The Rectus Sheath and Muscles of the Abdominal Body Wall: Implications for surgical incisions

Various clinical procedures require incisions of the body wall at different locations. The consequences of these incisions depends on the direction of muscle fibers and the location of neurovascular bundles. Accordingly, this exercise focuses on the anatomy of the abdominal body wall.

Objectives

Orientation

The rectus abdominis muscle runs on both sides of the midline from the rib cage to the pubis. Notice how the muscle fibers run from superior to inferior. Note that the midline is not covered by muscle. Confirm this by rotating the image to 25 degrees, 50 degrees, and 90 degrees. To understand the sheath of this muscle, we have to consider the lateral muscles of the abdominal wall.

Wrapping around the lateral body wall are three flat muscles: the external oblique, internal oblique and the transverse abdominis. Mouse over them on the cross sectional image. In the dissection window, note how the fibers of the external oblique travel inferiorly as they approach the rectus abdominis. Remove the right external oblique to expose the internal oblique In the dissection window, note how the fibers of the internal oblique travel superiorly as they approach the rectus abdominis. Switch back and forth between this and the previous link to convince yourself that the fibers of the external and internal obliques are roughly perpendicular to each other. Repeat this for the transverse abdominis . Be sure to examine the sectional windows as you add and subtract muscles.

What is the advantage of this crisscrossing pattern?

It's the same idea as plywood. By orienting each plie (muscle layer) in a different direction, force is exerted in multiple directions when the muscle contracts to creating a very strong wall.

How can make an incision here without injuring the muscles?

The surgeon splits the fibers of the muscles layer-by-layer. First, the external oblique is incision parallel to its fibers and the fibers are spread apart. This is a muscle-sparing incision, because very few fibers are damaged. To avoid dividing (cutting across) the fibers of the internal oblique, the orientation of the incision must be turned 90 degrees, and so on.

Below is a demonstration of the fiber-splitting technique.

The McBurney incision creates a window into the abdomen to access the appendix. The following image shows how McBurney's point is located along a line between the anterior iliac spine and the umbilicus. The appendix is the worm-like structure seen here below the cecum (brown).

Besides being a muscle-sparing incision, the McBurney incision places few nerves and vessels at risk of injury. View a dermatome map to see how a window-like incision at the "X" would affect only the T11 or T12 neurovascular bundle.

The Rectus abdominis and its sheath

Observe the fibers of the rectus abdominis, which lies parallel to the midline.

Which function of this muscle is enabled by this orientation?

Flexion (forward bending) of the trunk.

The rectus sheath is derived from the three lateral muscles discussed above. They are attached to the midline by a broad flat tendon. Typically, the tendons of muscles are tubular, but when they are broad and flat, we call them an aponeurosis. Add the aponeuroses now. Notice you can no longer see the fibers of the rectus, but there is still a gap at the midline. Confirm this by rotating the image 25 degrees to the left. The fibers of the aponeuroses of the right and left sides actually cross the midline to interweave with one another and form a very strong structure called the linea alba. The linea alba is hard to see in the sectional images, but has been placed in the crosshairs. Use mouse rollovers to define the extent of this thin structure.

The Anterior Rectus Sheath

Let's look at the rectus sheath a little more closely, by removing the layers of aponeurosis one by one. First, remove the external oblique and its aponeurosis on the left. Note how the rectus abdominis is still covered anteriorly by the aponeurosis of the internal oblique. Second, remove the internal oblique and its aponeurosis from the left, and the external oblique and its aponeurosis on the right. Note that the view of the fibers of the left rectus abdominis is unobstructed superiorly, but not inferiorly. The reason is that the aponeurosis of the transverse abdominis passes anteriorly to the rectus inferiorly, but posterior to the rectus superiorly. In contrast, the aponeuroses of the external oblique and internal oblique pass anterior to the entire length of the rectus abdominis. Remove the transversus abdominis and its aponeurosis. Quiz yourself by using mouse-overs to reveal the identity of all the structures in the dissection window.

The Posterior Rectus Sheath

If we remove the skeletal system and take a posterior view of the abdominal muscles, we can see the posterior rectus sheath. Mouse over the superior part of the rectus to identify the aponeuroses of the transverse abdominis. Mouse over the inferior part of the rectus to confirm that the sheath is absent. The transition occurs at the arcuate line. Now remove the transversus abdominis and its aponeurosis from the left. Note the appearance of the internal oblique and its aponeurosis. It forms part of the posterior sheath!! The thinking is that the aponeurosis of the internal oblique is two layers. Both layers travels superficial to the rectus abdominis inferior to the arcuate line, but one layer travels deep to the rectus abdominis superior to the arcuate line. Remove the internal oblique and its aponeurosis.

An anatomosis of two vascular bundles is found between the posterior rectus sheath and the rectus abdominis. To see them, we will go back to an anterior view. In principle, we should be able to see the superior and inferior epigastric arteries and viens by removing the anterior rectus sheath and rectus abdominis. Unfortunately, the software is not segmented that way and we removed all the rectus sheath. For clarity, the peritoneal fat was highlighted as a backdrop. Now, view the vessels from the posterior side with the fat and aponeuroses removed. The source of the superior epigastric is the subclavian via the intercostal artery. The source of the inferior epigastric is the external iliac. Both vessels become too thin to trace to their anastomosis with the other. Notice that they travel inferiorly/superiorly and do not cross the midline. These vessels also anastomose with the intercostal vessels from T7 to T12 as they course along the anterior abdomenal wall. (They are too small to see here, but T10 heads towards the umbilicus and T12 heads towards the pubic symphysis.) Consequently, the intercostal neurovascular bundles also fail to cross the midline.

Thought questions

A patient suffers abdominal trauma and the surgeon must enter the abdomen safely and quickly. Where can an incision be made that will not divide (cut) nerves, vessels, or muscles?

The midline through the linea alba.

The rectus abdominis can be used for reconstructing a breast following a mastectomy. The muscle is detached form its inferior insertion and the inferior epigastric vessels are ligated and divided. The muscle is then slid under the skin into the chest. How will the tissue receive a blood supply?

The superior epigastric vessels are still intact.

A Kocher incision is made parallel to the right subcostal margin (where the costal cartilages of ribs 7-10 fuse and travel superiomedially to meet the sternum). This gives access to the gall bladder and part of the liver. For greater access to the liver, the incision (Chevron incision) is lengthened from midaxillary line to midaxillary line, one inch inferior and parallel to the subcostal margin. Are these incisions muscle sparing or muscle dividing?

Unlike the McBurney incision (which separates muscle fibers), these long incisions must divide (transect) the external oblique and transversus abdominis (and for the Chevron, the rectus abdominis) muscles. As a result, a scar will form to reconnect the divided muscles, thereby weakening them.

Below is a demonstration.

Will the Chevron and Kocher Incisions divide any neurovascular bundles?

The neurovascular bundles of T7 - T10 will be divided, resulting in a loss of sensation along the anterior abdomen and atrophy of muscles distal to the incision.

To hide the scar of a Caesarean section, the Pfannenstiel incision is made transversely through the skin, just above the pubic symphysis (below the bikini line). The skin is spread apart and the direction of the incision is rotated 90 degrees to divide the midline. Why is the deep incision made in the superior/inferior direction?

The deep incision divides the linea alba, thereby avoiding the rectus abdominis and any neurovascular structures.

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