CHAPTER 7

FEMALE PELVIS

INTRODUCTION TO PELVIS

• The main function of the pelvis is to allow movement of the body.
• A woman’s pelvis is adapted for childbearing in that in comparison to the male pelvis the brim is rounder and wider.
• The pelvis contains and protects the reproductive organs as well as the bladder and rectum.
• While sitting, the weight of the body is taken on by ischial tuberosities.
• Pelvis helps in transmitting weight of the body to the legs.

NORMAL FEMALE PELVIS

The female pelvis provides the basic framework of the birth canal .

BONES

The pelvis girdle, a basin-shaped cavity, present between the lower section of the vertebral column and the lower limbs. The pelvis consists of four bones (Fig. 1).

• 2 innominate bones
• 1 sacrum
• 1 coccyx bone

Innominate Bones

Each innominate bone is made by the fusion of the ilium, the ischium, and the pubis.

Ilium

It is the large flared-out part of the innominate bone. The concave anterior surface of the ilium is called iliac fossa. A bony prominence is at the front of the iliac crest known as the anterior superior iliac spine and just below it is the anterior inferior iliac spine. Similar bony prominences are there at the posterior part known as posterior superior iliac spine and posterior inferior iliac spine .

Pubic Bone

The pubic bone constitutes the anterior part of the innominate bone. It has two structures, the superior ramus and the inferior ramus. The two pubic bones are joined together with the help of a ligament called symphysis pubis. The two inferior ramus come together and make an arch called the pubic arch. The space enclosed by the pubic bone, the rami and the ischium is called the obturator foramen.

Acetabulum

It is a cup-shaped structure present in the innominate bone. It lodges the head of the femur. All three bones fused to form acetabulum, i.e., ilium, ischium, and pubis in the ratio of 2:2:1.

Sacrum

The sacrum is a wedge-shaped bone formed by the fusion of five sacral vertebrae. The upper border of the first sacral vertebra is known as sacral promontory. The anterior surface of the sacrum is concave, lateral side of the sacrum has a wing or ala. Cauda equine emerges from the four pairs of foramina present in the sacrum to supply the pelvic organs. The posterior surface of the sacrum provides the site for the attachment of the muscles .

Coccyx

It is a vestigial tail. It is formed by the fusion of four coccygeal vertebrae to develop into a small triangular structure.

anterior view of the female pelvis

posterior view of the female pelvis

JOINTS IN FEMALE PELVIS

Female pelvis has four joints

• Two sacroiliac joints
• One sacrococcygeal joint
• One symphysis pubis

PELVIS LIGAMENTS

The pelvic joints are held together with the help of pelvic ligaments and are designed to reduce the movements.

• Sacroiliac ligaments: Connecting the sacrum and ilium.
• Sacrospinous ligaments: Connecting the sacrum with the spinous process.
• Sacrotuberous ligaments: Connecting the sacrum with the ischial tuberosity.
• Sacrococcygeal ligaments: Connecting the sacrum with the coccyx.
• Interpubic ligaments: Connecting both the pubic bones together

FALSE AND TRUE PELVIS

The obstetric pelvis is divided into false and true pelvis by the pelvic brim or inlet.

Greater Pelvis (False)

It is the part of the pelvis which is present above the pelvic brim or the iliac crest. Its function is to support the growing uterus during pregnancy by distributing weight of abdominal organs and gravid uterus.

Lesser Pelvis (True)

It is the part of the pelvis that has an obstetrical significance as the fetus has to pass through this canal. It is divided into the inlet, cavity and outlet.

• Pelvic inlet: Lies at the level of the pelvic brim.
• Pelvic cavity/mid pelvis: Lies at the level of ischial spine.
• Pelvic outlet: Lies at the level of ischial tuberosity.

INLET

The landmarks of the inlet or the brim are as follow :

Sacral promontory

Sacral wing or ala

Sacroiliac joint

Ilio pectineal line

Ilio pectineal eminence

Superior ramus of pubic bone

Upper inner border of pubic bone

Upper inner border of symphysis pubis

PLANE

It is the imaginary flat surface of the pelvic brim which is bounded by the bony points.

INCLINATION

The pelvic brim is not located in the horizontal plane when the subject stands erect. Thus it makes a 55-60° to the horizontal plane .

DIAMETERS

External Conjugate

It is the space between the upper margin of the pubic symphysis to the tip of the S1 vertebral spine .

True Conjugate

It is the space between the midpoint of the sacral promontory to the upper margin of the pubic symphysis. It corresponds to the anterior-posterior diameter. It is also known as anatomical conjugate. It measures about 11 cm.

Diagonal Conjugate

It is the space from the midpoint of the sacral promontory to the lower margin of the pubic symphysis. It measures about 12.5 cm.103

Obstetrical Conjugate

It is the shortest distance between the sacral promontory and the pubic symphysis. It measures about 10 cm.

Oblique Diameter

It is measured from the sacroiliac joint of one side to the iliopectineal eminence of the other side. It measures about 12 cm .

Transverse Diameter

It is the maximum width of the pelvic brim. It measures about 13 cm.

SACROCOTYLOID

It is the distance between the midpoint of the sacral promontory to iliopubic eminence. It measures about 9.5 cm.

CAVITY

The cavity is the part of the pelvis which is bounded above by the inlet and below by the outlet. It is almost round in shape.

Plane

It extends from the posterior surface of the pubic symphysis to the junction of the second and third sacral vertebrae.

Diameters

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• Anterioposterior: It goes from the middle of the pubic symphysis to the junction of the 2nd and 3rd sacral vertebrae.

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• Oblique diameter: It goes from the lower end of the sacroiliac joint of a single side to the middle of the obturator membrane of the different side.
• Transverse: It is the best width of the pelvic cavity measured between the points of sacrosciatic notches and obturator foramina.
• Anterior-posterior, transverse and oblique diameters of the cavity are the same. They measure about 12 cm each.

OUTLET

It is the segment of the pelvis bounded above by the cavity and below by the anatomical outlet. It is oval in shape.

Plane

This plane is also known as the plane of the least pelvic dimension. It extends from the lower border of the symphysis pubis to the tip of the ischial spines and posteriorly to meet the tip of the 5th sacral vertebrae.

Inclination

The pelvic outlet forms an angle of 15° with the horizontal surface

Diameters

• Anterioposterior: It is 13 cm, measured from the tip of the sacrum to the lower border of the pubic symphysis.
• Oblique: It is 12 cm, measured from the junction of the ischiopubic ramus of a single side to the middle of sacrotuberous ligament of the opposite side.
• Transverse: It is 11 cm, and measures between the two ischial tuberosities.
• Bituberous diameter: 11 cm, measures between inner border of ischial tuberosities
• Bispinous diameter: 10.5 cm, it measures between the tip of two ischial spines

PELVIS VARIATION

The size and the shape of the female pelvis varies as got influenced by various factors such as developmental, sexual or evolutionary factors. On the basis of shape, the female pelvis is divided into four parent types

• Gynecoid pelvis
• Anthropoid pelvis
• Android pelvis
• Platypelloid pelvis