Calot's triangle and biliary anatomy: the hepatocystic triangle, the cystic artery and safe cholecystectomy

One triangle, two definitions

Strictly, Calot's triangle and the hepatocystic triangle are not the same space. In his 1891 thesis Jean-François Calot described a triangle formed by the cystic duct, the common hepatic duct and the cystic artery. Modern surgical usage, and the critical view of safety, take the upper side as the inferior surface of the liver, which makes a larger triangle with the cystic artery inside it rather than along its edge. That larger space, the hepatocystic triangle, is what 'Calot's triangle' means in theatre and in almost every exam question. Its lower, lateral side is the cystic duct, running from the neck of the gallbladder downwards and to the left to join the right side of the common hepatic duct, which forms the medial side. Both its front and its back are covered by peritoneum, so the triangle is opened by dividing the peritoneum on each surface before the contents can be seen. In the operative view, with the fundus pushed up over the liver and Hartmann's pouch drawn laterally, it opens out into the shape its name suggests.

The cystic artery and the arteries that impersonate it

The cystic artery is usually a branch of the right hepatic artery, given off inside the triangle after the right hepatic artery has passed behind the common hepatic duct. It runs to the neck of the gallbladder above and slightly behind the cystic duct, often close to the cystic lymph node, and divides into a superficial branch on the peritoneal surface and a deep branch between the gallbladder and the cystic plate. It is the gallbladder's main arterial supply. Venous return is not by a single cystic vein but by small veins that pass straight into the liver through the gallbladder bed or join the portal vein. Variation is common enough that it should be expected. The artery can arise from the left hepatic artery, the hepatic artery proper, the gastroduodenal artery or a replaced right hepatic artery, and an artery that arises outside the triangle reaches the gallbladder by crossing in front of the common hepatic or common bile duct. There may be two cystic arteries, so finding one does not mean the dissection is finished.

Caterpillar (Moynihan's) hump

A tortuous right hepatic artery that loops forward into the triangle close to the gallbladder neck and gives off a very short cystic artery. The hump is easily mistaken for the cystic artery itself; clipping it devascularises the right liver.

Replaced or accessory right hepatic artery

Arises from the superior mesenteric artery (in roughly 10–20% of people, depending on the series), runs up behind the portal vein and to the right of the bile duct in the back of the free edge, and can enter the triangle from below. A large pulsatile vessel running up the right side of the triangle at cholecystectomy is this artery until proved otherwise.

Low-lying or anterior cystic artery

A cystic artery from the hepatic artery proper, the left hepatic or the gastroduodenal artery crosses in front of the bile duct to reach the gallbladder. It lies outside the triangle and is divided only after it has been traced to the gallbladder wall.

Replaced left hepatic artery

Arises from the left gastric artery and runs in the lesser omentum towards the fissure for the ligamentum venosum. It plays no part in the triangle, but it is not controlled by a Pringle manoeuvre.

The biliary tree from the porta hepatis to the papilla

The right and left hepatic ducts leave the liver at the porta hepatis, the transverse fissure between the quadrate lobe in front and the caudate lobe behind. There the ducts are the most anterior structures, the branches of the hepatic artery lie in the middle and the portal vein divides behind them. The ducts unite just outside the liver to form the common hepatic duct, which descends for a few centimetres before the cystic duct joins it to form the common bile duct, about 7.5 cm long. The bile duct runs down in the free edge of the lesser omentum, passes behind the first part of the duodenum, lies in a groove on or a tunnel through the back of the head of the pancreas, and enters the posteromedial wall of the second part of the duodenum obliquely. Within the wall it usually joins the main pancreatic duct to form the hepatopancreatic ampulla (of Vater), which opens at the major duodenal papilla under the control of the sphincter of Oddi. Small subvesical ducts (of Luschka) run in the cystic plate and drain liver tissue, not the gallbladder; dividing one as the gallbladder is lifted off its bed causes a bile leak despite secure clips on the cystic duct. An aberrant right posterior sectoral duct may join the common hepatic duct low down or open into the cystic duct itself, and is at risk whenever the cystic duct is divided close to its junction.

The free edge of the lesser omentum and the epiploic foramen

The common bile duct, the hepatic artery proper and the portal vein travel to the liver together in the hepatoduodenal ligament, the thickened free right edge of the lesser omentum. The bile duct lies anteriorly on the right, the hepatic artery proper anteriorly on the left and the portal vein behind both, with lymphatics, nodes and the hepatic nerve plexus around them. Behind the free edge is the epiploic foramen (of Winslow), the only natural communication between the greater sac and the lesser sac. A finger passed into the foramen has the portal triad in front of it and the inferior vena cava behind. Pinching the free edge between finger and thumb, or across it with a soft clamp, is the Pringle manoeuvre, which stops hepatic arterial and portal venous inflow to the liver. Bleeding that continues despite a Pringle manoeuvre is coming from the hepatic veins or the retrohepatic inferior vena cava, or from a replaced left hepatic artery that runs in the lesser omentum away from the free edge.

Anterior

The free edge of the lesser omentum, containing the bile duct, the hepatic artery proper and the portal vein.

Posterior

The inferior vena cava, covered by parietal peritoneum, with the right crus of the diaphragm.

Superior

The caudate process of the caudate lobe of the liver.

Inferior

The first part of the duodenum.

Safe cholecystectomy and the critical view of safety

Most bile duct injuries are not technical slips but errors of perception: the surgeon is convinced that the common bile duct is the cystic duct. In Strasberg's classical injury the common bile duct is clipped and divided in that belief, and the common hepatic duct is then divided higher up as the 'gallbladder' is freed, removing a segment of the extrahepatic biliary tree, often together with the right hepatic artery. Misidentification is encouraged by acute or chronic inflammation, a short cystic duct, a stone impacted in Hartmann's pouch and, above all, by strong upward traction on the fundus without drawing Hartmann's pouch laterally, which pulls the cystic duct and the common bile duct into one straight line. Lateral and downward traction on the infundibulum does the opposite: it opens the triangle and sets the cystic duct at an angle to the common hepatic duct. Strasberg's critical view of safety is the standard of identification before anything is clipped or divided.

1. The hepatocystic triangle is cleared of fat and fibrous tissue. The common bile duct and common hepatic duct do not need to be exposed: the point is to see what enters the gallbladder.
2. The lower third of the gallbladder is separated from the liver bed, exposing the cystic plate, so that the back of the triangle can be seen.
3. Two structures, and only two, are seen entering the gallbladder: the cystic duct and the cystic artery. Only then are they clipped and divided.

Two further landmarks help when the anatomy is uncertain. Rouvière's sulcus, a cleft on the underside of the right lobe present in most livers, marks the plane of the right portal pedicle: the cystic duct and artery lie above and in front of it and the common bile duct below, so dissection starts above the sulcus and stays above an imaginary line drawn from the sulcus to the base of segment IV. The cystic lymph node sits beside the cystic duct, and the cystic artery is usually close to it. When inflammation prevents the critical view, the safe course is to stop and choose a bail-out, such as intraoperative cholangiography, a subtotal (fenestrating or reconstituting) cholecystectomy, a cholecystostomy or conversion, rather than persisting in the triangle. A stone impacted in Hartmann's pouch or the cystic duct can also compress or erode into the common hepatic duct and cause obstructive jaundice (Mirizzi syndrome), which is exactly the setting in which the anatomy is most distorted.

Surface anatomy and biliary pain

The fundus of the gallbladder lies at the tip of the right ninth costal cartilage, where the lateral border of the rectus sheath (the linea semilunaris) meets the costal margin. Pressing there while the patient breathes in, and finding that pain stops the breath as the inflamed fundus descends onto the fingers, is Murphy's sign. Visceral pain from the gallbladder travels with sympathetic fibres through the coeliac plexus to the lower thoracic segments of the cord and is felt in the epigastrium and right upper quadrant, often radiating round towards the right scapula. When inflammation reaches the parietal peritoneum the pain localises over the gallbladder, and irritation of the diaphragmatic peritoneum is referred through the phrenic nerve (C3–C5) to the right shoulder tip. The gallbladder lies against the duodenum and the hepatic flexure of the colon, which is why a large gallstone can ulcerate into the bowel and cause gallstone ileus, usually impacting in the terminal ileum.