maternal pelvis

function of pelvis

transmits bodyweight to legs which enables mobility

gives flexibility of movement when upright

enables parturition

protects the reproductive organs

how many bones does the pelvis comprise of

4

what are the 4 bones

2 innominate bones

sacrum

coccyx

these bones enclose the pelvic cavity

where are the innominate bones joined

in the front (anteriorly) at the symphysis pubis

where do the four bones connect at

2 sacroiliac joints

symphysis pubis

sacrococcygeal joint

what are the 3 components of each innominate bone

ilium

ischium

pubis

pelvic ligaments

interpubic ligaments

sacroiliac ligaments

sacrococcygeal ligaments

sacrotuberous ligaments

sacrospinous ligaments

where is the ischium

lower part of pelvis (L- shaped)

what does the ischium connect to

the ilium at the back (posteriorly) and the pubis anteriorly

what thickened area of bone does the ischium have

ischial tuberosity

what is the ischial tuberosity

thickened area of bone in ischium

structure that the body rests on when a person is in a seated position

what is behind and slightly above the ischial tuberosity

ischial spine

ischial spine

when assessing a woman’s progress in labour and descent of foetus through the pelvis is estimated in relation to the level of the ischial spines

what are the components of the pubis

main body

2 arm-like structures which protrude out

what are the parts that protrude out in the pubis

rami (on either side)

superior (upper) ramus

inferior (lower) ramus

what does the inferior ramus attach to

ischium

what does the superior ramus attach to

ilium at the iliopectineal eminence

how much of the acetabulum does the superior ramus form

one-fifth

where do the two pubic bones meet

symphysis pubis

what to the two inferior rami form

the pubic arch

what does the suprapubic angle need to be and why

at least 90

in order to allow the baby to pass underneath it

what is the obturator foramen

triangular shaped space enclosed by the body of the pubic bone, rami and ischium

each innominate bone has a large cup shaped socket on it’s surface what’s it called

acetabulum

what is the acetabulum

it is with this that the head of the femur (thigh bone) articulates to form a person’s hip joint

there are 2 curves on the lower aspect of the innominate bone

greater sciatic bone

lesser sciatic bone

describe the greater sciatic notch

wide and rounded extending from the posterior inferior iliac spine up to the ischial spine

what can the size of the notch be used for

assessing the overall shape and size of the pelvis

where is the lesser sciatic notch

between the ischial spine and ischial tuberosity

what does the sacrum form

the back wall of the pelvis

what does the sacrum comprise of

5 vertebrae that are fused together to form a wedge shaped bone

what is the protrusion on the first vertebrae known as

the sacral promontory

what happens if the sacral promontory is too pronounced

it can impede fetal descent through the pelvis

what are on each side of the first sacral vertebra

widened wings of bone- sacral alae

what is the sacral bone perforated by

fur sets of foramina (holes)

how do the four sacral nerves pass

through the perforation

what shape is the anterior surface of the sacrum

concave

what role does the sacrum have

guiding the baby through the maternal pelvis and as part of this navigating around the curve of carus

foetus descends during labour and birth

what is the coccyx

small triangular bone that articulates with the lower end of the sacrum

what does the coccyx comprise of

4 fused vertebrae

what happens with the coccyx during childbirth

moves backwards to help enlarge the pelvic outlet, which assists the baby’s passage to facilitate a vaginal birth

what does the coccyx provide attachment points for

pelvic ligaments, the muscle fibres of the anal sphincter, and the ischiococcygeus muscle of pelvic floor

what is the true pelvis

bony canal that the foetus needs to pass through during the normal mechanism of childbirth (negotiates passage)

what does the true pelvis incorporate

the portions of the pelvis that are below the oblique plane of the pelvic brim

what is the false pelvis

portion of the pelvis that is above the pelvic brim- no impact on birth

what does the true pelvis comprise of

brim, cavity, and an outlet

what does the the pelvic brim separate

the false pelvis, which lies above, from the true pelvis that is below

what is the shape of the brim

round with the exception of the sacral promontory, which protrudes into it posteriorly

what does the pubic bone form

the anterior border of pelvic brim

what is the iliac bones, its lateral borders and the posterior border formed by

the promontory and sacral alae

what are the 3 diameters that are measured (pelvic brim)

anterior posterior diameter

oblique diameter (left and right)

transverse diameter

note down all the landmarks 1-8

1) sacral promontory

2) sacral ala

3) sacroiliac joint

4) iliopectineal line

5) iliopectineal eminence

6) superior ramus of pubic bone

7) upper inner border of the body of the pubic bone

8) upper inner border of the symphysis pubis

diameters of pelvic brim

what is the pelvic cavity

extends from the pelvic brim to the pelvic outlet

what does the pelvic cavity form

curve of carus

what is the curve of carus

foetus has to navigate in order to be born and has no specific landmarks

in the anteroposterior view, what does the cavity look like

wedge shaped- shallow at the front and deep at the back

what does the cavity look like if viewed from above in a gynaecoid pelvis

circular in shape and designed to facilitate the descent and rotation of the presenting part

what are the boundaries of the cavity

curve of sacrum

sacroiliac joints

sacrospinous ligaments

ischia

superior pubic ramus

inferior pubic ramus

bodies of the pubes

symphysis pubis

what is the shape of the pelvic outlet

either ovoid or diamond shaped space

what is the perimeter of the pelvic outlet partially comprised of

ligaments

what can the pelvic outlet be described by

anatomical structure

obstetric dimension- space available through which the baby must pass during birth

what are the anatomical boundaries of the pelvic outlet

tip of coccyx

sacrotuberous ligaments

ischial tuberosities

pubic arch

what is the obstetric outlet bounded by

inner border of the base of sacrum- as a result of the coccyx being deflected outwards during childbirth, thus enlarging the outlet

sacrospinous ligaments

ischial spines

lower inner border of the symphysis pubis

the bones of the pelvic outlet are also points of attachment for

the muscles of the pelvic floor and perineum

pelvic measurement of the brim- anteroposterior

11cm

pelvic measurement of brim- right and left oblique

12cm

pelvic measurement of brim- transverse

13cm

pelvic measurement of cavity- anteroposterior

12cm

pelvic measurement of cavity- right and left oblique

12cm

pelvic measurement of cavity- transverse

12cm

pelvic measurement of outlet- anteroposterior

13cm

pelvic measurement of outlet- anteroposterior

13cm

pelvic measurement of outlet- right and left oblique

12cm

pelvic measurement of outlet- transverse

11cm

why are the various diameters significant

help with the successful passage of foetus through the bony pelvic structure

what are pelvic planes

hypothetical flat surfaces on the pelvis, which is located at the brim, cavity and outlet

taking the pelvic brim as our landmark, what’s the plane of the brim

at an angle of 55 degrees to the horizontal

what does the term plane describe

relationship between the pelvis and a flat surface

hypothetical angles are then created in relation to the degree of tilt of a particular individual, which provides a representation of the angles in relation to the planes of the pelvis

curve of carus

imaginary line through which a foetus rotates as it passes through the pelvis

what does the shape of the pelvis determine

the availability of pelvic diameters during childbirth

gynaecoid

classic

inlet is transversely oval

roomier, shallow pelvic cavity with a broad well curved sacrum

sub-pubic angle of 90

blunt ischial spines

android

more masc. in it’s shape and diameters

heart shaped inlet

funnel shaped deep cavity

sacrum is straight—> contracted pelvic outlet

sub-pubic arch has an angle less than 90

ischial spines are prominent—> hinder internal rotation of fetal head—> deep transverse arrest

anthropoid

results from high assimilation i.e the sacral body is assimilated to the fifth lumbar vertebra

pelvic brim is long, narrow and oval in shape

sacrum is long and concave

sub-pubic angle is very wide

ischial spines aren’t prominent

platypelloid

wide pelvis that is characteristically flattened at the brim with the promontory of the sacrum pushed forwards—> forms a kidney shaped pelvic brim

sacrum is flat

pelvic cavity is shallow

sub-pubic angle is greater than 90 degrees

ischial spines are blunt—> fetal descent through cavity is usually unproblematic

gynaecoid- shape of brim

round

gynaecoid- subpubic arch

85-90 degrees

gynaecoid- ischial spines

not prominent, blunt

gynaecoid- sacrum

deep and curved

android- shape of brim

triangular, heart shaped

android- subpubic arch

60-75 degrees (narrow)

android- ischial spines

prominent and narrow interspinous diameter

android- sacrum

straight- flattened and long

anthropoid- shape of brim

oval (widest in the anteroposterior diameter)

anthropoid- subpubic arch

more than 90 degrees

anthropoid- ischial spines

not prominent but may have narrowed interspinous diameter

anthropoid- sacrum

long and narrow- may be slightly curved

platypelloid- shape of brim

bean shaped- flattened

platypelloid- subpubic arch

more than 90 degrees

platypelloid- ischial spines

blunted, usually widely separated- not prominent