The Bony Pelvis
The bony pelvis form the bony canal through which the fetus must pass during the normal birth process. If the canal is of the normal shape, and size, the baby of the normal size will negotiate it without difficulties,because pelvis vary in size and shape it is important that the midwife recognizes the normal pelvis so as to be able to detect deviation from the normal. One of the ways of estimating the progress of labor is by assessing the relationship of the fetus to certain pelvic landmarks.
· It connects the spine to the lower limbs
· It protects the female reproductive organs, bladder, the urethra ,colon, rectum and anal canal
· It allows movement of the body especially walking and running
· It permits sitting and kneeling
· It forms a bony passage for the fetus during labor
· It transmits the weight of the trunk to the legs and holds the two femurs
· Protects the pelvic organs and to a lesser extent the abdominal contents
· The Sacrum transmits cauda equina and distributes nerves to various parts of the pelvis
· Two innominate bones (hip bones),
· One sacrum and
· One coccyx
InnominateBones: each innominate bone is composed of three parts - the ilium, ischium and the pubis
· Ilium – large flared out part
· Ischium – the thick lower part
· Pubic bone – forms the anterior part
Sacrum is a wedge shaped bone consisting of five fused vertebrae. Upper border of the first sacral vertebral juts forward, known as SacralPromontory which is the most importantlandmark in the female pelvis. Anterior surface is concave, referred to as Hollow ofthe Sacrum. Lateral sacrum extends into a wing or ala. Posterior surface is roughened toreceive attachment of muscles. Two pairs of holes, or foramina, pierce the sacrum through which, nerve from the cauda equina emerge to supply pelvic organs.
Coccyx is a vestigial tail. It consists of four fused vertebrae forming a small triangular bone. The coccyx bends backwards at this joint during parturition to increase the anterior posterior diameter of the pelvic outlet.
· One symphysis pubis – formed at the joint of two pubic bones, united by a pad of cartilage known as the symphysis pubis
· Two (right and left) sacroiliac joints – is the strongest joint in the body articulates sacrum to ilium. Normally there are little or no movements in these joints, but during pregnancy especially towards the end there is a certain degree of movement due to the relaxation of the ligaments of the joints. This may give rise to difficulties in walking and backache, especially the multiparous women. There is little widening during labour , commonly referred to as “give” of the pelvis.
· One sacrococcygeal joint – join the base of the coccyx to the tip of the sacrum
· Inter pubic ligaments at the symphysis pubis
· Sacroiliac ligaments.
· Sacrotuberous ligament
· Sacrococcygeal ligaments.
· Inguinal ligament
The pelvis is divided into two parts, the true and the false pelvis. The false is the part above the brim. It has little importance in obstetrics
The true pelvis is the bony canal through whichfetus must pass during birth. It consists of brim, cavity and outlet. Brim is round except where sacral promontory projects into it. Commencing posteriorly the pelvic brim includes the following important landmarks.
· Sacral promontory
· Sacral ala or wing
· Sacroiliac joints
· Iliopectineal line
· Iliopectineal eminence
· Superior ramus of the pubic bone
· Upper inner border of the body of the pubic bone
· Upper inner border of the symphysis pubis
Diameters of the brim
· Antero-posterior diameter – from sacral promontory to upper most border of symphysis pubis 12cm. A measurement to the posterior border of the upper surface to a point 1.25cm lower is called the obstetrical conjugate, 11cm. It is the available space for the passage of the fetus hence it is called the true conjugate
· Diagonal conjugate is anteroposterior diameter from the lower border of the symphysis pubis to the centre of the sacral promontory measured vaginally for pelvic assessment 12-13 cm.
· Oblique diameter – from sacroiliac joint to the iliopectineal eminence on the opposite side (right and left). It measures 12cm
· Transverse diameter – it is between the points furthest apart on iliopectineal lines and measures 13cm. The fetal head commonly enters in transverse diameter of the pelvic brim
· Sacrocotyloid diameter – from sacral promontory to the iliopectineal eminence on each side, measures 9 – 9.5 cm
· The pelvic cavity extends from the pelvic brim above to the outlet below. Anterior wall is formed by pubic bones and symphysis pubis - depth is 4cm. The cavity is circular in shape and is considered to be 12cm all round.
Diameter of the outlet:
There are two Pelvic outlets: described as Anatomical Outlet and Obstetrical Outlet. Theanteroposterior diameter of outlet – from the lower border of the symphysis pubis to the sacrococcygeal joints 13cm
The oblique diameter of outlet - from the oburatorforamen to the sacrospinous ligament 12cm
The transverse diameter of outlet - is takenbetween two ischial spines 10 -11 cm which is the narrowest diameter of the pelvis
Pelvic inclination there is difference in theinclination of the pelvis when the woman is standing, sitting and recumbent position. The inclination of the outlet is 11O, cavity 30O, brim 60O, almost 90O in Negro woman
Pelvic planes these are imaginary flat surfacesat the brim, cavity and outlet of the pelvic canal. The fetus will enter at right angle to the plane according to the inclination.
Axis of the pelvic canal a line drawn exactly halfway between anterior wall and posterior wall of the pelvic canal to the plan of the outlet, cavity and the brim the curve it makes is known as the curve of Carus, the path which the fetus takes as it travels trough the birth canal.
· Gynaecoid pelvis – ideal pelvis for child bearing
· Android pelvis – resembles a male pelvis
· Anthropoid pelvis – has long oval brim in which anteroposterior diameter is longer than transverse diameters. Labor does not present any difficulties but favors occipitoanterior or occipitoposterior positions
· Platypelloid pelvis - flat with kidney shaped brim
Comparison of Male and Female Pelvis