FETAL SKULL IN ENGLISH

                                               

                                       FETAL SKULL IN ENGLISH

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FETAL SKULL-

DIVISION OF THE FETAL SKULL

The fetal skull is divided into the vault, the base and the face.

The vault is the large, dome-shaped part above an imaginary line drawn between the orbital ridges and the nape of the neck.

The base comprises bones that are firmly united to protect the vital centres in the medulla oblongata.

The face is composed of 14 small bones that are also firmly united and non-compressible.

BONES OF THE VAULT

—  The bones of the vault are laid down in membrane.

—  They harden from the center outwards in a process known as ossification.

—  Ossification is incomplete at birth, leaving small gaps between the bones, known as the sutures and fontanelles.

The bones of the vault consist of:

—  The occipital bone, which 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 of occipital bone is the occipital protuberance.

—  The two parietal bones, which lie on either side of vault

—  The two frontal bones, which form the forehead or sinciput. The frontal bone fuse into a single bone by 8 yrs of age.

—  The upper part of the temporal bones on both sides of the head forms part of the vault.

Sutures and fontanelles

—  Sutures: they are cranial joints formed where two bones meet.

—  Fontanelles : it is formed when two or more suture meet.

—  Moulding : the sutures and fontanelle described above permit a degree of overlapping of the skull bones during labour.

SUTURES

—  The lambdoidal suture : it separates the occipital bone from the two parietal bones.

—   The sagittal suture  : it lies between the two parietal bones.

—  The coronal suture it separates the frontal bones from the parietal bones, passing from one temple to the other.

—   The frontal suture :  it runs between the halves of the  two frontal bone. 

FONTANELLES

—  The posterior fontanelle or lambda (shaped like the Greek letter lambda ) is situated at the junction of the lambdoidal and sagittal sutures. It is small, tri- angular in shape and can be recognized vaginally because a suture leaves from each of the three angles. It normally closes by 6 weeks of age.

—  The anterior fontanelle or bregma : it  is found at the junction of the sagittal, coronal and frontal sutures. It is broad, kite-shaped and recognizable vaginally because a suture leaves from each of the four corners. It measures 3-4 cm long and 1.5-2 cm wide and normally closes by 18 months of age. 

Regions and Landmarks of the Fetal Skull

—  The skull is further separated into regions, and within these, there are important landmarks.

—  These landmarks are useful to the midwife when undertaking a vaginal examination, as they help to ascertain   the position of the fetal head.

—  The occiput region lies between the foramen mentum and the posterior fontanelle

—  The part below the occipital protuberance (landmark) is known as the sub occipital region.

—  The vertex region is bounded by the posterior fontanelle, the two parietal eminences and the anteriorfontanelle.

—  The forehead/sinciput region extends from the anterior fontanelle and the coronal suture to the orbital ridges.

—  The face extends from the orbital ridges and the root of the nose to the junction of the chin or mentum (landmark) and the neck. The point between the eye-brows is known as the glabella

Diameters of the Fetal Skull-

—  Knowledge of the diameters of the skull alongside the diameters of the pelvis allows the midwife to determine the relationship between the fetal head and the mother's pelvis.

—  There are six longitudinal diameters  and two transverse diameters in fetal skull

LONGITUDINAL DIAMETERS-

1. The suboccipitobregmatic (SOB) diameter (9.5 cm) measured from below the occipital protuberance to the centre of the anterior fontanelle or bregma.
2. The suboccipitofrontal (SOF) diameter (10 cm) measured from below the occipital protuberance to the centre of the frontal suture.
3. The occipitofrontal (OF) diameter (11.5 cm) measured from the occipital protuberance to the glabella..
4. The mentovertical (MV) diameter (13.5 cm) measured from the point of the chin to the highest point on the vertex
5. The submentovertical (SMV) diameter (11.5 cm) measured from the point where the chin joins the neck to the highest point on the vertex•
6.  The submentobregmatic (SMB) diameter (9.5 cm) measured from the point where the chin joins the neck to the centre of the bregma (anterior fontanelle).

TRANSVERSE DIAMETERS-

1.      The biparietal diameter (9.5 cm) the diameter between the two parietal eminences.

2.      The bitemporal diameter (8.2 cm) - the diameter between the two furthest points of the coronal suture at the temples.

 

Diameters of the Fetal trunk-

—  Biacromial diameter 12 cm This is the distance between the acromion processes on the two shoulder blades and is the dimension that needs to pass through the maternal pelvis for the shoulders to be born. The articulation of the clavicles on the sternum allows forward movement of the shoulders, which may reduce the diameter slightly.

—  Bitrochanteric diameter 10 cm This is measured between the greater trochanters of the femurs and is the presenting diameter in breech presentation

—   

Presenting diameters-

—  The presenting diameters of the head are those that are at right-angles to the curve of Carus of the maternal pelvis. There are always two:

—   a longitudinal diameter and

—  a transverse diameter.

—  The presenting diameters determine the presentation of the fetal head,

—  Some presenting diameters are more favourable than others for easy passage through the maternal pelvis and this will depend on the attitude of the fetal head.

—  This term attitude is used to describe the degree of flexion or extension of the fetal head on the neck. The attitude of the head determines which diameters will present in labour 

 

Presentation of the fetal head-

—  1. Vertex presentation. When the head is well flexed the sub-occipitobregmatic diameter (9.5 cm) and the biparietal diameter (9.5 cm) present. It is the most favourable shape for  dilating the cervix and birth of the head.

—  2. Face presentation. When the head is completely extended the presenting diameters are the submentobregmatic (9.5 cm) and the bitemporal (8.2 cm). The sub-mentovertical diameter (11.5 cm) will distend the vaginal orifice.

—  3. Brow presentation. When the head is partially extended, the mentovertical diameter (13.5 cm) and the bitemporal diameter (8.2 cm) present. If this presentation persists, normal vaginal birth is not possible

 

Moulding-

—  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 in shape is possible because the bones of the vault allow a slight degree of bending and the skull bones are able to override at the sutures