The retroperitoneum is the space just posterior to the peritoneal cavity and anterior to the paraspinous musculature. Superiorly, the retroperitoneum is bordered by the diaphragm, and inferiorly, it forms a natural extension into the pelvis (ie, the pelvic diaphragm). The paraspinous musculature includes the psoas major, psoas minor, quadratus lumborum, obturator internus (not shown), and pyriformis (not shown) muscles.
