Planes of the lesser pelvis: general overview
Anatomically, four planes are distinguished in the lesser pelvis cavity: the pelvic inlet plane, the plane of the greatest pelvic dimension (wide part), the plane of the least pelvic dimension (narrow part), and the pelvic outlet plane. Each plane is characterized by its own bony landmarks and a set of obstetrically significant dimensions — anteroposterior, transverse, and oblique.
For the pelvic inlet plane, the following are determined: anteroposterior dimension (true conjugate, conjugata vera) — about 11 cm, transverse dimension — about 13 cm, oblique dimensions — 12 cm each. In addition to the true conjugate, the anatomical conjugate is distinguished (from the sacral promontory to the upper inner margin of the pubic symphysis). The terms pubic symphysis and pubic symphysis (joint) denote the same structure.
Boundaries of the wide part of the pelvic cavity
The wide part of the lesser pelvis cavity has the largest transverse diameter compared to other segments. Its boundaries run as follows:
- Posteriorly — the joint between the II and III sacral vertebrae (anterior surface of the sacrum at the level of the second to third sacral vertebrae).
- Anteriorly — the midpoint of the inner surface of the pubic symphysis.
- Laterally — the midpoint of the acetabulum on each side.
Thus, the plane of the wide part is displaced below the inlet plane: while the true and anatomical conjugates are oriented toward the upper margin of the symphysis and the sacral promontory, the wide part passes through the middle of the sacrum and the middle of the symphysis.
Dimensions of the wide part
Only two true dimensions are distinguished in the wide part — the anteroposterior and the transverse. Oblique dimensions are not described separately here; they are conventionally assumed to be approximately 13 cm.
- Anteroposterior dimension — from the junction of the II and III sacral vertebrae to the midpoint of the inner surface of the pubic symphysis. It coincides with the posterior and anterior boundaries of the plane. It is equal to 12.5 cm (according to some sources, 12 cm).
- Transverse dimension — between the midpoints of the acetabula of the right and left hip bones. It coincides with the lateral boundaries of the plane. It is equal to 12.5 cm.
- Oblique dimensions — are not separately distinguished; a conventional value of about 13 cm.
Important pattern: the anteroposterior and transverse dimensions of any pelvic cavity plane anatomically coincide with its anteroposterior and lateral boundaries, respectively. This rule simplifies remembering the landmarks for all four planes.
Comparison with the inlet plane
During the transition from the inlet plane to the wide part, the ratio of dimensions changes. At the inlet, the transverse dimension dominates (13 cm) over the anteroposterior one (11 cm) — the pelvis here is transversely "stretched". In the wide part, the anteroposterior and transverse dimensions equalize (12.5 cm each), and the plane acquires an approximately circular shape. This anatomically corresponds to the most spacious compartment of the lesser pelvis cavity, in which internal rotation of the fetal head occurs during labor.
Clinical and anatomical significance
Knowledge of the boundaries and dimensions of the wide part is necessary to understand the biomechanics of labor and assess the correspondence between the pelvic dimensions and the fetal presenting part. A reduction in the dimensions of the wide part plane is less common than narrowing of the inlet or narrow part, but must be taken into account when diagnosing a contracted pelvis. Bony landmarks (II–III sacral vertebrae, midpoint of the symphysis, midpoints of the acetabula) serve as reference points during pelvimetry and the interpretation of diagnostic imaging data.
Key points
- The wide part of the lesser pelvis cavity is the most spacious section of the pelvis with an approximately circular shape of the plane.
- Posterior boundary is the junction of the II and III sacral vertebrae; anterior boundary is the midpoint of the pubic symphysis.
- Lateral boundaries pass through the midpoints of the acetabula.
- The anteroposterior dimension of the wide part is 12.5 cm and coincides with its anteroposterior boundary.
- The transverse dimension is 12.5 cm and runs between the midpoints of the acetabula.
- There are no true oblique dimensions; the conventional value is about 13 cm.
- True conjugate (inlet plane) — 11 cm, transverse dimension of the inlet — 13 cm, oblique dimensions — 12 cm.
- Pubic symphysis and pubic articulation are synonymous terms.