An infant’s skull is not a single, solid bone but a collection of bony plates connected by flexible, fibrous tissues called cranial sutures. The skull is made up of several major bones, including two frontal bones, two parietal bones, and one occipital bone. The sutures hold these plates together, allowing for the dynamic changes the skull undergoes during birth and the first years of life. This design is fundamental to both a safe delivery and healthy development.
The Purpose of Cranial Sutures
Cranial sutures serve two main functions. Their primary role is to allow the infant’s head to safely navigate the birth canal. The flexibility of the sutures enables the skull bones to overlap, a process called molding, which temporarily changes the head’s shape to facilitate delivery. This adaptability is why some newborns have a slightly cone-shaped head immediately after birth, which typically rounds out over the next few days.
Following birth, the sutures accommodate the rapid growth of the brain during infancy and early childhood. They act as expansion joints, permitting the skull to enlarge as the brain triples in size. At the intersections of these sutures are wider, membrane-covered gaps known as fontanelles, or “soft spots.” The most prominent of these are the anterior fontanelle on the top of the head and the smaller posterior fontanelle at the back.
Normal Suture Closure Timeline
The process of cranial sutures hardening into solid bone, known as ossification, follows a variable timeline that extends from infancy into adulthood. The fontanelles are the first to close. The posterior fontanelle at the back of the head closes by one to two months of age, while the larger anterior fontanelle at the top closes between 7 and 19 months.
The metopic suture, located in the middle of the forehead, is one of the earliest to fuse, typically between three and nine months of age. Other major sutures, like the sagittal suture along the top of the head and the coronal sutures running from ear to ear, begin to fuse in early adulthood. For instance, the sagittal suture may close around age 22, the coronal suture around age 24, and the lambdoid suture at the back of the head around age 26. Some sutures may not fully solidify until later in life.
Complications from Premature Fusion
When one or more cranial sutures fuses earlier than normal, it results in a condition called craniosynostosis. This premature closure prevents the skull from expanding symmetrically. As the brain continues to grow, it pushes against the fused area, forcing the skull to grow where the sutures remain open. This compensatory growth leads to abnormal head shapes, depending on which suture has closed early.
Sagittal synostosis, the most common type, restricts growth sideways, resulting in a long, narrow head. Coronal synostosis can cause a flattening of the forehead on one side. It is important to differentiate craniosynostosis from a common condition called positional plagiocephaly, or “flat head syndrome”. Positional plagiocephaly is not caused by fused sutures but by external pressure, such as a baby sleeping in the same position. Unlike craniosynostosis, it does not restrict brain growth and is a cosmetic issue addressed with non-surgical methods.
A distinction between the two conditions is the position of the ears and forehead. In positional plagiocephaly, the head takes on a parallelogram shape, with the ear on the flattened side often pushed forward. In lambdoid craniosynostosis, which also causes flattening at the back of the head, the skull develops a trapezoid shape, and the ear on the affected side is typically pulled backward. A physical exam can often distinguish them, but imaging tests may be used to confirm if sutures are open or fused.
Concerns with Delayed Closure
Delayed closure of sutures and fontanelles can also signal a problem. An anterior fontanelle that remains open long past the 18-month timeframe can be an indicator of an underlying medical condition. This delay might be associated with issues that affect bone development or the pressure inside the skull.
Several conditions can be linked to delayed fontanelle closure, including genetic syndromes like Down syndrome, congenital hypothyroidism, or rickets. Abnormally large fontanelles or delayed closure can also be a sign of increased intracranial pressure, an abnormal buildup of fluid in the brain. Fontanelles and sutures that do not close within their expected ranges warrant a medical evaluation to investigate potential causes.