THE FETAL SKULL Dr. Basima Al- Ghazali

The skull divided into; the vault, face and base. By the time of birth the bones of the face and base are all firmly united, but the bones of the vault are not so well ossified, being joined only by un ossified membranes at the sutures. During labour the bones of the vault can undergo moulding, by which the shape of the skull can be altered by overriding the cranial bones with reduction of some of its diameters. The bones which form the vault are the parietal bones, and parts of the occipital, frontal, and temporal bones.

SUTURES : This lies in between two parietal bone. Sagittal suture:- This lies in between the frontal and parietal bone on either side. Coronal suture:- This lies in between two frontal bone. Frontal suture:- It lies in between the parietal and occipital bone on either side. Lambdoid suture:-

FONTANELLES The points of junction of the various sutures are termed fontanelle

Anterior fontanelle or bregma:- It is a diamond shaped area of unossified membrane formed by the junction of 4 suture. The suture are:- * Anteriorly:- frontal suture * Posteriorly:- sagittal suture * Laterally, on both side:-coronal suture. It is felt on fetal head surface as a soft shallow depression. It ossifies by 18 months after birth. Clinical importance:- 1. Degree of flexion can be assessed from its position. If on vaginal examination it is felt easily, it indicates the head is not well flexed. 2. It helps in the moulding of head. 3. From its position, internal rotation of the head can be assessed. 4. ICP can be roughly assessed from its condition after birth. Depression in dehydration and bulging in raised ICP.

Posterior fontanelle or lambda:- It is the triangular depressed area at the junction of the three suture. The suture are:- * Anteriorly:-sagittal suture * Posteriorly:-2 lambdoid sutures at both side. It ossifies as term. Clinical importance:- From its relation of the maternal pelvis, position of vertex is determined. Internal rotation can be assessed from its location. Degree of flexion can be assessed from its position. On vaginal examination if it is felt easily and anterior fontanelle is not felt, this indicates good flexion of the fetal head.

The area of the fetal skull bounded by the two parietal eminences and the anterior and posterior fontanelles is termed the vertex .it is the part of the head which present during labour.

DIAMETERS OF THE FETAL SKULL The diameters of the fetal skull which are important in the mechanism of labour may be divided into vertical, longitudinal, and transverse.

VERTICAL AND LONGITUDINAL DIAMETERS; The fetal head is ovoid in shape VERTICAL AND LONGITUDINAL DIAMETERS; The fetal head is ovoid in shape .In normal labour the head is well flexed so that the least diameters of the ovoid ,namely the suboccipito bregmatic and biparietal (transverse) diameters are those which engage .

Diameter of skull The engaging diameter of the fetal skull depends on the degree of the flexion of the presenting part. A. The antero-posterior diameter which may be engaged are:- 1- Sub-occipito bregmatic:- It extends from the nape of the neck (sub occipital region)to the centre of anterior fontanelle. Length:-9.5cm Attitude:-complete flexion Presentation:-Vertex. Clinical importance:- Smallest diameter.

2- If the head is less well flexed the suboccipito frontal diameter is involved. This is taken from the sub occipital region to the prominence of the forehead and measures 10cm.This is the diameter of the head which pass through the vulval orifice at the moment of delivery of the head. Presentation is vertex.

3- With further extension of the head the occipito frontal diameter engages .This measured from the root of the nose ( glabella) to the posterior fontanelle and is 11.5cm.This diameter meets the pelvis with a persistent occipito posterior position. Clinical importance:- This engaging diameter may give rise to prolonged labour.

4- The greatest longitudinal diameter is the mentovertical, which is taken from the chin to the furthest point of the vertex (centre of sagital suture ) and measures 13cm.This is the diameter which is thrown across the pelvis in a brow presentation and is too large to pass through a normal pelvis. Clinical importance:- In this engaging diameter, baby has to be delivered by caesarean section.

5- Beyond this point further extention of the head so that the face presents results in a smaller vertical diameter,i.e. the other end of the ovoid presents.The submento bregmatic diameter is taken from below the chin to the anterior fontanelle and measures 9.5cm.

6.Sub-mento vertical:- It extends from the junction of the floor of the mouth and neck to the center of the sagittal suture, Length:-11.5cm Attitude: -Incomplete extension. Presentation:-Face Clinical importance:- In this engaging diameter, baby has to be delivered by caesarean section.

B. The transverse diameter are:- 1. Bi parietal diameter:- It extend between 2 parietal eminences. Length:-9.5cm Attitude:-irrespective of position of head this diameter always engages. 2. Bi temporal diameter:- Distance between the anterior-inferior ends of the coronal suture. Length:- 8.5 cm

FETAL SKULL CHANGES IN LABOUR Moulding:-It is the changes in shape of the head in vertex presentation during labour while passing through the resistant birth canal.

Mechanism:- 1. Overlapping of cranial bones at the membranous joints due to compression of the engaging diameter of the head. 2. It is physiological, harmless and disappears within a few hours after birth.

PROCESSES OF THE DURA MATER The great folds of the dura mater, the falx cerebri and the tentorium cerebelli, act in some degree as internal ligaments, resisting too great deformation of the fetal head both in the longitudinal and the transverse directions . If moulding is excessive ,or if the fetal head is subjected to severe and sudden stresses , these parts of the dura mater are liable to be torn; some of the great venous sinuses are then in danger of rupture. These are the inferior longitudinal sinus, running in the free edge of the falx cerebri and receiving the great cerebral veins of Galen from the brain, and the straight sinus running between the falx cerebri and the tentorium cerebelli.

CAPUT SUCCEDANEUM It is localized area of edema on fetal scalp on vertex presentation due to pressure effect of dilating cervical ring and vaginal introitus. Characteristics:- It is physiological, present at birth and disappears within 24 hours. It is soft, diffuse and pits on pressure. No underlying skull bone fracture.

Cephalhematoma It is a collection of blood between periosteum and skull bone which is limited by the periosteal attachments at the suture lines. Characteristics:- Appears after 12 hours of birth. Limited by suture lines. Tends to grow larger. Disappears within 6-8 weeks. It is circumscribed, soft and non pitting. May be associated with skull bone fracture. Treatment:- No treatment required. The blood is absorbed and the swelling subside.

DIAMETERS OF THE FETAL TRUNK The biacromial diameter is taken between the parts furthest apart on the shoulders and is 12cm. The bitrochanteric diameter measures 10cm (for breech ).