Relationship Between Frankfort "Plane" and Inferences About Chin Growth

Objective: To investigate the impact of small angular differences in the orientation Frankfort plane and the perceived forward displacement of chin point during growth.

Problem statement: Over a number of years our laboratory has consistently tended to report the forward displacement of pogonion relative to superimposition on anterior cranial base to be smaller than that reported by other investigators.  It has recently been posited that one reason for the difference may be the manner in which porion, the posterior point determining Frankfort line, is located. Method: We have recently conducted a graphical and numerical analytic test of this hypothesis using previously reported data on mandibular growth from a group of subjects with implants of the Bjork type (AJODO 102:134-42 102:227-38).  For the same data set  and the same superimposition on anterior cranial base (n = 20, age interval from 8.5 to 15.5 years), Frankfort plane was computed using three different positions for porion: 1) at machine porion, 2) 5 mm below machine porion, 3)  5 mm above machine porion.  The location of orbitale was held constant in all cases.  Results: Graphical and numerical analysis indicate that for constant location of orbitale, higher locations of porion will lead to the perception of a larger magnitude of forward movement of pogonion relative to the forward displacement of nasion.  The larger the actual downward movement of pogonion, the larger this difference will become.  Hence the disparity will be greater the longer the time interval between films in growing subjects.  (See Table 1.)   Conclusion: This experiment demonstrates the critical impact of differences in the orientation of the coordinate frame of reference upon inferences drawn from cephalometric data.  Conclusions must be based on a firm understanding of local geometry.