal opening.
Indirect Inguinal Hernia
Indirect inguinal hernias are the more common of the two types. Indirect inguinal hernia are caused by the failure of the processus vaginalis to regress because the peritoneal sac (and potentially loops of bowel) enters the inguinal canal via the deep inguinal ring.
The degree to which the sac herniates depends on the amount of processus vaginalis still present.
Large herniations are possible in which the peritoneal sac and its contents may traverse the entire inguinal canal, emerge through the superficial inguinal ring, and reach the scrotum.
Direct Inguinal Hernia
In contrast to the indirect hernia, direct inguinal hernias are acquired, usually in adulthood, due to weakening in the abdominal musculature.
The peritoneal sac bulges into the inguinal canal via the posterior wall medial to the epigastric vessels and can enter the superficial inguinal ring. The sac is not covered with the coverings of the contents of the canal.
Mid-Inguinal Point and Midpoint of the Inguinal Ligament
These two terms are mentioned frequently in this article, and are often (mistakenly) used interchangeably:
- Mid-inguinal point – halfway between the pubic symphysis and the anterior superior iliac spine. The femoral pulse can be palpated here.
- Midpoint of the inguinal ligament – halfway between the pubic tubercle and the anterior superior iliac spine (the two attachments of the inguinal ligament). The opening to the inguinal canal is located just above this point.