Tracing craniosynostosis to its developmental stage through bone center displacement.

In metopic and coronal suture synostosis, the involved bone centers are abnormally situated just next to the affected suture. Bone centers are the starting point of ossification during embryogenesis from which bone growth spreads radially. In this paper, we describe a similar observation for sagittal suture synostosis, with both parietal bone centers located almost completely cranially. The (reduced) distance between the bone centers of a synostotic suture reflects the time during embryogenesis at which fusion took place. We suggest that in craniosynostosis the bone centers arise in their normal position, and initial outgrowth is undisturbed until the bone fronts meet. It is during this developmental stage that fusion occurs instead of suture formation. Due to the fusion, growth can only occur at the free bony rims from then on. The bone centers remain located at a fixed distance from one another in the middle of the fused bones, becoming relatively more displaced with time. This implies that the distance between the involved bone centers directly indicates the developmental period during which sutural growth was arrested. The same phenomenon of bone center displacement is found in types of craniosynostosis with and without fibroblast growth factor receptor (FGFR) or TWIST gene mutations.