A New Measure of Posterior Morphology in Sagittal Craniosynostosis: The Occipital Bullet Index

Background: Sagittal craniosynostosis (SC) is associated with scaphocephaly, an elongated narrow head shape. Assessment of regional severity in the scaphocephalic head is limited by the use of serial CT imaging or complex computer programing. Three-dimensional measurements of cranial surface morphology provide a radiation-free alternative for assessing cranial shape. This study describes the creation of an Occipital Bulleting Index (OBI), a novel tool using surface morphology to assess the regional severity in patients with SC. Methods: Surface imaging from CT scans or 3D photographs of 360 individuals with sagittal craniosynostosis and 221 normocephalic individuals were compared to identify differences in morphology. Cartesian grids were created on each individual's surface mesh using equidistant axial and sagittal planes. Area under the curve (AUC) analyses were performed to identify trends in regional morphology and create measures capturing population differences. Results: The largest differences were located in the medial regions posteriorly. Using these population trends, a measure was created maximizing AUC. The Occipital Bullet index has an AUC of 0.72 with a sensitivity of 74% and a specificity of 61%. When the Frontal Bossing Index is applied in tandem, the two have a sensitivity of 94.7% and a specificity of 93.1%. Correlation between the two scores in individuals with SC was found to be negligible with an intraclass correlation coefficient of 0.018. Severity was found to be independent of age under 24 months, sex, and imaging modality. Conclusions: This index creates a tool for differentiating control head shapes from those with sagittal craniosynostosis, and has the potential to allow for objective evaluation of the regional severity, outcomes of different surgical techniques, and tracking shape changes in individuals over time, without the need for radiation.