Figure 2.

The coordinate system. A, The X-Z horizontal plane includes the origin, which is the junction of the incisive papilla and palatine raphe and is parallel to the occlusal plane constructed by bilateral mesio-buccal cusp tips of the first molars and the midpoint of the central incisors. B, The X-Y sagittal plane, which is perpendicular to the horizontal plane, is made up of the origin point and one arbitrary point on the mid-palatal suture. C, The Y-Z frontal plane is the section inclusive of the origin and perpendicular to both the sagittal and the horizontal planes. The measuring points were the midpoint on the edge of the upper central incisors and the mesio-buccal cusp tips of the upper first molars.