The calvarium is the anatomical term for the skullcap, representing the dome-shaped roof of the cranium. It forms the superior portion of the neurocranium, which encases and protects the brain. The calvarium is essentially a protective shell that covers the largest organ of the central nervous system.
This bony covering sits above the cranial base, establishing the boundaries of the cranial cavity where the brain resides. Its design reflects a balance between the need for rigid protection in adulthood and flexible growth during infancy, achieved through its unique composition and developmental process.
What is the Calvarium Made Of?
The calvarium is composed of four main bones: the superior parts of the frontal bone, a portion of the occipital bone, and the two large, paired parietal bones. These flat bones originate through intramembranous ossification, where bone tissue forms directly within connective tissue membranes. In adults, these bones are rigidly joined together by specialized, interlocking fibrous joints known as sutures.
Three primary sutures connect the bones of the calvarium: the coronal, sagittal, and lambdoid sutures. The coronal suture runs transversely across the top of the skull, marking the boundary between the frontal bone and the two parietal bones. The sagittal suture runs down the midline of the head, connecting the two parietal bones.
Posteriorly, the lambdoid suture separates the parietal bones from the superior portion of the occipital bone. These sutures form complex, wavy lines that interlock like puzzle pieces, creating a strong, fixed articulation that provides structural integrity to the skullcap. In adults, these joints are immovable, ensuring a solid defense for the underlying brain tissue.
The Calvarium’s Role in Protection
The primary function of the calvarium is to act as a robust shield, protecting the brain tissue from mechanical trauma. Its strength comes from its specific layered architecture, which is effective at dissipating forces. Most calvarial bones consist of three distinct layers fused together to create this protective system.
The outermost layer is a dense, compact bone called the outer table, which provides initial resistance to impact. Beneath this is the diploë, a layer of cancellous or spongy bone that contains red bone marrow and diploic veins. This middle, spongy layer functions to absorb and spread out the energy of a blow, preventing the full force from transmitting to the inner layer.
The innermost layer, known as the inner table, is a thin sheet of compact bone that faces the meninges. Because the inner table is thinner and more brittle than the outer table, extreme force can sometimes cause it to fracture independently, a phenomenon known as a contrecoup injury. The curved shape of the calvarium further enhances its protective capacity, as a dome structure resists external pressure better than a flat plane.
Development and Changes in Infancy
The development of the calvarium in infancy is characterized by soft spots, or fontanelles, which are temporary gaps between the cranial bones. These fontanelles are dense, fibrous membranes that connect the separate bony plates of the skull. This flexible arrangement is a necessary adaptation for two major developmental processes.
The flexibility provided by the fontanelles allows the bony plates to slightly overlap during childbirth, a process called molding, which permits the infant’s head to navigate the narrow birth canal. Without this pliability, the risk of serious injury would be significantly higher. Following birth, the fontanelles accommodate the rapid growth of the brain, which nearly triples its volume during the first year of life.
There are six fontanelles in a newborn skull, but the two most noticeable are the anterior and posterior fontanelles. The small, triangular posterior fontanelle, located at the junction of the sagittal and lambdoid sutures, is typically the first to close, usually within the first one to three months after birth. The diamond-shaped anterior fontanelle, situated at the intersection of the coronal and sagittal sutures, remains open longer.
The anterior fontanelle generally closes through ossification between 12 and 18 months of age. Clinicians routinely examine the fontanelles during checkups, as their size and tension provide insights into an infant’s hydration status and intracranial pressure. Premature closure of the sutures and fontanelles, known as craniosynostosis, can restrict normal brain growth and may require surgical intervention.
Medical Issues and Surgical Access
The calvarium is susceptible to various forms of trauma, with fractures being a common medical issue following high-impact accidents. Fractures range from a simple linear fracture, which runs straight through the bone without displacing the pieces, to a depressed fracture, where the bone is pushed inward toward the brain. Depressed fractures are concerning because they can directly compress or lacerate the underlying brain tissue or meninges.
The unique structure of the calvarium necessitates specific surgical approaches to access the brain. A craniotomy is a neurosurgical procedure where a section of the calvarium, known as a bone flap, is temporarily removed to allow a surgeon to operate. The flap is carefully cut using specialized tools and is then replaced and secured with plates and screws once the procedure is complete.
A distinct procedure is the craniectomy, where the bone flap is not immediately replaced. This technique is often performed as a decompressive measure to relieve high intracranial pressure caused by brain swelling from a stroke or traumatic injury. By removing a portion of the skullcap, the brain is given space to expand. The bone piece is typically stored and reattached in a subsequent procedure called a cranioplasty.