In the context of craniosynostosis, the word “sutures” refers to the long fibrous tissues between the individual plates that make up the skull. (These sutures connect the plates, as the sutures you think of as “stitches” connect separated parts of skin after surgery or an injury.)
Parents of newborns are aware of the “soft spot” in a baby’s head – that’s where four of those plates meet. As the baby’s brain grows rapidly in the first year to three years after birth, those sutures stretch to allow the plates to expand until the skull reaches its final size. Once the brain has finished its growth, the sutures fuse the plates together into a complete skull.
In some cases, one or more of the sutures fuses too soon, stopping the skull plates from expanding in that direction. The brain keeps expanding, so the skull plates have to move in whatever direction they still can. For example, if the sagittal suture fuses, the skull cannot expand right and left. The growing brain pushes forward and back instead, resulting in a narrow, elongated shape.
Dr. Imahiyerobo explains the anatomy of an infant skull, and how sutures allow for brain growth