Hepatic, biliary and pancreatic anatomy

Divisions of the liver based on the hepatic artery

The proper hepatic artery arises as a branch of the common hepatic artery. The primary (first-order) division of the proper hepatic artery is into the right and left hepatic arteries ( Fig. 1.2 ). These branches supply arterial inflow to the right and left hemilivers ( Fig. 1.3 ). The plane between the two distinct zones of vascular supply is called a watershed. The border or watershed of the first-order division is called the midplane of the liver . It intersects the gallbladder fossa and the fossa for the inferior vena cava (IVC) ( Fig. 1.4 ). The right hemiliver usually has a larger volume than the left hemiliver (60:40), although this is variable.

The second-order divisions ( Figs. 1.2 and 1.4 ) of the hepatic artery supply four distinct zones of the liver. Each is referred to as a section . The right liver is divided into two sections, the right anterior section and the right posterior section . These sections are supplied by the right anterior sectional hepatic artery and the right posterior sectional hepatic artery ( Fig. 1.2 ). The plane between these sections is the right intersectional plane , which does not have any surface markings to indicate its position. The left liver is also divided into two sections, the left medial section and the left lateral section ( Fig. 1.4 ), which are supplied by the left medial sectional hepatic artery and the left lateral sectional hepatic artery ( Fig. 1.2 ). The plane between these sections is referred to as the left intersectional plane , which is marked on the surface of the liver by the umbilical fissure and the line of attachment of the falciform ligament. The third-order divisions of the hepatic artery divide the right and left hemilivers into segments (Sg) 2–8 ( Figs. 1.2 and 1.5 ). Each of the segments has its own feeding segmental artery. The left lateral section is divided into Sg2 and Sg3. The ramification of vessels within the left medial section does not permit subdivision of this section into segments, each with its own arterial blood supply. Therefore, the left medial section and Sg4 are synonymous. However, Sg4 is arbitrarily divided into superior (4a) and inferior (4b) parts without an exact anatomical plane of separation. The right anterior section is divided into two segments, Sg5 and Sg8. The right posterior section is divided into Sg6 and Sg7. The planes between segments are referred to as intersegmental planes. The ramifications of the bile ducts are identical to that described for the arteries, as are the zones of the liver drained by the respective ducts.

Segment 1 (caudate lobe) is a distinct portion of the liver, separate from the right and left hemilivers ( Fig. 1.6 ). It is appropriately referred to as a lobe since it is demarcated by visible fissures. It consists of three parts: the bulbous left part (Spigelian lobe), which wraps around the left side of the vena cava and is readily visible through the lesser omentum; the paracaval portion, which lies anterior to the vena cava; and the caudate process, on the right. The caudate process merges indistinctly with the right hemiliver. The caudate lobe is situated posterior to the hilum and the portal veins (PVs). Lying anterior and superior to the paracaval portion are the hepatic veins, which limit the upper extent of the caudate lobe ^, ( Fig. 1.6 ). The caudate lobe receives vascular supply from both right and left hepatic arteries and PVs. Caudate bile ducts drain into both right and left hepatic ducts. The caudate lobe is drained by several short caudate veins that enter the IVC directly from the caudate lobe. Their number and size are variable, and they must be ligated when mobilising the caudate lobe from the vena cava. Commonly, these veins enter the IVC on either side of the midplane of the vessel, an anatomical feature that allows the creation of a tunnel behind the liver on the surface of the IVC without encountering the caudate veins. A ‘hanging manoeuvre’ can be performed by lifting up on a tape placed through this tunnel (see below).

Resectional terminology

The terminology of hepatic resections is based upon the terminology of hepatic anatomy. Resection of one side of the liver is called a hepatectomy or hemihepatectomy ( Fig. 1.3 ). Resection of the right side of the liver is a right hepatectomy or hemihepatectomy and resection of the left side of the liver is a left hemihepatectomy or hepatectomy. Resection of a liver section is referred to as a sectionectomy ( Fig. 1.4 ). Resection of the liver to the left side of the umbilical fissure is a left lateral sectionectomy. The other sectionectomies are named accordingly, e.g. right anterior sectionectomy. Resection of the right hemiliver plus Sg4 is referred to as a right trisectionectomy ( Fig. 1.4 ). Similarly, resection of the left hemiliver plus the right anterior section is referred to as a left trisectionectomy.

Resection of one of the numbered segments is referred to as a segmentectomy ( Fig. 1.5 ).

Hepatic arteries and liver resections

In the prevailing anatomical pattern, the coeliac artery terminates to divide into left gastric, splenic and common hepatic arteries. The common hepatic artery runs for 2–3 cm anteriorly and to the right to ramify into gastroduodenal and proper hepatic arteries. The proper hepatic artery enters the hepatoduodenal ligament and normally runs for 2–3 cm along the left side of the common bile duct (CBD) and terminates by dividing into the right and left hepatic arteries, the right immediately passing behind the common hepatic duct (CHD). The four sectional arteries arise from the right and left arteries 1–2 cm from the liver ( Fig. 1.7 ). While this is the commonest pattern, variations from this pattern are also very common. Thus it is imperative that the surgeon does not make assumptions regarding hepatic arteries based on size or position, but instead on complete dissection, trial clamping and intraoperative imaging. ‘Replaced’ and ‘aberrant’ arteries are surgically important anomalies. ‘Replaced’ means that the artery supplying a particular volume of liver is in an unusual location and also that it is the sole supply to that volume of liver. ‘Aberrant’ means the structure is in an unusual location. While the definition of ‘aberrant’ does not state whether the structure provides sole supply, it is usually considered to be synonymous with ‘replaced’ in respect to these arteries. ‘Accessory’ refers to an artery that is additional, i.e. is present in addition to the normal structure and as a result is not the sole supply to a volume. Consequently, ligation of an accessory artery does not result in ischaemia.

In about 25% of individuals, part or all of the liver is supplied by a replaced (or aberrant) artery. The replaced right hepatic artery arises from the superior mesenteric artery (SMA). It runs from left to right behind the lower end of the CBD to emerge and course on its right posterior border. It may supply a segment, section or the entire right hemiliver. Rarely, this artery supplies the entire liver and then it is called a replaced hepatic artery ( Fig. 1.8 ). The replaced left hepatic artery arises from the left gastric artery and courses in the lesser omentum in conjunction with vagal branches to the liver (hepatic nerve). As with the right artery, it may supply a segment, section (usually the left lateral section), hemiliver or very rarely the whole liver. Sometimes left hepatic arteries arising from the left gastric artery are actually accessory rather than replaced and exist in conjunction with normally situated left hepatic arteries. Knowledge of these particular arterial variations is of importance not only in hepatobiliary surgery, including transplantation, but also in gastric surgery and pancreatic surgery. Transection of the left gastric artery at its origin during gastrectomy may cause ischaemic necrosis of the left hemiliver if a replaced left artery is present. The same may occur on the right side as a result of injury to a replaced right artery. Also, and of particular importance, these vessels must be preserved and perfused during donor hepatectomy for transplantation.

Replaced arteries may confer an advantage during surgery. For instance, when a replaced left artery supplies the left lateral section, it is possible to resect the entire proper hepatic artery when performing a right trisectionectomy for hilar cholangiocarcinoma. The replaced right artery is sometimes invaded by pancreatic head tumours and is in danger of injury during pancreatico-duodenectomy. This is only a brief description of replaced arteries and there are many variations of replaced arteries, especially on the right, depending on the relationship of the artery to the pancreatic head and neck, the bile duct and the PV.

In performing hepatectomies by the standard technique of isolating individual structures instead of pedicles, it is critical to correctly identify the particular artery(ies) supplying the volume of liver to be resected. One important anatomical point is that an artery located to the right side of the bile duct always supplies the right side of the liver, but arteries found on the left side of the bile duct may supply either side of the liver. Therefore, when using the individual vessel ligation method, it is important to be aware of the position of the CHD. A trial occlusion of an artery with an atraumatic clamp should always be performed in order to be sure that there is a good pulse to the future remnant liver.

Bile ducts and liver resections

Prevailing pattern and important variations of bile ducts draining the right hemiliver

Normally only a short portion of the right hepatic duct, approximately 1 cm, is in an extrahepatic position. The prevailing pattern of bile duct drainage from the right liver is shown in Fig. 1.9a . The segmental ducts from Sg6 and Sg7 (called B6 , B7 ) unite to form the right posterior sectional bile duct and the segmental ducts from Sg5 and Sg8 ( B5 , B8 ) unite to form the right anterior sectional bile duct ( Fig. 1.9a ). The sectional ducts unite to form the right hepatic duct , which unites with the left hepatic duct at the confluence to form the CHD .

There are two important sets of biliary anomalies on the right side of the liver. The first involves insertion of a right sectional duct into the left bile duct. This is a common anomaly. The right posterior sectional duct inserts into the left hepatic duct in 20% of individuals ( Fig. 1.9b ) and the right anterior bile duct does so in 6% ( Fig. 1.9c ). In these situations there is no right hepatic duct. A right sectional bile duct inserting into the left hepatic duct is in danger of injury during left hepatectomy if the left duct is divided at its termination. Therefore, when performing left hepatectomy, the left hepatic duct should be divided close to the umbilical fissure to avoid injury to a right sectional duct.

The second important anomaly is insertion of a right bile duct into the biliary tree at a lower level than the prevailing site of confluence. Low union may affect the right hepatic duct, a sectional right duct, a segmental duct or a subsegmental duct. A right bile duct unites with the CHD below the prevailing site of confluence in about 2% of individuals. Sometimes the duct unites with the cystic duct and then with the CHD. The latter anomaly places the aberrant duct at great risk of injury during laparoscopic cholecystectomy.

Very rarely the right hepatic duct terminates in the gallbladder. This may be congenital or acquired. In the latter case, a gallstone has effaced a cystic duct which united with the right hepatic duct, giving the appearance that it joins the gallbladder. An extremely rare anomaly is the absent CHD. In these cases, the right and left hepatic duct enters the gallbladder and the duct emerging from the gallbladder runs downward to join with the duodenum. In the presence of these anomalies, which would be extremely difficult to detect, a complete cholecystectomy will result in ductal injury. These ducts should not be confused with ducts of Luschka (see below).

The right posterior sectional duct normally hooks over the origin of the right anterior sectional PV (‘Hjortsjo’s crook’), where it is in danger of being injured if the right anterior sectional pedicle is clamped too close to its origin ( Fig. 1.10 ).

Prevailing pattern and important variations of bile ducts draining the left hemiliver

The prevailing pattern of bile duct drainage from the left liver is shown in Fig. 1.11a . It is present in only 30% of individuals, i.e. variations (anomalies) are present in the majority of individuals. In the prevailing pattern, the segmental ducts from Sg2 and Sg3 ( B2 , B3 ) unite to form the left lateral sectional bile duct . This duct passes behind the umbilical portion of the PV and unites with the duct from Sg4 ( B4 ; also called the left medial sectional duct since section and segment are synonymous for this volume of liver). The site of union of these ducts to form the left hepatic duct lies about one-third of the distance between the umbilical fissure and the midplane of the liver. The left hepatic duct continues from this point for 2–3 cm along the base of Sg4 to its confluence with the right hepatic duct. Note that it is in an extrahepatic position and that it has a much longer extrahepatic course than the right bile duct. The extrahepatic position of the left hepatic duct is a key anatomical feature, which makes this section of duct the prime site for high biliary–enteric anastomoses.

The main anomalies of the left ductal system involve variations in the site of insertion of B4 ( Fig. 1.11b ), multiple ducts coming from B4 ( Fig. 1.11c ) and primary union of B3 and B4 with subsequent union of B2 ( Fig. 1.11d ). B4 may join the left lateral sectional duct to the left or right of its point of union in the prevailing pattern ( Fig. 1.11b ); in the former case, the insertion of B4 is at the umbilical fissure and in the latter the insertion may occur at any place to the right of the prevailing location up to the point where the left hepatic duct normally unites with the right hepatic duct. In the latter instance, which according to Couinaud is present in 8% of individuals, there is no left hepatic duct. Instead there is a confluence of three ducts, the left lateral sectional duct, B4 and the right hepatic duct, to form the CHD. These variations are important in split liver transplantation and in diagnosis and repair of biliary injuries.

The bile duct to Sg3 has been used to perform biliary bypass and can be isolated by following the superior surface of the ligamentum teres down to isolate the portal pedicle to Sg3. The technique is less commonly used now that internal endoscopic stenting has been developed.

Portal veins and liver resections

On the right side of the liver, the PV divisions correspond to those of the hepatic artery and bile duct, and they supply the same hepatic volumes. Therefore, there is a right PV that supplies the entire right hemiliver ( Fig. 1.12 ). It divides into two sectional and four segmental veins, as do the arteries and bile ducts. On the left side of the liver, however, the left PV is quite unusual because of the fact that its structure was adapted to function in utero as a conduit between the umbilical vein and the ductus venosus, whilst postnatally the direction of flow is reversed. The left PV consists of a horizontal or transverse portion , which is located under Sg4, and a vertical part or umbilical portion , which is situated in the umbilical fissure ( Fig. 1.13 ). Unlike the right PV, neither portion of the left PV actually enters the liver, but rather they lie directly on its surface. Often the umbilical portion is hidden by a bridge of tissue passing between left medial and lateral sections. This bridge of liver tissue may be as thick as 2 cm or be only a fibrous band. The junction of the transverse and umbilical portions of the left PV is marked by the attachment of a stout cord—the ligamentum venosum. This structure, the remnant of the fetal ductus venosus, runs in the groove between the left lateral section and the caudate lobe and attaches to the left hepatic vein/IVC junction.