PELVIS

female pelvis :-

 pelvis is composed of four bones—two innominate bones, sacrum

and coccyx. 

These are united together by four joints—

two sacroiliac joints,

 sacrococcygeal joint 

symphysis pubis.

The pelvis is anatomically divided into a false pelvis and a true pelvis, the boundary line being the brim of the pelvis. The bony landmarks on the brim of the pelvis from anterior to posterior on each side are—

upper border of symphysis pubis,

pubic crest

, pubic tubercle

, pectineal line,

iliopubic eminence

, iliopectineal line,

sacroiliac articulation, 

anterior border of

the ala of sacrum

  sacral promontory

                   FALSE PELVIS

The false pelvis is formed by the iliac portions of the innominate bones and is limited above by the iliac crests. It has got Mlittle obstetric significance except that its measurements can to a certain extent, predict the size and configuration of the true pelvis. Its only obstetric function is to support the enlarged uterus during pregnancy. Its boundaries are: posteriorly— lumbar vertebrae, laterally—iliac fossa and anteriorly—anterior abdominal wall.   

       TRUE PELVIS

This part of the pelvis is chiefly of concern to the obstetricians, as it forms the canal through which the fetus has to pass. It is shallow in front, formed by symphysis pubis and measures 4 cm (1 ½") and deep posteriorly, formed by the sacrum and coccyx and measures 11.5 cm (4 ½"). For descriptive purpose, 

it is divided into ---  inlet, cavity and outlet.

       INLET -Shape: It is almost round (gynecoid) with the anteroposterior diameter being the shortest. Other different shapes of the inlet are anthropoid, android and platypelloid.

In the erect posture, the pelvis is tilted forward. As such, the plane of the inlet makes an

angle of about 55° with the horizontal and is called angle of inclination. Another way of measuring the inclination radiographically is to take the angle between the plane of the inlet and the front of the body of the fifth lumbar vertebra. The angle is normally about 135°.

DIAMETERS -

Anteroposterior (Syn: true conjugate, anatomical conjugate, conjugate vera): It is the distance between the midpoint of the sacral promontory to the inner margin of the upper border of symphysis pubis. It measures 11 cm (4 ¼").

Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent

bony projection in the midline on the inner surface of the symphysis pubis . The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4").

Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint

on the sacral promontory. It measures 12 cm (4 ¾").

Oblique diameters:-There are two oblique diameters— right and left. Each one extends from one sacroiliac joint to the opposite iliopubic eminence and measures 12 cm (4 ¾"). Right or left denotes the sacroiliac joint from which it starts .

Transverse diameter: It is the distance

between the two farthest points on the pelvic

brim over the iliopectineal lines. It measures

13 cm (5 ¼").

CAVITY -

Cavity is the segment of the pelvis bounded above by the inlet and below by plane of least pelvic dimensions.

Shape: It is almost round.

Plane: The plane extends from the midpoint of

posterior surface of symphysis pubis to the junction of second and third sacral vertebrae.

Diameters: 

Anteroposterior (12 cm or 4 ¾"): It measures from the midpoint on the posterior surface

of the symphysis pubis to the junction of second and third sacral vertebrae (Fig. 9.9).

Transverse (12 cm or 4 ¾"): It cannot be precisely measured as the points lie over the soft tissues

covering the sacrosciatic notches and obturator foramina.

OUTLET -

Obstetrical outlet: It is the segment of the pelvis bounded above by the plane of least pelvic dimensions and below by the anatomical outlet Its anterior wall is deficient at the pubic arch;

its lateral walls are formed by ischial bones and

the posterior wall includes whole of the coccyx.

Shape: It is anteroposteriorly oval.

Plane: The plane is otherwise known as

plane of least pelvic dimensions or narrow

pelvic plane. The plane extends from the

lower border of the symphysis pubis to the tip

of ischial spines and posteriorly to meet the tip

of the fifth sacral vertebra.

Diameters:

Transverse—Syn: Bispinous

(10.5 cm or 4 1/5"): It is the distance between

the tip of two ischial spines.

Anteroposterior (11 cm or 4 ¼"): It extends

from the inferior border of the symphysis pubis to the tip of the sacrum (Fig. 9.9).

Posterior sagittal (5 cm or 2"): It is the distance between the tip of the sacrum and the midpoint of

bispinous diameter.

Axis: It is represented by a line joining the center of

the plane with the sacral promontory. Its direction is

almost vertical.

Anatomical Outlet: It is otherwise known as bony outlet. It is bounded in front by the lower border of the symphysis pubis; laterally by the ischiopubic rami, ischial tuberosity and sacrotuberous ligament and posteriorly by the tip of coccyx.

PHYSIOLOGICAL ENLARGEMENT OF PELVIS DURING PREGNANCY AND LABOR

Imaging studies show an increase in width and mobility of the symphysis pubis during pregnancy which returns to

normal following delivery. The pubic bones may separate by 5–10 mm. Similar changes also occur in sacroiliac joints.

There is gliding movement of the symphysis pubis near term. Relaxation of the pelvic joints is due to progesterone

and relaxin. There is increase of the anteroposterior diameter of the inlet during labor by the rotatory movement

of the sacroiliac joints. In dorsal lithotomy position, the anteroposterior diameter of the outlet may be increased to 1.5–2 cm. Furthermore, the coccyx is pushed back while the head descends down to the perineum

.