Parietal Bones

1. Overview

The parietal bones are a pair of large, curved, quadrilateral bones forming a significant portion of the superior and lateral walls of the cranium. These bones are essential components of the neurocranium, enclosing and protecting the brain, particularly the parietal lobes. The right and left parietal bones meet at the top of the skull and contribute to its overall strength and structure.

2. Location

The parietal bones are located on the upper lateral sides of the skull:

• Superiorly: Meet each other at the sagittal suture.

• Anteriorly: Articulate with the frontal bone at the coronal suture.

• Posteriorly: Articulate with the occipital bone at the lambdoid suture.

• Inferiorly: Articulate with the temporal and sphenoid bones at the squamosal and sphenoparietal sutures, respectively.

3. Structure

Each parietal bone is thin, flat, and curved, contributing to the cranial vault:

• External surface: Convex and smooth, provides attachment for the temporalis muscle; marked by parietal eminence (a rounded prominence).

• Internal surface: Concave and grooved for meningeal vessels, especially the middle meningeal artery.

• Four borders: Frontal (anterior), occipital (posterior), temporal (inferior), and sagittal (medial).

• Four angles: Frontal, sphenoidal, occipital, and mastoid angles, which connect with corresponding bones.

The bone consists of two layers of compact bone (inner and outer tables) with cancellous bone (diploë) in between, providing strength and lightness.

4. Function

The parietal bones serve several crucial roles in cranial protection and form:

• Protect the brain: Enclose the parietal lobes of the brain, shielding them from external trauma.

• Structural support: Contribute to the cranial vault, maintaining the shape and strength of the skull.

• Muscle attachment: Provide surfaces for temporalis and other muscles of mastication and facial expression.

• Formation of sutures: Interlock with surrounding bones to form immovable joints that stabilize the skull.

5. Physiological role(s)

Although passive in terms of direct physiological processes, the parietal bones support vital functions by:

• Enabling cerebral protection: Encase brain tissue, especially the parietal lobes responsible for sensory perception and spatial orientation.

• Assisting thermoregulation: The scalp and underlying skull dissipate heat produced by brain metabolism.

• Providing pathways: Grooves on the inner surface house vessels like the middle meningeal artery, essential for cranial perfusion.

6. Clinical Significance

Parietal bones are involved in several clinical and surgical conditions:

• Skull fractures:

  • Parietal bones are commonly fractured in head trauma due to their exposure; may result in epidural hematomas if the middle meningeal artery is torn.

• Craniotomy procedures:

  • Parietal regions are often accessed surgically to reach underlying brain tissue during tumor resections or hemorrhage evacuation.

• Hydrocephalus and cranial expansion:

  • In pediatric patients, parietal bones may expand or bulge in conditions like hydrocephalus or craniosynostosis.

• Parietal foramina anomalies:

  • Small openings near the sagittal suture may transmit emissary veins; enlarged foramina can be associated with syndromes like cleidocranial dysostosis.

• Decompression surgery:

  • Increased intracranial pressure may require removal of a parietal bone flap (decompressive craniectomy).