The skull, or cranium, is the bony structure that forms the head in humans and other vertebrates. Its primary function is to protect the delicate brain tissue within and to serve as the supporting framework for the face. While the general structure is consistent across all people, the specific shape of the human skull is remarkably diverse, reflecting a complex interplay of genetic, developmental, and environmental influences. Understanding the causes of these differences, from subtle population variations to significant abnormalities, requires examining the skull’s formation from infancy through adulthood.
The Basis of Shape: Development and Function
The skull begins its formation as several separate bony plates connected by flexible, fibrous joints called sutures. At the points where multiple sutures meet, there are wider membranous gaps known as fontanelles, commonly referred to as soft spots. This initial flexibility is a mechanism that serves two important biological purposes for a newborn. First, the separate plates allow the skull to temporarily compress and overlap, which facilitates the passage of the head through the mother’s narrow birth canal during labor.
Second, the open sutures and fontanelles accommodate the rapid growth of the brain during the first few years of life. The brain volume increases significantly in infancy, and the flexible skull structure ensures there is always enough space for this expansion. As the child grows, the sutures gradually fuse and harden, a process that continues until the brain reaches its adult size. The precise final shape of the adult skull is ultimately a result of genetic programming interacting with these early developmental and mechanical forces.
Classifying Human Skull Variation
Anthropologists and biologists have historically used a measurement known as the cephalic index to categorize the natural diversity in human skull shapes. This index is a ratio calculated by dividing the maximum width of the head by its maximum length and multiplying the result by one hundred. It serves as a scientific way to describe the proportional relationship between the side-to-side and front-to-back dimensions of the cranium.
Based on this ratio, three broad, non-pathological categories have been established to describe adult skull variation. People whose index is below 75 are classified as dolichocephalic, meaning their skull is relatively long and narrow when viewed from above. Those with an index of 80 or above are called brachycephalic, possessing a head that is comparatively short and wide. A skull with an index falling in the intermediate range, typically between 75 and 80, is designated as mesocephalic.
Causes of Abnormal Skull Shapes
Significant deviations in skull shape are often caused by a congenital condition called craniosynostosis, which involves the premature fusion of one or more of the cranial sutures. This early closing effectively locks the skull bones together, preventing the head from expanding perpendicularly to the fused suture. While the brain continues to grow, the skull is forced to expand parallel to the closed suture, leading to a distinctly misshapen head.
The implications of craniosynostosis vary depending on which sutures are involved and how many are affected, but the condition can restrict overall head growth. In cases where multiple sutures fuse, the resulting lack of space can lead to increased intracranial pressure. Elevated pressure inside the skull is a serious concern that may impair brain development, cause problems with learning and vision, and sometimes necessitate surgical intervention to reshape the skull. The cause of craniosynostosis is often unknown, though some cases are linked to specific genetic syndromes.
A misshapen head can also be caused by positional deformation, or plagiocephaly, which is distinct from the pathology of craniosynostosis. Positional deformation is caused by external pressure on the still-soft infant skull rather than a problem with the sutures themselves. A thorough medical evaluation is necessary to differentiate between these two causes because craniosynostosis requires specialized surgical management.
Factors Influencing Modern Infant Skull Shape
The prevalence of acquired, non-pathological misshapen heads in infants has increased since the 1990s following recommendations that babies sleep on their backs. This practice is highly effective at reducing the risk of Sudden Infant Death Syndrome (SIDS). However, infants spend a significant amount of time with the back of their skull resting on a flat surface. Since the infant skull remains soft and malleable, this constant external pressure can lead to deformational plagiocephaly, characterized by a flattening on one side of the back of the head, or brachycephaly, which is a symmetrical flattening across the back.
Fortunately, these positional changes are largely manageable through simple, non-invasive techniques.
Management Strategies
- Repositioning involves altering the infant’s head placement while they are awake and adjusting their orientation in the crib to encourage turning their head to the non-flattened side.
- Providing supervised “Tummy Time” during the day is a preventative measure that relieves pressure on the back of the head.
- Tummy Time also strengthens the neck and shoulder muscles, which helps the baby develop the ability to move their head freely.
- For moderate to severe cases of positional flattening that do not respond to repositioning, a custom-fitted cranial orthotic helmet may be used to gently guide the skull’s growth into a more rounded shape.