Review: wide plane of the lesser pelvis cavity
Before studying the narrow plane, it is necessary to briefly review the characteristics of the wide plane of the lesser pelvis cavity. This plane is bounded anteriorly by the middle of the inner surface of the pubic symphysis, posteriorly by the junction between the 2nd and 3rd sacral vertebrae, and laterally by the midpoints of the acetabula.
Two main diameters are distinguished in the wide plane:
- Anteroposterior diameter — from the junction of the 2nd and 3rd sacral vertebrae to the middle of the inner surface of the pubic symphysis, equal to 12.5 cm.
- Transverse diameter — between the midpoints of the acetabula, equal to 12.5 cm.
A feature of the wide plane is the absence of distinct oblique diameters, as the lateral walls in this plane are smooth.
Boundaries of the narrow plane of the lesser pelvis cavity
The narrow plane of the lesser pelvis cavity (plana interspinalis) lies inferior to the wide plane and is characterized by the smallest transverse diameters among all planes of the pelvic cavity. Its bony landmarks are strictly defined and used both in anatomical description and in obstetric practice.
- Anteriorly — lower margin of the pubic symphysis (symphysis pubica).
- Posteriorly — sacrococcygeal joint (articulatio sacrococcygea).
- Laterally — ischial spines (spinae ischiadicae).
Thus, the narrow plane passes through two fixed bony points posteriorly and anteriorly and through the symmetrical ischial spines laterally, forming a characteristic geometry important for understanding the biomechanics of labor.
Diameters of the narrow plane
Two classic diameters are distinguished in the narrow plane of the lesser pelvis cavity: anteroposterior and transverse. Oblique diameters are traditionally not considered in this plane.
- Anteroposterior diameter — from the sacrococcygeal joint to the lower margin of the pubic symphysis. Normally, it is 9.5 cm.
- Transverse diameter — between the ischial spines (interspinarum). It is 11.5 cm.
The anteroposterior diameter coincides with the anteroposterior boundary of the plane, and the transverse diameter with its lateral boundary, which is characteristic of all planes of the pelvic cavity.
Clinical significance in obstetrics
The narrow plane is of particular importance in obstetrics, since the presenting part of the fetus passes precisely through it during the second stage of labor. The anteroposterior diameter of 9.5 cm is one of the smallest in the pelvic cavity and potentially limits the descent of the fetal head.
However, during labor, an important biomechanical phenomenon occurs: the coccyx deviates posteriorly at the sacrococcygeal joint due to its mobility. This leads to an increase in the anteroposterior diameter by approximately 2 cm, and the actual anteroposterior diameter during the passage of the fetus reaches about 11.5 cm. This anatomical and functional expansion makes the physiological course of labor possible.
The transverse diameter between the ischial spines (interspinarum) remains practically unchanged, which is why it often becomes a critical indicator in assessing pelvic capacity and diagnosing a clinically narrow pelvis.
Key points
- The narrow plane of the lesser pelvis cavity is bounded anteriorly by the lower margin of the pubic symphysis, posteriorly by the sacrococcygeal joint, and laterally by the ischial spines.
- The normal anteroposterior diameter of the narrow plane is 9.5 cm.
- The transverse diameter (interspinarum) between the ischial spines is 11.5 cm.
- Oblique diameters are not distinguished in the narrow plane.
- During labor, the coccyx deviates posteriorly, increasing the anteroposterior diameter by about 2 cm.
- The narrow plane is the anatomically most restricted plane of the pelvic cavity.
- The transverse diameter between the ischial spines is of key importance in diagnosing a narrow pelvis.