The lesser omentum is a thin, double-layered fold of peritoneum that hangs between the liver above and the stomach and first part of the duodenum below, forming a curtain-like membrane in the upper abdomen. It is one of the body’s most surgically important sheets of tissue because it houses critical blood vessels, bile ducts, and nerves that supply the liver, and because it forms part of the boundary of a hidden compartment called the lesser sac. Despite its delicate appearance, the lesser omentum plays a role in conditions ranging from pancreatitis and cancer spread to rare internal hernias.
Where It Sits and What It Looks Like
The lesser omentum is tucked beneath the liver and is not immediately visible when you open the abdomen; a surgeon typically has to lift the liver to see it. A study of the structure in albino rats confirmed that it becomes visible only after the liver is retracted upward, and that it takes the form of two named ligaments with structured tissue running between thin, translucent membranes.1PubMed. Morphological differences between the lesser and the greater omenta in albino rats Those two ligaments are the hepatogastric ligament, which connects the liver to the lesser curvature of the stomach, and the hepatoduodenal ligament, which connects the liver to the first portion of the duodenum. Together they form a continuous sheet, but the hepatoduodenal portion is thicker and far more consequential in surgery.
The hepatogastric ligament is the thinner, more membranous part. In many people it is partly translucent, and surgeons sometimes describe being able to see organs behind it. It can contain an aberrant left hepatic artery and carries the hepatic branch of the vagus nerve, both of which matter during stomach and esophageal surgery. The hepatoduodenal ligament, by contrast, is a compact bundle. Its free right edge forms one of the borders of a natural opening called the epiploic foramen, or foramen of Winslow, which is the only passage between the main peritoneal cavity and the lesser sac behind the stomach.
What Runs Through the Hepatoduodenal Ligament
The hepatoduodenal ligament is sometimes called the “portal triad” region because three vital structures travel inside it on their way to or from the liver: the common bile duct, the hepatic artery proper, and the portal vein. The bile duct sits to the right, the hepatic artery to the left, and the portal vein lies behind both. Surgeons who operate on the liver, pancreas, or bile ducts interact with these structures constantly, and knowing exactly where they are within the lesser omentum can be the difference between a routine operation and a catastrophic bleed.
The classic textbook arrangement does not always hold. In some people the portal vein sits in front rather than behind, and the hepatic arteries may branch in unexpected ways. A report of two patients undergoing pancreatic surgery found that the portal vein was positioned anteriorly in the portal triad, and that both the right and left hepatic arteries arose directly from the celiac trunk rather than following the standard branching pattern.2PubMed Central. A deviant anterior portal vein in the hepatoduodenal ligament with aberrant origin of hepatic arteries directly from the celiac trunk Variations like these are not just anatomical curiosities; a surgeon who expects the portal vein behind the duct and artery could inadvertently injure it if it has migrated forward.
The Epiploic Foramen and the Lesser Sac
The free right edge of the hepatoduodenal ligament forms the front wall of the epiploic foramen. This small opening, usually only wide enough to admit one or two fingers, connects the greater peritoneal cavity with the lesser sac, a pouch-like space tucked behind the stomach. The lesser sac matters clinically because it can trap fluid, blood, bile, or pus when disease strikes nearby organs. In pancreatitis, for example, inflammatory fluid commonly collects in the lesser sac and can extend into the lesser omentum itself.3PubMed. Acute pancreatitis in children: CT findings of intra- and extrapancreatic fluid collections
The lesser sac is also a conduit for the spread of disease within the abdomen. Fluid collections in this space can arise from many causes, including bile leaks, blood from injured vessels, and inflammatory exudates. Masses, whether cancerous or not, and even internal hernias can involve the lesser sac.4AJR Am J Roentgenol. The Lesser Sac and Foramen of Winslow: Anatomy, Embryology, and CT Appearance of Pathologic Processes Because the foramen of Winslow is the only way in and out, a loop of bowel that slips through it can become trapped and strangled, which is a surgical emergency.
How the Lesser Omentum Develops
During embryonic life, the gut tube is suspended by mesenteries, sheets of tissue that anchor it to the body wall. The stomach starts out in the midline and then rotates roughly ninety degrees clockwise (when viewed from above). That rotation pulls the mesentery on the stomach’s back (dorsal) side to the left, where it eventually becomes the greater omentum. The mesentery on the front (ventral) side stays connected between the liver and the stomach, becoming the lesser omentum and the falciform ligament that attaches the liver to the front abdominal wall. The lesser omentum is therefore one of the few structures in the abdomen derived from the ventral mesentery; most abdominal mesenteries come from the dorsal side.
This embryological origin also explains some rare anatomical variants. Reports exist of an extra fold within the lesser omentum called a cystoduodenal ligament, running between the gallbladder and the duodenum rather than following the usual hepatogastric or hepatoduodenal pattern. When present, this fold can alter the shape and boundaries of the epiploic foramen, changing the surgical landscape in unpredictable ways.
The Vagus Nerve and Why Surgeons Care About It
A branch of the vagus nerve, called the hepatic branch, travels through the hepatogastric ligament on its way to the liver and gallbladder. This nerve helps regulate bile secretion, digestion, and glucose metabolism. During operations on the upper stomach or esophagus, surgeons routinely divide the hepatogastric ligament to get access to the area behind the stomach, and in the process the hepatic branch of the vagus nerve is often cut.5Foregut: The Journal of the American Foregut Society. A Systematic Review and Meta-Analysis on the Impact of Preserving the Hepatic Branch of the Vagus Nerve in Upper Gastrointestinal Surgeries
Whether cutting this nerve actually matters has been debated. Some researchers have explored whether preserving it leads to better outcomes after surgery. In the context of gastric bypass for obesity, one study found that cutting the neurovascular bundle in the lesser omentum was associated with more complications, while the expected benefit of greater weight loss did not materialize.6PubMed. Transection versus preservation of the neurovascular bundle of the lesser omentum in primary Roux-en-Y gastric bypass surgery The evidence is still evolving, but the trend in recent surgical research leans toward preserving the hepatic branch when it is technically feasible, especially in operations where dividing it is not strictly necessary.
The Pringle Maneuver and Bleeding Control
One of the most important surgical techniques involving the lesser omentum is the Pringle maneuver. When a patient is bleeding heavily from the liver during surgery, a surgeon can clamp or compress the hepatoduodenal ligament to temporarily stop all blood flow into the liver. Because the hepatic artery and portal vein both run through this ligament, squeezing it shuts off the liver’s incoming blood supply, which usually slows or stops the hemorrhage enough to let the surgeon work.
The technique has been adapted for minimally invasive liver surgery using several approaches: a tourniquet looped around the ligament and tightened, a vascular clamp applied directly, or a specialized looping device threaded through the tissue.7PubMed Central. The Pringle maneuver in the modern era: A review of techniques for hepatic inflow occlusion in minimally invasive liver resection The Pringle maneuver is a temporary measure since the liver cannot tolerate having its blood supply cut off for long periods, but in acute bleeding it can be lifesaving. Its effectiveness depends entirely on the fact that the hepatoduodenal ligament bundles the liver’s main inflow vessels into one compact, accessible strip of tissue.
Aberrant Arteries in the Lesser Omentum
Roughly one in four people has an aberrant left hepatic artery, a vessel that branches off the left gastric artery and courses through the hepatogastric portion of the lesser omentum on its way to the left lobe of the liver. This vessel can supply a meaningful fraction of the left lobe’s blood, so accidentally cutting it during stomach surgery can cause liver damage. In a study comparing patients whose aberrant artery was preserved during stomach cancer surgery with those whose artery was divided, the group that lost the artery showed significantly higher levels of liver enzymes on the first and third days after surgery, indicating liver cell injury.8PubMed. The significance of the aberrant left hepatic artery arising from the left gastric artery at curative gastrectomy for gastric cancer
Identifying this artery during laparoscopic (keyhole) stomach surgery can be tricky, especially in patients with a higher body mass index. One study noted that the group of patients whose aberrant artery ended up being ligated had a significantly higher average BMI, suggesting that excess tissue around the lesser omentum made the vessel harder to spot and protect.9PubMed Central. Should an Aberrant Left Hepatic Artery Arising from the Left Gastric Artery Be Preserved during Laparoscopic Gastrectomy for Early Gastric Cancer Treatment? Despite the enzyme spike, long-term outcomes such as five-year survival and complication rates did not differ significantly between the two groups in the earlier study, so the liver damage appears to be mostly temporary.10PubMed. The significance of the aberrant left hepatic artery arising from the left gastric artery at curative gastrectomy for gastric cancer Still, most surgeons now try to preserve the artery when possible, on the principle that transient liver injury is best avoided.
Internal Hernias Through the Lesser Omentum
Internal hernias, where loops of bowel slip through an abnormal opening inside the abdomen and become trapped, are uncommon. When they do involve the lesser omentum, they come in two main flavors. The first is herniation through the epiploic foramen, where a loop of small bowel passes through the natural opening at the free edge of the hepatoduodenal ligament and enters the lesser sac. Risk factors include an unusually long small-bowel mesentery, a mobile right colon, and anything that increases abdominal pressure.11PubMed Central. Lesser sac herniation: a rare cause of acute abdomen and bowel perforation On CT imaging, telltale signs include bowel loops in the space behind the stomach, mesenteric fat and vessels sitting behind the portal vein, and the cecum displaced from its normal position.
The second type involves a defect, either congenital or acquired, in the body of the lesser omentum itself. In one reported case, a three-centimeter hole in the lesser omentum allowed loops of jejunum to herniate through, causing congestion of the bowel that required emergency surgery.12PubMed Central. Congenital Defect in Lesser Omentum Leading to Internal Hernia in Adult: A Rare Case Report In another case, a ninety-centimeter segment of ileum herniated through an opening in the lesser omentum, became strangulated, and perforated into an abscess in the lesser sac, requiring bowel resection.13Sao Paulo Medical Journal. Strangulated internal hernia through the lesser omentum with intestinal necrosis: a case report These cases are rare enough that the diagnosis is almost never suspected before surgery. The key clinical lesson is that a relatively young patient with bowel obstruction who has never had abdominal surgery and has no external hernia should prompt a surgeon to think about internal hernias, including those involving the lesser omentum.
Cancer Spread Along the Lesser Omentum
The lesser omentum is a highway for the spread of gastric cancer. The tissue between its peritoneal layers is part of the subperitoneal space, a continuous compartment that extends throughout the abdomen’s ligaments and mesenteries. Because this space is uninterrupted, cancer cells, infection, or inflammation can travel along it in many directions.14PubMed Central. Pathways of abdominal tumour spread: the role of the subperitoneal space In gastric cancer specifically, tumors on the lesser curvature of the stomach can spread along the hepatogastric and hepatoduodenal ligaments toward the liver hilum, carrying cancer cells to lymph nodes embedded in the lesser omentum along the way.15PubMed. Ligaments and Lymphatic Pathways in Gastric Adenocarcinoma
This is why surgeons performing gastrectomy for cancer routinely remove the lesser omentum along with the stomach. Leaving it behind would risk leaving behind cancer cells that have already migrated into the ligament’s lymphatic channels. The same spread patterns can carry pancreatic or biliary tract cancers toward or through the lesser omentum, making it relevant in the staging and surgical planning of several upper abdominal malignancies.
Primary Tumors Arising in the Lesser Omentum
Tumors that originate within the lesser omentum itself, rather than arriving there from a nearby organ, are genuinely rare. When they do occur, they tend to be mesenchymal tumors, meaning they arise from the connective tissue, fat, or smooth muscle cells within the omentum’s layers. One reported case involved a large leiomyosarcoma, a malignant smooth-muscle tumor measuring roughly twelve by eight centimeters, found in a seventy-two-year-old woman who presented with a palpable mass and weight loss. Imaging showed a cystic mass between the stomach and liver, and surgery confirmed it originated from the lesser omentum.16Korean Journal of Internal Medicine. A case of primary leiomyosarcoma of the lesser omentum
More benign primary tumors also turn up. A case of an infarcted lipoma (a fatty tumor whose blood supply had been cut off) in the lesser omentum was described in a patient with insidious epigastric pain and signs of peritonitis but a normal white-blood-cell count. CT showed a focal inflammatory mass with surrounding fat stranding near the lesser curvature. In most such cases, the diagnosis is only confirmed at surgery, though some patients can be managed with pain relief alone since the condition is self-limiting.17PubMed. Infarction of a lipoma of the lesser omentum: laparoscopic treatment of a rare condition Another recent case report documented a gastrointestinal stromal tumor (GIST) measuring about ten centimeters across arising within the lesser omentum in a young adult, identified on CT by the absence of a “claw sign” (which would indicate the mass arose from an adjacent organ) and preserved fat planes between the tumor and both the stomach and liver.18PubMed Central. Computed tomography features of a primary extragastrointestinal stromal tumor of the lesser omentum with spontaneous calcification in a young adult
How Imaging Captures the Lesser Omentum
CT scanning is the primary tool for evaluating disease involving the lesser omentum. Under normal conditions the lesser omentum is thin enough that it may not be visible at all on a standard scan; it becomes apparent when disease thickens it, fills it with fluid, or deposits a mass within its layers. Modern multi-detector CT with the ability to reconstruct images in different planes allows radiologists to trace the complex anatomy of the peritoneal cavity and identify which ligament or fold a lesion is arising from.19PubMed. Greater and lesser omenta: normal anatomy and pathologic processes
For the conditions described in earlier sections, CT findings serve as the diagnostic roadmap. In pancreatitis, fluid tracking into the lesser omentum shows up as a low-density stripe between the liver and stomach. In internal hernias, displaced bowel loops and mesenteric vessels in abnormal positions behind the portal triad raise suspicion. In tumors, the key imaging question is whether the mass genuinely arose from the lesser omentum or merely invaded it from the stomach, liver, or pancreas. Preserved fat planes between the mass and adjacent organs, along with the absence of signs that the mass is “growing out of” an organ, help radiologists pin down a lesser-omental origin. Getting this distinction right matters for surgical planning, because a primary lesser-omental tumor is resected differently from a gastric cancer that has merely invaded the omentum.
Anatomical Variants That Change the Surgical Landscape
Beyond the aberrant left hepatic artery already discussed, cadaveric studies have turned up other lesser-omental variations. One unusual finding is a cystoduodenal ligament, a peritoneal fold running from the gallbladder to the duodenum within the lesser omentum, distinct from the usual hepatogastric and hepatoduodenal ligaments. When present, this fold alters the boundaries of the omental bursa and can shift the position of the epiploic foramen away from its classically described location behind the free edge of the hepatoduodenal ligament. For surgeons who rely on anatomical landmarks to navigate the area safely, encountering such a variant unexpectedly could create confusion during a procedure around the biliary tract or portal triad.
Variations in the lesser omentum’s contents are actually more common than many surgical trainees realize. Estimates of aberrant hepatic artery frequency vary across studies, but the combined incidence of various hepatic artery anomalies is high enough that experienced surgeons treat “standard” anatomy as the most common pattern rather than a guarantee. Preoperative imaging, particularly CT angiography, has made it possible to map the vascular anatomy of the lesser omentum before the first incision, reducing the risk of surprise encounters during surgery.