I gladly oblige when parents ask me to check their baby’s soft spot; and then I pronounce it’s just fine. But the truth is that I don’t know what I’m checking. Nobody does. The tough tissue of the soft spot is so strong that it really won’t even stretch much if there is a brain problem1. And the pulsations from the soft spot, which are admittedly a little freaky, are not understood at all. What we do know is that the flexible spaces (sutures) between the bones of the skull and their intersections (the fontanels) allow the skull to withstand significant mechanical forces.

Everyone who has seen a newborn cone-head is familiar with the distortion that can occur during childbirth. The sutures allow for skull flexibility during birth and for the skull to stretch as the brain grows more rapidly in size than bone, which expands more slowly. The flexible sutures of the skull help kids endure countless falls and head injuries. Any force that hits the head is cushioned by the added elasticity of the suture lines and fontanels. Studies of the strength of the fibrous, flexible sutures have shown that these skull “joints” in kids deform 30 times more than kid’s bone and 243 times more than adult skull bone before they break. These inborn shock absorbers allow kids to be dropped by their siblings, fall out of shopping carts and roll off of beds with hardly a mark.

The thickness of the skull plays a role here as well. And studies have shown that the parietal bones on the sides of the head are thinner and stiffer than the bones of the forehead and back of the skull. So you should worry if your child is hit above the ear. But worry less about blows to the forehead or back of the head.

Head bonks & head-butts

Early studies of the strength and mechanical function of skull sutures were done on animals. Researchers soon realized that animals have different dietary habits and behaviors than humans. Our skulls don’t need to withstand hours of chomping grass and we generally don’t head-butt each other like goats. Goats skulls were particularly interesting to study because of the humongous forces they can withstand. As such, sutures likely have evolved to serve loading forces specific to each species.

Fortunately, sutures don’t fuse completely and continue to aid us throughout our lives. Cranial sutures (the ones in your dome) are very active when a child’s brain is growing. The brain grows rapidly in early childhood and it is this very growth in size that sends a chemical signal to the skull to make more bone on the edges of the bony skull plates. Throughout early childhood these sutures close and form a nice round skull.

There are sutures in all the facial bones as well; and these are most active during adolescence as the face shape changes and matures.

Grown adults have open sutures throughout the face that function as shock absorbers. For example the sutures near the jaw accommodate the cyclic loading from chewing so we don’t get stress fractures from bagels, for example. There are several sutures in the skull that don’t close until after age 70.

Researchers are examining the mechanical properties of sutures to discover precisely how they function so well to protect our brains. So, when we finally uncover the mysteries of the anterior fontanel we may unlock the secret to designing concussion-preventing, energy-absorbing helmets. Or perhaps it would be more informative if we continue to examine head-butting goats.

 

1. An increase in internal pressure from 10 mmHg (normal infant intracranial pressure) to 50 mmHg (infants with severe head trauma) enlarges the suture by 0.03–0.14% which is not detectable on exam of a child. Even if the entire skull case was assumed to be composed of suture material, the total volume change of the skull case would be 0.01–1.1%. This implies that a pressure increase of 40 mmHg in an infant deforms the suture only a minimal amount.

Coats, B., & Margulies, S. S. (2006). Material properties of human infant skull and suture at high rates. Journal of neurotrauma, 23(8), 1222-1232.