General overview of the pelvis
Pelvis (pelvis) is a bony structure formed by the hip bones, sacrum, and coccyx. From anatomical and obstetrical viewpoints, it is conventionally divided into two parts: the greater pelvis (pelvis major) and the lesser pelvis (pelvis minor). This division is of fundamental importance because the lesser pelvis constitutes the birth canal and contains the urogenital organs and distal intestines.
The structure of the male and female pelvis is generally similar, but the female pelvis has a number of features: it is wider and lower, the iliac wings are more flared, the pelvic inlet is closer to an oval shape, and the subpubic angle is more obtuse. These features are adapted to the reproductive function.
Greater pelvis
The greater pelvis is the upper, wider part of the pelvic ring. Its boundaries are:
- posteriorly — bodies of the lower lumbar vertebrae;
- laterally — iliac wings (alae ossium ilii);
- anteriorly — the greater pelvis has no bony wall, being bounded by the anterior abdominal wall.
The greater pelvis serves as a receptacle for the lower abdominal organs (loops of the small intestine, sigmoid colon, and filled urinary bladder). In obstetrics, its measurement is used for indirect estimation of the dimensions of the lesser pelvis.
Boundary between the greater and lesser pelvis
The greater and lesser pelvis are separated by an imaginary plane passing along the so-called terminal line (linea terminalis) (linea terminalis). It is formed by:
- posteriorly — the sacral promontory (promontorium);
- laterally — the arcuate lines of the iliac bones (linea arcuata) and the pecten of the pubic bones (pecten ossis pubis);
- anteriorly — the upper margin of the pubic symphysis (symphysis pubica).
This same line also serves as the boundary of the plane of the pelvic inlet.
Walls of the lesser pelvis
The lesser pelvis has the shape of a truncated canal, unevenly narrowed at different levels. Its bony walls are:
- anterior — pubic symphysis and pubic rami;
- posterior — sacrum (os sacrum) and coccyx (os coccygis);
- lateral — inner surfaces of the hip bones, the acetabular region, and the ischial bones with their spines and tuberosities.
The cavity of the lesser pelvis is uneven: it is wider above, has a characteristic dilation in the middle part, and noticeably narrows toward the outlet. It is this irregularity that led to the identification of several planes.
Planes of the lesser pelvis
The lesser pelvis contains four classical obstetrical planes sequentially traversed by the fetal presenting part:
- Plane of the inlet of the lesser pelvis — bounded by the sacral promontory, terminal lines, and the upper margin of the pubic symphysis.
- Plane of the wide part of the pelvic cavity — corresponds to the most spacious level; passes through the middle of the inner surface of the acetabula and the articulation of the II and III sacral vertebrae.
- Plane of the narrow part of the pelvic cavity — passes through the lower margin of the pubic symphysis, ischial spines, and the sacrococcygeal joint.
- Plane of the outlet of the lesser pelvis — bounded by the lower margin of the symphysis, ischial tuberosities, and the tip of the coccyx.
Each plane is characterized by its own direct, transverse, and oblique dimensions, knowledge of which is essential for assessing the pelvis in obstetrics and understanding the biomechanism of labor.
Clinical significance
The anatomy of the female pelvis forms the basis of obstetric practice. The dimensions of the greater pelvis (distantia spinarum, cristarum, trochanterica, conjugata externa) are used for indirect assessment of internal dimensions. The parameters of the lesser pelvis planes determine whether the fetus can pass through the birth canal, while the shape and inclination of the planes determine the features of the birth biomechanism. Narrowing of any of the planes underlies the classification of the anatomically narrow pelvis.
Key points
- The pelvis is divided into the greater (pelvis major) and lesser (pelvis minor) pelvis.
- The boundary between them is the terminal line — linea terminalis.
- The line runs from the sacral promontory along the arcuate lines of the iliac bones to the upper margin of the pubic symphysis.
- The walls of the lesser pelvis are formed by the symphysis, hip bones, sacrum, and coccyx.
- There are four planes in the lesser pelvis: the inlet, the wide part, the narrow part, and the outlet.
- Each plane has its own dimensions, which are important for obstetrics.
- The features of the female pelvis (width, inlet shape, obtuse subpubic angle) ensure reproductive function.