上顎大臼歯部の口内法撮影に関する幾何学的検討

抄録

In order to obtain the image with the least amount of distortion in the intraoral radiography for upper molar teeth, the authors performed the geometric investigations which are based on some anatomic matters, on the angulation between teeth and the film.
The anatomic matters measured by using extracted teeth, plaster models of upper jaw and dry skulls were as follows:
1.The rectangle of 21mm×12mm represents mesiodistally the mean outward form of the upper first molar tooth.It includes the long axis of tooth which almost bisects the rectangle longitudinally, the buccal root axis and the palatal root axis, the latter two of which are at the angle of 17 degrees to the long axis of tooth.
2.The angle formed by the long axis of the upper first molar tooth and the occlusal surface was 84.6°±4.6°in the male and 82.1°±4.8° in the female.
3.The angle formed by the long axis of the upper first molar tooth and the film was 48.0°±4.6°in the male and 47.4°±5.1°in the female.
4.The height from the occlusal surface to the median line of palate at the region of upper first molar teeth was 21.6mm±x2.4mm in the male and 19.8±2.2mm in the female.
5.The perpendicular from zygomaxillare to the occlusal surface was 37.8mm±3.9mm in length.The distance between the central pit of the upper first molar tooth and the line running parallel to the line which puts together buccal cusps of upper molar teeth, and passing through the foot of the perpendicular above mentioned.
6.We presumed that the rate of application of the paralleling technic on the radiography of upper molar teeth to the Japanese would be below 3%.
On the base of the anatomic matters above mentioned, we calculated the amount of the distortion by use of the bisecting technic for three cases of the angle formed by the film and the long axis of tooth, the palatal root axis and the buccal root axis.
Then, we clarified that the amount of the distortion in the radiography for upper molar teeth is less than 10%, if the angle formed by the long axis of tooth and the film is kept below 25°and the bisecting technic is employed for the angle formed by the buccal root axis and the film.
We calculated the vertical angle in the relation to the occlusal plane.The result of the calculation has shown that the vertical angle varies between 20°and 40°, and that superimposition of the shadow of the malar bone and zygomatic process over the roots of the maxillary posterior teeth may occur at angles more than about 34°.