The angle between the Frankfort horizontal and the sella-nasion line. Changes in porion and orbitale position during growth.

Aim: The starting point of this study was the statement made in the literature that the angle between the two reference planes Frankfort horizontal (FH) and sella-nasion line (SN) changes relatively little during growth. The growth-induced relocation of the orbitale (Or; anterior reference point of the FH) in relation to SN is known from implant studies, whereas the relocation of the porion (Po; posterior reference point of the FH) has been the subject of only little research. The present study was aimed at analyzing the factors contributing to the almost constant angle between FH and SN.

Material And Method: The study material consisted of two groups of macerated skulls and the relevant lateral cephalograms. The first group comprised 32 skulls of individuals aged 2.5 to 5 years, and the second group ten skulls of individuals aged 18 to 20 years. A diagram showing the growth-dependent changes was prepared with reference to the mean values for the two groups. The cephalograms were superimposed on the anterior cranial base line at sella point (S).

Results: A 3.1 degrees increase in the angle between FH and SN during growth was recorded in our investigations. The distance between Or and SN increased by 3.9 mm while Po remained vertically almost constant with respect to SN. In sagittal direction the distance between Or and S also increased, while Po was displaced to almost the same extent in the opposite direction. The increasing vertical distance between the anterior and posterior reference points of FH and SN was largely compensated by the sagittal developments of the reference points Po and Or, so that the angle between these two planes changed very little. The relatively stable angle between FH and SN thus showed by no means a constant relationship of the four reference points to one another.