Couinaud liver segment

The Couinaud classification (pronounced kwee-NO) is currently the most widely used system to describe functional liver anatomy. It is the preferred anatomy classification system as it divides the liver into eight independent functional units (termed segments) rather than relying on the traditional morphological description based on the external appearance of the liver. The segments are numbered in Roman numerals I to VIII.

The delineation of the segments is based on the fact that each segment has its own dual vascular inflow, biliary drainage and lymphatic drainage. In general, each segment can be visualised as wedge-shaped with the apex directed towards the hepatic hilum (porta hepatis). At the apex a single segmental branch of the portal vein, hepatic artery and bile duct enter; whereas along the boundaries of each segment there is venous outflow through the hepatic veins so that a hepatic vein drains two adjacent segments (and each segment has multiple draining hepatic veins). These veins run in 3 vertical planes radiating from the intrahepatic IVC that separate 4 sections of the liver (a section is two segments on top of each other):right hepatic vein located in the right intersegmental fissure, divides the right lobe into right lateral (posterior) and right medial (anterior) sections.

Middle hepatic vein lies in the main lobar fissure, divides the liver into right and left lobes (or right and left hemiliver): this vertical plane runs from the inferior vena cava to the gallbladder fossa and is known as Cantle’s  line. To the right is the right medial section and to the left is the left medial section.

The classification system uses the vascular supply in the liver to separate the functional units (numbered I to VIII):

.Unit I is the caudate lobe and is situated posterior l and it may receive its supply from both the right and the left branches of portal vein. It contains one or more hepatic veins which drain directly into the IVC.

The remainder of the units (II to VIII) are numbered in a clockwise fashion:

.units II and III lie medial to the falciform ligament with II superior to the portal venous supply and III inferior

.unit IV lies lateral to the falciform ligament and is subdivided into IVa (superior) and IVb (inferior)

Units V to VIII make up the right part of the liver:

.unit V is the most medial and inferior

.unit VI is located more posteriorly

.unit VII is located above unit VI

.unit VIII sits above unit V in the superio-medial position


Reference

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