The role of calvarial sutures in determining skull shape was questioned by Moss11 in 1959. His skepticism was based on several observations: that cranial vault deformities can occur in the absence of a fused suture and that skull growth in the rat was not altered by experimental removal of a normal cranial vault suture; moreover, in cephalometric data, he found cranial base abnormalities associated with craniosynostosis.11 He described abnormalities in the planum, orbital, and foramino-clival angles in patients with sagittal or bicoronal synostosis. Moss, therefore, proposed that cranial base abnormalities were primarily responsible for the characteristic forms of craniosynostosis rather than the result of premature closure of a vault suture. He emphasized that the principal influence for growth of the skull was brain development.9
Although consistent cranial base abnormalities have not been described in sagittal and lambdoid synostosis, multiple deformities are seen in coronal synostosis.3,4 For example, in bilateral coronal synostosis several abnormalities have been described, including: protrusion of the greater wing of the sphenoid bone; a short anterior cranial base and clivus; an enlarged sella turcica; frontalization of the orbital roof; and deficient nasion-sella-basion angle. The cranial base is markedly asymmetrical in unilateral coronal synostosis. The anterior cranial base is short and the slope of the lesser wing of the sphenoid bone is pointed upward and laterally ipsilateral to the stenotic coronal suture. The cranial base angle is also increased and the sphenoidal-petrosal angle is narrowed due to ipsilateral anterior displacement of the petrous ridge.
Despite cranial base involvement, premature closure of a cranial vault suture alone can be responsible for the characteristic deformities seen in each form of craniosynostosis. Isolated restriction of growth in a normal cranial vault suture of a growing animal can cause permanent cranial vault, cranial base, and facial skeleton abnormalities.13 Clinical observations show that these skull base changes are similar to cranial base maldevelopment in humans with craniosynostosis. Calvarial growth is a result of a complex interaction between the cranial vault, skull base, and facial skeleton. Thus, there is a theoretical basis for predicting the clinical form of each single suture synostosis by making reference to the vault suture alone.
To begin with, the observed calvarial deformities cannot be totally explained by restricted growth perpendicular to the fused suture. For example, multiple stereotyped deformities are seen in unilateral craniosynostosis (Fig. 1). Virchow's guidelines17 predict a deficit in growth in ipsilateral frontal and parietal bones perpendicular to the fused suture. He also implied that compensatory growth must occur at the remaining sutures to accommodate a growing brain. Virchow, however, did not establish a specific compensatory growth pattern that fully predicts the observed calvarial deformities. If the normal sutures could effectively accommodate for growth restriction caused by a prematurely fused suture, then the skull could conform to a shape similar to the underlying round brain. Indeed, the twofold purpose of cranial sutures is to allow molding during birth and to facilitate growth of a basically round head to accommodate the human round brain. It is of considerable interest that closure of a single suture should result in such a profoundly abnormal skull shape that secondarily changes the shape of the brain itself. In unilateral coronal synostosis, a normal calvarial shape could still develop if bone deposition along the metopic suture would increase and compensate for growth restriction at the coronal suture. Instead, contrary to the basic notion that brain growth primarily influences skull shape, enhanced compensatory growth results in an ipsilateral temporal and contralateral frontal boss. Although many of the characteristics of this compensation have been described, no unified theory is available to predict the outcome of compensatory growth. A new understanding is needed to explain how skull growth patterns change in craniosynostosis. By understanding these growth patterns, we might also understand the pathological mechanism of premature sutural stenosis.
The authors thank Lucille Staiger for her excellent technical assistance and Craig Luce for the illustrations in this manuscript.
Preliminary results of this article were presented by John A. Jane, M.D., Ph.D., as part of the Olivecrona Lecture at the Karolinska Institute in 1985.