The Fetal Skull

Question 1

Describe the different divisions of the fetal skull

Answer 1

• the vault
  - -> large, dome-shaped part above an imaginary line drawn between the orbital ridges and the nape of the neck
• the base
  - -> compromises bones that are firmly united to protect the vital centres in the medulla oblongata
• the face
  - -> composed of 14 small bones that are firmly united and no-compressible

Question 2

Describe the bones present in the vault

Answer 2

• the bones of the vault are laid down in membrane
• they harden from the centre outwards by ossification
  - -> ossification is incomplete at birth leaving small gaps between the bones (sutures and fontanelles)
• the ossification centre on each bone appears as a protuberance
• occipital bone
  - -> lies at the back of the head, part of it contributes to the base of the skull as it contains the foramen magnum which protects the spinal cord as it leaves the skull, the ossification centre is the occipital protuberance
• parietal bones
  - -> lie on either side of the skull, the ossification centre of each of these is called the parietal eminence
• frontal bones
  - -> form the forehead (sinciput), the ossification centre of each bone is the frontal eminence
• upper part of the temporal bone
  - -> on both sides of the head forms part of the vault

Question 3

Describe the different sutures and fontanelles present in the fetal skull

Answer 3

• sutures are the cranial joints formed where two bones meet
• where two or more sutures meet a fontanelle is formed
• lamdoidal 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
• posterior fontanelle (lambda) -> situated at the junction of the lamdoidal and sagittal sutures, it is small, triangular in shape and can be recognised vaginally as a suture leaves from each of the three angles
• anterior fontanelle (bregma) -> found at the junction of the sagittal, coronal and frontal sutures, it is broad, kite-shaped and recognised vaginally as a suture leaves from each of the 4 corners

Question 4

Describe the regions and landmarks of the foetal skull

Answer 4

• occiput region -> lies between the foramen magnum and the posterior fontanelle, the part below the occipital protuberance I known as the sub-occipital region
• vertex region -> bounded by the posterior fontanelle and the two parietal eminences and the anterior fontanelle
• forehead/ sinciput region -> extends from the anterior fontanelle and the coronal suture to the orbital ridges
• the face extends from the orbital regions and the root of the nose to the junction of the chin (mentum) and the neck, the point between the eyebrows is known as the glabella

Question 5

What are the different diameters of the fetal skull?

Answer 5

• sub-occipitobregmatic (SOB) diameter -> 9.5cm measured from below the occipital protuberance to the centre of the anterior fontanelle (bregma)
• sub-occipitofrontal (SOF) diameter -> 10cm measured from below the occipito protuberance to the centre of the frontal suture
• occiptofrontal (OF) -> 11.5cm measured from the occipito protuberance t the glabella
• mentovertical (MV) -> 13.5cm measured from the point of the chin to the highest point of the vertex
• sub-mento vertical (SMV) -> 11.5cm measured from the point where the chin joins the neck to the highest point of the vertex
• sub-mentobregmatic (SMB) -> 9.5cm measured from the point where the chin joins the neck to the centre of the anterior fontanelle (bregma)
• biparietal diameter -> 9.5cm the diameter between the two parietal eminences
• bitemporal diameter -> 8.2cm the diameter between the two furthest points of the coronal suture at the temples

Question 6

What is vertex presentation?

Answer 6

• when the head is well flexed the sub-occiptobregmatic diameter (9.5cm) and the biparietal diameter (9.5cm) present
• as these two diameters are the same length the presenting area is circular which is the most favourable shape for dilating the cervix and birth of the head
• the diameter that distends the vaginal orifice is the sub-occiptofrontal diameter (10cm)
• when the head is deflexed the presenting diameters are the occiptofrontal (11.5cm) and the biparietal (9.5cm)
  - -> this often occurs when the occiput is in a posterior position, if it remains so the diameter distending the vaginal orifice will be the occipitofrontal (11.5cm)

Question 7

What is face presentation?

Answer 7

• when the head is completely extended the presenting diameters are the sub-mentobregmatic (9.5cm) and the bitemporal (8.2cm)
• the sub-mentovertical diameter (11.5cm) will distend the vagina

Question 8

What is brow presentation?

Answer 8

• when the head is partially extended the mentovertical (13.5cm) and the bitemporal (8.2cm) present
• if this presentation persists, vaginal birth is unlikely

Question 9

What is moulding?

Answer 9

• the term moulding is used to describe the change in shape of the fetal head that takes place during its passage through the birth canal
• alteration is possible because the bones of the vault allow for a slight degree of bending and the skull bones are able to override the sutures
• the overriding allows for considerable reduction in the size of the presenting diameters while the diameter at right angles to them is able to lengthen owing to the give of the skull bones
• the shortening of the fetal head diameters may be as much as 1.25cm
• the moulding is also a protective mechanism and prevents the fetal brain from being compressed as long as it is not excessive, too rapid or in an unfavourable direction
• venous sinuses -> closely associated with the intracranial membranes, if they are torn due to excessive moulding or precipitate labour then there is a danger of bleeding, a tear of the tentorium cerrebelli may result in bleeding from the great cerebral vein

Question 10

Describe the internal anatomy of the fetal skull

Answer 10

• the vault of the skull protects the two cerebral hemispheres that make up the substance of the brain
• covering the brain are meninges which provide protection and blood to the cerebrum
• folds of the outermost layer of the meninges (dura mater) dip in between the hemispheres of the cerebrum
• the fold of dura mater situated between the two cerebral hemispheres is the falx cerebri
• a second fold runs horizontal to this between the cerebral hemispheres and the cerebellum - this is the tentorium cerebellum
• two sinuses containing blood run along the margins of both the falx cerebri and the tentorium cerebelli
• the superior sagittal sinus runs along the upper aspect of the falx cerebri and the inferior sagittal sinus runs along the lower aspect of the falx
• the straight sinus runs along the margin of the falx cerebri and the tentorium cerebelli
• two lateral sinuses pass along the outer aspect of the tentorium cerebelli
• the great vein of Galen which is situated at the junction of the falx cerebri and the tentorium cerebelli drains into the straight sinus