PARIETAL BONES

Key Concepts:

• Parietal Bone: Flat, arched bone forming part of the skull vault.
• Parietal Tuber: Point of maximum convexity on the outer surface of the parietal bone.
• Superior and Inferior Temporal Lines: Arched lines below the parietal tuber for muscle attachment.
• Parietal Foramen: Opening near the superior border for emissary vein and meningeal artery.
• Sagittal Sulcus: Groove along the superior border of the inner surface lodging the superior sagittal sinus.
• Granular Pits: Pitted areas on the inner surface for arachnoid granulations.
• Sigmoid Sulcus: Groove on the inner surface near the posteroinferior angle lodging the sigmoid sinus.
• Bregma: Meeting point of sagittal and coronal sutures.
• Lambda: Meeting point of sagittal and lambdoid sutures.
• Pterion: H-shaped suture formed by frontal, parietal, temporal, and sphenoid bones.
• Asterion: Meeting point of parietal, occipital, and temporal bones.

1. General Overview of Parietal Bones

• Parietal bones are flat, arched plates that form a significant portion of the skull vault when joined at the midline.
• Each parietal bone is quadrilateral in shape, possessing an outer surface, an inner surface, four borders, and four angles.

2. Outer Surface

• The outer surface is smooth and convex, with the parietal tuber representing the point of maximum convexity.
  • The parietal tuber is significant for determining the maximum transverse diameter of the skull (interparietal diameter).
  • Ossification starts from the parietal tuber in membranous bone development.
• Below the parietal tuber are the superior and inferior temporal lines.
  • The superior temporal line provides attachment for the temporal fascia and epicranial aponeurosis.
  • The inferior temporal line and the area below it form part of the temporal fossa floor and provide origin for the temporalis muscle.
• The parietal foramen, located near the posterior end of the superior border, allows passage for the emissary vein and the meningeal branch of the occipital artery.
  • The emissary vein connects the veins of the scalp with the superior sagittal sinus.

3. Inner Surface

• The inner surface exhibits vascular markings and surface irregularities due to the gyri of the brain and the middle meningeal vessels.
• Two sets of vascular markings are present:
  • One set behind the anterior angle, running upward and backward, produced by the anterior division of the middle meningeal vessels (mainly veins).
  • Another set near the posterior angle, produced by the posterior division of the middle meningeal vessels (mainly veins).
• Along the superior border, a half groove forms the sagittal sulcus when joined with the opposite parietal bone, lodging the superior sagittal sinus.
• In aged skulls, pitted areas (granular pits) are observed on either side of the sagittal sulcus, resulting from arachnoid granulations that absorb CSF into the superior sagittal sinus.
• A wide groove along the posteroinferior angle lodges the sigmoid sinus.
• The inner surface at its four angles is related to vascular structures:
  • Superior sagittal sinus along the superior border.
  • Sigmoid sinus along the posteroinferior angle.
  • Middle meningeal vessels along the anteroinferior angle.

4. Imaginary Lines on Inner Surface

• Two imaginary lines can be drawn on the inner surface to approximate brain structures:
  • Anterior Line: 3-4 cm behind the anterosuperior angle, running downward and forward (approximately 7.5 cm long), coinciding with the central sulcus. Frontal lobe lies in front, parietal lobe behind.
  • Posterior Line: From the anteroinferior angle to the inner aspect of the parietal tuber, coinciding with the posterior ramus of the lateral sulcus. Parietal lobe lies in front, occipital and temporal lobes behind and below.

5. Borders

• Superior Border: Longest, serrated, forms the sagittal suture when joined with its counterpart (bregma to lambda).
• Inferior Border: Beveled at the anteroinferior angle, connecting with the tip of the greater wing of the sphenoid. The middle part is harsh and connects with the squamous part of the temporal bone. The posterior part connects with the upper border of the mastoid temporal bone.
• Anterior Border: Beveled, straight, extends from the anterosuperior to the anteroinferior angle, joining with the squamous part of the frontal bone to form part of the coronal suture.
• Posterior Border: Dented, articulates with the superolateral border of the squamous part of the occipital bone, forming part of the lambdoid suture.

6. Angles

• Anterosuperior Angle: 90-degree angle, forms bregma at the meeting point of the coronal and sagittal sutures.
• Posterosuperior Angle: Forms lambda at the meeting point of the sagittal and lambdoid sutures.
• Anteroinferior Angle: Prolonged (sphenoidal angle), articulates with the frontal, sphenoid, and temporal bones, forming the pterion (H-shaped suture). Deep to the pterion lies the stem of the lateral sulcus and middle meningeal vessels.
• Posteroinferior Angle: Connects with the mastoid temporal and squamous occipital bones, forming the asterion. Deep to the asterion lies the sigmoid sinus.

7. Side Determination

• Outer surface is convex, inner surface is concave.
• Superior border is the longest.
• Inferior border has a notch.
• Anteroinferior angle is prolonged.

8. Synthesis/Conclusion

The parietal bones are crucial components of the skull vault, characterized by distinct surface features, borders, and angles that serve as landmarks for anatomical and surgical purposes. Understanding these features, including the parietal tuber, temporal lines, vascular markings, and the articulation points at bregma, lambda, pterion, and asterion, is essential for comprehending the structural and functional relationships of the skull and its contents. The detailed description of the borders and angles, along with the side determination method, provides a comprehensive understanding of the parietal bones' morphology and their role in forming the skull.