Pelvic inlet

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Overview of the bony pelvis.

The pelvic inlet, also known as the superior pelvic aperture, refers to the circular superior opening of the pelvis that leads into the pelvic cavity. It is outlined by the pelvic brim, a bony oval ring formed by the superior surfaces and bony edges of the promontory and ala of the sacrum, along with the iliac and pubic bones, which are joined at the midline by the pubic symphysis. Together, these elements delineate the rim of the pelvic inlet.

In the anatomical position, the pelvic inlet is oriented more vertically than horizontally, with the inlet angled about 50-60 degrees forward and facing slightly upwards. The oblique plane of the pelvic inlet serves as an imaginary boundary, rather than a physical division, between the abdominal and pelvic cavities. This plane also marks the demarcation between the greater (false) pelvis above and the lesser (true) pelvis below.

The pelvic inlet exhibits unique differences between males and females. It is narrow and heart-shaped in males, whereas in females, it features a rounded oval shape with a wide transverse diameter. These structural differences, along with several others pertaining to the bony pelvis, are closely related to their specific functions, particularly with regard to childbirth in females.

The pelvic inlet transmits several structures between the abdomen and the pelvic cavity, including elements of the reproductive, urinary, and lower digestive systems, as well as neurovasculature. Additionally, in females, it is adapted to allow the passage of a newborn's head during childbirth. It also contributes to the structural stability of the pelvis, providing attachment points for various muscles and ligaments.