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Hepatic, biliary and pancreatic anatomy
The aim of this chapter is to provide the basic anatomical foundation for performing liver, biliary and pancreatic surgery. Surgically unimportant anatomical features are omitted, but anatomical distortions due to pathological processes are included. A key point of hepato-pancreato-biliary (HPB) surgical anatomy is that whilst there is a prevailing pattern of anatomy, i.e. a pattern that is most commonly found, variations from the prevailing pattern termed anomalies are frequent. Every surgical operation in this area should be conducted with this fact in mind.
Liver
Overview of hepatic anatomy and terminology
Modern hepatic anatomy is concerned mainly with internal vascular and biliary structures rather than surface markings. Ramifications of the hepatic artery and bile ducts are regular and virtually identical. The portal vein on the left side of the liver is a vessel with unusual morphology, consequent to its need to perform different functions in the foetus and in the postnatal period. Consequently, the Brisbane 2000 Terminology of Hepatic Anatomy and Resections of the International Hepato-Pancreato-Biliary Association used in this chapter is primarily based on hepatic artery and bile duct ramifications.1
Divisions of the liver based on the hepatic artery
The primary (first-order) division of the proper hepatic artery is into the right and left hepatic arteries (Fig. 2.1). These branches supply arterial inflow to the right and left hemilivers or livers (Fig. 2.2). 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. 2.2). The right liver usually has a larger volume than the left liver (60:40), although this is variable.
Figure 2.1 Ramification of the hepatic artery in the liver. The prevailing pattern is shown. The first-order division of the proper hepatic artery is into the right (a) and left (b) hepatic arteries, which supply right and left hemilivers (Fig. 2.2), respectively. The second-order division of the hepatic arteries, supplies the four sections (Fig. 2.3). The third-order division, shown in orange, supplies the segments (Fig. 2.4). Since the left medial section and segment 4 are the same, the artery is shown as being both sectional and segmental (red/orange). The caudate lobe is supplied by branches from (a) and (b). Bile duct anatomy and nomenclature is similar to that of the hepatic artery. © Washington University in St Louis.
Figure 2.2 Nomenclature for first-order division anatomy (hemilivers or livers) and resections. © Washington University in St Louis.
The second-order divisions (Figs 2.1 and 2.3) 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. 2.1). The plane between these sections is the right intersectional plane. The right intersectional plane 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. 2.3), which are supplied by the left medial sectional hepatic artery and the left lateral sectional hepatic artery (Fig. 2.1). The plane between these sections is referred to as the left intersectional plane. It does have surface markings indicating its position – the umbilical fissure and the line of attachment of the falciform ligament to the anterior surface of the liver.
Figure 2.3 Nomenclature for second-order division anatomy (sections) and resections including extended resections. © Washington University in St Louis.
The third-order divisions of the hepatic artery divide the right and left hemilivers into segments (Sg) 2–8 (Figs 2.1 and 2.4). Each of the segments has its own feeding segmental artery. The left lateral section is divided into Sg2 and Sg3. The pattern or 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 based on internal ramification of vessels. 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.
Figure 2.4 Nomenclature for third-order division anatomy (segments) and resections. © Washington University in St Louis.
Segment 1 (caudate lobe) is a distinct portion of the liver, separate from the right and left hemilivers (Fig. 2.5). It is appropriately referred to as a lobe since it is demarcated by visible fissures. It consists of three parts: the bulbous left part (Spiegelian lobe), which grips 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. Lying anterior and superior to the paracaval portion are the hepatic veins, which limit the upper extent of the caudate lobe2,3 (Fig. 2.5). The caudate receives vascular supply from both right and left hepatic arteries (and portal veins). Caudate bile ducts drain into both right and left hepatic ducts.3 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. Occasionally caudate veins are quite short and wide, and therefore must be isolated and divided cautiously. Commonly, these veins enter the IVC on either side of the midplane of the vessel, an anatomical feature that normally allows the creation of a tunnel behind the liver on the surface of the IVC without encountering the caudate veins. The ‘hanging manoeuvre’ is performed by lifting up on a tape placed through this tunnel (see below).
Figure 2.5 Schematic representation of the anatomy of the caudate lobe. The caudate lobe consists of three parts: the caudate process (CP), on the right, the paracaval portion anterior to the vena cava (PC) and the bulbous left part (Spiegelian lobe, SL). IVC, inferior vena cava; PV, portal vein; RHV, MHV, LHV, right hepatic, middle hepatic and left hepatic veins, respectively. © Washington University in St Louis.
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. 2.2). 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. 2.3). 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. 2.3). 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. 2.4). Resection of the caudate lobe can be referred to as a caudate lobectomy or resection of Sg1. It is always appropriate to refer to a resection by the numbered segments. For instance, it would be appropriate to call a left lateral sectionectomy a resection of Sg2 and Sg3.
Surgical anatomy for liver resections
Hepatic arteries and liver resections
In the prevailing anatomical pattern, the coeliac artery terminates to divide into splenic and common hepatic arteries. Rarely, the hepatic artery arises directly from the aorta. 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 and terminates by dividing into the right and left hepatic arteries, the right immediately passing behind the common hepatic duct. The terms “common” and “proper” in respect to hepatic arteries while correct are not intuitive and the arteries are sometimes confused in the literature. The four sectional arteries arise from the right and left arteries 1–2 cm from the liver. While this is the commonest pattern, variations from this pattern are also very common (Fig. 2.6). The surgeon is wise not to make assumptions regarding hepatic arteries based on size or position, but rely instead on complete dissection, trial occlusions and radiological support. When an artery appears unusually large it is especially important to dissect until identification is unquestionable.
Figure 2.6 A most dangerous arterial anatomy. The right hepatic artery (RHA) arises from the gastroduodenal artery (GDA). There is no proper hepatic artery. The left hepatic artery (LHA) could easily be mistaken for the proper hepatic artery. Ligation of the GDA would lead to arterial devascularisation of the right liver. © Washington University in St Louis.
‘Replaced’ 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 patients, 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. It runs from left to right behind the lower end of the common bile duct 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. 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, these vessels need to be preserved and perfused during donor hepatectomy. Sometimes there is no proper hepatic artery because the entire liver is supplied by right or left replaced arteries or both. This anomaly may be suspected when, on opening the peritoneum at the base of the right side of the hepatoduodenal ligament, the portal vein is immediately apparent instead of the hepatic artery.
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 cholangicarcinoma. The replaced right artery is sometimes invaded by pancreatic head tumours and is in danger of injury during pancreato-duodenectomy. This is only a brief description of replaced arteries. There are many variations of replaced arteries, especially on the right, depending on the relationships of the artery to the pancreatic head and neck, the bile duct and the portal vein.4
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 common hepatic duct. 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 side of the liver to be retained.
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, about 1 cm, is in an extrahepatic position. The prevailing pattern of bile duct drainage from the right liver is shown in Fig. 2.7a. 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. 2.7a). The sectional ducts unite to form the right hepatic duct, which unites with the left hepatic duct at the confluence to form the common hepatic duct.
Figure 2.7 Prevailing pattern (a) and important variations (b–d) of bile ducts draining the right hemiliver (see text). © Washington University in St Louis.
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. 2.7b) and the right anterior bile duct does so in 6% (Fig. 2.7c). 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 (usually the anterior one), a segmental duct or a subsegmental duct. A right bile duct unites with the common hepatic duct below the prevailing site of confluence in about 2% of individuals. Sometimes the duct unites with the cystic duct and then with the common hepatic duct. 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 common hepatic duct. 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.5 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 portal vein (‘Hjortsjo’s crook’),6 where it is in danger of being injured if the right anterior sectional pedicle is clamped too close to its origin (Fig. 2.8).
