The lesser sac (omental bursa) is the peritoneal space posterior to the stomach and lesser omentum, anterior to the pancreas and posterior abdominal wall, bounded superiorly by the caudate lobe of the liver. It communicates with the greater peritoneal cavity through the epiploic foramen (foramen of Winslow) — a narrow opening behind the hepatoduodenal ligament containing the portal triad (portal vein, hepatic artery, bile duct). The Pringle manoeuvre occludes the hepatoduodenal ligament to control hepatic inflow through this foramen.
The lesser sac is a critical space for acute pancreatitis fluid collections (acute peripancreatic fluid collections, pseudocysts), which develop between the inflamed pancreas and the posterior stomach wall. Pancreatic pseudocysts within the lesser sac are drained by endoscopic cystogastrostomy (through the posterior stomach wall) or by laparoscopic cystogastrostomy. Penetrating peptic ulcers may erode into the lesser sac. The lesser sac provides surgical access to the posterior stomach, the pancreas, and the coeliac axis during gastrectomy, distal pancreatectomy, and Whipple procedures through the gastrocolic ligament.
Acute pancreatitis produces peripancreatic fluid collections that mature into walled-off pseudocysts within the lesser sac between the pancreatic body and the posterior gastric wall; endoscopic ultrasound-guided transmural cystogastrostomy places a lumen-apposing metal stent through the posterior stomach into the pseudocyst for internal drainage.
