The Couinaud classification (pronounced kwee-NO) is used to describe functional liver anatomy. It is preferred over morphological liver anatomy since it allows the division of the liver into eight independent functional units (segments) rather than relying on the traditional morphological description based on the external appearance of the organ. They are numbered I to VIII.
The separation of the segments is based on the fact that each has its own dual vascular inflow, biliary drainage and lymphatic drainage. Most of these segments are wedged shaped with the apex directed towards the hilum. At the apex of each segment there is a branch of the portal vein, hepatic artery and bile duct; in the periphery of each segment there is vascular outflow through the hepatic veins, which run in 3 vertical planes that separate the segments:
- right hepatic vein located in the right intersegmental fissure, divides the right lobe into anterior and posterior segments
- 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 also known as Cantlie's line
- left hepatic vein located in the left intersegmental fissure, divides the left lobe (cephalic aspect) into a medial and lateral part
A fourth horizontal plane, the portal plane where the portal vein bifurcates and becomes horizontal divides the liver into upper and lower segments. The left and right portal veins branch superiorly and inferiorly to project into the apex of each segment.
- segment I is the caudate lobe situated posteriorly. It may receive its supply from both the right and the left branches of portal vein. It is drainded directly into the IVC by one or more small hepatic veins
The remainder of the segments (II to VIII) are numbered in a clockwise fashion starting superiorly in the left hemiliver :
- segments II and III lie lateral to the falciform ligament and left hepatic vein with II superior to the portal plane and III inferior
- segment IV lies medial to the falciform ligament and is subdivided into IVa (superior) and IVb (inferior). Easy tip: IVa above and IVb below the portal plane.
Segment V to VIII make up the right hemiliver:
- segment V is the most medial and inferior
- segment VI is located more posteriorly, with
- segment VII above it
- segment VIII sits above unit V in the superio-medial position
The division of the liver into self-contained units means that each segment can be resected without damaging those remaining. For the liver to remain viable, resections occurs along the vessels that define the boundaries of these segments. This means, that resection-lines parallel the hepatic veins and there is preservation of the centrally located portal veins, bile ducts, and hepatic arteries.
History and etymology
It was first described by C Couinaud in 1957. The notion of the Couinaud liver segments being based on the arrondissements (administrative districts) of Paris is a radiological urban myth 4.