The evaluation of horizontal reference planes of adult Chinese in natural head position.

A group of 112 adult females (average age 25.66) and 104 adult males (average age 25.62) were subjected to cephalometric evaluation in their natural head position. No significant differences were found in the nasionsella (SN) plane, Frankfort horizontal (FH) plane or palatal planes between male and female subjects. The SN plane was found to be located at 7.26 degrees above the true horizontal on the average, and the FH plane differed on the average 1.92 degrees from the true horizontal with the line diverging forward and upward. The palatal plane was found to be located in a downward and forward direction at 1.17 degrees from the true horizontal plane on the average. The orbitale (Or) was found to be located at 2.34 mm on the average above the true horizontal plane depicted from the porion (Po) in this study, which implies that the racial characteristic of a high zygomatic bone in Chinese may have some influence upon the position of Or, and this should be taken into consideration when the FH plane is used as the true horizontal during face bow transfer. The mandibular plane (MP) angle, occlusal plane (OP) angle and the gonial angle (GA) of males were found to be smaller than those of females, with a significant difference at the level of P less than 0.001, P less than 0.01 and P less than 0.01, respectively. The upper facial divergent angles of males and females were quite similar, as reflected by the similarity in the SN-FH angles and SN-PP angles between the sexes A significant difference was found to exist in the lower facial divergent angles between males and females. The SN-MP angle, PP-OP angle and PP-MP angle were all significantly smaller in males, with a P less than 0.001. The OP-MP angle was also smaller in males with P less than 0.05.