Kim et al. the cause of biliary stone. The commonest cause of foreign bodies in the CBD is residual objects from a previous operation, like portions of tubes, suture material or surgical clips, which act as a nidus for stone formation. Another cause is ingested foreign bodies such as food materials. Ingested foreign bodies in the biliary tree are usually associated with a previous biliary tree operation. However, there have been very few case of ingested foreign bodies, such as the fish bones in our patients, acting as a nidus for stone formation in the CBD without any previous history of operation (2, 3). Most ingested foreign bodies pass through the gastroin- testinal tract uneventfully within 1 week. Gastrointestinal perforation following the ingestion of a foreign body usually occurs in the case of a material with a sharp, pointed end. Perforation may occur at any site in the gastrointestinal tract, but the ileocecal or rectosigmoid regions are the most commonly affected areas (4). There have been many reports of ingested fish bones penetrating the aerodigestive tract and migrating into various parts of the thoracic cage and abdominal cavity, resulting in retropharyngeal abscess, mediastinitis, pleural empyema, liver abscess and perivesical abscess (5 9). Orda et al. (3) reported one case of CBD stone caused by a fish bone, which occurred in a patient who had no history of previous abdominal surgery. In their case, the presence of a choledochoduodenal fistula, which acted as the route of migration for a fish bone originating from the intestinal tract, was demonstrated by endoscopic retrograde cholangiopancreatography (ERCP). In contrast to their case, no choledochoenteric fistula was observed on the cholangiograms obtained either before or after stone removal in our cases. Prochazka et al. (2) reported two cases of choledocholithiasis caused by foreign material in patients who had not undergone any prior operation. They mentioned manometric studies of the sphincter of Oddi in patients with choledocholithiasis, and in a comparative study they found that the patients with choledocholithiasis had sequences of antegrade phasic waves in 18 5% and retrograde waves in 53 9%, while the control group of patients without choledocholithiasis had antegrade waves in 60 4% and retrograde waves only in 26 3%. They concluded that the presence of foreign material within the stones in patients without a history of previous operation could be explained by a possible reflux from the duodenum. Because there was no demonstrable choledo- choenteric fistula on the cholangiogram in our cases, we retrospectively speculated that the fish bone in the CBD might be the result of reflux from the duodenum through the ampulla of Vater, although we did not perform a manometric study. Although their visibility varies depending on the fish species, location and orientation, ingested fish bones are poorly identified by plain radiography. CT is useful for diagnosing fish bone impaction (10). One of our cases demonstrated a unique bright dot of bone density in the stone on CT. In this particular case, the cholangiograms of the stone formed from the nidus of fish bone showed filling defects with an unusually elongated shape, because the stone was formed over a linear fish bone nidus. When CT shows a stone containing a bright dot of bone density or the cholangiogram shows an unusually elongated filling defect in the biliary tree, the possibility of a stone formed over fish bone should be considered. A B Fig. 2. A 67-year-old man with common bile duct stone. A. Cholangiogram shows an elongated filling defect in the common bile duct. B. Photograph of specimen extracted from common bile duct using a stone basket shows a friable pigment stone, that was formed around a fish bone. 212 Korean J Radiol 5(3), September 2004