Female Bony Pelvis And Fetal Skull

Dr Isameldin Elamin MD DOWH MBBSAssistant professor Obstetrics & Gynaecology Department

The bony pelvis is made up of four bones: ◦Sacrum.◦Coccyx.◦Two innominate bone.

Innominate bone composed of: Ilium. Ischium. Pubis.

Bony pelvis

They are held by three joints: Sacroiliac joints. Symphysis pubis. Sacrococcygeal joint. Union with the vertebral column lead to:◦Stabilizes the pelvis.◦Transmit weight to lower extremities.

Bony pelvis…CONT.

The sacrum:◦Five fused vertebrae.

Promontory: ◦Edge of the first sacral vertebra.◦Protrudes into the cavity of the pelvis.

Bony pelvis…CONT.

The sacrum articulates with: ◦ Ilium. ◦Coccyx. ◦Sacrospinous ligaments.◦Sacrotuberous ligaments.

The coccyx:◦Three to five rudimentary vertebrae.◦Articulates with the sacrum.

Bony pelvis…CONT.

The pelvis is divided into: False pelvis. ◦Bordered by:

The lumbar vertebrae. Iliac fossa. Abdominal wall.

◦ It supports the pregnant uterus.

Bony pelvis…CONT.

True pelvis.◦Bony canal formed by:

Sacrum Coccyx

ischium Pubis.

◦ It is the area of concern to the obstetrician.

Bony pelvis…CONT.

The pelvic brim or inlet bounded by:◦Symphysis pubis.◦Upper margin of the pubic bone.◦ Ileopectineal line and the Ala of the sacrum.◦Promontory of the sacrum.

Planes of pelvis

Transverse diameter is 13.5 cm. Anterior–posterior (AP) diameter 11 cm. The angle of the inlet is normally 60º. (Inclination

angle). In some women it may be as much as 90º.

Planes of pelvis

The pelvic mid-cavity bounded by:◦ Middle of the symphysis pubis.◦ Pubic bone.◦ Obturator fascia .◦ Ischial bone and spines.◦ The junction of the second and third sections of the sacrum. ◦ The cavity is almost round.◦ Transverse and anterior diameters are similar at 12 cm.

The ischial spines:◦ Palpable vaginally.◦ Used to assess the descent of the head. (Station). ◦ Used as landmarks to the pudendal block.

Planes of pelvis…CONT.

The pelvic outlet is bounded by:◦Lower margin of the symphysis pubis.◦Descending ramus of the pubic bone.◦ Ischial tuberosity.◦Sacrotuberous ligament.◦Last piece of the sacrum.

AP diameter is 13.5 cm. Transverse diameter is 11 cm.

Planes of pelvis…CONT.

Measurements related to:◦Maternal stature.◦Previous pelvic fractures.◦Metabolic bone disease, such as rickets.

Pelvic ligaments become more flexible towards labour.

X-rays or CT scans now uncommon to be used for predicting the outcome of labour.

Planes of pelvis…CONT.

Caldwell and Moloy classification:◦Gynaecoid pelvis. ◦Android pelvis.

◦Anthropoid pelvis. ◦Platypelloid pelvis.

Types of pelvis

The gynaecoid pelvis:◦ Most favourable for labour.◦ Most common.

An android pelvis:◦ Deep transverse arrest.

The anthropoid:◦ The antero-posterior diameter is larger than the transverse◦ Occipito-posterior (OP) position common.

A platypelloid pelvis:◦ Obstructed labour.

Types of pelvis…CONT.

Formed by the two levator-Ani muscles with their fascia.

The pelvic floor

The perineal body:

◦ Found between the vagina and the anus.

◦ It is a condensation of fibro muscular tissue.

◦ It receives attachments of: Bulbo-cavernous muscles.

The superficial and deep transverse perineal muscles.

External anal sphincter.

◦ It is involved in a second-degree perineal tear and

an episiotomy.

The perineum

Formed by:◦Bones, sutures and fontanelles.

Fetal skull is made up of:◦The vault.◦The face and the base. ◦The sutures are formed where the bones meet.

The fetal skull

The vault are formed by:◦The parietal bones.◦Part of occipital bones.◦Part of frontal bones.◦Part of temporal bones.

There are four sutures: ◦The sagittal.◦Frontal.◦Coronal.◦Lambdoidal sutures.

The fetal skull…CONT.

Fontanelles are the junctions of the varioussutures.

There are 2 fontanelles:◦The anterior fontanelle, or bregma: ◦Diamond shaped.◦At the junction of:

The sagittal suture. Frontal suture.

Coronal sutures.

The fetal skull…CONT.

The posterior fontanelle:◦Triangular shaped.◦At the junction of:◦The sagittal suture.◦The lambdoidal sutures.◦Between:

The two parietal bones. The occipital bone.

The fetal skull…CONT.

Clinical importance sutures and fontanelles :◦Overlap during labour.◦Bones are compressible.◦Moulding occur easily. ◦Severe moulding is a sign of cephalo-pelvic

disproportion. CPD. The ‘vertex’ is the area bounded by the two

parietal eminences and the anterior and posterior fontanelles.

The fetal skull…CONT.

The attitude is the degree of flexion and extension. There are different longitudinal diameters: Suboccipito-bregmatic diameter.◦ Well-flexed fetal head. ◦ It measures 9.5 cm.◦ From suboccipital region to anterior fontanelle (bregma).

Suboccipito-frontal diameter:◦ Less well flexed.◦ Found in the OP position. ◦ From the suboccipital region to the prominence of the

forehead. ◦ It measures 10 cm.

The diameters of the skull:

Occipitofrontal diameter:◦ Further extension of the head.◦ Measured from the root of the nose to the posterior fontanelle.

it measures 11.5 cm. Mento-vertical diameter:◦ It is the greatest diameter.◦ From chin to vertex and ◦ Measures 13 cm. ◦ Known as a brow presentation.◦ Too large to pass through the normal pelvis.

Submentobregmatic diameter:◦ Measured from below the chin to the anterior fontanelle.◦ It is 9.5 cm.◦ It is the face presentation.

The diameters of the skull:

The true conjugate (anatomic conjugate):

◦ From the middle of the sacral promontory to the superior

surface of the pubic symphysis.

The obstetric conjugate:

◦ From the middle of the sacral promontory to the closest point

on the convex posterior surface of the symphysis pubis.

◦ The shortest distance between the sacral promontory and

the symphysis pubis

How to estimate anteroposterior diameter of the inlet?

The diagonal conjugate:◦From the lower border of the pubis to the sacral

promontory . The obstetric conjugate is then estimated by

subtracting 1.5 to 2 cm from diagonal conjugate. Obstetric conjugate is adequate when:◦Middle finger cannot reach the sacral promontory.◦ If the diagonal conjugate is greater than or equal to

11.5 cm.

How to estimate anteroposterior diameter of the inlet? …CONT.

Obstetrics by ten teachers 19 editions. Essential of obstetrics and gynaecology. Hacker & Moore, fifth edition http://www.uptodate.com.

Further reading

Thank you