A 62-year-old woman with a history of recurrent cholangitis presents with obstructive jaundice. Imaging shows a solitary stone impacted in the common bile duct. At what most common site is the stone likely to be lodged?

Explanation

## Most Common Site of Choledocholithiasis **Key Point:** The ampulla of Vater is the most common site of impaction for choledocholiths, accounting for 60–70% of CBD stone impactions. This is the narrowest and most dependent portion of the biliary tree. ### Anatomical Basis for Stone Impaction at the Ampulla | Site | Diameter | Frequency of Impaction | Reason | |------|----------|------------------------|--------| | **Ampulla of Vater** | 2–3 mm | 60–70% | Narrowest point; gravity-dependent | | **Intrapancreatic CBD** | 3–4 mm | 20–30% | Narrow, surrounded by pancreatic tissue | | **Supraduodenal CBD** | 4–5 mm | 5–10% | Widest segment; stones pass through | | **Hepatic bifurcation** | Variable | Rare | Proximal; stones lodge distally | **High-Yield:** The CBD has three anatomically distinct segments: 1. **Supraduodenal** (widest, 4–5 mm) — stones rarely lodge here 2. **Intrapancreatic** (narrow, 3–4 mm) — second most common site 3. **Ampullary** (narrowest, 2–3 mm) — most common site of impaction ### Why the Ampulla Is the Narrowest Point 1. **Anatomical constriction**: The ampulla of Vater is where the CBD enters the duodenum; it is the terminal, most constricted segment. 2. **Gravity and flow dynamics**: The ampulla is the most dependent (lowest) point; stones naturally gravitate here and lodge due to reduced flow. 3. **Sphincter of Oddi**: The muscular sphincter creates additional resistance, trapping stones. **Clinical Pearl:** A stone impacted at the ampulla may cause acute pancreatitis if it temporarily obstructs the pancreatic duct orifice, leading to reflux of bile into the pancreatic duct. This is the most common cause of gallstone pancreatitis. ### Anatomical Diagram of CBD Segments ```mermaid flowchart TD A[Common Bile Duct]:::outcome --> B[Supraduodenal segment<br/>4-5 mm diameter]:::action B --> C[Intrapancreatic segment<br/>3-4 mm diameter]:::action C --> D[Ampulla of Vater<br/>2-3 mm diameter<br/>NARROWEST]:::urgent D --> E[Duodenum]:::outcome F[Stone impaction<br/>frequency] --> G[Ampulla: 60-70%]:::action F --> H[Intrapancreatic: 20-30%]:::action F --> I[Supraduodenal: 5-10%]:::action ``` **Mnemonic:** **AMPS** — Ampulla Most Probable Site: - **A**mpulla of Vater (60–70%) - **M**ost common site of stone impaction - **P**ancreatic segment (intrapancreatic, second most common) - **S**upraduodenal (widest, stones pass through) ### Clinical Implications - **Recurrent cholangitis**: Stones at the ampulla are prone to impaction and re-obstruction, causing recurrent infections. - **ERCP with sphincterotomy**: The procedure targets the ampullary sphincter to allow stone extraction. - **Acute pancreatitis**: Ampullary stones are the most common cause of gallstone pancreatitis. [cite:Sabiston Textbook of Surgery 21e Ch 55; Bailey & Love 28e Ch 70]