A Novel Quantitative Image-Based Method for Evaluating Cranial Symmetry and Its Usefulness in Patients Undergoing Surgery for Unicoronal Synostosis

Background: Unicoronal synostosis presents with cranial asymmetry. Fixed points are difficult to identify; surgical results are therefore difficult to evaluate. The aim of this study was to develop a computer-based method for evaluation of forehead symmetry to enable evaluation of surgical results in unicoronal synostosis. Methods: The MATLAB tool was programmed to segment computed tomographic images, leaving the outermost contour. Cephalometric images were segmented manually due to lower contrast. A center-point (O) and an end-point were manually defined in the midline of the forehead and at the nonfused coronal suture, respectively. The program then found a point (p) on the fused side, at the same distance from the O as the end-point. The contours of the left and right side of the forehead were thereafter superimposed, and the position of minimal area mismatch of the sides was identified. To correct for growth between preoperative images and follow-up, the number of mismatching pixels was related to the area outlined by the contour of the forehead, the end-point and p. Two quantities, the relative symmetry change and the absolute symmetry change, were defined and evaluated by repeated measurements on spherical and elliptical phantoms and 15 patients. Results: Measurements with the MATLAB program were reliable with an SD of 0.26% to 5.39% for the expected range of differences. The SD was lower for measurements on computed tomographic images than for measurements on cephalometric images. The SD was also lower in patients with large surgical improvement than in patients with little improvement. The results support the use of relative symmetry change to evaluate surgical results. Conclusions: Our new computer-based method is capable of measuring forehead symmetry with good precision. This method can be used for systematic evaluation of surgical outcome for unicoronal synostosis and other asymmetric skull deformities.