A 52-year-old woman with a 10-year history of recurrent biliary colic undergoes imaging for obstructive jaundice. MRCP reveals a 15 mm stone impacted in the common bile duct at the level of the ampulla. What is the most common site of impaction of choledocholiths in the common bile duct?

## Most Common Site of Choledocholiths Impaction **Key Point:** The distal third of the common bile duct, particularly at or just above the ampulla of Vater, is the most common site of choledocholiths impaction. This accounts for 60–70% of all choledocholiths and is due to the anatomical narrowing at the ampulla. ### Anatomical Basis for Impaction Sites | Site | Frequency | Anatomical Reason | |------|-----------|-------------------| | **Distal third (at/below ampulla)** | 60–70% (most common) | Narrowest point of CBD; physiological sphincter (Sphincter of Oddi) | | **Middle third** | 20–25% | Wider diameter; less common impaction | | **Proximal third** | 5–10% | Widest portion; stones rarely lodge here | | **Intrahepatic ducts** | 2–5% | Rare; suggests primary intrahepatic stone formation | **High-Yield:** The **ampulla of Vater** is the narrowest and most physiologically resistant point in the biliary tree. Stones >10 mm are particularly prone to impaction here, explaining why this is the most common site of obstruction. ### Clinical Pearl: Ampullary Anatomy The ampulla of Vater is the junction where the common bile duct and pancreatic duct converge and enter the duodenum. Key features: - Diameter: 2–3 mm (narrowest point in CBD) - Sphincter of Oddi surrounds the ampulla, creating a pressure gradient - Stones >10 mm have difficulty passing through - Impaction here causes acute pancreatitis in 5–10% of cases ### Mnemonic: "DAMP" for Choledocholiths **D** — Distal (most common site) **A** — Ampulla (where stones lodge) **M** — Middle (less common) **P** — Proximal (rare) ### Anatomical Diagram: Common Bile Duct Anatomy ```mermaid flowchart TD A[Hepatic Ducts]:::outcome --> B[Common Hepatic Duct]:::outcome B --> C[Cystic Duct insertion]:::outcome C --> D[Proximal CBD<br/>Widest segment]:::outcome D --> E[Middle CBD<br/>Intermediate diameter]:::outcome E --> F[Distal CBD<br/>Narrowest segment]:::outcome F --> G[Ampulla of Vater<br/>Most common impaction site]:::urgent G --> H[Duodenum]:::outcome style G fill:#ff6b6b ``` **Warning:** Do not confuse the most common site of impaction (distal third) with the most common site of stone *formation* (gallbladder). Stones form in the gallbladder but migrate to and lodge in the distal CBD.