Surgical anatomy of the medial segment (S4) of the liver with special reference to bile ducts and vessels

BACKGROUND/AIMS: Resection of the inferior area of the medial segment (S4a) plus S5 with preservation of the superior,area of the medial segment (S4b) is being,performed to manage hilar bile duct carcinoma and pT2 type gallbl adder carcinoma, and thus, attention has been focused on the surgical anato my of the medial segment of the liver to identify the specific vessels and bile ducts of the areas of that segment to be resected and to be preserved. METHODOLOGY: Anatomical study of the bile duct, portal vein, middle hepatic vein, and middle hepatic artery to the medial segment branches of the live r (S4) was,performed in a total of 171 specimens comprised of 71 adult cada vers, and 100 liver casts. RESULTS: 1) Two main types of bile duct branches of the medial segment (B4) were recognized. Type I included the branches which joined to the left hep atic duct on the hilar duct side (35.5%), and type II included the branches that joined on the peripheral side (54.6%). Several subtypes were also fou nd in both types. The B2-B3 confluence was mostly on the left (41.7%) or po sterior (42.7%) to the umbilical portion (VP) of the portal vein, and to th e right of the UP (hilar side) in only 15.6%. 2) The portal vein of the med ial segment branches (P4): P4a branched from the right angle and upper righ t border of the UP in every specimen. The most common morphology was 1 larg e and 2-3 small branches (41%). P4b was almost always found to branch poste rior to the UP and lower than P4a, and the most common morphology was 1 lar ge and 0-1 small branches (57.8%). 3) The middle hepatic vein: In 83.2% a c ommon trunk was observed at the confluence with the inferior vena cava, and 8 types of the middle hepatic vein were recognized. 4) The middle hepatic- artery: It arose from the left hepatic artery in 61.5%, from of the right h epatic artery in 27.5%, from the proper hepatic artery in 5.5%, and from bo th the left and the right hepatic artery in 5.5%. CONCLUSIONS: The detailed vascular and bile duct anatomy of S4 is described . This study should be helpful in identifying the specific vessels and bile ducts of the areas of the medial segment to be resected and to be preserve d, thereby facilitating resection Of the medial segment.