Fish Bone as a Nidus for Common Bile Duct Stone Formation IN) was inserted through the sheath catheter beyond the stone. The stone was trapped by rotating the basket and was then extracted through the sheath catheter. There was no evidence of residual stone or choledochoenteric fistula on the completion cholangiogram. The postoperative course was uneventful. The stone was friable, brittle, elongated and pigmented, containing a nidus of fish bone 1.5 cm in length (Fig. 1D). Case 2 A 67-year-old man who had no previous history of abdominal surgery was admitted to our institution because of epigastric pain and fever. Serum bilirubin was 10 mg%, and was mostly conjugated. CT showed mild dilatation of the intra- and extra-hepatic ducts, but there was no demonstrable obstructive cause. US demonstrated the presence of a CBD stone. PTC showed an elongated filling defect in the CBD (Fig. 2A). After improvement of the cholangitic symptoms, stone removal was performed through a PTBD tract under fluoroscopic guidance. A 1.5 cm sized nidus consisting of fish bone was found in the removed stone (Fig. 2B). DISCUSSION The pathogenesis of stone formation in the biliary duct remains unknown. With regard to the formation of stones in the biliary duct, there are a variety of predisposing factors, including benign or malignant strictures, bacterial infection or parasites, metabolic changes and unusual dietary habits, as well as anatomical conditions in the bilioduodenal region. Foreign bodies in the CBD are rarely A B C D Fig. 1. A 75-year-old woman with common bile duct stone. A. Ultrasonography demonstrates a hyperechoic stone with acoustic shadowing (arrow) in the common bile duct. B. Computed tomography shows a common bile duct stone containing a bright dot of bone density (arrow). C. Cholangiogram shows an elongated filling defect in the common bile duct. D. Photograph of specimen extracted from common bile duct using a stone basket shows a friable pigment stone (thick arrow), that was formed around a fish bone (thin arrow). Korean J Radiol 5(3), September 2004 211