The area where the bones of the skull join together is a specialized type of joint known as a cranial suture. These junctions are not flexible like the joints in your elbow or knee; instead, they are bands of dense, fibrous connective tissue that connect the flat, bony plates of the cranium. Their function is to create a protective, enclosed space for the brain while allowing for specific developmental requirements. These protective seams maintain the structural integrity of the head.
The Name and Structure of Cranial Joints
Cranial sutures are anatomically classified as fibrous joints, specifically categorized as synarthroses in their mature form, meaning they permit little to no movement. This structure is unique to the skull bones, with the exception of the mandible, or lower jaw. The bones are held together by a thin layer of dense connective tissue rich in collagen fibers that forms a tight union between the adjacent bony plates.
These joints possess a distinctive, interlocking structure, often described as serrated or wavy, similar to puzzle pieces fitted tightly together. This convoluted pattern increases the surface area where the bones meet, significantly contributing to the overall strength and resilience of the skull. The three most prominent sutures include the coronal, which separates the frontal bone from the two parietal bones; the sagittal, which runs down the midline between the two parietal bones; and the lambdoid suture, which joins the parietal bones to the occipital bone at the back of the head.
Mobility and Function in Infancy
The flexibility of the cranial sutures is temporary and serves two purposes during the neonatal period. The first function occurs during childbirth, where the sutures allow the bony plates of the skull to slightly overlap or mold, temporarily reducing the head’s diameter. This compression facilitates safe passage through the relatively narrow birth canal without causing damage to the delicate brain tissue.
The second, more prolonged function is to accommodate the rapid growth of the brain during the first years of life. The sutures act as growth sites, permitting the skull to enlarge to match the increasing volume of the brain. The points where several sutures intersect are particularly noticeable in infants as fontanelles, commonly called “soft spots.”
The anterior fontanelle, the largest soft spot located near the front of the head where the coronal and sagittal sutures meet, remains open for a prolonged period. This fontanelle typically closes between 7 and 19 months of age, providing the longest window for brain growth and skull expansion. The posterior fontanelle, located at the back where the lambdoid and sagittal sutures intersect, closes much earlier, often within the first two months of life. Monitoring the tension and closure of these fontanelles is one method healthcare providers use to evaluate an infant’s growth and internal cranial pressure.
Fusion and Protection in Adulthood
As brain growth slows and eventually stops, the temporary flexibility of the sutures is replaced by ossification, or fusion. During this phase, the fibrous connective tissue within the suture begins to harden and is converted into solid bone, a process also known as synostosis. This natural fusion permanently unites the adjacent cranial bones, turning the formerly flexible joint into a rigid structure.
The timeline for fusion varies among the different sutures and individuals, but the process generally begins in early adulthood and continues across the lifespan. For instance, the sagittal suture often begins to fuse around age 22, the coronal suture around age 24, and the lambdoid suture around age 26. This complete fusion results in a unified, bony case that provides maximum structural integrity and protection for the mature brain against external forces.
When the Joints Close Too Early
A condition called craniosynostosis occurs when one or more cranial sutures fuse prematurely, before the brain has finished its growth phase. This early closure restricts the skull’s expansion perpendicular to the affected suture, preventing the skull from growing normally in that direction. Since the brain continues to grow, it is forced to push the skull outward at the remaining open sutures, leading to a compensatory and often abnormal head shape.
The specific shape is determined by which suture closes first; for example, premature fusion of the sagittal suture can lead to a long, narrow head shape known as scaphocephaly. If the coronal suture fuses prematurely on one side, it can cause an asymmetrical flattening of the forehead and orbit, a condition called plagiocephaly. This restriction of growth can increase pressure within the skull. Craniosynostosis often requires specialized surgical intervention to reopen the fused joint and allow the brain the necessary space to develop properly.