Influence of the Choice of the Measurement Plane on Infant Head Shape Indices: An Examination Using a Statistical Shape Model

The cranial vault asymmetry index (CVAI) and the cephalic index (CI) quantify infant head shape, but the convention for setting the height of the measurement plane varies across facilities. From a public statistical shape model of the infant head, 2000 synthetic shapes were generated, 500 in each of 4 classes. Taking the plane through the bilateral tragi and the sellion as reference, CVAI and CI were obtained at level 3, the third of 10 equal divisions to the vertex, at the eyebrow-height horizontal plane and from the top-view silhouette, and compared within each individual. Relative to level 3, the eyebrow-height plane gave a CI larger by 6.14 and a CVAI smaller by 0.75 in all 4 classes. Every individual showed the larger CI; 71% showed the smaller CVAI, with limits of agreement crossing 0. In sensitivity analyses, the sign of the CI difference remained consistent, whereas that of the CVAI reversed when the plane was raised to 20 mm above the canthus. The CVAI difference increased with deformation, behaving as compression (slope 0.833), not a fixed offset; regression removed the systematic deviation in a confirmation sample but narrowed the limits of agreement only from 5.72 to 4.77 points. The top-view silhouette lay close to level 3 rather than the eyebrow-height plane, with its CVAI differing by <0.5 and its CI by 1.59 at 40 cm. The measurement convention, including the plane, should be stated and unified; without a 3-dimensional device, a top-view photograph is the closest substitute. The height of the measurement plane systematically alters infant head shape indices; the effect scales with deformation, and a top-view silhouette approximates the conventional 3-dimensional plane.