Quantification of Head Shape and Cranioplasty Outcomes: Six-compartment Volume Method Applied to Sagittal Synostosis

Abstract

Background: Premature fusion of the sagittal (midline) suture between 2 parietal bones is the most common form of craniosynostosis. Surgical correction is mandated to improve head shape and to decrease the risk of raised intracranial pressure. This study evaluated the utility of 3-dimensional (3D) imaging to quantify the volumetric changes of surgical correction. Currently there is no standardized method used to quantify the outcomes of surgery for craniosynostosis, with the cranial index (width: length ratio) being commonly used.

Methods: A method for quantification of head shape using 3D imaging is described in which the cranium is divided up into 6 compartments and the volumes of 6 compartments are quantified and analyzed. The method is size invariant, meaning that it can be used to assess the long-term postoperative outcomes of patients through growth. The method is applied to a cohort of sagittal synostosis patients and a normal cohort, and is used to follow up a smaller group of synostotic patients 1, 2, and 3 years postoperatively.

Results: Statistical analysis of the results shows that the 6-compartment volume quantification method is more accurate in separating normal from synostotic patient head shapes than the cranial index.

Conclusions: Spring-mediated cranioplasty does not return head shape back to normal, but results in significant improvements in the first year following surgery compared with the preoperative sagittal synostosis head shape. 3D imaging can be a valuable tool in assessing the volumetric changes due to surgery and growth in craniosynstosis patients.

Fig. 1.

Steps in CT segmentation and reconstruction.

Fig. 2.

Anatomical features used to determine divisions of the head: pituitary fossa, anterior fontanelle, and posterior fontanelle on the normal case.

Fig. 3.

Angles for division of the head and labeling of compartments.

Fig. 4.

Least squares linear regression plot of Ln transformed total volume vs Ln transformed age for normal and sagittal cases. Red points and line represent the sagittal synostosis cases, and blue points and line represent the normal cases. Translucent bands show the 95% confidence intervals (to dashed lines), with the dotted lines showing the 99% confidence intervals. Note that these confidence intervals overlap considerably, showing that there is no significant difference between the regression lines. The best fit lines were (sagittal) y = 12.34 + 0.31x (R² = 0.88) and (normal) y = 12.31 + 0.30x (R² = 0.85).

Fig. 5.

CI frequency distribution of normals (dark blue) vs preoperative (red) vs postoperative 1 (orange) vs postoperative 2 (cyan) vs postoperative 3 (green).

Fig. 6.

A scatter plot of PC1 (x axis) vs PC2 (y axis) scores for the PCA of the 6-compartment volumes with convex hulls showing the distribution of each of the groups. Normals = blue (mean represented by N). Preoperative = red (mean represented by S). Postoperative 1 = orange (mean represented by S-Po1). Postoperative 2 = cyan (mean represented by S-Po2). Postoperative 3 = green (mean represented by S-Po3). The polygon shapes denote the convex hulls for each of the groups.

Fig. 7.

A frequency histogram of PC1 scores. Normals are in blue, preoperative sagittal in red. Postoperative sagittal are in orange (1 year postoperatively), cyan (2 years postoperatively), and green (3 years postoperatively). The preoperative sagittal (negative PC1 scores) and normal (positive PC1 scores) at either end of the PC1 axis with minimal overlap (only 1 preoperative sagittal synostosis case fell within the convex hull of the normal in figure 6). P1 overlaps both the controls and preoperative groups sitting between the 2, whereas P2 has a lie back toward the preoperative group. ANOVA of the PC1 scores showed no statistical difference between preoperative and P3, although the sample size for the P3 group was relatively low (P3, n = 5). The middle volumes (49.54% combined) and the anterior volumes (48.69% combined) were evenly weighted in PC1 with the posterior compartment volumes contributing only 1.77% of the weighting.

Fig. 8.

Summed left- and right-hand side anterior middle and posterior compartments to illustrate the shape difference between normal and sagittal synostosis head shapes captured by the 6-compartment volume distribution method. Mean percentage differences between normal and sagittal synostosis heads are shown on a single normal and sagittal synostosis (skin) head shape.