The omental bursa or lesser sac is a hollow space that is formed by the greater and lesser omentum and its adjacent organs. It communicates with the greater sac via the epiploic foramen of winslow, which is known as the general cavity of the abdomen that sits within the peritoneum, but outside the lesser sac.
The borders of the omental bursa are demarcated anteriorly by the quadrate lobe of the liver, the gastrocolic ligament and the lesser omentum. To the left it is limited by the left kidney and the left adrenal gland. Posteriorly it is walled off by the pancreas. Lastly, to the right, the epiploic foramen and lesser omentum can be found and the greater sac beyond that.
The cavity itself is almost completely closed, save its communication with the greater sac and the entrance through the omental foramen and is filled with a capillary film. The greater part of the omental bursa consists of its superior recess which extends cranially between the esophagus and the inferior vena cava. The splenic recess extends to the left between the splenic ligaments and the stomach. Finally, the inferior recess of the omental bursa extends caudally between the stomach and the transverse colon. Other anatomical landmarks of note include a varied number of small peritoneal folds, recesses and fossae which seem to accumulate mostly around the cecum and the duodenum.
During embryonic development, the peritoneum is anchored to the gut in the midline of the abdomen anteriorly, with the dorsal mesentery securing it posteriorly. The mesenteric layers develop in an anterior direction around the upper alimentary canal, carrying the blood supply and creating the ventral mesentery. Due to the growth of the organs, they gradually become larger and have to shift in order to fit into the abdominal cavity. The stomach rotates 90 degrees, the spleen is displaced to the left and the liver moves to the right. The peritoneum twists with these movements which lead to the formation of the falciform ligament, the lesser omentum and the coronary ligaments of the liver. Throughout this entire process, the cavity of the lesser sac is created.
The lesser sac has seven distinctly categorized pathological groups under which its potential disorders may be listed:
- Congenital anomalies include duplication cysts and cystic lymphangiomas.
- A hematoma or a biloma are classed as traumatic injuries.
- Inflammatory states could be due to an abscess, a pseudocyst or even acute pancreatitis.
- Neoplastic changes may lead to the growth of a stromal tumor, a leiomyoblastoma, a leiomyosarcoma, a liposarcoma, a schwannoma, both benign and malignant pancreatic neoplasms that may have endocrine involvement or not, hepatic tumors and desmoid tumors.
- A hydatid cyst indicates a parasitic infestation.
- The only infective cause of a lesser sac disorder as yet known of is tuberculosis.
- Mechanical irritation could potentially be caused by hernias of the cecum, transverse colon, small intestine and gallbladder.