Normal sutural fusion and the etiology of single sutural craniosynostosis: the microspicule hypothesis

Single sutural craniosynostosis is a disorder wherein a calvarial suture fuses prematurely, resulting in an abnormally shaped head. Children afflicted are normal neurologically, and the bone bridging the sutures is normal histologically. The mechanism of normal or pathologic sutural fusion is unknown. These facts prompted the authors to reexamine normal sutural anatomy and the concepts of skull growth in an animal model. Histochemical staining to identify osteoblasts and osteoclasts and tetracycline labeling were performed on neonatal rabbit calvarial sutures. Osteoblasts are found on all bony surfaces including the sutural edges, but do not extend across the sutural space. Thus the periosteum per se does not bridge the suture. Osteoclasts are found only in the diploic space. Thus, there is no mechanism to remove bone at/or within the suture. Tetracycline labeling revealed immense bone production at the suture as compared with dural and periosteal surfaces. Microscopic spicules of bone bridging the suture were identified. Based upon the above observations, we propose a new hypothesis for the mechanism of normal and pathologic sutural fusion. Bony microspicules normally and intermittently form and bridge the suture. As there is no mechanism to remove these spicules, we propose that normal mechanical forces cause them to fracture. A spicule that fails to fracture functions as a scaffold, upon which more bone is deposited, resulting in sutural fusion. Single sutural craniosynostosis results when a normal process occurs prematurely.