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I./1.2.: Intrahepatic anatomy of the liver – lobes and segments of the liver
I./1.2.1.: Principles of the description of internal hepatic anatomy
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Still up till present, the division of the liver into left and right lobes according to the asymetric outer hepatic morphology is in use. In the past, The border between the larger right lobe and the much smaller left lobe was considered to be in a plane determined by the hepatic attachment of falciform ligament, the fissure for ligamentum venosum and fissure for round ligament. However, the correct knowledge of the branching patterns of the intrahepatic vascular structures and the surgical practice required a new anatomical terminology which equally represents a new terminology for liver resections. The bases of the new division of the liver into its units are the branching patterns of the portal structures.
Surgical techniques of the hepatic interventions have made a great progress that resulted in an urgent need to make an end of the confusion concerning the anatomical and surgical terminological inexactitude and to create a uniform terminology. The world Hepato-Pancreato-Biliary Association established the Brisbane 2000 terminology of liver anatomy and liver resections.
According to the widely accepted new terminology, the boundary between the right and left hepatic lobes is a line that connects the middle of gallbladder bed and the left side of the inferior vena cava, known as Rex-Cantlie line. This distribution is based on the portions of the liver drained by the right and left hepatic ducts. The quadrate and caudate lobes belong to the left lobe, since both are drained by the left hepatic duct.
A new expression: the section was introduced to describe the intralobar portions which become especially imortant in partial liver resections and the partial liver transplantations. Subdivisions of the the sections, the liver segments are of surgical importance.
I./1.2.2.: Subdivisions of the right hemi-liver
In the right hemi-liver two sections can be distinguished according to the division of right branch of the portal vein: an anterior section (antero-medial sector) and a posterior section (posterolateral sector).
The branching pattern of the sectoral portal vein subdivides the anterior section into the segment V (anterior and inferior) and VIII (anterior and superior). Segments VI (posterior and inferior) and VII (posterior and superior) are subdivisions of the posterior section of the right hemi-liver.
I./1.2.3.: Subdivisions of the left hemi-liver
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In the left hemi-liver, two sections are distinguished according to the branching pattern of the left hepatic duct, since the primary division of the left portal vein does not follow the branching pattern of the left bile duct. Thus the new terminology divides the left hemi-liver into the left medial section and the left lateral section. The traditional Couinaud’s terminology described the medial section as part of the right lobe. The segments of the medial section are: the segment IV (left medial) and segment I (posterior-caudate lobe). The segment II (left lateral superior) and segment III (left lateral inferior) form the lateral section. According to Couinaud’s terminology, the lateral sector corresponded to the lateral lobe of the liver.
I./1.2.4.: Intrahepatic biliary tract
The site of confluence of the horizontally coursing shorter right hepatic duct and the longer left hepatic duct is located extrahepatic, in the porta hepatis, forming the common hepatic duct. The right hepatic duct branches into a right posterior sectoral duct and a right anterior sectoral duct. The anterior duct drains the anterior section of the right hemi-liver (V, VIII), while the right posterior duct drains the bile of the posterior section (VI, VII). In the left liver, the hepatic duct is formed by the junction of the segmental ducts II and III, while the duct of the segment IV generally joins the left hepatic duct.
I./1.2.5.: Proper hepatic artery
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The common hepatic artery originates from the celiac trunk, after giving off the gastroduodenal artery, it continues as the proper hepatic artery that turns upward in the hepatoduodenal ligament. In most cases, it bifurcates in front of the porta hepatis, however, bifurcation may occur anywhere from the distal portion of the hepatoduodenal ligament up to the porta. Most frequently, the bifurcation is situated in the left half of the hilum, thus the right hepatic artery has a longer course in the porta hepatis. About 50-70% of cases, the segment IV has a separate blood supply from the common hepatic, or the right hepatic artery, known as middle hepatic artery, which is a residual of the embryonic middle hepatic artery given off by the common hepatic artery.
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The right hepatic artery courses to the right (towards the triangle of Calot) in the hilar plate, posterior to the common hepatic duct and the right hepatic duct. The triangle of Calot is a surgically important landmark whose boundaries include the common hepatic duct medially, the cystic duct laterally and the cystic artery proximally (in most cases it branches off the right hepatic artery). This triangular space is dissected in order to identify, divide, and ligate the cystic duct and artery.
The left hepatic artery courses with the left hepatic duct and left portal vein on the undersurface of the left liver towards the basal part of the fissure for round ligament, where it enters the parenchyma. The arteries for the segments IV, II, III display intrahepatic course. The caudate lobe and caudate process receive blood supply both from the left and right hepatic arteries.
The above described main anatomic variation was observed in 60% of cases. Aberrant right hepatic artery may arise from the superior mesenteric artery, the aberrant left hepatic artery may originate from the left gastric artery. Accessory branches from the superior mesenteric and left hepatic arteries are frequently observed. Rarely the common hepatic artery is given off from the superior mesenteric artery.
I./1.2.6.: Portal vein
The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein behind the neck of the pancreas and also receives venous blood from the inferior mesenteric, gastric and cystic veins. The portal vein courses between the layers of the hepatoduodenal ligament where the right and left gastric vein (coronary vein) join it. At the porta hepatis, the portal vein bifurcates into right and left branches before entering the liver. In general, portal veins are found posterior to hepatic arteries and the bile ducts in their lobar and segmental distribution.
The right portal vein is situated anterior to the caudate process and it enters the liver parenchyma through the hilar plate and soon divides into anterior (for segments V and VIII) and posterior (for segments VI and VII) branches which further bifurcate into superior and inferior branches.
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The substantially longer left portal vein displays a 3-5 cm long transverse and an umbilical portion running with the left hepatic artery, below the left hepatic duct. At the level of the fissure for the round ligament (umbilical fissure) the wide umbilical part turns sharply in the direction of the round ligament lying into the umbilical fissure then joins the round ligament. In the umbilical fissure it gives off an intrahepatic branch for the segment II, then together branches for segments III and IV. The widened umbilical portion of the left portal vein is known as Rex’ sinus or portal sinus. For surgeons and for ultra sound detection, the identification of the Rex’ sinus is an important point of orientation. The caudate lobe (segment I) is mostly supplied by branches (two or more) from the transverse part of the left portal vein, however, the caudate lobe may receive branches also from the right portal vein or from main portal vein.
I./1.2.7.: Hepatic veins
Three large venous trunks assure the venous drainage of the liver: the right, left and middle hepatic veins which open in acute angle into the suprahepatic part of the inferior vena cava. In addition to these three veins, numerous smaller accessory veins may directly enter the inferior vena cava on the posterior hepatic surface. The large veins divide the liver parenchyma into four sectors. The course of these veins defines three planes, known as portal scissures: right left and main portal scissures. The left hepatic vein defines the left portal scissure dividing the left hemi-liver into an anterior sector (segments IV and III) and a posterior sector (segment II); the middle hepatic vein separates the segment IV from the segments V and VIII; the right hepatic vein separates the anteriorly located segments V and VIII from the posteriorly situated segments VI and VII.
The right hepatic vein is the longest one formed by an anterior and a posterior tributaries; the anterior branch drains the segments V and VI, while the posterior branch drains mainly the segment VII. The right main trunk has a 1 cm long extrahepatic portion befor it enters the inferior vena cava.
The middle hepatic vein courses along the Rex-Cantlie line, in the main portal scissure, and in 85% of cases a common trunk of the middle and left hepatic veins opens into the left anterior part of the inferior vena cava. The plane which is defined by the course of the middle hepatic vein corresponds to the border between the right and left hemilivers (true functinal lobes). This vein receives branches on the left from the segment IV and on the right from the segments VIII and V.
The left hepatic vein lies in the left scissure, in line with or just to the right of the falciform ligament. It is formed by the junction of the transversely running vein from the segment II and the sagitally coursing vein from segment III. Occasionally the segment IV is also drained by the left hepatic vein.
The venous drainage of the caudate lobe is unique, since its vein/s directly empty into the inferior vena cava.
There are some small veins draining additionally the the segments VI and VII which also directly empty into the inferior vena cava.
In 20-25% of cases one or more large accessory veins are present, which drain large part of segment VI and/or VII and open into the inferior vena cava through its anterior surface. This vein/s have a short extrahepatic portion, they are known as inferior hepatic vein/s (Makuuchi vein). During liver sugery, the identification of Makuuchi vein is essential.
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