Anatomical variations of cystic duct insertion and their relationship with choledocholithiasis: an MRCP study

The frequency of insertion variations of cystic duct (CD) is inconsistent between studies caused to some extent by the way they give the relative frequency of the variations. Moreover, certain insertion variations have been reported to be associated with choledocholithiasis. This study aimed to assess the frequency of CD insertion variations with a comprehensive way of classification in an unselected population in whom MRCP was performed. Moreover, the relationship between the types of variant insertions and choledocholithiasis using MRCP was also assessed. Patients undergoing magnetic resonance cholangiopancreatography (MRCP) were reviewed retrospectively by two radiologists who were blinded to the clinical data. The normal insertion was defined as the union through middle one third of the lateral border of the extrahepatic bile duct. The transverse site (lateral, medial, anterior, and posterior) and the craniocaudal level (high, mid and low) of insertions and their intersections were assessed using axial and coronal slices, respectively. In addition, the frequencies of the CD insertion variations were compared between choledocholithiasis and control (no-choledocholithiasis) groups. A total of 307 patients (124 with choledocholithiasis and 183 controls) were analyzed. A true variant insertion was found in 149 (48.5%) cases. The insertion variations were less frequent in the choledocholithiasis group [50 (40.3%) vs. 99 (54.1%), respectively, p = 0.018]. The frequencies of craniocaudal level of insertion differed significantly between groups (p = 0.014) that was driven by a lower rate of low medial insertion (1.6% vs. 9.8%, respectively) in the choledocholithiasis group. The frequencies of transverse site of insertion were similar between groups (p = 0.314). The low medial insertion was 80.7% less likely associated with choledocholithiasis even after adjustment for age (Odds ratio: 0.193, 95% Confidence interval: 0.039–0.954, p = 0.044). The interreader agreement for insertion assessment was good (Cohen’s Kappa: 0.748, p < 0.001). The prevalence of CD insertion variations in an unselected population undergoing MRCP is quite high and a mid-posterior insertion is the most common variant type. Insertion variations of CD, the low medial insertion in particular, are less common in patients with choledocholithiasis than controls.