Common bile duct
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In ultrasound imaging, it is not always possible to confidently see where the cystic duct enters the common hepatic duct to form the common bile duct. Therefore it is common practice to use the term common duct (CD) as a term conflating the common hepatic and common bile ducts. Unfortunately, this is in conflict with Gray's Anatomy, which refers to the short duct formed by the union of the common bile and pancreatic ducts as the common duct.
The CBD is formed from the combination of the common hepatic duct and cystic duct, lying at the free edge of the lesser omentum with the hepatic artery on its left and the portal vein lying posteriorly. The CBD passes behind the first part of the duodenum together with the gastroduodenal artery, then behind the pancreas to join with the pancreatic duct and the ampulla of Vater 10.
The CBD is approximately 8 cm long and usually <6-7 mm wide in internal diameter in adults. It joins the pancreatic duct at the ampulla of Vater, which drains into the second part of the duodenum through the major duodenal papilla.
Of note, for decades, what had been labeled CBD in much radiology literature is now known, which is usually the common hepatic duct (CHD). Thus, the oft-quoted normal value of less than 6 mm (measured inner aspect of wall to inner aspect of wall, typically by ultrasound) in adults actually refers to the CHD in most cases. The CHD is best measured when the patient is fasting, at the porta hepatis, typically parallel and anterior to the portal vein. In recent years, 7 mm has been proposed as a better cut-off by some investigators.
Also for decades, it had been thought that the CHD (then termed the CBD, more often than not erroneously) could increase as much as 4 mm after cholecystectomy, and by age as much as 1 mm per decade after age 60. This has not been supported by subsequent studies, which indicate that the CHD diameter may increase only by 0.1-0.2 mm per decade, and increases only about 1 mm after cholecystectomy.
The diameter of the actual CBD is far more variable, not readily affording a valid cut-off value. By ultrasound, it is seen more inferiorly, usually having exited the porta toward pancreas.
The common bile duct is supplied by a network of arteries from several sources:
from below: posterior superior pancreaticoduodenal artery, retroduodenal artery, and retroportal artery
There are four main relationships of the CBD with the pancreatic head 2:
partially covered posteriorly (most common: ~50%)
completely covered (30%)
completely uncovered (16.5%)
CBD may pass laterally to the pancreatic head (least common)
Studies have found that complete separation of the CBD and pancreatic duct with two openings in the duodenum occur in 20% of autopsied specimens and 37% in ERCP studies 6,7. Other authors have reported that overall the finding is rare, with only several cases reported in the literature 8,9.
In about 2/3 of the individuals, common bile duct passes in front of right hepatic artery, in about 1/4 of the cases, it passes behind the right hepatic artery. Meanwhile, in 12% of the cases, the right hepatic artery arises from the superior mesenteric artery and passes behind the common bile duct 10.