Frontal sections by Polytome, 3 and 4cm posterior to nasion, were analysed in order to calculate the spatial distribution of the ethmoidomaxillary plate (EMP). 80 male adults and 68 female adults, all without the previous histories of sinus surgeries, were included in the study. Both the medial and lateral borders of EMP were analysed in order to calculate the angles between the borders and the anthropological base plane (Frankfort plane). These angles of the medial and lateral borders are as follows respectively; 8.0° and 15.6° in the male, 0.6° and 11.3° in the female, and 9.6° and 14.6° by the average of both the male and female.
A labelling study of EMP by dry skulls was performed in order to determine the changing positions of the overlapping petrous ridge by various positioning in PA view. Each of the three corners of EMP was labelled with a tiny lead ball in the maxillary sinus. When the central X-ray hits the EMP tangentially, these labels will assume linear arrangement among each other. This linear arrangement is seen when the central X-ray makes an angle of 0° to -15° to the anthropological base plane. In order to avoid the overlapping of the petrous ridge, an angle exceeding -10° is necessary. These results suggest that EMP will be best demonstrated by -15° angulation of the central X-ray in the PA view. This is in accordance to the previous reports that Caldwell view is suited for the demonstration of EMP.
