Applied Surgical Anatomy of the Anterior Approach to the Lumbar Spine

Applied Surgical Anatomy of the Anterior Approach to the Lumbar Spine ‌

Overview

The anterior approach to the lumbar spine involves three stages of dissection. The superficial stage consists of cutting the skin and subcutaneous tissues down to the peritoneum. Below the skin lies the linea alba, a fibrous structure in the midline that is identified most easily in the upper abdomen. Cutting the linea alba in the lower half of the abdomen exposes the rectus muscle, which can be separated by finger pressure. Beneath it is the posterior rectus sheath and peritoneum.

The anatomy of the intermediate stage, which involves packing away the bowel, is the anatomy of the abdominal cavity and is not included in this book.

The deep stage of dissection consists of mobilizing the retroperitoneal structures that lie anterior to the L4-5 and L5-S1 disc spaces. These structures include the aorta, vena cava, common iliac vessels, lumbar vessels, ureter, and presacral plexus.

Landmarks and Incision

Landmarks

The umbilicus lies superficial to the linea alba. It usually is about halfway between the pubic symphysis and the infrasternal notch, although it may be pulled lower in obese patients.

The linea alba is marked externally by a groove in the midline of the abdomen. It divides one side of the rectus abdominis muscle from the other. In the upper abdomen, it actually separates the two muscles; cutting through it leads directly down to the peritoneum, with neither muscle being exposed. Below the umbilicus, the linea alba is less distinct; it does not separate the two rectus muscles.

The pubic symphysis is the articulation between the two pubic bones in the midline of the body. It is a relatively immobile joint (secondary cartilaginous; Fig. 6-30).

Figure 6-30 Superficial aspect of the distal rectus sheath. Note that the fibers of the external oblique appear laterally.

Incision

The midline longitudinal incision arches around the umbilicus. Because the skin is mobile and loosely attached to the tissues immediately beneath it, it heals with a thin scar. The cleavage or tension lines below the umbilicus appear in a chevron pattern, with the apex of the V in the midline.

The skin of the anterior abdominal wall is supplied segmentally from T7 in the region of the xiphoid to T12 just above the inguinal ligament. These segmental nerves do not cross the midline. Therefore, midline incisions do not cut any major cutaneous nerves.

Superficial Surgical Dissection and Its Dangers

The long, flat rectus abdominis muscle extends along the length of the entire abdomen, split into two muscles in the upper half by the linea alba. The muscle is enclosed in a fascial sheath. Above the umbilicus, the sheath has three elements: The aponeurosis of the internal oblique splits to enclose the rectus muscle; the aponeurosis of the external oblique passes in front of the rectus to form part of the anterior sheath; and the aponeurosis of the transversus abdominis fascia passes behind to form part of the posterior sheath. The inferior margin of the posterior sheath is known as the semicircular line (semicircular fold of Douglas). Below the umbilicus, all three aponeuroses pass anteriorly, leaving a thin film of tissue posteriorly (Figs. 6-31 and 6-32).

The arrangement of the rectus sheath and the linea alba means that, in the upper half of the incision, the approach through fibrous tissue leads directly down to the peritoneum, whereas in the lower half, it leads to the rectus abdominis muscle. Because of this, it is easier to open the abdomen in the lower half of the incision (Fig. 6-33; see Fig. 6-32).

Figure 6-31 The anterior portion of the rectus sheath is resected, revealing the fibers of the rectus abdominis muscle. Distal to the semicircular line, the linea alba (which is shown elevated by sutures) overlies the muscle fibers of the rectus abdominis but does not separate them. Proximal to the semicircular line, the linea alba separates the rectus abdominis muscles by attaching to the posterior rectus sheath, which begins at the semicircular line.

Figure 6-32 A: The rectus abdominis muscle has been resected. The posterior aspect of the rectus sheath ends just distal to the umbilicus. Its distal edge is called the semicircular line. The linea alba attaches to the posterior rectus sheath, thus separating the rectus abdominis muscles proximal to the semicircular line. B: Cross section above the semicircular line. Note that the rectus abdominis muscles are enveloped by the posterior and anterior rectus sheaths and separated from each other by the linea alba. C: Cross section below the semicircular line. The rectus sheath exists only anteriorly. Posteriorly is the transversalis fascia and peritoneum.

Figure 6-33 The posterior rectus sheath has been removed to reveal the peritoneum and the abdominal viscera.

The inferior epigastric artery supplies blood to the lower half of the rectus abdominis muscle. The artery lies between the muscle and the posterior part of the rectus sheath. If the surgical plane remains in the midline, this vessel should escape injury. If the artery is damaged when the rectus muscle is mobilized, it can be tied with impunity.

Deep Surgical Dissection and Its Dangers

Deep surgical dissection consists of freeing the distal ends of the aorta and the vena cava from the vertebrae in the L4-5 vertebral area. The aorta divides on the anterior surface of the L4 vertebra into the two common iliac arteries. Just below this bifurcation, the common iliac vessels divide in turn at about the S1 level into the internal and external iliac vessels. The

internal iliac is the more medial of the two (Fig. 6-34).

The aorta and vena cava are held firmly onto the anterior parts of the lower lumbar vertebrae by the lumbar vessels. These segmental vessels must be mobilized to permit the aorta and vena cava to be moved (see Fig. 6-12 ). Because the arterial structures are easier to dissect and mor e muscular than are the thin-walled venous structures, the preferred approach to the L4-5 disc space is from the left, the more arterial side. The median sacral artery originates from the aorta at its bifurcation at L4 and runs in the midline, over the sacral promontory and down into the hollow of the sacrum (see Fig. 6-35). The lumbosacral disc usually lies in the V that is formed by the two common iliac vessels. Nevertheless, the level at which the vessels bifurcate may vary; on rare occasions, they may have to be mobilized to expose the L5-S1 disc space.

Note that the left common iliac vein lies below the left common iliac artery, whereas the right common iliac artery lies below and medial to the right common iliac vein. Therefore, special care must be taken when mobilizing the left side of the vascular V, because the vessel closest to the surgery is the thin-walled vein, not the artery (Fig. 6-35; see Fig. 6-34).

A diffuse plexus of nerves exists in the presacral area. The nerves anterior to the L5-S1 disc are part of the superior hypogastric plexus which receives sympathetic innervation from T11 to L3 via the sympathetic chain and a plexus of nerves around the aorta. Injury to these nerves may cause ejaculatory dysfunction in men (retrograde ejaculation). More distally the plexus of nerve has parasympathetic contributions from the pelvic nerves (S2, S3, S4) which are important for erectile function in men. These nerves are more at risk with surgeries anterior to the mid and lower sacrum as well as low rectal and prostate procedures (see Figs. 6-34 and 6-35).16

Figure 6-34 The abdominal viscera have been retracted proximally, and the retroperitoneum has been resected to reveal the great vessels at their bifurcation, the ureters, and the presacral (superior hypogastric) plexus.