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01 CHAPTER Handbook of Complex Abdominal Wall Anatomy and physiology of the abdominal wall Álvaro Robín Valle de Lersundi, MD, PhD Arturo Cruz Cidoncha, MD, PhD 1.1. Anatomy of the abdominal wall 1.1.1. Introduction The abdominal wall is delimited by muscle structures than can be classified in 5 anatomical areas: lateral, anterior, posterior, diaphragmatic and perineal (Table 1.1). We will describe the first four due to their relevance in surgical repair of complex abdominal wall. These groups of muscles are enclosed by several bone structures: last ribs, chondrocostal joints, xyphoid, pelvis and costal apophysis of lumbar vertebrae. Layers of the anterior and lateral abdominal wall include skin, subcutaneous tissue, superficial fascia, deep fascia, muscles, extraperitoneal fascia and peritoneum. Table 1.1. Muscular limits of the abdominal wall POSTERIOR ∙ Quadratus lumborum ∙ Psoas ∙ Iliac muscle LATERAL ∙ External oblique ∙ Internal oblique ∙ Transversus abdominis ANTERIOR ∙ Rectus abdominis ∙ Piramidalis SUPERIOR ∙ Diaphragm INFERIOR ∙ Perineal muscles 1.1.2. Muscles of the abdominal wall CONTENTS 1.1. Anatomy of the abdominal wall 1.2. Physiology y gy Muscles of the anterolateral wall Rectus abdominis The rectus abdominis (m. rectus abdominis) is a long and thick muscle that is extended from the anterolateral thorax to the pubis close to the midline (Figure 1.1). 1 Handbook of Complex Abdominal Wall Figure 1.1. External oblique Rectus abdominis The external oblique muscle (m. obliquus externus abdominis) is the most superficial and thickest of the three lateral abdominal wall muscles (Figure 1.2). Figure 1.2. External oblique muscle Cranially, the rectus abdominis muscles originates from 3 digitations that insert on the 5th-7th costal cartilages, the xyphoid process and costoxyphoid ligament. Caudally, it inserts on the anterior surface of the superior border of the pubis, symphysis and pubic crest. The muscle is packed between the anterior and posterior rectus sheaths, except on the lower third of the abdomen where it is only covered by the anterior sheath. Laterally, the rectus sheath merges within the aponeurosis of the external oblique muscles to form the linea semilunaris. Three to four horizontal tendinous structures, that closely adhered to the anterior rectus sheath, interrupt the rectus abdominis along its length. These intersections make difficult the dissection between the anterior rectus sheath and the muscle and explain why hematomas and abscesses mainly extend over the posterior rectus sheath. Table 1.2. Relevant characteristics of the rectus abdominis Characteristics ∙ Originates on the anterior surface of 5, 6, 7 costal cartilages and xyphoid ∙ Insertion on the anterior surface of pubis and symphysis of pubis ∙ Packed in their sheaths except in the lower third where the posterior sheath is missing ∙ Difficulty of dissection between the anterior rectus sheath and the muscle ∙ The most important muscle for flexion of the trunk ∙ Used for pedicled muscle flaps Its muscle fibers originate from the anterior surface of the lower eight ribs that intertwine with insertions of the serratus anterior and latissimus dorsi. It runs vertically and medially and the muscular tendinous boundary descends in such a way that, approaching the midline and also below the anterosuperior iliac spine, the muscle becomes completely aponeurosis. Inferiorly the aponeurosis bends in itself to form the inguinal ligament, which extends between the anterior superior iliac spine and pubic tubercle. Anteriorly the aponeurosis joins the aponeurosis of internal oblique and contributes to form the anterior rectus sheath and linea alba, crossing the midline with fibers of the other side. Insertion on the pubis is made through the pillars of the superficial inguinal ring, the inguinal ligament and Gimbernat´s ligament. Table 1.3. Relevant characteristics of external oblique Pyramidalis Characteristics The pyramidalis (m. pyramidalis) is a small muscle that can be found inferiorly overlaying the rectus (Figure 1.1). It is a rudimentary muscle that can be missing in 20% of subjects. It originates from the pubis and inserts into the lateral borders of linea alba, inferior to the umbilicus. 2 ∙ ∙ ∙ ∙ ∙ ∙ Originates from the anterior surface of the last 8 ribs The thickest muscle of the lateral abdominal wall Inferior and medial direction of fibers of muscle and aponeurosis The aponeurosis contributes to form anterior rectus sheath Boundary between muscle and aponeurosis It bends in itself to form the inguinal ligament Anatomy and physiology of the abdominal wall Some inferior fibers form the cremaster muscle at the level of the inguinal canal. Internal oblique The internal oblique muscle (m. obliquus internus abdominis) lies between the external oblique muscle and transversus abdominis (Figure 1.3). Posteriorly, it is covered by the latissimus dorsi. Figure 1.3. CHAPTER 01 There is an avascular layer of loose connective tissue between external oblique and internal oblique muscles that allows easy dissection between these muscle layers. Transversus abdominis Internal oblique muscle The transversus abdominis muscle (m. transversus abdominis) is the deepest of the three lateral abdominal wall muscles and runs in a horizontal direction (Figure 1.4). Fleshy in the middle and tendinous at the extremities, it covers the entire lateral half of the abdominal wall, from the spine to the linea alba. Figure 1.4. Transversus abdominis It originates from the thoracolumbar fascia (fascia toracolumbaris), anterior two thirds of the iliac crest, and lateral half of the inguinal ligament. The internal oblique muscle runs in a superomedial direction, perpendicular to the external oblique muscle. At the level of linea semilunaris, it becomes a wide aponeurosis. In the superior two-thirds the aponeurosis splits medially to form the anterior and the posterior rectus sheaths. In the inferior third, the aponeurosis does not split, joins the anterior aponeurosis of external oblique muscle and only comprises the anterior rectus sheath. The inferior border of the posterior rectus sheath is the arcuate line. The internal oblique inserts on the inferior border of the 10th to 12th ribs superiorly. Inferiorly, the internal oblique inserts with aponeurotic fibers from the transversus abdominis, forming the conjoined tendon, which inserts on the pubic crest. Table 1.4. Relevant characteristics of internal oblique Characteristics ∙ Originates from thoracolumbar fascia, iliac crest and inguinal ligament. ∙ Fibers run perpendicular to the external oblique muscle ∙ Easy dissection between the muscle layers of external oblique and internal oblique ∙ The aponeurosis does not form the posterior rectus sheath below the arcuate line ∙ It has insertions on the inferior border of 10th-12th ribs ∙ Involved in the composition of the conjoined tendon It originates from the thoraco-lumbar fascia, the iliac crest, the inguinal ligament and from the inner surface of the lower six costal cartilages, interdigitating with the insertions of fibers of the diaphragm (Figure 1.5). Posteriorly, the transversus abdominis also starts from a wide aponeurosis of insertion in the thoraco-lumbar fascia, over the cuadratus lumborum, which has been termed posterior aponeurosis of the transversus abdominis. In posterior component separation technique, the mesh can be laid on this posterior aponeurosis (Figure 1.6). The muscle runs horizontally and ends medially in a broad flat aponeurosis in the linea semilunaris, with a shape of a curve line with medial concavity. Above the arcuate line, the aponeurosis joins the internal oblique aponeurosis and the linea alba. In the superior third of the abdomen the fibers of the muscle extend behind the rectus abdominis muscle approaching the midline. Below the arcuate line, the aponeurosis of the muscle contributes to form the conjoined tendon with the internal oblique. At this level, the differentiation between the fibers of internal oblique and transversus is quite difficult. 3 Handbook of Complex Abdominal Wall Figure 1.5. Anatomical relationships between transversus abdominis and diaphragm It is not an easy task to separate the layer of the transversus abdominis from the internal oblique because the neurovascular bundles are intercalated in this space. Muscles of the posterior wall Quadratus lumborum The quadratus lumborum (m. Quadratus lumborum) is a flat and quadrangular muscle situated between the last rib and the iliac crest (Figure 1.7). It originates in the iliac crest and iliolumbar ligament and its fibers ascend to insert on the 12th rib and transverse apophysis of the first lumbar vertebrae. The lateral fibers are more vertical than the medial ones that are oblique. It is covered by the posterior aponeurosis of transversus abdominis muscle. Figure 1.6. Extension of the mesh following the plane behind the transversus abdominis and laying on the posterior aponeurosis of transversus abdominis and quadratus lumborum. 1: transversus abdominis; 2: mesh; 3: thoracolumbar fascia; 4: posterior fascia of transversus abdominis; 5: quadratus lumborum; 6: psoas Figure 1.7. Quadratus lumborum and psoas iliacus Iliopsoas Table 1.5. Relevant characteristics of transversus abdominis Characteristics ∙ Originates from thoracolumbar fascia, iliac crest, inguinal ligament and inner side of last 6 costal cartilages ∙ Fibers run horizontal ∙ The muscle fibers reach behind the rectus abdominis near the midline in the upper third ∙ In the retrocostal space, the retromuscular dissection can be extended between the peritoneum and diaphragm ∙ The lumbar neuro-vascular bundles come across the space between transversus abdomins and internal oblique ∙ Involved in the composition of the conjoined tendon 4 The iliopsoas muscle (m. iliopsoas) is made of 2 muscles: psoas major and iliacus. The iliacus muscle (m. iliacus) originates in the iliac fossa and its fibers descend to form the tendon of the psoas that inserts on lesser trochanter. The psoas major (m. psoas major) starts on the bodies and transverse apophysis of 1-4 lumbar vertebrae. Its fibers come down to join iliacus muscle to form the tendon of the psoas. It is interesting to remember the anatomic relationships of these muscles with the lumbar plexus. The iliohypogastric and ilioinguinal nerves run between quadratus lumborum and lateral border of psoas major. The lateral femoral cutaneous nerve arises from the convex border of the psoas major and the genitofermoral nerve emerges in the medial border of the psoas major at the level of the 4th lumbar vertebra. The obturator nerve runs the medial border of the psoas major before entering the lesser pelvis. The femoral nerve comes posteriorly between the psoas major and the iliacus muscle. CHAPTER 01 Anatomy and physiology of the abdominal wall complex way to form the anterior and posterior rectus sheaths and, weaving in the center, they constitute the linea alba (Figura 1.9). Figure 1.9. Myofascial limits of lateral abdominal muscles. T: transversus; IO: internal oblique; EO: external Oblique Diaphragm It is a dome-shaped flat and thin muscle that separates thorax from abdomen. The origins of the diaphragm are found along the lumbar vertebrae, inferior border of the ribs and sternum. It has a peripheral part of muscle and a central part (central tendon) of flat aponeurosis. The peripheral muscle is divided according to its origin: sternal, costal and lumbar. The sternal origin is made up of 2 small muscles that are attached to the posterior side of the xyphoid. The costal origin is made up of several wide muscles that originate in the internal surface of the 7-12 ribs. These fibers interlace with insertions of the transversus abdomnis in the last 6 ribs. The lumbar origin is made up of the pillars of the diaphragm. In lateral hernias, in the retrocostal dissection, following the retromuscular layer behind the transversus abdominis, a space between the diaphragm and the peritoneum can easily be created allowing the extension of the mesh in this position (Figure 1.5). 1.1.3. Other components of the abdominal wall The muscles previously described are enveloped by a fascia and has an aponeurosis of insertion. Linea alba The linea alba (linea alba) is a tendinous median line formed by the crossing and fusion of the anterior and posterior rectus sheaths made up of the intersection of the bilaminar aponeuroses of the three abdominal flat muscles. Different ways of decussation in the midline have been described without any surgical relevance so far. Rectus sheaths Commonly, the fascia has a thin connective tissue that makes the dissection difficult to perform (Figure 1.8). On the contrary, the aponeurosis works like an authentic tendon of insertion and has a very strong and resistant tissue. Figure 1.8. Drawing showing the concept of fascia and aponeurosis. F: fascia; M: muscle; A: aponeurosis The bilaminar aponeuroses of the three muscles of the anterolateral abdominal wall are flat tendons that contribute in a Rectus sheaths (vagina musculi recti abdominis) are fibrous sheaths that envelope recti abdominis on each side. In the superior two-thirds the rectus sheath is formed anteriorly by the external oblique aponeurosis and the anterior aponeurosis of the internal oblique muscle and, posteriorly, by the posterior aponeurosis of the internal oblique muscle and the aponeurosis of the transversus muscle. In the lower third, the aponeurosis of the external oblique, internal oblique and transversus abdominis pass anteriorly to form the anterior rectus sheath. At this level the rectus abdominis is only covered posteriorly by the transversalis fascia. The semicircular line that divides these areas of the abdomen is the arcuate line of Douglas (linea arcuata) and can be found to the mid-point between the umbilicus and the pubis. The semilunar line (linea semilunaris) or line of Spiegel is made up of the fusion of the aponeurosis of the 3 lateral flat muscles on the lateral border of the rectus muscle. It can be identified from the 9th costochondral arch to the pubis tubercle. 5 Handbook of Complex Abdominal Wall Fascia transversalis It is simply the inner fascia of the transversus abdominis muscle and was so-called by Sir Astley Cooper. The fascia transversalis is the deepest parietal layer situated over the parietal peritoneum. It is a fine and resistant lamina of connective tissue and is part of the intraabdominal fascia (fascia endoabdominalis). It is covered by the internal surface of the transversus abdominis muscle that closely adheres to it. obturator arteries or veins (obturatoria accesoria) or anastomotic vessels that connect external iliac or inferior epigastric vessels with the obturator vessels (r. obturatorius or corona mortis). Hemostasia can be achieved by electrocautery, other energy devices or with the stitch used for mesh fixation. Superficial fascias Between the skin and muscles there is a single layer of thin connective tissue above the umbilicus. When making the transversus abdominis release in the posterior component separation technique, the surgeon may make the dissection over or under the fascia transversalis. If the dissection over this fascia is made, the peritoneum will be protected by the fascia and will be more difficult to be torn. However the plane is irrigated by very small vessels that easily bleed. If the dissection is made under the fascia transversalis, this plane will be avascular but opening of the peritoneum will be more probable to occur. Below the umbilicus, the fascia is divided in a fatty outer layer (Camper´s fascia) and a deeper membranous layer (Scarpa´s fascia). Posteriorly, the fascia transversalis joins the thoracolumbar fascia (fascia thoracolumbalis) and, superiorly, blends the diaphragmatic fascia (fascia diaphragmatica). It is denser at the level of the posterior wall of the inguinal area and continues inferiorly with the endopelvic (fascia pelvica) and iliac fascias (fascia iliaca). Knowledge of the blood supply to the abdominal wall is important when planning incisions and surgical repair of complex abdominal wall. Cooper ligament The Cooper´s pecten ligament (lig. pectineum) is a fixed and rigid structure made up of the thickening of the upper branch of pubis periosteum, the sheath of the pectineous muscle, the ileopubic tract (inguinal ligament) and the insertion aponeurosis of the transversus abdominis muscle (Figure 1.10). Figure 1.10. Dissection on the cadaver of Cooper ligaments This fascia can be used to appropriately close the subcutaneous tissue after incisional hernias repair. 1.1.4. Vascular supply There is an inferior superficial vascularization of the abdominal wall provided by a superficial system made up of superficial epigastric, superficial circumflex and superficial pudendal arteries, branches of the femoral artery. The rest of the vascular supply to the abdominal wall is provided by deep vascular axes: • A vertical axis of the epigastric vessels along the posterior surface of the rectus muscle. • A horizontal axis made up of the last 6 intercostal and lumbar arteries. • A vertical lateral axis formed by ascending branches of the deep circumflex artery. Inferior epigastric artery The inferior epigastric artery irrigates the rectus muscle and the subcutaneous tissue and skin by means of a medial and lateral row of musculo-cutaneous perforators. It arises from the medial side of the external iliac artery, behind the deep inguinal ring, pierces the transversalis fascia and ascends cranially and medially enveloped in perivascular fatty tissue as far as the linea arcuata where it passes between the rectus muscle and the posterior rectus sheath. This ligament is the one of the most important sites for mesh fixation in incisional hernias. It can be easily found following laterally the retropubic space close to the iliac vessels. The surgeons who use this ligament to fix the mesh should be aware that some vessels could be found crossing the Cooper ligament in almost 20% of patients. These vessels are aberrant 6 When making the lateral retrorectus dissection below the linea arcuata, the surgeon should coagulate 2-3 medial branches to the midline from the inferior epigastric artery that may cause bleeding and the potential ligation of the inferior epigastric vessels. To avoid injury to these vessels, the dissection should elevate the fatty tissue that surrounds the vessels leaving below the peritoneum and the fascia transversalis. The deep epigastric artery finally divides above the umbilicus in 1-3 branches, which anastomose the superior epigastric artery branches. CHAPTER 01 Anatomy and physiology of the abdominal wall Several musculocutaneous perforators arise from the deep epigastric artery giving a metameric subdivision of lateral and medial rows of perforators, usually 4 above and 3 below the umbilicus. These perforators supply the subcutaneous tissue and skin. The genitofemoral nerve arises from the first and second lumbar nerves and contributes to the sensory innervation of the root of the external genital organs. It is also the motor nerve of the cremaster. Superficial epigastric artery The lateral cutaneous nerve of the thigh (the laterla femorcutaneous nerve), originating from the second lumbar, is not involved in the superficial innervation of the abdominal wall. It arises the external iliac artery, distal to inguinal ligament and ascends about 2 cm laterally to the linea semilunaris over the muscles in the subcutaneous tissue between the fascias of Camper and Scarpa. Intercostal and lumbar arteries These vessels come laterally along with the nerves in the space between the internal oblique and transversus muscle. They give several posterior perforators, medially and laterally. Some diaphragmatic branches from the internal thoracic artery anastomose the superior intercostal vessels. The lumbar arteries also anastomose branches of the deep circumflex iliac artery. 1.2. Physiology Veins The abdominal wall is a myofascial complex that can resist the continuous variation in intraabdominal pressure. The sheaths of the muscles are responsible for abdominal wall continence in rest, while the muscle contraction protects the wall from the sudden changes in intraabdominal pressure. The venous system is patterned parallel to arteries, accompanying the perforators and, subsequently, the main veins, by means of the thoracophrenic, phrenic, lower intercostal, lumbar and epigastric veins. The abdominal wall must be considered as a unitary functional system. Andres de Laguna wrote in 1535 that “this wall of the body is made up of eight muscles by means of which it attracts, holds back, prepares, expels and accomplishes many other functions”. They finally drain into the azygous, subclavian and iliac veins. 1.2.1. Protection 1.1.5. Innervation The lower intercostal nerves and the nerves of the lumbar plexus suply the innervation of the anterolateral abdominal wall. The last 6 intercostal nerves (T7-T12) and the iliohypogastric and ilioinguinal nerves run with the homologous vessels in the layer between the internal oblique and transversus abdominis. They all give motor and sensory innervation. The T7-T12 nerves divide in two branches. One branch runs along the cartilaginous costal margin, terminates in the wall of the thorax and innervates the internal intercostal space. These intercostal nerves are in a relatively protected position under the chondrocostal margin. The other abdominal branch crosses the cartilaginous costal margin and penetrates in the space between the internal oblique and transversus muscle. When the nerves come close to the linea semilunaris, they pierce the posterior rectus sheath slightly medial to its lateral margin, to innervate the rectus muscle. This lateral row of perforators must be preserved during the posterior component separation technique. The two abdominogenital nerves (iliohypogastric and ilioinguinal nerves) supply sensory innervation to the oblique area of the abdominal wall comprising the lower part of the iliac fossa, the inguinal region and part of the external genital organs. The motor branch of the iliohypogastric nerve reaches the inferior part of the rectus and the pyramidalis, while the ilioinguinal nerve terminates in the flat abdominal muscles. The muscles on both sides of the abdominal wall work synkinetically with their aponeuroses containing and protecting the abdominal contents. It also maintains the correct anti-gravitational position. 1.2.2. Body actions This myoaponeurotic system participates in several functions: • Flexion, extension and rotation of the trunk. In association with the diaphragm, lattissimus dorsi, trapezius, iliopsoas and quadratus lumborum muscles, combines upper and lower body movements, allowing coordinated movement, overall balance of the spine and weight shifts. • Increases of intraabdominal pressure by muscle contractions: - Micturition - Defecation - Cough - Valsalva - Delivery • Pelvic stabilization: Together with the psoas and the quadratus lumborum, the rectus abdominis muscle stabilizes the pelvis during walking, running and jumping. 1.2.3. Respiration Together with the diaphragm, the abdominal wall muscles work as a functional system in respiration. Abdominal muscular contraction pushes the viscera upwards and participates in expiration, both at rest and forced. The essential component is the transversus ab- 7 Handbook of Complex Abdominal Wall dominis muscle, which acts as an antagonist to the diaphragm. While the diaphragm contracts in inspiration, the muscles of the abdominal wall relax and vice versa. So that, the diaphragm and abdominal wall muscles work antagonistically during respiration, except in coughing or other actions that need an increase of intraabdominal pressure, when they act synergistically. In fact, the voluntary contraction of the abdominal wall muscles blocks respiration. 1.2.4. Specific functions In the inguinal region, the myoaponeurotic arch of the transversus abdominis and internal oblique protects the posterior wall of the inguinal canal. 1.2.5. Consequences of large abdominal wall defects The external oblique and internal oblique muscles are wall tensors and costal depressors; their main contribution is protection of viscera and help in the flexion and rotation of the trunk. The first consequence of loss of abdominal wall continence is the lack of protection of viscera and the decrease in intraabdominal pressure. The transversus abdominis muscle is also a tensor of the wall and a costal depressor of the ribs; it participates in expiration. In the presence of a large parietal defect, there is a creation a second cavity with respiratory, visceral, vascular and vertebral consequences. This second cavity has also been named loss of domain or abdominal volet. The rectus abdominis muscle is a tensor, act in flexion of the trunk; it contributes to stabilizing the pelvis during walking, protects the abdominal viscera and is active during forced respiration. The pyramidalis muscle is a tensor of the linea alba. The modification of synergism makes the great incisional hernias a respiratory disease with major concerns in ventilation, diffusion and perfusion. The quadratus lumborum muscle flexes the spinal column ipsilaterally. The mechanical separation of muscles also causes deterioration of the muscular part and sheaths leading to muscular atrophy and fibrosis. The psoas major muscle flexes and turns the thigh laterally and inclines the spine laterally. The alteration on the vascularization of viscera may produce impaired vena cava and portal venous return. Table 1.6. Summary of abdominal wall muscles Name 8 The diaphragm muscle is the main respiratory muscle; it flattens during contraction enlarging the thoracic cage. Origin Insertion Innervation Function Rectus abdominis ∙ Anterior surface 5-7 costochondral archs ∙ Xyphoid ∙ Pubis ∙ 7-12 intercostal ∙ ∙ ∙ ∙ Pyramidalis ∙ Pubis ∙ Linea alba, between umbilicus and pubis ∙ 12 intercostal ∙ Tension of linea alba External oblique ∙ External surface 5-12 ribs ∙ Rectus sheath and linea alba ∙ 5-12 intercostal ∙ Iliac crest ∙ ∙ ∙ ∙ Internal oblique ∙ Thoraco-lumbar fascia, iliac crest and inguinal ligament ∙ Rectus sheath ∙ 10-12 ribs ∙ 8-12 intercostal ∙ Iliohipogastric ∙ Ilioinguinal ∙ Descend of ribs ∙ Lateral and ventral flexion of the trunk ∙ Tension of the abdomen Transversus abdominis ∙ ∙ ∙ ∙ ∙ Linea alba ∙ Rectus sheaths ∙ ∙ ∙ ∙ 7-12 intercostal Iliohipogastric Ilioinguinal Genito-femoral ∙ Tension of the abdomen Psoas ∙ Iliac fossa ∙ 1-4 lumbar vertebrae ∙ Trochanter minor ∙ Femoral ∙ Lumbar plexus ∙ Flexion, rotation of thigh Quadratus lumborum ∙ Iliac crest ∙ Lumbar vertebrae ∙ 12 rib ∙ ∙ ∙ ∙ ∙ Descend of ribs ∙ Lateral flexion Diaphragm ∙ Vertebrae ∙ Arcuate ligaments ∙ 7-12 costal arches ∙ Phrenic center ∙ Phrenic Inner surface 7-12 ribs Thoraco-lumbar fascia Iliac crest Inguinal ligament 7-12 intercostal Iliohipogastric Ilioinguinal Genito-femoral Flexion of trunk Descend of thorax Pelvis elevation Tension of the wall Rotation of trunk Descend of thorax Lateral flexion Tension of the wall ∙ Separation of thorax from abdomen ∙ Respiration CHAPTER 01 Anatomy and physiology of the abdominal wall References Chevrel JP. 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