Deep and Superficial Inferior Epigastric Artery Perforator Flaps

Arterial Supply of the Flap ( Figures 57.1 ; see Fig 11.2 , Fig 11.3 , Fig 11.6 )

The lower abdominal flap can be vascularized by either the deep or the superficial inferior epigastric artery. The superficial circumflex iliac artery only vascularizes the lateral part of the lower abdomen. By definition, the DIEAP flap is perfused only via one or more perforators of the DIEA.

The blood supply of the anterior trunk is founded on the presence of two epigastric arcades, each linking the external iliac artery and the subclavian artery through the rectus abdominis muscle. Caudally, the deep inferior epigastric artery (DIEA) originates from the external iliac artery just proximal to the inguinal ligament. After running superomedially for a short distance, it approaches the posterior aspect of the rectus abdominis muscle and divides into two main branches. These branches enter the rectus muscle and follow a cranial intramuscular course to feed the muscle and overlying skin. The cephalad portion of the epigastric arcade is made of the internal mammary artery, which arises from the subclavian artery. The internal mammary artery continues in the abdomen as the superior epigastric artery (SEA) to finally coalesce in a watershed area midway between the xiphoid process and the umbilicus. In this region, multiple anastomotic channels exist between the superior and inferior epigastric systems. Laterally, the epigastric arcade anastomoses with the terminal branches of the lower six intercostal arteries as well as with the ascending branch of the deep circumflex iliac artery.

It is evident that the DIEA is relatively more important than the SEA. First, the DIEA is larger, with an external diameter of 3.4 mm at its point of origin as compared with 1.6 mm for the SEA. Second and more importantly, the vast majority of cutaneous perforators are located in the periumbilical region, inferior to the watershed anastomotic zone, and therefore in the DIEA territory. The branching pattern of the DIEA is summarized in Figure 57.2 . There are muscular branches to the rectus abdominis, myocutaneous perforators with major contribution to the muscle and minor supply to the skin, and, conversely, perforators that are mainly directed at the skin. Obviously, the latter constitute the target vessels to vascularize a DIEAP flap. Occasionally, one can see perforators going around the edge of the rectus abdominis and not through the muscle itself. These pararectal fasciocutaneous perforators pass either medially just next to the linea alba or laterally from the lateral segmental vessels through the external oblique aponeurosis.

After piercing the anterior rectus sheath, the different perforators take on a variable course through the subcutaneous fat. Some ascend vertically to the overlying skin, whereas others will have a more horizontal trajectory. Perforators usually arborize in a fractal manner until they join the subdermal plexus to supply it. The subdermal plexus effectively constitutes an intricate network of microvessels spanning the entire abdominal skin and allowing innumerable redundant connections. Through the subdermal plexus, we find the second level of anastomotic channels between the source arteries. This is where perforators from the DIEA will meet cutaneous branches of the SEA, lower intercostal arteries, deep and superficial circumflex iliac arteries, superficial inferior epigastric artery, superficial external pudendal artery, and contralateral DIEA perforators. The subdermal plexus is the supporting vascular framework of lower abdominal skin flaps and the reason why the same skin/fat island can be perfused by either the deep or the superficial inferior epigastric system. In fact, the subdermal plexus can be conceptualized as an interface between the relatively scarce vessels originating from the deep fascia and the dense network of terminal dermal microvessels. It may act as a random equilibrating system that distributes blood evenly across surrounding cutaneous areas. If all vessels feeding into the subdermal plexus are ligated except for one perforator, the area of irrigated skin would be determined by the capacity of the perforator to provide pressure into the subdermal system. Therefore, the diameter and branching pattern of the perforator become key elements in defining the clinical territory of perforator flaps. Of course, further anatomic and physiologic studies are warranted to compare this theory to the angiosome and choke vessel concept in order to help elucidate the intricacies of skin perfusion.

The superficial inferior epigastric artery (SIEA) is the second important blood supply provider to the lower abdominal skin. The SIEA is a direct cutaneous vessel, originating from the common femoral artery 2–3 cm below the inguinal ligament in 17% of cases or from a common trunk with the superficial circumflex iliac artery in 48% of patients. The artery is absent or hypoplastic in 35% of specimens, in which case it is often substituted by a large ascending branch from the superficial circumflex iliac artery. The SIEA courses in the subcutaneous fat towards the umbilicus where it often anastomoses with large periumbilical perforating arteries.

Dominant:

deep inferior epigastric artery

• Length: 16 cm (range 14–18 cm)

• Diameter: 3.5 mm (range 2–4 mm)

After taking off from the external iliac artery, the DIEA travels superomedially in between the peritoneum and a very thin fibrous layer that is an extension of transverse fascia. In its lower portion, it gives off its first branches directed to the pubic bone, the lower rectus abdominis, and the pyramidalis muscle. A variable amount of peritoneal branches can emerge, the most cranial one providing vascularization to the umbilical stalk. The DIEA then enters the rectus muscle by passing anterior to the arcuate line. As it penetrates the muscle, the DIEA generally divides into two major branches. The lateral branch is dominant in 50% of cases and the medial branch in 7%. There is equal caliber of both branches in 15% and one central axis with multiple side branches in 28%. During their cranial course, the medial and lateral divisions of the DIEA give off several muscular and myocutaneous side branches. Overall, although small-caliber cutaneous perforators are numerous, there is an average of five perforators with a diameter of 0.5 mm originating from each DIEA. Sometimes, perforators as large as 1.5 mm can be encountered. Box 57.1 presents key points on the typical location of perforators.

Box 57.1

Topographic Distribution of Musculocutaneous Perforators from the DIEA

• More than 90% of the major perforators are located within a 6 cm radius lateral and inferior to the umbilicus.

• There is a higher concentration of perforators in the middle and medial thirds of the muscle.

• The largest periumbilical perforators take origin from the terminal branches of the DIEA and are often located within the lowest tendinous intersection of the rectus abdominis.

• The lowest fifth of the muscle contains only sparse perforators and the veins are rarely of adequate size.

• Medial and lateral pararectal fasciocutaneous perforators are often of small caliber.

• The distribution of perforators on both sides of the abdomen is rarely symmetrical.

When the pedicle is dissected down to the lower lateral rectus border, a typical pedicle length of 10–14 cm is obtained, depending on the position of the perforator. Although this length is generally sufficient for the purpose of free tissue transfer, it can be increased by up to 4 cm if the pedicle is dissected to its origin. At this point, the diameter of the artery is 3–3.5 mm.

Dominant:

superficial inferior epigastric artery

• Length: 5 cm (range 4–7 cm)

• Diameter: 1.6 mm (range 0.75–3.5 mm)

After piercing the deep fascia, the SIEA passes superiorly and laterally in the femoral triangle, crossing the inguinal ligament at its midpoint and lying deep to Scarpa's fascia. The vessel continues on superomedially towards the umbilicus, penetrating Scarpa's fascia, well above the inguinal ligament to lie in the superficial subcutaneous tissue ( Fig. 57.3 ).

A recent anatomic study showed that the SIEA was identified in 72% of specimens, with an average diameter of 1.6 mm (0.75–3.5 mm) at the level of the inguinal ligament. In 58% of cases, the artery was present bilaterally. The length of the pedicle ranged from 4 cm to 7 cm.

Minor:

superior epigastric artery

• Length: 6 cm (range 4–8 cm)

• Diameter: 2 mm (range 0.5–2.5 mm)

Secondary arterial inputs to the lower abdominal flap come from the SEA, lower intercostal arteries, superficial and deep circumflex iliac arteries, and superficial external pudendal artery. Since the DIEAP and SIEA flaps are elevated as skin islands isolated on their respective dominant pedicle, the minor contributors are ligated during harvesting of the flap. If indicated, however, a minor pedicle can be dissected and anastomosed in addition to the dominant pedicle to extend the territory of the flap.

Venous Drainage of the Flap (see Figs 11.6 and 11.8 )

The venous anatomy of the lower abdominal area is similar to the arterial anatomy in that the different venae comitantes closely follow their paired arteries to the proximal vessels. Indeed, the small veins perforating the anterior rectus sheath drain into the deep inferior epigastric veins (DIEV) and then on to the external iliac vein. Similarly, the SIEA possesses two venae comitantes closely related to it, which drain into the common femoral vein or occasionally into the saphenous bulb. However, these veins are often small and can even be absent. On the other hand, there is a constant and often large superficial inferior epigastric vein (SIEV) located medial to the SIEA and more superficial ( Fig. 57.3 ). The SIEV is not accompanied by an artery and is a tributary of the greater saphenous vein close to the fossa ovale. It actually drains an extensive polygonal network of subcutaneous veins in the anterior abdominal wall. There exist anastomotic connections between this superficial system and perforating veins from the DIEV. The SIEV is considered by certain authors to be the preferential drainage path of the abdominal panniculus in the normal, non-surgical condition.

Flap Innervation (see Fig 11.2 , Fig 11.9 , Fig 11.10 )

The motor and sensory innervation to the abdominal wall is provided mainly by the intercostal nerves 7–12 with some contribution from the iliohypogastric and ilio-inguinal nerves. As shown in Table 57.1 , the lateral muscles are supplied by all eight nerves, whereas the more medial rectus abdominis and pyramidalis are innervated solely by the intercostals. The sensory innervation to the abdominal skin is derived from the lateral and anterior cutaneous branches of the intercostal nerves giving rise to segmental dermatomes. A small skin area immediately superior to the pubis is supplied by the anterior cutaneous branch of the iliohypogastric nerve.

Sensory:

The DIEAP flap can be innervated using one of the sensory intercostal branches entering the panniculus on its way to the skin, usually in company with a perforator from the DIEA (see Fig 11.2, Fig 11.9, Fig 11.10 ).

After traveling in the plane between the internal oblique and the transversus abdominis muscles along with their respective intercostal arteries and veins, the mixed intercostal nerves T7–T12 pierce the lateral edge of the rectus sheath and then run on the posterior surface of the muscle for a short distance. Less frequent the mixed nerves enter the muscle at the lateral border or its anterior surface. Posteriorly, they enter the rectus abdominis usually at the junction of the middle and lateral thirds ( Fig. 57.4 ). At that point, they arborize into muscular motor branches to the medial and lateral muscle and a pure sensory branch that perforates the anterior rectus sheath together with the perforator vessels to supply the overlying skin. The SIEA flap is not used as an innervated flap because harvesting a sensory nerve entails an intramuscular dissection.

Motor:

No muscle is included with these flaps.