Plane representing the boundary between the pelvic and abdominal cavities
The pelvic inlet or superior aperture of the pelvis is a planar surface which defines the boundary between the pelvic cavity and the abdominal cavity (or, according to some authors, between two parts of the pelvic cavity, called lesser pelvis and greater pelvis). It is a major target of measurements of pelvimetry.
Its position and orientation relative to the skeleton of the pelvis is anatomically defined by its edge, the pelvic brim. The pelvic brim is an approximately apple-shaped line passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis.
Occasionally, the terms pelvic inlet and pelvic brim are used interchangeably.
Boundaries
The edge of the pelvic inlet (pelvic brim) is formed as follows:
Diameters
The diameters or conjugates of the pelvis are measured at the pelvic inlet and outlet and as oblique diameters.
| Name |
Description |
Average measurement in female
|
| Anteroposterior or conjugate diameter or conjugata vera |
Extends from the upper margin of the pubic symphysis to the sacrococcygeal joint; |
about 110 mm.
|
| Transverse diameter |
Extends across the greatest width of the superior aperture, from the middle of the brim on one side to the same point on the opposite; |
about 135 mm.
|
| Oblique diameter |
Extends from the iliopectineal eminence of one side to the sacroiliac articulation of the opposite side; |
about 125 mm.
|
| Anatomical conjugate |
Extends from the pubic symphysis to the promontory; |
about 120 mm.
|
| Diagonal conjugate |
Extends from lower margin of the pubic symphysis to the sacral promontory; |
about 130 mm.
|
| Straight conjugate |
Extends from the lower border of the pubic symphysis to the tip of coccyx. The coccyx can bend posteriorly and expand the diameter with 25 mm; |
about 95 mm (+ 25 mm).
|
| Median conjugate |
Extends from the lower border of the pubic symphysis to the lower border of the sacrum; |
about 115 mm.
|
Two diameters may be measured from the outside of the body using a pelvimeter
| Name |
Description |
Average measurement in female
|
| Interspinous distance |
Extends between the anterior superior iliac spines; |
about 260 mm.
|
| Intercristal distance |
Extends between the furthest later points of the two iliac crests; |
about 290 mm.
|
