The calvarium is the upper portion of the skull, commonly known as the skullcap. Its primary role is to form a durable, protective case enclosing the brain, shielding it from external impact. The calvarium constitutes the roof of the cranial cavity, providing a strong barrier for one of the body’s most delicate organs.
Composition and Structure of the Calvarium
The adult calvarium is not a single, solid bone but is formed from several distinct bones. These include the frontal bone at the front, the two parietal bones that form the sides and roof, and the posterior occipital bone. These flat bones are composed of a spongy bone layer, known as diploë, sandwiched between two hard layers of compact bone. This construction provides strength while minimizing weight.
These individual bones are connected by fibrous joints called sutures. The coronal suture joins the frontal bone with the two parietal bones. The sagittal suture runs down the midline, connecting the two parietal bones to each other. At the back of the skull, the lambdoid suture connects both parietal bones to the occipital bone. These interlocking joints provide the calvarium with its structural integrity.
Development from Infancy to Adulthood
At birth, the bones of the calvarium are not yet fused. They are separated by flexible, membranous gaps known as fontanelles, or “soft spots.” The most prominent are the large, diamond-shaped anterior fontanelle at the front and the smaller, triangular posterior fontanelle at the back. This arrangement allows the bony plates of the skull to overlap during birth, facilitating the head’s passage through the birth canal.
Following birth, the fontanelles accommodate the rapid growth of the brain. The bones of the calvarium grow outward as the fontanelles gradually close. This happens as the bones expand and ossify, a process where soft tissue hardens into bone. The posterior fontanelle closes by three months of age, while the larger anterior fontanelle closes around 18 months. The sutures also begin to fuse, a process continuing into adulthood.
Common Calvarial Fractures
A calvarial fracture is a break in one or more of the bones that form the skullcap, resulting from direct trauma to the head. The nature of the fracture often depends on the force and location of the impact.
One common type is a linear fracture, a simple break in the bone that travels in a straight line where the pieces have not shifted. Another type is a depressed fracture, caused by a forceful blow from a smaller object. In a depressed fracture, a fragment of the skull is pushed inward, potentially pressing on the underlying brain tissue.