Anatomy of pelvis in relation to obstetrics and of the fetal skull

Dr Mu Mu WinSenior Lecturer

Faculty of MedicineUiTM

The bony pelvis

Bones forming the pelvis:

1.hip bones, left and right

a.pubic

b.ilium

c.ischium

2.sacrum

3.coccyx

Aspects of pelvic architecture

In a normal pelvis:

• Anterior superior iliac spines and the pubic symphysis are in the same coronal plane

The bony pelvis

Anatomical position of the pelvis

• Longitudinal axix of the symphysis is parallel to sacrum

• Tip of the coccyx and the upper margin of the pubic symphysis lie in the horizontal

plane

The pelvic inclination• Angle that any pelvic plane

makes with the horizontal line

Plane of the pelvic inlet(brim)

• Is directed downward and forward from the sacral promontry to the pubic symphysis

• Forms an angle of about 60 degrees with the horizontal plane

• In negros , this angle may approach 90 degrees and the fetal head may be slow to engage during labour

Plane of

inlet

Horizontal plane

Plane of outlet

Vertical plane

The pelvic inclination

Plane of the plevic outlet

• inclined about 25 degrees to the horizontal line

Plane of

inlet

Horizontal plane

Plane of outlet

Vertical plane

The pelvic axis

Axis of the pelvic cavity(Axis of the birth canal)

• The axis of the birth canal is the path followed by the fetal head in its course through the pelvic cavity

• It extends downward and backward in the axis of the inlet (ie. at a right angle to the plane of the inlet ) as far as the ischial spine

• The axis turns downward and forward ,at a right angle and parallel to the plane of the inlet

The bony pelvis

Joints of the pelvis:

• I .Lumbo-sacral joints

• ii. Sacro-iliac joints

• iiii.Sacro-coccygeal joints

• Iv.Pubic symphysis

The bony pelvis

Divisions of the pelvis:

1. Pelvis major (False pelvis , Greater pelvis)

i. Ala of the sacrum

ii. Iliac fossa

2. Pelvis minor (True pelvis , Lesser pelvis )

i. an upper pelvic apature

(pelvic inlet)

ii. a cavity ( pelvic cavity)

iii.a lower pelvic aparture

( pelvic outlet)

Planes and diameters of the pelvis

True pelvis has three planes of obstetrics significance

• The inlet

• Planes of least dimensions – or the mid plane

• The outlet

The pelvic cavity extends from the inlet to the outlet

Plane of

inlet

Horizontal plane

Plane of outlet

Vertical plane

Planes and diameters of the pelvisPelvic inlet

• Pelvic brim is the boundary line between the pelvic major and the pelvic minor (ie. The boundary line between the abdominal and pelvic cavities)

• The pelvic cavity is composed of:

a. promontry of sacrum

b. anterior border of ala of sacrum

c. arcuate line of ilium

d. pectinial line of pubis

e. pubic crest

f. upper end of pubic symphysis

Planes and diameter of the pelvisDiameter of the pelvic inlet

• 6 diameters of the inlet are customarily described

a. Anatomical conjugate (true conjugate,)

• Antero-posterior diameter extending from middle of sacrum promontry to middle of the upper margin of the symphysis pubis

• Normally 11 cm, of no obstetric significance

Planes and diameter of the pelvisb. Obstetric Conjugate

• Obstetrically important antero posterior diameter

• Shortest distance from the sacral promontry and the symphysis pubis

• Generally drawn from the middle of the sacral promontry to the closest point on the convest posterior surface of the symphysis pubis

• Approx; 11 cm

• Represent the actual space available to the fetus in negiotiating the pelvic inlet

• If OC less than 10 cm, it is considered contracted pelvis

Planes and Diameter of the Pelvis

C. Diagonal Conjugate

• Extend from the midpoint of sacral promontory to the midpoint of the inferior margin of the symphysis pubis

• Approx 12.5 cm

• It is the only diameter of the inlet that can be measured clinically

• By subtracting 1.5 cm from the DC, approx length of the OC can be obtained

Diagonal conjugate

Planes and diameter of the pelvisd. Transverse diameter

• Widest distance between the iliopectineal line which is perpendicular to the AP diameter

• Approx; 13.5 cm

e. Oblique diameter

• Extend from one sacroiliac joint to opposite iliopectineal eminence

• Designated right or left according to the sacroiliac joint from which it originates

• Approx; 12.75 cm

Oblique diameter

Transverse diameter

Anteroposteriordiameter

Planes and diameter of the pelvis

Midplane (plane of the least dimensions)

• Bounded;

• A. anterior – middle of the symphysis pubis

• B. lateral- pubic bone, obturator fascia, inner aspect of the ischial bones and spines

• C. posterior- junction of the 2nd and 3rd sections of the sacrum

Planes and diameter of the pelvis

Midplane

• A.Transverse diameter

• B. anterioposterior diameter

• Distances similar at 12 cm

• Ischial spines are palpable vaginally

Assessing descent of the fetal head by vaginal examination

Planes and diameter of the pelvis

Pelvic outlet

Boundaries of the pelvic outlet:

• Lower margin of the symphysis pubis,

• on each side by the descending ramus of the pubic bone, the ischial tuberosity and the sacrotuberous ligament,

• Last piece of the sacrum

Planes and diameter of the pelvisPelvic oulet • Anterioposterior diameter of

the outlet_ measures from inferior margin of the symphysis pubis to last piece of the sacrum.

• Approxi; 13.5 cm• Because the coccyx is usually

pushed out of the way by the advancing presenting part ,its not included in measurements of the outlet for obstetrics purpose

• Bituberous diameter –distance between inner aspects of the ischial tuberosities

• Transverse diameter is 11 cm

Classification of the pelvic type

Based on the shape of the pelvic inlet

• A. Anthropoid

• B. Platypelloid

• C. Android

• D. Gynaecoid

Classification of pelvic type

A. Gynaecoid(50%)

• Normal female pelvis and ideal for childbearing

• Has a round or transverse oval inlet

• Transverse diameter is greater than anterioposterior diameter

• 13.5 cm > 11 cm

• Forepelvis is wide and round

• Side walls are straight

• Sacraum usually well curve

• Wide sacrosciatic notch

• Ischial spines are everted (blunt)

• Pubic arch is wide

• Engagement occurs in the transverse or oblique anterior position followed by descend, anterior rotation and spontaneous vaginal delivery

B. Android (20%)

• Typical male type pelvis• Bone structure is heavy in comparison with other 3 pelvic types• Inlet is heart shaped or wedge- shape with a narrow and angulated

forepelvis• Prominent sacral promontry• Sacrum bone is long, flat and inclined forward

• Side walls are convergent producing a funnel pelvis

• Sacrosciatic notch is narrow

• Ischial spines are inverted and prominent

• Pubic arch is narrow

• Engagement usually occurs in transverse or posterior postion

• Frequent outcome is deep transverse arrest or arrest as an occipitoposterior with failure of rotation

Deep transverse arrest

Differences between Gynaecoid and Android Type

C. Anthropoid (25%)

• Inlet is oval, with the AP diameter is much longer than transverse diameter

• All the AP diameters are longer and all transverse diameters are shorter than in comparison with the average gynaecoid pelvis

• Forepelvis is oval and more narrow than in gynaecoid pelvis

• Side walls are generally straight

• Ischial spines are usually not encroaching

• Pubic arch is normal or relatively narrow but well shaped

• sacrum has an average curvature with a wide sacrosciatic notch ,thus creating an increased space in the post pelvis

• Engagement usually occurs in the anterioposterior or oblique diameter and occipitoposterior are common

• Fetuses in OP usually descend and deliver without rotating

• Progress is good for spontaneous vaginal delivery with increased frequency of OP deliveries

D. Platypelloid type (<3%)

• Flat pelvis _ rare

• Inlet is transverse oval (transverse diameter is longer than AP diameter)

• Characteristics of this pelvis are those of a gynaecoid pelvis that has been compressed in the anteriorposterior direction

• All the transverse diameters are long and all AP diameters are short

• If engagement occour ,it is transverse position , often with marked acyclinism

• Frequently there is associated with an increased risk of obstructed labour (eg; Brown presentation)

Clinical pelvimetry

Pelvimetry:Pelvimetry:

Measurement of the dimensions and Measurement of the dimensions and capacity of the pelviscapacity of the pelvis

• more accurate accomplished by more accurate accomplished by radiographic pelvimetry, however radiographic pelvimetry, however risks of radiation to fetusrisks of radiation to fetus

• Clinical pelvimetry – entails using Clinical pelvimetry – entails using hands to measure : hands to measure :

– certain pelvic diametercertain pelvic diameter

– Pelvic architecturePelvic architecture

– Predict the adequacy of the Predict the adequacy of the pelvis for a particular fetuspelvis for a particular fetus

Diagonal

Conjugate

• Fetal skull bone

Fetal relationship

• Engagement –the fetal is engaged if the widest leading part (typically the widest circumference of the head) is negotiating the inlet.

• Station –Relationship of the leading bony part of the fetus to the maternal ischial spines. If at the level of spines, it is at “zero” 0 station, if it passed it by 2 cm, it is at +2 station.

• Attitude– Relationship of the fetal head to spine,flexed, “ neutral” (military) or extended attitudes are possible.

• Position– Relationship of the presenting part to maternal pelvis, ie. ROP=Rt occiput posterior, LOA=Lt occiput anterior

Fetal relationship

• Presentation –Relationship between the leading fetal part and the pelvic inlet; cephalic, breech or shoulder presentation

• Lie – Relationship between the longitudinal axis of fetus and mother; longitudinal , oblique and transverse

• Caput or Caput succedaneum: edema typically formed by the tissue overlying the fetal skull during the vaginal delivery process.

Different positions of the fetal head