**4. Consideration regarding extrahepatic bile duct anatomy of CBDE**

The cystic duct is a connection between the gallbladder and the common bile duct. Generally speaking, its length is about 3 cm and its diameter is 0.2–0.3 cm. It consists of two parts, which include 5–12 consecutive half-moon mucosal folds called the Heister spiral valves and a smooth portion close to the CBD. Its elasticity and the smoothness of its interior are similar to the CBD. The muscles of the spiral folds are arranged like an annular valve, which can drive the bile flow by contraction and relaxation. Furthermore, the cystic duct itself functions like a sphincter and

can coordinate gallbladder filling. The complex anatomy of this structure could be the explanation of difficulties during extraction of CBD stones through the cystic approach.

The diameter of the confluence between the cystic and hepatic ducts is wider than the diameter of the CBD. That is why we can use it to do a microincision on this part of the bile duct and to provide access to CBD. The diameter of the cystic duct can expand up to 1 cm or more when the CBD is obstructed, and the expansion is more obvious at the confluence. The anatomical features of the cystic duct and CBD create actually favorable conditions for LTCBDE if there are no anatomical abnormalities or variations [58].
