Introduction:
Fetal skull is the bony part which protects the fetal brain & has to pass through the maternal pelvis to be born.
Parts of fetal skull:
- Vault
- Face
- Base
Bones of vault:
- Frontal bones-2
- Parietal bones-2
- Occipital bone-1
- Temporal bones-2
Sutures of vault:
- Frontal suture: Between 2 frontal bones
- Coronal suture: Between frontal & parietal bone
- Saggital suture: Between 2 parietal bones
- Lamdoidal suture: Between parietal & occipital bone
- Squamous suture: Between Temporal & parietal bone on either side.
Regions of fetal skull:
- Vertex: A quadrangular area bounded anteriorly by bregma & coronal suture Posteriorly by lamda & lamdoidal sutures & laterally by parietal eminence.
- Brow: An area bounded by bregma & coronal sutures on one side & root of nose & supra orbital ridges on either side.
- Face: Area bounded by root of nose & supra orbital ridges on one side & by the junction of floor of mouth with neck on the other side.
- Sinciput: An area lying in front of anterior fontanel & corresponds to area of brow.
- Occiput: Area limited to occipital bone.
Landmarks of fetal skull:
- Vertex
- Face
- Brow
- Glabella
- Occiput
- Sinciput
- Mentum
- Parietal eminence
Fontanelles of fetal skull:
Wide gap in the suture line is called fontanel.
ANTERIOR FONTANEL:
- Also called as bregma
- Shape: diamond
- Bounded by:
- Anteriorly- Frontal bone
- Posteriorly- Saggital suture
- Laterally- Coronal suture
- Floor is made by membrane
- Measures- 3-4 * 1.5-2.5 cm.
- Ossification at 18 months
Importance of anterior fontanel:
- Denotes degree of flexion by palpation during vaginal examination.
- Facilitates moulding
- Helps in accommodating fetal brain growth as the brain becomes almost double in size at the first year of life.
- Palpation of it reflects intracranial status:
- Depressed- Dehydration
- Elevated: Raised intracranial tension
- Blood collection or exchange transfussion in case of scalp infusion.
- Withdrawal of CSF .
POSTERIOR FONTANEL:
- Formed by junction of 3 sutures
- bounded by:
- Anteriorly- saggital suture
- Laterally- lamdoidal suture
- Shape- Triangular
- Measures about- 1.2 * 1.2 cm
- Ossification- 3 months.
Saggital fontanelle & Posteriolateral fontanelle both on either side have no clinical implications.
Moulding:
- Sliding of bones under each other thereby reducing diameters of fetal skull is called as moulding.
- During normal delivery an moulding of 4 mm diameter is common.
- Grade 1- Bones are not touching each other & no overlapping.
- Grade 2- Overlapping but easily separated.
- Grade 3- Fixed overlapping
Diameters of fetal skull:
1] Anterioposterior diameters
2] Transverse diameters
1] Anterio-posterior diameters:
- Suboccipito bregmatic diameter: Below the occipital protuberance to the center of bregma- 9.5 cm
- Subocciputo frontal diameter: Below the occipital protuberance to the center of frontal suture- 10 cm
- Occipito frontal diameter: From occipital protuberance to glabella- 11.5 cm
- Mento vertical diameter: Loingest diameter. From the midpoint of chin to highest point of vertex- 14cm
- Submento vertical diameter: From the point where chin joints the neck to highest point of vertex- 11.5cm.
- Submento bregmatic diameter: From the point where chin joints the neck to the center of bregma- 9.5cm.
2] Transverse diameters:
- Biparietal diameter: Longest transverse diameter. distance between to parietal eminences- 9.5 cm.
- Bitemporal diameter: Distance between 2 farthest point of coronal suture- 8 cm.
- Supra-sub parietal diameter: From the point below 1 parietal eminence to the point placed above the other parietal eminence on opposite side - 8.5 cm.
- Bimastoid diameter: Distance between 2 mastoid process- 7.5 cm.
Clinical Implications:
- The sutures & fontanels are used for identification of presentation & position of fetus.
- The engaging diameter is an important factor that determines the progress of labor.
