Pelvic Inlet
Pelvic inlet is the upper pelvic opening bounded by the sacral promontory, iliopectineal lines, and pubic crest.
The pelvic inlet, also known as the superior pelvic aperture, is an obliquely oriented, oval-shaped opening that marks the boundary between the abdominal cavity above and the true pelvic cavity below. It serves as the entrance to the lesser pelvis and plays a critical role in both obstetrics and pelvic anatomy. Bounded by bony landmarks, the pelvic inlet defines the superior margin of the birth canal and is important for evaluating pelvic shape and size, especially in females.
Structure
The pelvic inlet is formed by an imaginary plane passing through several anatomical landmarks of the pelvic bones. Its boundaries are continuous and rigid, defined entirely by bone.
Bony Boundaries of the Pelvic Inlet
| Boundary | Landmark |
|---|---|
| Posterior | Promontory and alae of the sacrum |
| Lateral | Arcuate lines of the ilium |
| Anterolateral | Pectineal lines of the pubis |
| Anterior | Pubic crest and upper border of the pubic symphysis |
Location
The pelvic inlet is located at the junction between the greater (false) pelvis and the lesser (true) pelvis. It is tilted forward relative to the horizontal plane due to the natural inclination of the pelvis in anatomical position. Superior to the inlet lies the abdominal cavity, while inferiorly begins the true pelvic cavity housing pelvic viscera.
Function
- Divides Pelvic Regions: Separates the greater pelvis (supporting abdominal organs) from the lesser pelvis (containing pelvic organs)
- Marks Obstetric Boundary: Acts as the upper boundary of the birth canal; its dimensions are crucial during childbirth
- Supports Visceral Structures: Helps transmit the weight of the abdominal and pelvic organs through the bony pelvis
Clinical Significance
- Obstetric Pelvimetry: The size and shape of the pelvic inlet are assessed via imaging or manual examination to determine if a vaginal delivery is feasible
- Pelvic Types: The inlet's shape helps define pelvic types (gynecoid, android, anthropoid, platypelloid), which influence labor and delivery outcomes
- Cephalopelvic Disproportion: A mismatch between fetal head size and pelvic inlet size may necessitate cesarean delivery
- Radiological Landmark: The pelvic brim (which outlines the inlet) serves as a reference in CT and MRI for delineating abdominal vs pelvic pathology
Last updated on May 9, 2025
