Fetal Skull

Matthew Idowu OLATUBI

Introduction

• The skull contain the brain (delicate)• Maybe subjected to pressure as the head passes

through the birth canal• Large in relation to the fetal body and mother’s

pelvis• The head is the most difficult to be born whether

it comes first or last.• Understanding of the landmarks and the

measurements of the fetal skull is important torecognize presentations and positions– Helps in making appropriate decision

Ossification

• Skull bones originate in 2 different ways• Face is laid in cartilage and is almost completely

ossified at birth• The bones of the vault are laid down in

membrane and are much flatter and more pliable• Ossify from the centre outwards and the process

is incomplete at birth leaving small gaps to formsutures and fontanelles

• Ossification centre on each bone appears as aboss or protuberance

• The fetal head consist of :

– The scalp,

– The skull, and

– Its contents

The Scalp

• The scalp is the outer soft part of the head

• It consist of:

– The skin on which the hair grows

– Connective tissue on which the skin rests

– Aponeurosis with some muscle fibres which aresituated mainly to the sides of the head

– Loose connective tissue

– Periosteum which covers the bones of the skull

Skull

• The skull is the skeleton of the fetal head

• consists of

• the dome-shaped portion called the vault,

• Underneath which is the base of the skull, and

• The face: composed of 14 small bones

Bones of the Vault

• There are five main bones in the vault

• The occipital bone

• Two parietal bones

• Two frontal bone

The Occipital Bone

• Lies at the back of the head and forms theregion of the occiput

• Part of it contributes to the base of the skull asit contains the foramen magnum

– Protect the spinal cord as it leaves the skull

• At the centre is occipital protuberence

Two parietal bones

• Lie on either side of the skull

• The ossification centre of each is called theparietal eminence

Two Frontal Bones

• Form the forehead or sinciput

• At the centre of each is a frontal boss orfrontal eminence

• The frontal bones fuse into a single by 8 yearsof age

• In addition to these five bones, the upper partof the temporal bone is also flat and forms asmall part of the vault

The sutures

• They are cranial joints and are formed wheretwo bones adjoin.

• The are of great obstetrical importance, theyallow overlapping of the bones of the vaultduring labour

• The overlapping is called moulding

• The sutures closes immediately after birth

Sutures Most obstetrical important sutures are• Lambdoidal suture: separates the occipital bone

from the two parietal bones• Sagittal suture: Lies between the two parietal bones• Coronal suture: separates the frontal bones from the

parietal bones, passing from one temple to the other• Frontal suture: runs between the two halves of the

frontal bone– Whereas the frontal suture becomes obliterated in time– The other sutures eventually become fixed joints

• Ossification of the skull is not complete until earlyadulthood

Fontanelles

• Fontanelles are membranous spaces at thepoint of junction of the sutures.

• Anterior fontanelle or bregma:

• Posterior fontanelle or lambda

Anterior fontanelle

• Found at the junction of the sagittal, coronaland frontal sutures

• It is broad, kite shaped and recognisablevaginally because a suture leaves from each ofthe four corners

• Measures 3 – 4 cm long and 1.5 – 2 cm wide

• Closes at 18 months

• Pulsations or cerebral vessels can be feltthrough it

Posterior fontanelle

• Shaped like Greek letter lambda – λ

• Situated at the junction of the lambdoidal andsagittal sutures

• It is small, triangular in shape and can berecognized vaginally because a suture leavesfrom each of the three angles

• Normally closes by 6 weeks age

Regions of the skull

• Occiput: region between the foramenmagnum and the posterior fontanelle– The part below the occipital protuberance is called

suboccipital region

– The protuberance can be seen and felt as aprominent point on the posterior aspect of theskull

• Vertex: bounded by the posterior fontanelle,the two parietal eminences and the anteriorfontanelle

Regions of the skull

• Sinciput or brow: extend from the antfontanelle and the coronal suture to theorbital ridges

• The face: small in newborn baby– Extend from the orbital ridges and the root of the

nose to the junction of the chin and the neck

– The point between the eyebrows is known as theglabella

– The chin is termed the mentum and it animportant landmark

Diameters of the fetal skull Two transverse diameters

• Biparietal – 9.5 cm – between the two parietaleminences

• Bitemporal: 8.2cm – between the furthest points ofthe coronal suture at the temples

Ant-post and longitudinal diameters

• Suboccipitobregmatic: 9.5cm – below the occipitalprotuberance to the centre of the bregma

• Suboccipitofrontal: 10cm – from below the occipitalprotuberance to the center of frontal suture

Diameters of the fetal skull

• Occipitofrontal: 11.5cm from the occipitalprotuberance to the glabella

• Mentovertical: 13.5cm point of the chin to thehighest point of the vertex, slightely rearer theposterior than to the anterior

• Submentovertical: 11.5cm point where the chinjoins the neck to the highest point on the vertex

• Submentobregmatic: 9.5cm from the point wherethe chin joins the neck to the center of thebregma

Attitude of the Fetal head

• Describe the degree of flexion or extension ofthe head on the neck

• Determines which diameter will present inlabour and influence the outcome

Attitude of the head

Presentation Engaging diameter and length

Complete flexion Vertex SOB 9.5cm

Poorly flexed head or deflexed head

Vertex OP position

SOF 10cm

Head in military attitude

Crown persisitentOP

OF 11.4

Head partially extended

Brow Mentovertical13.3cm

Head fully extended

Sub-mentobregmatic

9.5

Mouding • Changes in the shape of the fetal head that take

place during the passage through the birth canal

– Bones of the vault allow a slight degree of bending

– Skull bones are able to override at the sutures

• It allows for reduction in the size of the presentingdiameters while the diameter at the right angles tothem is able to lengthen owing to the give of theskull bones

• It is a protective mechanism and prevent the fetalbrain from being compressed as long as it is notexcessive, too rapid of in unfavourable direction

Mouding

• The skull of preterm babies are softer andwith wider sutures may mould excessively

• The skull of the post-term infants does notmould well and its greater hardness tend tomake labour more difficult

Assignment

• Read about intracranial membranes andsinuses