Abdominal wall anatomy

INTRODUCTION

By anterolateral abdominal wall we mean that area limited by superiorly by the costal arch and the xiphoid process of the sternum, laterally by the middle axillary line and inferiorly by the iliac crests, the pubis and the pubic symphysis.

The abdominal wall is a structure organized in layers and is remarkably resistant considering its relative thinness. characterizes it.

Its multi-planar organization also fully fulfills the function of containment as well as of movement.

THE ABDOMINAL WALL BY LAYERS

Skin

The outer covering of the anterolateral abdominal wall includes skin, soft tissue, lymphatic and vascular structures, and segmental nerves. The outermost layer consists of the skin and subcutaneous fat (which can vary greatly in thickness depending on the individual).

The panniculus adiposus

The subcutaneous adipose layer is interspersed with various connective layers up to the muscle-tendon component of the abdominal wall such as the superficial adipose fascia (of Camper) and the deep membranous fascia (of Scarpa).

Muscles and tendons

The structural layers consist of the rectus abdominis muscles, external oblique muscles, internal oblique muscles, transverse and pyramidal muscles accompanied by their covering and boundary tendon structures.

The last layer of the abdominal wall

To conclude, immediately behind the musculotendinous structural layers is the pre-peritoneal space and the parietal peritoneum, which is a very thin membrane representing the last layer of the abdominal wall.

THE PARIETAL PERITONEUM AND PRE-PERION SPACE

The parietal peritoneum

The parietal peritoneum is the last layer of the abdominal wall before entering the abdominal cavity.

It is a very thin and transparent membrane, which is why you can see the small capillaries that run through it and see in transparency the thin adipose layer located in some regions of the pre-peritoneal space.

The parietal peritoneum is a tissue that is very fast in its cellular regeneration and at the histological level is classified as a serous epithelial tissue.

The endoabdominal parietal peritoneum slope is that which is in contact with the viscera and is extremely smooth and slippery.

This feature allows the viscera to not adhere to the abdominal wall.

In situations where the body is confronted with intra-abdominal infection(peritonitis) or where foreign bodies or nets are placed on the intra-abdominal surface of the parietal peritoneum following surgery ( laparoscopic repairs with intra-abdominal nets), the naturally non-stick function of the peritoneum may be hindered, with the risk of adhesion formation.

Because of its thinness, the peritoneal membrane is a large exchange surface at the cellular level and also conveys important protective immune functions such as the secretion of opsonins.

The pre-peritoneal space

The pre-peritoneal space is that space between the muscle-tendon barrier and the parietal peritoneum. It is a thin space and its thickness varies according to the region of the abdominal wall and the amount of pre-peritoneal fat present. The pre-peritoneal space is in fact The pre-peritoneal space is in fact occupied by a veil of adipose of a few millimeters, more represented in the subxiphoid region and in the inguinal regions. In some regions the pre-peritoneal space is devoid of adipose, especially in the retromuscular regions of the rectus, which makes it a virtual space in these areas, i.e. a collaborated space not occupied by any tissue.

The very thin thickness of the pre-peritoneal space and the fragility of the parietal peritoneum have for a long time limited the surgeon's access to this advantageous layer of work, fortunately this technical limit is now largely overcome thanks to the characteristics of the latest generation of robotic instruments.

R-TAPP Surgery

R-TAPP (robotic transabdominal preperitoneal) abdominal wall surgery takes advantage of the preperitoneal plane of work through millimeter access routes and the use of robotic instrumentation.

The advantages are those of placing the repair in an isolated space protected from contact with the viscera, poorly innervated at the pain level, without having to cut anteriorly the skin, without having to dissect the subcutaneous panniculus adiposus and without having to cut the muscular-tendinous barrier; all manoeuvres that had to be performed as a necessity in traditional surgery.

THE ANTERIOR AND LATERAL MUSCLES OF THE ABDOMEN

The rectus abdominis, pyramidal, external oblique, internal oblique, and transverse abdominis muscles bilaterally represent the anterior and lateral muscles of the abdomen that act in synergy to accomplish a variety of functions, such as generating positive pressure within the abdominal cavity.

Basic muscle tone provides containment to the viscera, maintains the overall shape of the abdomen, and contributes to proper posture.

In general, the state of contractility and release of the various components of the abdominal wall is profoundly correlated and synergistic with respiration.

Finally, some important functions such as defecation and emptying of the urinary bladder, childbirth, coughing and vomiting need an abdominal overpressure properly generated when needed, by the muscles of the abdominal wall.

THE RECTUS ABDOMINIS MUSCLES

Description

The rectus abdominis muscles are two large ribbons of muscle equipped with multiple horizontal segmentations and enveloped by flat tendinous surfaces called fasciae or aponeuroses.

These tendon fasciae protect the rectus muscles and keep them anchored to neighboring structures in a complex structural architecture both macroscopic and microscopic as we will see later.

The rectus abdominis muscles and their fasciae insert superiorly on the sixth and seventh costal cartilages; below, however, the rectus abdominis muscles insert on the pubis.

The central (medial) margin of each rectus abdominis muscle is bordered by the linea alba

The lateral margin of the rectus muscles borders laterally with the oblique muscles where the so-called semilunar line is outlined, especially in lean and muscular subjects.

The sheaths lining the rectus muscles

The rectus abdominis muscles are lined anteriorly and posteriorly by tendon sheaths.

These sheaths protect the muscle and help keep it in place during contraction.

The anterior sheath is continuous and thick over its entire surface from the ribs to the pubis. The posterior sheath of the rectus muscles, on the other hand, at a certain point becomes thinner or sometimes breaks off sharply in the subumbilical-pubic region, giving rise to the so-called Douglas line, below which the muscle is covered only by the fascia transversalis, extraperitoneal connective tissue and parietal peritoneum.

The sheaths of the rectus muscles are a direct continuation of the sheaths that line and lateral abdominal muscles (the external oblique muscle, the internal oblique muscle, and the transverse muscle). Each aponeurosis is split into two surfaces giving rise to a multilayered system where the tendinous fibers of the different surfaces are oriented at 90-degree angles (which greatly increases resistance to physical loads). Near the midline of the abdomen, the fibers of the sheaths also cross in the anteroposterior direction in addition to their 90-degree multiplanar angulation, giving rise to a robust tendon raphe named linea alba.

Vascularization

The rectus abdominis muscles are vascularized primarily by the inferior epigastric arteries and to a lesser extent by the superior epigastric arteries. There are also small terminal branches of the last three posterior intercostal arteries, the subcostal artery, the lumbar arteries, and the deep circumflex iliac artery that may provide some vascular contribution.

Innervation

The rectus abdominis muscles are innervated by the anterior terminal branches of the last seven thoracic spinal nerves.

Function

The rectus muscles give containment and tone to the abdominal wall, they also antagonize the lats allowing for torso flexion and correct posture.

PYRAMIDAL MUSCLES

The pyramidal muscles, located in front of the rectus muscles in their lower portion, are triangular in shape and are inserted by tendinous fibers on the anterosuperior margin of the pubis. They are also covered by the anterior sheath of the rectus muscles. Decreasing progressively upward, they reduce in width, insert medially on the linea alba, and terminate with an apex situated with some anatomical variability between the umbilicus and pubis.

Vascularization

The pyramidal muscle is vascularized by branches of the inferior epigastric artery.

Innervation

Innervation is from terminal branches of the subcostal nerve.

Actions

The pyramidal muscle acts by tensing the lower part of the abdominal wall and linea alba.

The linea alba

Linea alba is a tendon raphe, that is, a tendon whose fibers are centrally unified.

As the name suggests, it is white in color and has a precise microscopic weave of its fibers to increase its strength.

linea alba extends from thexiphoid apophysis of the sternum to the pubic symphysis.

Its tendinous fibers derive from the anterior and posterior sheaths of the rectus abdominis muscles, which in turn are a continuation of the sheaths of the other antero-lateral abdominal muscles.

The linea alba is sometimes externally perceptible in subjects with little panniculus adiposus and muscularity. Its width varies with respect to the level of measurement: it is wider above the umbilicus than below and reaches its maximum width at the level of the umbilicus (Rath et al. 1996).

The umbilicus

The umbilicus is a scarred fibrous remnant. It consists of skin and several fibrous layers that more internally adhere to the parietal peritoneum.

Its shape and insertion level compared to linea alba can vary subjectively, being on average lower in overweight subjects.

In fetal age, the umbilicus is transected by the vessels of the umbilical cord and closes a few days after birth. The remnants of the vessels conveyed by the umbilical cord remain visible on the posterior aspect of the abdominal wall even when atrophied.

THE EXTERNAL OBLIQUE MUSCLE

Description

The external oblique muscle is the most superficial and largest of the anterolateral abdominal muscles. It inserts on the last eight ribs interdigitating with the anterior dentate and great dorsal muscles. Medially, its sheath (or aponeurosis) continues into the sheaths of the rectus abdominis muscle. Inferiorly, however, the aponeurosis of the external oblique muscle becomes part of the inguinal ligament and inguinal canal.

Vascularization

The external oblique muscle is vascularized by branches of the posterior last intercostal arteries, the subcostal artery superiorly as well as the deep circumflex iliac artery inferiorly.

Innervation

The external oblique muscle is innervated by the terminal branches of the last five intercostal nerves and the subcostal nerve, arising from the anterior branches of the last six thoracic spinal nerves.

Function

The external oblique muscle takes part in generating intra-abdominal pressure and lateral torsion of the torso.

THE INGUINAL LIGAMENT

The inguinal ligament consists of the flap of the aponeurosis of the external oblique muscle extending from the anterior-superior iliac spine to the pubic tubercle. Being a flap of a flat tendon, it forms within it a shower that goes to form the floor of . At the level of the medial end of the inguinal ligament, near its insertion site on the pubic tubercle, some fibers extend posteriorly and laterally to insert on the pectine crest, constituting the lacunar ligament.

THE INTERNAL OBLIQUE MUSCLE

Description

The internal oblique muscle is located almost entirely posterior to the external oblique muscle, with respect to which it is thinner. It is inserted on the lateral portion of the inguinal ligament and on the iliac crest deeply compared to the insertion of the external oblique muscle. Posteriorly, the internal oblique muscle inserts on the thoracolumbar fascia.

Vascularization

The internal oblique muscle is vascularized by branches of the posterior intercostal arteries, subcostal artery, inferior epigastric artery, and deep circumflex iliac artery.

Innervation

The internal oblique muscle is innervated by the terminal branches of the last five intercostal nerves and the subcostal nerve. To a lesser extent it is also reached by the ileohypogastric and ileoinguinal nerves.

Function

The internal oblique muscle contributes to the maintenance of basic abdominal tone, increased intra-abdominal pressure, and lateral bending of the torso.

TRANSVERSE ABDOMINAL MUSCLE

Description

The transverse muscle is the deepest of the three of the lateral muscles of the abdominal wall. It inserts on the ileo-pectineal arch, iliac crest, thoracolumbar fascia and the last six costal cartilages where it interdigitates with the diaphragm. Anteriorly, the muscle becomes an aponeurosis whose fibers, when brought medially, cross with the other side at linea alba, with which they fuse. Some inferior fibers diverge downward and centrally together with fibers of the aponeurosis of the internal oblique muscle forming the conjoined tendon.

Vascularization

The transverse muscle of the abdomen is vascularized by branches of the posterior intercostal arteries, the subcostal artery, the superior and inferior epigastric arteries, the superficial and deep circumflex iliac arteries, and the posterior lumbar arteries.

Innervation

The transverse muscle of the abdomen is innervated by branches of the last five intercostal nerves, the subcostal nerve, and the ileohypogastric and ileoinguinal nerves.

Function

The transverse muscle of the abdomen primarily contributes to basic abdominal contractile tone and increased intraabdominal pressure.

THE CREMASTER MUSCLE

Description

The cremastery muscle (also known as the cremaster fascia) consists of rarefied muscle fibers, both striated and smooth, and connective tissue. envelop the spermatic cord or round ligament in women.

The cremasteric fascia originates primarily from the internal oblique and transverse muscles of the abdomen.

Vascularization

The remastere muscle is vascularized by a branch of the inferior epigastric artery.

Innervation

The cremastere muscle is innervated by the genital branch of the genitofemoral nerve.

Function

Contraction of the cremasterian muscle raises the testis as an involuntary reaction and probably for protective purposes. There is also the possibility that the cremastery muscle may play a role in thermoregulation of the testis.

THE INGUINAL CANAL

The inguinal canal is a natural passage between the muscular layers of the abdominal wall. It is an oblique pathway between an internal entry ring and a superficial exit ring. The inguinal canal allows the passage through the abdominal wall of the spermatic cord in men and of the round ligament in women, allowing these structures to reach the scrotum and the external genitalia respectively.

Thesuperficial inguinal ring is an orifice in the aponeurosis of the external oblique muscle located immediately above and lateral to the pubic crest. The ring is not round but is loosely triangular in shape, with the apex facing laterally toward the anterosuperior iliac spine.

Thedeep inguinal ring is an orifice in the transversalis fascia, located approximately 1 cm above the inguinal ligament lateral to the epigastric vessels and 1-2 cm in size (although physiologically, contraction of the internal oblique muscle may narrow its caliber to counteract any increase in intraabdominal pressure).

The inguinal canal develops obliquely from the deep inguinal ring to the superficial one: its length varies according to the age of the subject, although in the adult it is between 3 and 6 cm. The canal is delimited anteriorly by the aponeurosis of the external oblique muscle and by some fibers of the internal oblique muscle. Posteriorly, there is the reflex portion of the inguinal ligament, the conjoined tendon, and the fascia transversalis, which separate it from the pre-peritoneal fat and peritoneum. Superiorly, there are the arcuate fibers of the internal oblique and transverse muscles of the abdomen that form the conjoined tendon medially. Inferiorly, the floor of the inguinal canal is primarily formed by by the shower of the inguinal ligament.

In the newborn the two inguinal rings are almost superimposed and then progressively move apart with growth, determining a greater selective restraining effect on the increase of intra-abdominal pressure. intraabdominal pressure.

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