Pelvic outlet
The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet. It is an important component of pelvimetry.
Boundaries
It has the following boundaries:- anteriorly: the pubic arch
- laterally: the ischial tuberosities
- posterolaterally: the inferior margin of the sacrotuberous ligament
- posteriorly: the anterior border of the middle of the coccyx.
Notches
These eminences are separated by three notches:- one in front, the pubic arch, formed by the convergence of the inferior rami of the ischium and pubis on either side.
- The other notches, one on either side, are formed by the sacrum and coccyx behind, the ischium in front, and the ilium above; they are called the sciatic notches; in the natural state they are converted into foramina by the sacrotuberous and sacrospinous ligaments.
In situ
When the ligaments are in situ, the inferior aperture of the pelvis is lozenge-shaped, bounded as follows:- in front, by the pubic arcuate ligament and the inferior [rami of the pubes] and ischia
- laterally, by the ischial tuberosities
- behind, by the sacrotuberous ligaments and the tip of the coccyx.