Most common cause of gallbladder fistula is

Internal fistulas are spontaneous, rare and occur without a significant collection of bile External fistulas are common and are often caused by iatrogenic injury after operation, invasive procedures or trauma involving the biliary tract Etiology External fistula Bile leakage from the cystic duct remnant Central hepatectomy and caudate resection. Difficult cases of open cholecystectomy Hepatic cryotherapy or harmonic scalpel use Internal fistula Calculous biliary tract disease (90%) Duodenal ulcer (6%) Neoplasm, trauma, Parasitic infestation and congenital anomalies Ref: Sabiston 20th edition Pgno :1507