The suture connecting the frontal and parietal bones is called the Coronal Suture. This fibrous joint serves as the anatomical junction between the frontal bone (at the front of the cranium) and the two parietal bones located behind it. Cranial sutures are dense, fibrous connective tissue joints that maintain a flexible structure during infancy. This flexibility is necessary to accommodate the rapid growth of the brain and allow the skull to change shape during birth. The name “coronal” comes from the Latin word “corona,” meaning garland or crown, referring to its position on the skull where a crown would be worn.
The Coronal Suture: Location and Structure
The coronal suture runs transversely across the top of the skull, extending from one side of the cranium to the other. Its path begins laterally at the pterion, a region near the temple where the frontal, parietal, temporal, and sphenoid bones meet. From the pterion, the suture arches superiorly and medially, separating the frontal bone (the forehead) from the two paired parietal bones.
The suture terminates at the midline of the skull where it meets the sagittal suture, which runs front-to-back between the two parietal bones. This intersection point is known as the bregma, and in infants, it marks the anterior corner of the large anterior fontanelle, or “soft spot.” Structurally, the coronal suture is a type of fibrous joint called a syndesmosis, composed of dense connective tissue holding the bone edges together. The articulating edges of the bones are highly irregular and interlock with a serrated, saw-like appearance, which provides significant mechanical strength to the joint.
The Role of Sutures in Skull Development
The presence of open, flexible sutures is necessary for two fundamental processes in early human development: birth and brain growth. During childbirth, the fibrous nature of the sutures allows the bony plates of the skull to slightly overlap, a process called molding. This temporary change in skull shape is essential for reducing the head’s overall diameter, enabling it to pass safely through the narrow birth canal.
After birth, the primary function of the sutures shifts to accommodating the rapid expansion of the brain. The human brain grows significantly faster than the rest of the body, reaching approximately 50% of its adult size by one year of age. The sutures act as growth centers, allowing the skull bones to expand outward at their margins in response to the pressure from the growing brain tissue. The connective tissue within the suture acts as a membrane where new bone is gradually deposited, facilitating this controlled expansion until the brain reaches its adult size.
Clinical Significance: When Sutures Fuse
The coronal suture is intimately connected to the anterior fontanelle, the largest of the soft spots on an infant’s skull. This diamond-shaped fontanelle is formed where the coronal, sagittal, and metopic sutures converge, and it remains open at birth. The fontanelle typically closes as the surrounding bones expand and meet, with the average closure time ranging from about 13 to 24 months of age.
While the fontanelle closes early, the coronal suture itself is meant to remain patent until much later in life, often fusing around 24 years of age. Premature closure of a cranial suture is a condition called craniosynostosis, which occurs in about one out of every 2,000 live births. When the coronal suture fuses too early, the normal outward growth perpendicular to the suture is restricted.
If only one coronal suture closes prematurely, the condition is known as unilateral coronal synostosis, or anterior plagiocephaly, which results in a flattened forehead on the affected side and a bulging on the opposite side. If both the left and right coronal sutures fuse early, it is called bicoronal synostosis, which restricts growth in the front-to-back direction. This causes the skull to be abnormally short and wide, a head shape known as brachycephaly. Early diagnosis and surgical intervention are often necessary to reshape the skull, allowing the brain sufficient space to continue its development and to prevent complications like increased intracranial pressure.