The skull is not a single, solid structure at birth but is composed of several bony plates connected by specialized fibrous joints known as sutures. These sutures function as expansion joints, allowing the skull to change shape and grow. The coronal suture is one of the four major cranial sutures, named for its position across the head, resembling where a crown, or corona, would sit. This flexible seam is an integral part of the skull’s architecture, serving a dynamic purpose throughout life.
Structure and Location
The coronal suture forms a transverse line across the top of the head, extending from one side to the other in an ear-to-ear direction. It acts as the boundary between the frontal bone, which forms the forehead and eye sockets, and the two parietal bones, which form the upper sides and roof of the skull. This junction is a type of fibrous joint called a syndesmosis, composed of dense connective tissue that permits a small degree of movement between the adjacent bones.
Before fusion, the suture line displays a characteristic, interlocking, wavy pattern resembling the teeth of a saw. On a newborn’s head, the coronal suture meets the sagittal suture (which runs front-to-back) at the anterior fontanelle, commonly known as the soft spot. This intersection marks the largest membrane-covered space where multiple skull bones converge. The flexibility of this joint is a temporary state, essential for the brain’s massive growth spurt.
Function in Infant Skull Growth
The primary function of the coronal suture is to facilitate two processes in early life: birth and brain development. During passage through the narrow birth canal, the flexibility of the coronal and other sutures allows the bony plates of the skull to overlap and mold. This temporary compression, known as molding, protects the infant’s brain from trauma during delivery.
Following birth, the coronal suture permits the continuous and rapid expansion of the cranial vault to accommodate the developing brain. The brain undergoes its most significant growth during the first two years of life, and open sutures ensure the skull can enlarge evenly in all directions. The coronal suture specifically allows for growth in the front-to-back direction and contributes to the widening of the skull, ensuring symmetrical expansion and a typically shaped head.
Coronal Craniosynostosis
Coronal craniosynostosis is defined by the premature fusion, or closure, of the coronal suture before the brain has finished its major growth period. This early fusion restricts bone growth perpendicular to the affected suture, forcing the skull to compensate by growing parallel to the fused seam. The resulting head shape depends on whether one side or both sides of the suture fuse prematurely.
When only one side closes early, it is termed unicoronal craniosynostosis, the second most common type of the disorder. This unilateral fusion causes the forehead and brow to flatten on the affected side, while the opposite, unfused side often bulges out. The eye socket on the affected side may also become raised or have a different shape, leading to a facial asymmetry known as anterior plagiocephaly.
Bicoronal craniosynostosis involves the premature fusion of both the right and left coronal sutures, resulting in a head shape called brachycephaly. The skull cannot grow normally from front to back, leading to a head that appears abnormally short and wide, often with an excessively tall and flat forehead. Untreated craniosynostosis can lead to increased pressure inside the skull, which may cause developmental delays or vision problems. Treatment typically involves surgery to reopen the suture and reshape the skull, allowing the brain the space it needs to grow.
Changes Over the Lifespan
The dynamic, flexible state of the coronal suture in infancy eventually gives way to natural, non-pathological fusion, or synostosis, later in life. Unlike the metopic suture, which typically fuses early in childhood, the major cranial sutures, including the coronal suture, remain open throughout adolescence and well into adulthood. The fusion process involves the fibrous connective tissue being replaced by solid bone, turning the flexible joint into a rigid seam.
The timing of this adult-onset fusion is highly variable, but the coronal suture is often one of the first major sutures to begin this process. While some sources suggest fusion may begin in the mid-twenties, complete obliteration of the coronal suture is rare, even in advanced age. This gradual closure pattern is a factor sometimes used in forensic science to help estimate the age of skeletal remains. Forensic analysis uses the degree of suture closure on both the outer (ectocranial) and inner (endocranial) surfaces of the skull to provide an expected age range.