Identification of segments VI and VII of the liver based on the ramification patterns of the intrahepatic portal and hepatic veins

We describe the pattern of intrahepatic vessel ramification in the right po sterior hepatic sector in a population of 197 adults. Each specimen was dis sected from its visceral (inferior) surface in order to demonstrate variati ons in the distribution of the portal vein branches to the hepatic segments of the right lobe, especially to segments VI (S6) and VII (S7) as describe d by Couinaud. We also examine whether three hepatic veins, i.e.. the right hepatic vein (RHV), middle hepatic vein (MHV), and the short hepatic vein (SHV), aid the identification of segmental portal branches in the lower pos terior sector. Four major patterns of branching of the posterior sectorial trunk of the portal vein system are described. In group A (32.0%) a single posterior trunk formed an arch-like pattern sending multiple branches to S6 and S7 (P6 and P7). We named the multiple branches to the apparent S6 the inferoposterior portal branches. It was difficult to identify which of thes e branches were equivalent to P6. In group B (27.9%), the posterior sectori al trunk bifurcated to form P6 and P7. In most of the specimens in this gro up, therefore, we were able clearly to identify both S6 and S7 based on the portal vein system. In group C (6.6%), the trunk trifurcated to form P6, P 7, and an intermediate branch, which supplied both segments or a gray zone between them. Group D (33.5%) included variations of the anterior segmental branches, and in specimens of this group, the anteromedial border of the s ector was difficult to identify. Notably, the three-dimensional interdigita ting topographical relationship of the hepatic veins and the portal branche s was not evident in the lower posterior sector, since tributaries of the R HV and the portal branches followed similar courses and paralleled each oth er in the region and since the territory of the SHV was usually restricted to the superficial parenchyma near the inferior surface, In group A, tribut aries of the RHV/SHV (>3 mm in diameter) passed between the inferoposterior portal branches in only 22.2%/14.3% of the specimens. Thus the hepatic vei ns often did not reveal which of the multiple inferoposterior branches was P6. Moreover, in the subset of Group B in which the segments were identifie d based on the portal vein ramification, tributaries of the RHV/SHV (>3 mm in diameter) showed the intersegmental interdigitating arrangement in only 32.0%/6.0% of the specimens. In addition, a thick tributary of the MHV, som etimes arising from S6, did not run along, but penetrated the S5/S6 border plane from the lateral to the medial side. Therefore, the three hepatic vei ns (RHV, SHV, MHV) often did not aid the identification of the liver segmen ts in the region. Consequently, the less than ideal combinations of irregul ar configurations of the portal and hepatic venous systems suggest that the right posterior segments cannot be conclusively identified anatomically in 30-40% of cases. Other means of identification, such as the conventional p roportional manner (the upper and lower halves of the posterior sector roug hly correspond to S6 and S7) may be required. (C) 1999 Wiley-Liss, Inc.