Pelvic inlet plane: definition and boundaries
The bony pelvis is divided into the greater pelvis (pelvis major) and lesser pelvis (pelvis minor). The lesser pelvis contains four classic planes: the inlet plane, the plane of greatest pelvic dimension, the plane of least pelvic dimension, and the outlet plane. The first of these—the inlet plane—serves as the conventional boundary between the greater and lesser pelvis.
Boundaries of the inlet plane:
- posteriorly — sacral promontory (promontorium);
- anteriorly — upper margin of the pubic symphysis (symphysis pubica);
- laterally — arcuate (terminal) lines (linea terminalis), running along the arcuate line of the iliac bones and the crest of the pubic bones.
The shape of the inlet plane in women approximates a transverse oval, which is of fundamental obstetrical significance when assessing pelvic capacity.
Anteroposterior diameter: true and anatomical conjugate
Each plane of the lesser pelvis features an anteroposterior, transverse, and two oblique diameters. In the inlet plane, the anteroposterior diameter is represented by two measurements.
- True (obstetrical) conjugate — conjugata vera. Measured from the sacral promontory to the most posteriorly projecting point of the pubic symphysis. This point is usually located at the midpoint of the posterior surface of the symphysis. Length — 11 cm. The true conjugate is the key obstetrical diameter that determines whether the fetal head can pass through.
- Anatomical conjugate — conjugata anatomica. Measured from the sacral promontory to the upper inner margin of the pubic symphysis. Length — 11.5 cm. It is slightly larger than the true conjugate because it extends to a higher point on the symphysis.
The difference between these two measurements is due to the fact that the posterior surface of the symphysis has a slight protrusion, resulting in the shortest distance to the promontory being measured not from the upper margin, but from the middle part of the symphysis.
Transverse diameter of the inlet plane
The transverse diameter (diameter transversa) is defined as the distance between the two points on the terminal lines that are furthest apart from each other. Its length is 13 cm. This is the longest diameter of the inlet plane, and it is what primarily determines the transversely oval shape of the pelvic inlet.
Oblique diameters: right and left
There are two oblique diameters, and both run from the sacroiliac joint (articulatio sacroiliaca) on one side to the iliopubic eminence (eminentia iliopubica) on the opposite side.
- Right oblique diameter — from the right sacroiliac joint to the left iliopubic eminence.
- Left oblique diameter — from the left sacroiliac joint to the right iliopubic eminence.
Mnemonic rule: the side of the oblique diameter is determined by the sacroiliac joint from which it originates. The length of each oblique diameter is 12 cm.
Summary of inlet plane diameters
It is convenient to remember the numerical values using the rule of a 1 cm step starting from the true conjugate:
- True conjugate (anteroposterior diameter) — 11 cm.
- Anatomical conjugate — 11.5 cm.
- Oblique diameters (right and left) — 12 cm.
- Transverse diameter — 13 cm.
These values refer to the normal female pelvis and serve as benchmarks for the obstetrical assessment of its capacity. Narrowing of any of these diameters serves as a basis for diagnosing an anatomically contracted pelvis.
Key points
- The pelvic inlet is bounded by the sacral promontory, the upper margin of the symphysis, and the terminal lines.
- The inlet plane includes anteroposterior, transverse, and two oblique diameters.
- True conjugate (11 cm) runs from the promontory to the most prominent point of the posterior surface of the symphysis.
- Anatomical conjugate (11.5 cm) is measured to the upper inner margin of the symphysis.
- Transverse diameter (13 cm) — between the most distant points of the terminal lines.
- Oblique diameters (12 cm each) run from the sacroiliac joint to the opposite iliopubic eminence.
- The side of the oblique diameter is determined by the starting sacroiliac joint.
- Mnemonic rule: 11 — 11.5 — 12 — 13 cm.