The human skull changes shape as it grows, and physicians and biologists would like to understand the causes of these changes and ultimately predict them. Now a team has developed a model that predicts skull shape changes based on physics and geometry principles alone, without any reference to biochemistry or genetics. The model successfully describes the development of known skull deformities and helps explain the ways in which growing bones react to mechanical cues. It also provides a first step toward a model that could guide surgeons treating infants with abnormal skull growth.
The growth of the skull in harmony with the brain is a complex process. Biologists have traditionally focused on biochemistry and genetics as the main drivers of bone growth, but some researchers have felt that the mechanical stresses produced by the growing brain are equally important. How cells detect and react to mechanical signals such as pressure is a fundamental question that has perplexed biologists for at least a century [1, 2].
Mathematician Alain Goriely of Oxford University in the UK and biomechanical engineer Ellen Kuhl of Stanford University in California, along with colleagues, have now developed computational tools to simulate the growth and evolving shape of the cranial vault—the “brain space” within the skull—in a way that is entirely based on mechanical effects. A future version of the model could guide surgeons who must remove parts of the skull to treat infants whose heads are growing improperly. This type of surgery is currently “more an art and craft than science,” says Goriely.
The team incorporated two bone growth processes into their model. Most bone activity in a newborn’s skull occurs along the sutures, soft fibrous boundaries between adjacent bone plates. As new bone accretes on either side of the suture—starting with a layer of specialized, pre-bone cells —the plates push against one another, and the bones grow outward, away from this boundary. The second process, called remodeling, occurs as existing bone is removed from the inner surface of the plates, and new bone accretes on the outer surface. Remodeling allows the skull’s curvature to decrease as it enlarges and also causes an increase in bone thickness.
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