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Anterior abdominal wall
The anterior abdominal wall constitutes a curved hexagonal area defined superiorly by the costal arches (margins) and xiphisternal junction, laterally by the mid-axillary line, and inferiorly by an imaginary line running along the iliac crests, inguinal ligament and pubic symphysis. It is continuous with the posterior abdominal wall and paravertebral tissues, forming a flexible sheet of skin, muscle and connective tissue across the anterior and lateral aspects of the abdomen. It is also contiguous with the respiratory diaphragm and with the bony and myofascial structures of the thorax. These tissues form the complex structures of the myopectineal orifice and the inguinal canal, which connect the peritoneal cavity to the scrotum in males or to the labia majora in females. The anterior abdominal wall maintains the shape of the abdomen and aids in numerous physiological functions. However, its dysfunction is equally notable, as hernia repair remains the single most common type of operation performed by general surgeons.
Skin and Soft Tissue
The integument of the anterior abdominal wall comprises skin, soft tissues, lymphatic and vascular structures, and segmental nerves. The outer layer is formed from the skin and subcutaneous fat. The skin is non-specialized and variably hirsute, depending on sex and ancestry. All postpubertal individuals have some extension of the pubic hair on to the skin of the anterior abdominal wall, though this is commonly most pronounced in males, where the hair can extend to the umbilicus in a triangular pattern. The subcutaneous fat of the abdominal wall is highly variable in thickness, depending in part on sex and caloric intake.
Soft tissue
Superficial fascia
The superficial fascia of the abdominal wall lies between the dermis and the muscles, and is conventionally divided into a superficial fatty layer (Camper’s fascia) and a membranous layer (Scarpa’s fascia) deeper to this. In reality there are three layers, with a further layer of adipose tissue deep to the membranous layer (i.e. deep fatty layer) ( ). These three layers are particularly well defined in the child; the membranous layer remains well defined in adults. Lying within the superficial fascial planes are adipose tissue, blood vessels, lymphatics, nerves and, in the inguinal region, superficial inguinal nodes.
Superficial fatty layer
The superficial fatty layer contains a variable amount of fat that is partitioned by fibrous septa connecting the dermis with the deeper membranous layer. Inferiorly it is continuous with the superficial fascia of the thigh, and medially it is continuous over the linea alba. In males this layer continues over the external genitalia, where it becomes thin and pale red and contains very little adipose tissue. In the scrotum it also contains the smooth muscle fibres of dartos. In females it continues from the hypogastric region of the abdomen into the labia majora and perineum.
Membranous layer
The membranous layer is a variably developed entity composed of connective tissue and elastic fibres. In the adult its thickness varies over the anterior abdominal wall, becoming thinner in the proximal abdomen ( ). Measured histologically, it is between 0.5 and 1 mm thick but it appears thicker on computed tomography ( , ). It is loosely connected to the underlying aponeurosis of external abdominal oblique and the rectus sheath by oblique fibrous septa. Superiorly, it is continuous with the superficial fascia over the remainder of the trunk. In the midline it is adherent to the linea alba and pubic symphysis. Inferiorly, it fuses with the iliac crest, and extends superficial to the inguinal ligament and fuses with the fascia lata at the inguinal crease. In males it extends onto the dorsum of the penis, forming the fundiform ligament of the penis, and onto the scrotum where it becomes continuous with the membranous layer of the superficial fascia of the perineum (Colles’ fascia). In females it continues into the labia majora and is continuous with the superficial fascia of the perineum.
In boys the testis can frequently be retracted out of the scrotum into the loose areolar tissue between the membranous layer of superficial fascia over the inguinal canal and the aponeurosis of external abdominal oblique.
Deep fatty layer
The thickness of the deep fatty layer is more variable than the superficial fatty layer. It is thin or absent where the membranous layer fuses with bony prominences and the linea alba, and can become markedly thick in some morbidly obese individuals. Its adipocytes show different metabolic activities from those in the superficial fatty layer ( ). Liposuction preferentially removes this layer of fat with relative preservation of the superficial fatty layer in order to avoid skin dimpling and other skin contour irregularities ( ).
Transversalis fascia
The transversalis fascia is a thin layer of connective tissue lying between the deep surface of transversus abdominis and the extraperitoneal fat. It is part of the general layer of thin fascia between the peritoneum and the abdominal wall. Posteriorly, it fuses with the anterior layer of the thoracolumbar fascia; anteriorly, it forms a continuous sheet. Superiorly, it blends with the fascia covering the inferior surface of respiratory diaphragm. Inferiorly, it is continuous with the iliac and pelvic parietal fasciae. It is attached to the iliac crest between the origins of transversus abdominis and iliacus, and to the posterior margin of the inguinal ligament as a discrete thickening known as the iliopubic tract ( ); it consists of transverse fibres that fan out laterally towards the anterior superior iliac spine to blend with the iliopsoas fascia and run medially to the pubic bone. The iliopubic tract is a landmark structure during laparoscopic inguinal hernia repair, although it is often incorrectly described as the inguinal ligament. Medial to the femoral sheath the transveralis fascia is fused to the pubis behind the conjoint aponeurosis (conjoint ‘tendon’). An inferior extension of the fascia forms the anterior part of the femoral sheath. A further thickening of the transversalis fascia, the interfoveolar ligament, runs inferior to the inguinal ligament at the medial margin of the deep inguinal ring; it may contain muscle fibres.
The transversalis fascia continues as the internal spermatic fascia over the structures that pass through the deep inguinal ring (the testicular vessels and ductus deferens in males and the round ligament of the uterus in females).
Extraperitoneal connective tissue
The extraperitoneal connective tissue lying between the peritoneum and the fasciae lining the abdominal and pelvic cavities contains a variable amount of fat. This adipose tissue is the most common content of small congenital umbilical and epigastric hernias (see below). The fat is especially abundant on the posterior wall of the abdomen around the kidneys but scanty proximal to the iliac crest and in much of the pelvis. The inferior epigastric vessels themselves are contained within an envelope of fat as they travel from the external iliac vessels to rectus abdominis. This fat pad serves as an anatomical landmark during various types of abdominal hernia repairs. An abundance of fat in the extraperitoneal space also enables a peritoneal flap to be created easily: such a flap can be used to cover mesh during hernia surgery (a so-called extraperitoneal or ‘pre-peritoneal’ hernia repair). Areas with little fat, where the peritoneum and transversalis fascia are in direct contact with one another, are difficult to separate, commonly resulting in rents in the peritoneum that must be carefully repaired.
Vascular supply and lymphatic drainage
Understanding the blood supply of the anterior abdominal wall is critical when planning incisions, creating lipocutaneous or myofascial flaps or reconstructing the abdominal wall during ventral hernia repair ( Fig. 60.1 ). FLOAT NOT FOUND
Surgical zones have been defined to better understand this vascular distribution. Zone 1 is the central proximal abdomen. Superiorly, it receives blood supply from the superior epigastric artery, a branch of the internal thoracic artery. Inferiorly it is supplied by the inferior epigastric artery, a branch of the external iliac artery. As the superior and inferior epigastric arteries run posterior to rectus abdominis they supply musculocutaneous perforating vessels (the so-called periumbilical perforator vessels) to the overlying tissues. The superior and inferior epigastric arteries converge in the proximal umbilical region. Zone 2 encompasses the hypogastric region distal to the arcuate line (semicircular line of Douglas). The area is supplied medially by superficial and deep branches of the inferior epigastric artery. Laterally, the blood supply arises from the superficial circumflex iliac artery as a branch of the femoral artery. Zone 3 is the area proximal to the arcuate line and lateral to the semilunar line (linea semilunaris). It is perfused inferiorly by the deep circumflex iliac artery and superiorly by the musculophrenic artery, a branch of the internal thoracic artery.
Superior epigastric artery and veins
The superior epigastric artery is a terminal branch of the internal thoracic artery. It arises at the level of the sixth costal cartilage and descends between the costal and sternal parts of the diaphragm, accompanied by two or more veins that drain to the internal thoracic vein (see Fig. 60.1 ; Fig. 60.2 ). The vessels pass anterior to the distal fibres of transversus thoracis and the proximal fibres of transversus abdominis before entering the rectus sheath, where they run inferiorly behind rectus abdominis. They anastomose with the inferior epigastric arteries within rectus abdominis in one of several potential branching patterns ( ).
Vascular branches supply rectus abdominis itself and perforate the anterior layer of the rectus sheath to supply the abdominal skin and subcutaneous fat. These vessels generally penetrate rectus abdominis within a 10 cm circle around the umbilicus and are referred to clinically as ‘periumbilical perforator’ vessels. A branch given off in the proximal rectus sheath passes anterior to the xiphoid process of the sternum and anastomoses with a corresponding contralateral branch. This vessel can give rise to bleeding during surgical incisions that extend up to and alongside the xiphoid process. The superior epigastric artery also gives small branches to the anterior part of the diaphragm. On the right, small branches reach the falciform ligament, where they anastomose with branches from the hepatic artery.
Inferior epigastric artery and veins
The inferior epigastric artery (often referred to as the deep inferior epigastric artery in clinical practice in order to distinguish it from the superficial (inferior) epigastric artery) originates from the medial aspect of the external iliac artery just proximal to where the vessels pass deep to the inguinal ligament (see Figs 60.1 – 60.2 ; Fig. 60.3 ). Its accompanying veins, usually two, unite to form a single vein that drains into the external iliac vein ( ). The vessel curves forwards in the extraperitoneal tissue and ascends obliquely along the medial margin of the deep inguinal ring. It lies posterior to the spermatic cord, separated from it by the transversalis fascia. It pierces the transversalis fascia and enters the posterior layer of the rectus sheath by passing anterior to the arcuate line. In this part of its course it is visible through the parietal peritoneum of the anterior abdominal wall and with this covering forms the lateral umbilical fold. Disruption of the artery by surgical incisions (e.g. insertion of laparoscopic ports, surgical tacks or abdominal drains) can cause a haematoma, which can expand to considerable size because there is no adjacent tissue against which the bleeding can be tamponaded. FLOAT NOT FOUND
The inferior epigastric arteries ascend and anastomose with their superior counterparts as a single vessel in only about 30% of cases ( ). Branching into two vessels before anastomosing is the most common pattern, accounting for almost 60%, with a trifurcation in the remainder. The inferior epigastric arteries have an average diameter of approximately 3 mm at their origin, compared to an average diameter of 1.6 mm at the origin of the superior epigastric arteries, presumably explaining why the inferior epigastric arteries provide the ‘dominant’ blood supply to rectus abdominis. Preliminary ligation of the inferior epigastric artery is often performed when preparing a myocutaneous flap from the mid or lower rectus abdominis based on the superior epigastric artery; this encourages augmentation of the superior epigastric arterial supply.
Branches of the inferior epigastric artery anastomose with branches of the superior epigastric artery within rectus abdominis at a variable level proximal to the umbilicus ( ). Other branches anastomose with terminal branches of the distal five posterior intercostal, subcostal and lumbar arteries at the lateral border of the rectus sheath. Inferolaterally, branches anastomose with the deep circumflex iliac artery.
The ductus (vas) deferens in males or the round ligament of the uterus in females pass medially after hooking around the inferior epigastric artery at the deep inguinal ring. The inferior epigastric vessels form the lateral border of the inguinal triangle (Hesselbach’s triangle). The boundaries of this anatomical region are seen during laparoscopic inguinal hernia repair: the inferior border of the triangle is formed by the iliopubic tract/inguinal ligament and the medial border by the lateral margin of rectus abdominis. Hernias occurring within the triangle are termed direct inguinal hernias.
The inferior epigastric artery also gives off the cremasteric artery, a pubic branch and muscular and cutaneous branches. The cremasteric artery accompanies the spermatic cord in males, supplies cremaster and other coverings of the cord and anastomoses with the testicular artery. In females the artery is small and accompanies the round ligament of the uterus. A pubic branch, near the femoral ring, descends posterior to the pubis and anastomoses with the pubic branch of the obturator artery. The pubic branch of the inferior epigastric artery can be larger than the obturator artery and supply most of the territory of the obturator artery in the thigh, in which case it is referred to as the aberrant obturator artery ( ). It lies close to the medial border of the femoral ring and can be injured during medial dissection of the ring in femoral hernia repair, laparoscopic inguinal hernia repair, or with pelvic fractures. Muscular branches supply the abdominal muscles and peritoneum and anastomose with the circumflex iliac and lumbar arteries. Cutaneous branches arise within rectus abdominis, and perforate the aponeurosis of external abdominal oblique to supply the overlying skin. They anastomose with branches of the superficial epigastric artery, contributing to the ‘periumbilical perforator vessels’. These perforating vessels have been mapped in detail because they are particularly important in creating large lipocutaneous flaps on the abdominal wall: if this requires ligation of these vessels, the flap is at risk of necrosis ( ).
Occasionally, the inferior epigastric artery arises from the femoral artery. It then ascends anterior to the femoral vein, passing deep to the inguinal ligament and into the abdominal cavity before following its usual abdominal course. Rarely it arises from the external iliac artery in common with an aberrant obturator artery or from the obturator artery itself.
The superior and inferior epigastric arteries are important sources of collateral blood flow between the internal thoracic artery, the external iliac artery and the lumbar vessels when aortic blood flow is compromised. Additionally, small tributaries of the inferior epigastric vein draining the skin around the umbilicus anastomose with terminal branches of the umbilical vein that drain into the liver via the falciform ligament. These portosystemic communications can dilate substantially in patients with portal hypertension, resulting in portal venous blood draining into the systemic circulation via the inferior epigastric veins. Back pressure to the level of the skin leads to a pattern of dilated, serpiginous superficial veins radiating out from the umbilicus, referred to as the ‘caput medusae’.
Posterior intercostal, subcostal and lumbar arteries
In the paraspinal region, the tenth and eleventh posterior intercostal arteries, the subcostal artery and the lumbar arteries all pierce the posterior aponeurosis of transversus abdominis to enter the neurovascular plane of the abdominal wall running between transversus abdominis and internal abdominal oblique ( Fig. 60.4 ). The arteries on either side run forwards, giving off muscular branches to the overlying internal and external abdominal obliques, before anastomosing with lateral branches of the superior and inferior epigastric arteries at the lateral border of the rectus sheath, just medial to the semilunar line (see Fig. 60.4 ). The location of these arteries is of clinical importance when myofascial flaps are created during anterior abdominal wall reconstruction, not because of their vascular contributions but because of their accompanying nerves, which innervate rectus abdominis. Division of these nerves can result in a segmental denervation of portions of rectus abdominis and/or the lateral anterior abdominal wall musculature. Perforating vessels run vertically through the myofascial planes to supply the overlying skin and subcutaneous tissue. A small contribution to the supply of the distal abdominal muscles comes from branches of the deep circumflex iliac arteries.
The anterior abdominal wall is also supplied by branches of the femoral artery: namely, the superficial epigastric, superficial circumflex iliac and superficial external pudendal arteries, and by the deep circumflex iliac artery arising from the external iliac artery (see Fig. 76.4A ).
Lymphatic drainage
The highest concentration of lymphatic vessels in the anterior abdominal wall is found in the dermis ( ). Lymphatic vessels from the lumbar and gluteal regions run with the superficial circumflex iliac vessels, and those from the abdominal skin distal to the umbilicus run with the superficial epigastric vessels. Both drain into the superficial inguinal nodes. Lymphatic vessels from the proximal umbilical region drain to axillary and parasternal nodes. Enlargement of a lymph node in the umbilical region is rare but can herald an abdominal or pelvic malignancy.
The deep lymphatic vessels accompany the deeper arteries. Laterally, they run either with the lumbar arteries to drain into the lateral aortic nodes, or with the posterior intercostal and subcostal arteries to nodes in the posterior mediastinum. Lymphatics in the proximal anterior abdominal wall run with the superior epigastric vessels to parasternal nodes while those in the distal abdominal wall run with the deep circumflex iliac and inferior epigastric arteries to external iliac nodes. For further reading, see .
Segmental nerves
The ventral rami of the sixth to eleventh intercostal nerves, the subcostal nerve (twelfth thoracic) and first lumbar nerve (iliohypogastric and ilio-inguinal nerves) innervate the muscles and skin of the anterior abdominal wall ( ). The seventh to the twelfth thoracic ventral rami continue anteriorly into the abdominal wall ( Fig. 60.5 ). Approaching the costal arch, the seventh to tenth intercostal nerves curve medially across the deep surface of the costal cartilages between the digitations of the respiratory diaphragm and transversus abdominis. The subcostal nerve often gives a branch to the first lumbar ventral ramus (dorsolumbar nerve), and in these cases therefore contributes to the lumbar plexus ( Ch. 61 ). It accompanies the subcostal vessels along the inferior border of the twelfth rib, passing posterior to the lateral arcuate ligament and kidney, and anterior to the proximal part of quadratus lumborum.
All these segmental nerves run anteriorly within a thin layer of fascia in the neurovascular plane between transversus abdominis and internal abdominal oblique, where they branch and interconnect with adjacent nerves and give off branches to the lateral musculature ( ). Cutaneous branches supply the skin of the lateral and anterior abdominal walls. The nerves enter the rectus sheath at its lateral margin by penetrating the posterior lamina of internal abdominal oblique and penetrating rectus abdominis on its posterior aspect. Here they give off muscular branches to rectus abdominis, transversus abdominis and pyramidalis, and cutaneous branches that pierce the anterior layer of the rectus sheath to supply overlying skin.
The ninth intercostal nerve supplies skin proximal to the umbilicus, the tenth intercostal nerve supplies skin that consistently includes the umbilicus, and the eleventh intercostal nerve supplies skin distal to the umbilicus (see Fig. 24.10 ; Fig. 60.5 ). The subcostal nerve supplies the skin over the anterior gluteal region just distal to the iliac crest and the skin of the distal abdomen and inguinal region (overlapping with the L1 dermatome in this region) ( ). A typical dermatome map of the anterior abdominal wall is shown in Fig. 24.10 . The inferior intercostal nerves and subcostal nerve also provide sensory fibres to the costal parts of the diaphragm and parietal peritoneum.
The anterolateral abdominal wall muscles are innervated by several segmental nerves and injury to a single nerve rarely causes a clinically detectable loss of muscle tone. The overlap between sequential dermatomes means that significant cutaneous anaesthesia and muscle denervation are appreciated only after at least two sequential nerves have been surgically sectioned ( Fig. 60.6 ). FLOAT NOT FOUND
The transversus abdominis plane (TAP) blockade is a regional anaesthetic technique for abdominal surgery. It involves delivery of analgesic medication between internal abdominal oblique and transversus abdominis, thus providing local anaesthesia to the skin, muscles of the anterior abdominal wall and the parietal peritoneum, while avoiding the epidural-associated risks of central nervous system depression, urinary bladder retention, hypotension, or epidural catheter dislodgement. In abdominal surgeries, TAP blockade has been shown to reduce overall narcotic use (and narcotic-associated side effects), improve overall pain relief and reduce hospital length of stay.
TAP blockade can be performed ‘blind’(using anatomical landmarks and feeling a blunt needle ‘pop’ through myofascial planes) by using real-time sonography to visualize an appropriately positioned needle tip, or by direct visualization of a needle entering the correct plane with subsequent bulging of transversus abdominis as fluid enters this plane ( Fig. 60.7 ). FLOAT NOT FOUND
Muscles
Anterolateral muscles of the abdomen
Rectus abdominis, external and internal abdominal obliques, transversus abdominis and pyramidalis constitute the anterolateral abdominal musculature. They act together to perform a range of functions, some of which involve the generation of positive pressure within one or more body cavities. Although many of these activities can occur with no ‘forced assistance’, exhalation, defecation and micturition can be aided by the generation of positive intra-abdominal pressure. Parturition, coughing and vomiting always require such positive pressure. Under resting conditions, the tone developed within the muscles provides support for the abdominal viscera and retains the normal contour of the abdomen. The consequences of diminished muscular support can be seen in patients who have massive anterior abdominal wall hernias or congenital conditions such as ‘prune belly syndrome’ where there is deficiency or absence of these muscles.
Active contraction of the muscles is important for maintaining abdominal wall tone. The compression of the abdominal cavity required to increase intra-abdominal pressure is mainly effected by contraction of the respiratory diaphragm. The pelvic girdle, lumbar vertebrae and distal thorax provide rigidity to parts of the abdominal wall. During the generation of positive intra-abdominal pressure, the abdominal muscles hold the abdominal wall in a relatively fixed position rather than generating pressure directly: because most of the abdominal wall is muscular, the anterolateral abdominal wall muscles must contract synchronously to prevent displacement of the viscera.
The anterolateral abdominal muscles contribute little to the movements of the trunk during normal sitting and standing: these movements are controlled predominantly by the muscles of the back. However, movements of the trunk against resistance or when an individual is supine require the anterolateral abdominal muscles. Rectus abdominis is the most important in these situations, producing flexion of the trunk. If the pelvic girdle is fixed, the trunk is flexed. With a fixed thorax, contraction of rectus abdominis causes the pelvis to tilt and lift. Lateral flexion and rotation of the trunk against resistance is provided by unilateral contraction of external and internal abdominal obliques and transversus abdominis.
Rectus abdominis
Rectus abdominis is a paired, long, strap-like muscle that extends along the entire length of the anterior abdominal wall on either side of the linea alba ( Fig. 60.8 ). It varies widely in width and thickness from individual to individual but is generally widest in the proximal abdomen. The muscle fibres of rectus abdominis are usually interrupted by one to four fibrous bands (three being the most common) or tendinous intersections, which pass transversely or obliquely across the muscle. One is usually situated at the level of the umbilicus, another opposite the free end of the xiphoid process and the third about midway between the other two. They are rarely full-thickness and usually extend only half-way through the anterior thickness of the muscle, fusing with the fibres of the anterior layer of the rectus sheath. Occasionally, one or two incomplete intersections are present distal to the umbilicus. The intersections may represent the myosepta delineating the myotomes that form the muscle ( ).
The medial border of each rectus abdominis abuts the linea alba. Its lateral border is sometimes visible on the surface of the anterior abdominal wall as a gently curved groove, the semilunar line, which extends from the tip of the ninth costal cartilage to the pubic tubercle. In a muscular individual it is readily visible, even when the muscle is not actively contracting, but in many obese and non-obese individuals it can be completely obscured.
Attachments
Rectus abdominis arises by two tendons. The larger, lateral tendon is attached to the pubic crest and can extend beyond the pubic tubercle to the pecten pubis. The medial tendon interlaces with the contralateral muscle and blends with ligamentous fibres covering the front of the pubic symphysis. Additional fibres may arise from the distal part of the linea alba. The pubic attachment of the tendon of rectus abdominis and the anterior layer of the rectus sheath run over the anterior surface of the pubic symphysis and become continuous with the attachments of gracilis and adductor longus ( ). Superiorly, rectus abdominis is attached by three slips of muscle to the fifth, sixth and seventh costal cartilages. The most lateral fibres are usually attached to the anterior end of the fifth rib; occasionally this slip is absent or it extends to the third and fourth ribs. The most medial fibres are occasionally connected to the adjacent ligaments of the costosternal joint and the side of the xiphoid process.
Vascular supply
Rectus abdominis is supplied principally by the superior and inferior epigastric arteries, the latter being the dominant supply. Small terminal branches from the distal three posterior intercostal arteries, the subcostal artery, the lumbar arteries and the deep circumflex iliac artery can contribute, particularly at the lateral edges and the distal parts of the muscle, where they anastomose with small lateral branches of the epigastric arteries. Rectus abdominis provides a reliable and versatile myocutaneous flap, either pedicled or free, because of the excellent vascularity provided by the epigastric vessels and because the muscle belly can be separated from its surrounding sheath.
Without vascular division (a pedicled flap), the proximal half of the muscle can be used for breast reconstruction; the distal half can be transposed to the inguinal region or proximal thigh or rotated on its distal attachments into the perineum for reconstruction after radical pelvic and perineal resections. Because the inferior epigastric vessels are sufficiently large, the distal portion of rectus abdominis can be harvested as a free flap. The vessels are ligated at their junction with the external iliac vessels and are taken together with the distal portion of rectus abdominis. The overlying fascia, adipose tissue and skin can be taken as part of the flap as necessary to manage the soft tissue defect. The free flap is then revascularized by creating anastomoses between the epigastric vessels and the vessels near the defect that is being managed by placement of the flap.
Innervation
Rectus abdominis is innervated segmentally by the terminal branches of the ventral rami of the distal six or seven thoracic spinal nerves ( ). It can also receive a branch from the ilio-inguinal nerve ( ).
Actions
Rectus abdominis contributes to flexion of the trunk and the maintenance of abdominal wall tone required during straining.
Rectus sheath
Rectus abdominis on each side is enclosed by a fibrous sheath ( Figs 60.9 – 60.12 ). The anterior layer of the rectus sheath extends the entire length of the muscle and fuses with periosteum and ligaments at sites of the muscle’s attachments. The posterior layer of the rectus sheath is complete behind the proximal two-thirds of the muscle but absent distal to this level, which corresponds to approximately one-third of the distance between the umbilicus and the pubis ( ). The termination of the posterior layer of the rectus sheath is usually gradual but can be abrupt and marked by a clearly visible curved horizontal line known as the arcuate line.
The rectus sheath is formed from the aponeuroses of external and internal abdominal oblique and transversus abdominis. Each aponeurosis is bilaminar; the fibres from all three anterior leaves run obliquely upwards, whereas the posterior leaves run obliquely downwards at right angles to the anterior leaves. Proximal to the arcuate line, the anterior layer of the rectus sheath is composed of both leaves of the aponeurosis of external abdominal oblique and the anterior leaf of the aponeurosis of internal abdominal oblique fused together. The posterior layer of the rectus sheath is composed of the posterior leaf of the aponeurosis of internal abdominal oblique and both leaves of the aponeurosis of transversus abdominis. Thus, both the anterior and posterior layers of the rectus sheath consist of three layers of fibres, the middle layer running at right-angles to the other two. Distal to the arcuate line, all three aponeuroses from external and internal abdominal oblique and transversus abdominis pass into the anterior layer of the rectus sheath leaving only the transversalis fascia, extraperitoneal tissues (fat and connective tissue) and the parietal peritoneum to cover the posterior aspect of rectus abdominis (see Fig. 60.9 ). FLOAT NOT FOUND
At the midline, the anterior and posterior layers of the rectus sheath are closely approximated. Fibres from each layer decussate to the opposite side of the sheath, forming a continuous aponeurosis with the contralateral muscles; they also decussate anteroposteriorly, crossing from the anterior layer of the rectus sheath to the posterior layer of the rectus sheath. The dense fibrous line caused by this decussation is called the linea alba. External and internal abdominal oblique and transversus abdominis can therefore be regarded as digastric muscles with a central tendon comprising the linea alba ( ). The decussating fibres at the linea alba can be used to identify the midline during surgical incisions.
Linea alba and umbilicus
The linea alba is a tendinous raphe extending from the xiphoid process to the pubic symphysis and pubic crest. It lies between the left and right rectus abdominis and is formed by the interlacing and decussating aponeurotic fibres of external and internal abdominal oblique and transversus abdominis. At its distal end, the linea alba has two attachments to the pubis: superficial fibres are attached to the pubic symphysis, and deeper fibres form a triangular lamella that is attached behind rectus abdominis to the posterior surface of both pubic crests (adminiculum lineae albae). The pyramidalis is attached to the suprapubic part of the linea alba. The linea alba is visible externally in those who are lean and muscular as a shallow midline groove in the anterior abdominal wall. Its width varies along its length: it is wider superior to the umbilicus than inferior to it and generally widest at the level of the umbilicus ( ). It is wider and thinner in women, in adults aged over 50 years and in obese individuals and multiparous women ( ). Its tensile strength is proportional to its thickness and density ( ). The linea alba is relatively bloodless but it is crossed superficially from side to side by a few small blood vessels.
The umbilicus is a fibrous cicatrix that lies just distal to the midpoint of the linea alba and is covered by an adherent area of skin. It consists of skin, a fibrous layer (representing the area of fusion between the round ligament of the liver, the median umbilical ligament and two medial umbilical ligaments), the transversalis fascia, the umbilical fascia surrounding the urachal remnant, and peritoneum ( ). Its appearance and position are variable ( ). In adults, it tends to lie at a relatively more distal position with advancing age, in men and in individuals with a higher body mass index.
In the fetus the umbilicus transmits the umbilical vessels, urachus and, up to the third month of gestation, the vitelline or yolk stalk. It closes a few days after birth, but the vestiges of the vessels and urachus remain attached to its deep surface. The remnant of the embryonic left umbilical vein forms the round ligament of the liver. The obliterated umbilical arteries form the medial umbilical ligaments on the deep surface of the anterior abdominal wall: when covered with parietal peritoneum they form the medial umbilical folds. The partially obliterated remains of the urachus persist as the median umbilical ligament: when covered with parietal peritoneum, it forms the median umbilical fold. Both congenital and acquired umbilical hernias are common; most childhood umbilical hernias close spontaneously and do not require surgical repair. The umbilicus is the most common site for laparoscopic access to the peritoneal cavity.
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