Surgical Anatomy of the Abdomen and Pelvis

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Chapter 7 Surgical Anatomy of the Abdomen and Pelvis

ANTERIOR ABDOMINAL WALL

The abdominal wall is made up of four structural layers beneath the skin: (1) subcutaneous tissue and superficial fascial layers, (2) muscles and transversalis fascia, (3) deep fascia of the rectus sheath and the extraperitoneal fascia, and (4) parietal peritoneum (Fig. 7-1). Interspersed among these layers are several important nerves and blood vessels.

Muscles and Transversalis Fascia

The abdominal wall is made up of five pairs of muscles. In the midline, the rectus abdominis muscles extend along the whole length of the front of the abdomen from the xiphoid process and costal cartilages of the fifth through the seventh ribs to the pubic crest and pubic symphysis. This broad strap muscle is divided into four segments by three fibrous intersections attached to the anterior, but not the posterior, rectus sheath. This allows the inferior (deep) epigastric vessels to pass along the posterior surface of the muscle without encountering a barrier.

The pyramidalis muscle is a small triangular muscle that lies in front of the rectus abdominis at the lower part of the abdomen and is contained within its fascial sheath. It arises from the front of the pubic symphysis and the anterior pubic ligament bilaterally and inserts into the linea alba, between the umbilicus and pubic symphysis. This muscle is commonly absent on one or both sides.

There are three sets of lateral muscles. The external oblique muscle is the most external and arises from the lower eight ribs. The fibers run downward and forward to form aponeuroses that extend anteriorly. Aponeuroses are fibrous membranes resembling flattened tendons that bind muscles to each other or bones. Beneath the external oblique muscle, the internal oblique muscle arises from the lumbar fascia, the iliac crest, and the lateral two thirds of the inguinal ligament and runs upward and forward to form aponeuroses. The most internal of the lateral muscles is the transversus abdominis muscle. It arises from the lateral third of the inguinal ligament, from the anterior three fourths of the iliac crest, from the costal cartilages of the sixth through eighth ribs, interdigitating with the diaphragm, and from the lumbodorsal fascia and ends in front in a broad aponeurosis. Deep to the transversus abdominis muscle is a continuous layer of specialized investing fascia that lines the abdominal cavity and continues into the pelvic cavity, the transversalis fascia.

Deep Fascia of the Rectus Sheath and Extraperitoneal Fascia

The rectus abdominis muscle is enclosed anteriorly and posteriorly by fascia known as the rectus sheath. This sheath is formed from fusion of the aponeuroses of all three lateral abdominal muscles. These aponeuroses fuse lateral to the rectus abdominis muscles as the linea semilunares and again in the midline as the linea alba, which extends from the xiphoid process to the pubic symphysis. The arcuate line is a tranverse line midway between the umbilicus and pubic symphsis. Above this line, the aponeuroses of the lateral muscles split to enclose the rectus muscle both anterior and posterior; below this line these aponeuroses all pass anterior to the rectus muscle. Inferiorly, the aponeuroses of the external oblique inserts into the anterior superior iliac spine and stretches over to the pubic tubercle, forming the inguinal ligament.

The inguinal canal is about 4 cm long and runs parallel to the inguinal ligament. The inguinal canal has an anterior wall formed by the aponeurosis of the external oblique, an inferior wall formed by the inguinal ligament, a superior wall formed by arching fibers of the internal oblique and transversus abdominis muscles, and a posterior wall formed by the transversalis fascia. A defect, or more precisely a tubular evagination, of the transversalis fascia forms the deep inguinal ring, through which the round ligament enters the inguinal canal. This ring lies midway between the anterior superior iliac spine and the pubic symphysis. Medial to the deep inguinal ring are the inferior epigastric vessels. The opening of the aponeurosis of the external oblique superior to the pubic tubercle is the superficial inguinal ring. Through it the round ligament, the terminal part of the ilioinguinal nerve, and the genital branch of the genitofemoral nerve exit the inguinal canal (see Fig. 7-1).

Deep to the transversalis fascia and the rectus sheath is a layer of connective tissue separating the transversalis fascia from the parietal peritoneum, the extraperitoneal fascia. This layer contains varying amounts of fat, lines the abdominal cavity and is continuous with a similar layer lining the pelvic cavity. Viscera in the extraperitoneal fascia are referred to as retroperitoneal.

Nerves

There are four categories of nerves that supply the anterior abdominal wall, each of which contain both motor and sensory fibers. The thoracoabdominal nerves originate from T7–T11, travel anteroinferiorly between the internal oblique and transversus abdominis muscles, and have the following distribution:

The subcostal nerves originate from T12 and travel anteroinferiorly between the internal oblique and transversus abdominis muscles to innervate the abdominal wall inferior to the umbilicus.

The iliohypogastric nerve and ilioinguinal nerve both originate from L1. Like the thoracoabdominal and subcostal nerves, these nerves begin their course anteroinferiorly between the internal oblique and transversus abdominis muscles. However, at the anterior superior iliac spine, they both pierce the internal oblique muscle to travel between the internal and external oblique muscles. The iliohypogastric nerves innervate the abdominal wall lateral and inferior to the umbilicus. The ilioinguinal nerve enters the inguinal canal and emerges from the superficial inguinal ring and is sensory to the labia majora, inner thigh, and groin.

These nerves are particularly at risk in lower abdominal incisions, which are the most common causes of abdominal wall pain as a result of nerve entrapment by suture or scar tissue.1 For this reason, knowledge of the course of the ilioinguinal and iliohypogastric nerves in the anterior abdominal wall can help avoid injury during laparotomy and laparoscopic surgery. Data from cadaveric studies suggest that injury to these nerves can be minimized during laparoscopy by making transverse skin incisions and placing laparoscopic trocars at or above the level of the anterior superior iliac spine.2 In cases of chronic abdominal pain caused by these nerves, an injection of local anesthetic at a site approximately 3 cm medial to the anterior superior iliac spine will often provide relief.

Blood Vessels

The major vessels in the anterior abdominal wall can be divided into deep and superficial vessels (Fig. 7-2).3 The superficial vessels include the superficial epigastric and the superficial circumflex iliac vessels. These vessels are branches of the femoral artery and vein. They course bilaterally through the subcutaneous tissue of the abdominal wall, branching as they proceed toward the head of the patient.

image

Figure 7-2 Anterior abdominal wall blood vessels.

(Modified from Hurd WW, Bude RO, DeLancey JOL, Newman JS: The location of abdominal wall blood vessels in relationship to abdominal landmarks apparent at laparoscopy. Am J Obstet Gynecol 171:642–646, 1994.)

To avoid vessel injuries, these superficial vessels can often be seen before secondary laparscopic port placement by transillumination of the abdominal wall using the intra-abdominal laparoscopic light source.3 Injury to these vessels during trocar placement can result in a palpable hematoma that will be found to be located anterior to the fascia on computed tomography (CT) scan.4 In unusual cases, the hematoma can dissect down into the labia majora.

The deep vessels consist of the inferior epigastric artery and vein, which are also bilateral. These vessels originate from the external iliac artery and vein and course along the peritoneum until they dive deeply into the rectus abdominis muscles midway between the pubic symphysis and the umbilicus. The inferior epigastric vessels are the lateral border of an inguinal triangle called Hesselbach’s triangle. This triangle is bound medially by the rectus abdominis muscle and inferiorly by the inguinal ligament.

The course of the inferior epigastric vessels can often be visualized at laparoscopy as the lateral umbilical fold because of the absence of the posterior rectus sheath below the arcuate line (Fig. 7-3).5 Injury to these vessels can result in life-threatening hemorrhage that must be quickly controlled by occluding the lacerated vessels with electrosurgery or precisely placed sutures.

If these vessels cannot be visualized (usually because of excess tissue), trocars should be place approximately 8 cm lateral to the midline and 8 cm above the pubic symphysis.3 On the right side of the abdomen, this point approximates McBurney’s point, located one-third the distance from the anterior superior iliac spine to the umbilicus. The corresponding point on left is sometimes referred to as Hurd’s point.

Peritoneal Landmarks

Peritoneal Folds

Several useful landmarks can be used to guide the laparoscopic surgeon to avoid injury to important retroperitoneal structures. Two midline and two bilateral pairs of peritoneal folds can usually be seen on the anterior abdominal wall at laparoscopy (Fig. 7-4). The falciform ligament, which is the remnant of the ventral mesentery and contains the obliterated umbilical vein in its free edge, can be seen in the midline above the umbilicus extending to the liver. The median umbilical fold, which contains the urachus, can usually be seen in the midline below the umbilicus extending to the bladder. Although the urachus normally closes before birth, it should be avoided during secondary trocar placement, both because it can be difficult to penetrate and in rare cases can remain patent to the bladder.

On each side of the urachus lie the medial umbilical folds. These landmarks contain the obliterated umbilical arteries and extend from the umbilicus to the anterior division of the internal iliac artery. Lateral to these, the lateral umbilical folds can be seen in 82% of patients.5 These are the most important structures to the laparoscopist, because they contain the inferior epigastric vessels and knowing their location can help the laparoscopist avoid injury to these large vessels during placement of secondary laparoscopic ports.

Peritoneal pouches normally exist between the pelvic organs (see Fig. 7-4). The vesico-uterine pouch is located anteriorly between the uterus and bladder. The ventral margin of the bladder can be visualized in approximately half of patients behind the anterior abdominal wall peritoneum and is important for secondary trocar placement, especially after previous abdominal surgery.5 The dorsal bladder margin can often be visualized on the anterior uterus and is used as a landmark during dissections during hysterectomy.

The recto-uterine pouch (pouch of Douglas) is located between the anterior surface of the rectum and the posterior surface of the vagina, cervix, and uterus. Endometriosis often involves the recto-uterine pouch and in severe cases, completely obliterates it. Inferiorly, an extraperitoneal fascial plane called the rectovaginal septum extends from the recto-uterine pouch to the perineal body. It lies between the posterior wall of the vagina and anterior wall of the rectum, and when involved with endometriosis can be felt on pelvic examination as nodularity.

UPPER ABDOMEN

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