parietal bone : easy lecture note for viva

 

Parietal Bone

1. Anatomical Points:

• The superior border is the longest, thickest, and serrated.

• The superior border lies in the median plane.

• The external surface is convex.

• The anterior-inferior angle is acute and internally marked by a groove for the anterior division of the middle meningeal vessel.

2. Morphological Type:

• Pneumatic flat bone.

3. Structure of the Bone:

• Composed of an inner and outer table (compact bone) with a middle diploë (spongy bone).

4. Ossification:

• Intramembranous ossification.

5. Development:

• Derived from the neural crest.

6. Parts and Identifications:

Borders: Superior, anterior , posterior and inferior borders 

Articulations:

• Superior border: Articulates with the opposite parietal bone, forming the sagittal suture.

• Anterior border: Articulates with the frontal bone, forming the coronal suture.

• Posterior border: Articulates with the occipital bone, forming the lambdoid suture.

• Inferior border: Articulates with:

• The greater wing of the sphenoid bone.

• The squamous part of the temporal bone.

• The petrous part of the temporal bone.

Anterior and posterior borders: Straight and serrated.

Superior border

• Longest, thickest, and serrated. Internally marked by the sagittal sulcus, which lodges the superior sagittal sinus. Granular pits present on the sides of the sagittal sulcus lodge arachnoid granulations.

Arachnoid Granulations & CSF Circulation

Arachnoid Granulations:

• Small protrusions of the arachnoid mater into the dural venous sinuses.

• Primarily located along the superior sagittal sinus.

• Function: Facilitate cerebrospinal fluid (CSF) drainage into the venous system.

CSF Circulation:

1. Production: CSF is produced by the choroid plexus in the lateral, third, and fourth ventricles.

2. Flow Pathway:

• Lateral ventricles → Interventricular foramen (Foramen of Monro) → Third ventricle

• Third ventricle → Cerebral aqueduct (Aqueduct of Sylvius) → Fourth ventricle

• Fourth ventricle → Median aperture (Foramen of Magendie) & Lateral apertures (Foramina of Luschka) → Subarachnoid space → Arachnoid granulation (contain arachnoid villi ) → Superior sagittal sinus 

3. Absorption: CSF is absorbed into the dural venous sinuses via arachnoid granulations.

Clinical Significance:

• Blockage of CSF circulation mainly due to blockage of cerebral aqueduct can lead to hydrocephalus, causing increased intracranial pressure.

• Inferior border 

• Irregular and serrated. Internally marked by the posterior division of the middle meningeal artery (at the middle of the inferior border).

7. Pterion:

• Definition: The union of four bones:

1. Frontal bone

2. Anterior-inferior angle of the parietal bone

3. Greater wing of the sphenoid

4. Squamous part of the temporal bone

• Clinical Importance: The internal surface of the pterion is lodged by the anterior division of the middle meningeal artery.

• 

8. Angles of the Parietal Bone:

• Anterior superior angle: Forms the bregma.

• Posterior superior angle: Forms the lambda (called the posterior fontanelle in children).

• Anterior inferior angle: Forms the pterion and is internally marked by a groove for the anterior division of the middle meningeal artery.

• Posterior inferior angle: Forms the asterion and contains an internal groove that lodges the sigmoid sinus.

Fontanelle: Location and Duration of Closure

Fontanelles are soft, membranous gaps between the bones of an infant’s skull that allow for brain growth and flexibility during birth. There are six fontanelles in total, but the two most clinically important are:

1. Anterior Fontanelle

• Location: Between the frontal and parietal bones. 

• Closure: Typically closes by 12-18 months (may extend up to 2 years). Then this area known as bregma 

2. Posterior Fontanelle

• Location: Between the occipital and parietal bones.

• Closure: Closes by 6-8 weeks after birth.then this area is known as lambda 

Other Minor Fontanelles

• Mastoid (posterolateral) Fontanelle – Closes by 6-18 months.

• Sphenoidal (anterolateral) Fontanelle – Closes by 6 months.

Clinical Importance:

• Delayed closure may indicate conditions like hydrocephalus, hypothyroidism, or rickets.

• Early closure can be associated with craniosynostosis, leading to skull deformities.

9. External Surface Features:

• Parietal tuberosity: The point used to measure the maximum transverse diameter of the skull.

• Superior and inferior temporal lines: Serve as attachment sites for the temporalis muscle and fascia.

10. Internal Surface Features:

• Impressions of sulci and gyri of the cerebrum.

• Groove for the anterior division of the middle meningeal artery (at the internal surface of the anterior-inferior angle).

• Groove for the posterior division of the middle meningeal artery (at the middle of the inferior border).

• Sagittal sulcus: Located near the superior border.

• Sulcus for the sigmoid sinus: Located near the posterior-inferior angle.

11. Middle Meningeal Vessels:

• Groove identification: The parietal bone contains grooves for the branches of the middle meningeal artery.

• Origin of the middle meningeal artery: A branch of the maxillary artery.

• Entry into the cranial cavity: Through the foramen spinosum.