Surgical and radiologic anatomy *

Gross anatomy

The liver lies under the cover of the lower ribs, applied to the undersurface of the diaphragm, and is astride the inferior vena cava (IVC) posteriorly. Most of the bulk of the liver lies to the right, where the lower border coincides with the right costal margin, but it extends as a wedge to the left of the midline between the anterior surface of the stomach and the left dome of the diaphragm. The liver is invested with peritoneum except on the posterior surface, where the peritoneum reflects onto the diaphragm, forming the right and left triangular ligaments. The posterior surface of the liver is triangular in outline, with its base to the right, and here the liver lying between the upper and lower leaves of the triangular ligaments is bare and devoid of peritoneum. Anteriorly, the convex surface of the liver lies against the concavity of the diaphragm and is attached to it by the falciform ligament, left triangular ligament, and upper layer of the right triangular ligament ( Fig. 1.1 ).

Behind the liver, the IVC is embraced in a groove on its posterior surface ( Figs. 1.2 and 1.3 ). The right adrenal vein is a short vessel that enters the IVC behind the bare area (see Fig. 1.2 ). Usually, there are no vena caval tributaries posteriorly.

The hepatic veins ( Figs. 1.4 and 1.5 ) drain directly from the upper part of the posterior surface of the liver at an oblique angle directly into the vena cava. The right hepatic vein, which is larger than the left and middle hepatic veins, has a short extrahepatic course of approximately 1 cm. The left and middle hepatic veins may drain separately into the IVC, but usually they are joined after a short extrahepatic course to form a common venous channel approximately 2 cm long that traverses to the left of the anterior surface of the IVC below the diaphragm (see Figs. 1.4 and 1.5 ). There are other hepatic veins in addition to the three major hepatic veins. The umbilical vein is single in most cases, running beneath the falciform ligament between the middle and left hepatic veins and emptying into the terminal portion of the left hepatic vein. Additional posterior and inferior draining hepatic veins with a short course into the anterior surface of the IVC are frequent and may be large ( Figs. 1.5 to 1.7 ).

Hidden within the external gross appearance of the liver is a detailed internal anatomy, an understanding of which is essential to the performance of a precise hepatectomy. This internal anatomy has been called the functional anatomy of the liver .

Functional surgical anatomy

The internal architecture of the liver is composed of a series of segments combining to form sectors separated by scissurae containing the hepatic veins ( Fig. 1.8 ). The internal structure has been clarified by the publications of , , and . The description by Couinaud is the most complete and the most useful for the operating surgeon, and this description is generally used in this book.

Essentially, the three main hepatic veins within the scissurae divide the liver into four sectors, each of which receives a portal pedicle. The main portal scissura contains the middle hepatic vein and progresses from the middle of the gallbladder bed anteriorly to the left of the vena cava posteriorly. The right and left parts of the liver, demarcated by the main portal scissura, are independent in terms of portal and arterial vascularization and of biliary drainage ( Fig. 1.9 ). These right and left livers are themselves divided into two by the remaining portal scissurae. These four subdivisions in are termed sectors (see Figs. 1.8 to 1.10 ).

The right portal scissura separating the right liver into two sectors—anteromedial or anterior and posterolateral or posterior—is almost in the frontal plane with the body supine. The right hepatic vein runs within the right scissura. The left portal scissura divides the left liver into two sectors. The left portal scissura is not within the umbilical fissure because this fissure is not a portal scissura and contains a portal pedicle. The left portal scissura is located posterior to the ligamentum teres and within the left lobe of the liver along the course of the left hepatic vein. The anterior sector of the left liver is composed of a part of the right lobe (segment IV) that is to the left of the main portal scissura and of the anterior part of the left lobe (segment III) (see Figs. 1.8 to 1.10 ). The left posterior sector is the only sector composed of one segment (segment II) ( Fig. 1.11 ).

At the hilus of the liver, the right portal triad pursues a short course before entering the substance of the right liver. ( Figs. 1.6 and 1.12 ). In some cases, the right anterior and posterior pedicles arise independently, and their origins may be separated by 2 cm ( Figs. 1.12 and 1.13 ). Indeed, it may appear as though the left portal vein arises from the right anterior branch (see also Fig. 1.48 ). On the left side, however, the portal triad crosses over approximately 3 to 4 cm beneath the quadrate lobe embraced in a peritoneal sheath at the upper end of the gastrohepatic ligament and separated from the undersurface of the quadrate lobe (segment IV) by connective tissue (the hilar plate). This prolongation of the left portal pedicle turns anteriorly and caudally within the umbilical fissure, giving branches of supply to segments II and III and recurrent branches to segment IV (see Figs. 1.8 , 1.9 , 1.14 , and 1.15 ). Beneath the quadrate lobe, the pedicle is composed of the left branch of the portal vein and the left hepatic duct, but it is joined at the base of the umbilical fissure by the left branch of the hepatic artery.

The branching of the portal pedicle at the hilus (see Figs. 1.8 , 1.12 , 1.14 , and 1.15 ), the distribution of the branches to the caudate lobe (segment I) on the right and left side, and the distribution to the segments of the right (segments V to VIII) and left (segments II to IV) hemilivers follow a remarkably symmetric pattern and allow separation of segment IV into segment IVa superiorly and segment IVb inferiorly (see Fig. 1.8 ). This arrangement of subsegments mimics the distribution to segments V and VIII on the right side. The umbilical vein provides drainage of at least parts of segment IVb after ligation of the middle hepatic vein and is important in the performance of segmental resection.

The caudate lobe (segment I) is the dorsal portion of the liver lying posteriorly and embracing the retrohepatic IVC. The lobe lies between major vascular structures. On the left, the caudate lies between the IVC posteriorly and the left portal triad inferiorly and the IVC and the middle and left hepatic veins superiorly ( Figs. 1.16 and 1.17 ). The portion of the caudate on the right varies, but is usually quite small. The anterior surface within the parenchyma is covered by the posterior surface of segment IV, the limit being an oblique plane slanting from the left portal vein to the left hepatic vein. Thus there is a caudate lobe (segment I) with a constantly present left portion and a right portion of variable size (see Figs. 1.16 and 1.17 ).

The caudate lobe is supplied by blood vessels and drained by biliary tributaries from the right and left portal triads. Small vessels from the portal vein and tributaries joining the biliary ducts also are found, usually two on the left side and one on the right. The right portion of the caudate lobe, including the caudate process, predominantly receives portal venous blood from the right portal vein or the bifurcation of the main portal vein, whereas on the left side the portal supply arises from the left branch of the portal vein almost exclusively. Similarly, the arterial supply and biliary drainage of the right portion is most commonly associated with the right posterior sectoral vessels or pedicle and the left portion with the left main vessels. The hepatic venous drainage of the caudate is unique in that it is the only hepatic segment draining directly into the IVC. These veins sometimes can drain into the posterior aspect of the vena cava if there is a significant retrocaval caudate component.

The posterior edge of the caudate lobe on the left has a fibrous component, which fans out and attaches lightly to the crural area of the diaphragm, but extends posteriorly behind the vena cava to link with a similar component of fibrous tissue from the posterior surface of segment VII and embraces the vena cava (see Figs. 1.2 C and 1.16 ). This ligament may be replaced, in whole or in part, by hepatic tissue, and the caudate may completely encircle the IVC and contact segment VII on the right side. A papillary process of the caudate lobe can be mistaken for an enlarged lymph node on computed tomography (CT) scan ( Fig. 1.18 ). To summarize:

• 1.

  The liver is divided into two hemilivers by the main hepatic scissura within which runs the middle hepatic vein.

• 2.

  The left liver is divided into two sectors by the left portal scissura within which the left hepatic vein runs (see Fig. 1.8 ). The posterior sector comprises only one segment (segment II), which is the posterior part of the left lobe. This is the only sector that comprises one segment (this is referred to as the left posterior section according to Strasberg [2000]). The anterior sector is divided by the umbilical fissure into two segments—a medial segment (the quadrate lobe) (segment IV) and a lateral segment (segment III), which is the anterior part of the left lobe.

• 3.

  The right liver is divided into two sectors by the right portal scissura containing the right hepatic vein. Each of these two sectors is divided into two segments: an anterior sector (segment V inferiorly and segment VIII superiorly) and a posterior sector (segment VI inferiorly and segment VII superiorly) (see Figs. 1.8 and 1.9 ).

• 4.

  Segment I (the caudate lobe) lies posteriorly embracing the vena cava, its intraparenchymal anterior surface abutting the posterior surface of segment IV and merging with segments VI and VII on the right ( Figs. 1.16 and 1.19 ).

  

Further details of segmental anatomy important in sectoral or segmental resection are described in Chapter 5 .

Lymphatic drainage

The lymphatic drainage of the liver and gallbladder is mainly to nodes in the hepatoduodenal ligament and along the hepatic artery and is shown in Fig. 1.20 .

Intrahepatic bile duct anatomy

The right and left livers are drained by the right and the left hepatic ducts, whereas the dorsal lobe (caudate lobe) is drained by several ducts joining both the right and left hepatic ducts. The intrahepatic ducts are tributaries of the corresponding hepatic ducts, which form part of the major portal triads that penetrate the liver, invaginating Glisson’s capsule at the hilus. Bile ducts usually are located above the corresponding portal branches, whereas hepatic arterial branches are situated inferiorly to the veins. Each branch of the intrahepatic portal veins corresponds to one or two bile duct tributaries joining to form the right and left hepatic ductal systems converging at the liver hilus to constitute the common hepatic duct.

The left hepatic duct drains the three segments (II, III, and IV) that constitute the left liver ( Fig. 1.21 ). The left hepatic duct traverses beneath the left liver at the base of segment IV, just above and behind the left branch of the portal vein; crosses the anterior edge of that vein; and joins the right hepatic duct to constitute the hepatic ductal confluence.

The right hepatic duct drains segments V, VI, VII, and VIII and arises from the junction of two main sectoral ductal tributaries ( Fig. 1.22 ). The right posterior sectoral duct has an almost horizontal course and is constituted by the confluence of the ducts of segments VI and VII (see Figs. 1.21 and 1.22 ). The duct then runs to join the right anterior sectoral duct as it descends in a vertical manner. The right anterior sectoral duct is formed by the confluence of the ducts draining segments V and VIII. The junction of these two main right biliary channels usually occurs above the right branch of the portal vein. The right hepatic duct is short and joins the left hepatic duct to constitute the confluence lying in front of the right portal vein and forming the common hepatic duct.

The caudate lobe is divided into right and left portions and a caudate process. Separate ducts may drain these parts of the lobe, whereas in 26% there is a common duct between the right portion of the caudate lobe proper and the caudate process and an independent duct draining the left part of the caudate lobe.

Extrahepatic biliary anatomy

The extrahepatic bile ducts are represented by the extrahepatic segments of the right and left hepatic ducts joining to form the biliary confluence and the main biliary channel draining to the duodenum. The accessory biliary apparatus comprises the gallbladder and cystic duct ( Figs. 1.23 and 1.24 ). The confluence of the right and left hepatic ducts occurs at the right of the hilar fissure of the liver anterior to the portal venous bifurcation and overlying the origin of the right branch of the portal vein (see Fig. 1.23 ). The extrahepatic segment of the right duct is short, but the left duct has a much longer extrahepatic course. The biliary confluence is separated from the posterior aspect of the quadrate lobe of the liver by the hilar plate, which is the fusion of connective tissue enclosing the biliary and vascular elements with Glisson’s capsule ( Fig. 1.25 ). It is possible to open the connective tissue constituting the hilar plate and display the biliary convergence and left hepatic duct ( Fig. 1.26 ).

Main bile duct and sphincter of Oddi

The main bile duct (see Fig. 1.23 ) is divided into two segments: The upper segment is called the common hepatic duct and is situated above the cystic duct, which joins it to form the common bile duct (see Fig. 1.24 ). The common duct courses downward anterior to the portal vein and is closely applied to the hepatic artery, which runs upward on its left, giving rise to the right branch of the hepatic artery, which crosses the main bile duct, usually posteriorly. The cystic artery, arising from the right branch of the hepatic artery, may cross the common hepatic duct posteriorly or anteriorly. The common hepatic duct constitutes the left border of the triangle of Calot. The commonly accepted definition of Calot’s triangle recognizes, however, the inferior surface of the right lobe of the liver as the upper border and the cystic duct as the lower. In this triangle runs the cystic artery, often the right branch of the hepatic artery, and occasionally a bile duct, which should be displayed before cholecystectomy. If there is a replaced or accessory common or right hepatic artery ( Fig. 1.27 ), it usually runs behind the cystic duct.

The common variations in the relationship of the hepatic artery and origin and course of the cystic artery to the biliary apparatus are shown in Fig. 1.28 . The union between the cystic duct and the common hepatic duct may be located at various levels. At its lower extrahepatic portion, the common bile duct traverses the posterior aspect of the pancreas running in a groove or tunnel. The retropancreatic portion of the common bile duct approaches the second portion of the duodenum obliquely, accompanied by the terminal part of the pancreatic duct of Wirsung.