Pelvic inlet

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.

Pelvic inlet
Diameters of superior aperture of lesser pelvis (female).
Details
Identifiers
Latinapertura pelvis superior
TAA02.5.02.008
FMA17272
Anatomical terms of bone

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:

Anteriorly
by the pubic crest (or pubic symphysis)
Laterally
by the iliopectineal line		(same as other side)
Posteriorly
by the anterior margin of the base of the sacrum (or the ala of sacrum) and sacrovertebral angle (or sacral promontory)

Diameters

The diameters or conjugates of the pelvis are measured at the pelvic inlet and outlet and as oblique diameters.

NameDescriptionAverage measurement in female
Anteroposterior or conjugate diameter or conjugata veraExtends from the lower margin of the pubic symphysis to the sacrococcygeal joint;about 110 mm.
Transverse diameterExtends 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 diameterExtends from the iliopectineal eminence of one side to the sacroiliac articulation of the opposite side;about 125 mm.
Anatomical conjugateExtends from the pubic symphysis to the promontory;about 120 mm.
Diagonal conjugateExtends from inferior pubic ligament to promontory through the vagina;about 130 mm.
Straight conjugateExtends 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 conjugateExtends 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

NameDescriptionAverage measurement in female
Interspinous distanceExtends between the anterior superior iliac spines;about 260 mm.
Intercristal distanceExtends between the furthest later points of the two iliac crests;about 290 mm.

Additional images
  • Pelvis
  • Conjugata vera as measured on sagittal MRI
  • Low-dose CT scan of the transverse diameter of the pelvic inlet, as part of pelvimetry
  • Obstetric conjugate, as a measure of the pelvic inlet in the sagittal plane
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See also
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