Reproducibility of Natural Head Position assessed with stereophotogrammetry

The aim of this study was to assess the stability of Natural Head Position (NHP) over time using the 3dMDface System.

This was an experimental study. Three-dimensional facial images of 40 students were captured on two different occasions, with an interval of at least two weeks.

The images were taken using a stereophotogrammetric device (3dMD). The mirror positioned NHP was obtained in a standing position and then replicated in a sitting position for capturing. The self-balanced NHP was taken in a sitting position. Rapidform 3D software was used for position angle calculations. The angle changes between the positions were calculated for rotations around the x-, y- and z-axes.

The differences between NHP in the self-balanced and mirror positions recorded on the first and second occasions were 2.43 and 1.75 degrees, respectively, around the x-axis. The average changes in NHP around the x-axis between the self-balanced and mirror balanced positions exceeded 3 degrees at the two-week interval. The differences were smaller for the rotations around the y- and z-axes. Some subjects consistently tended to hold their heads in a more extended position when self-balanced, while others did this when mirror balanced. There was no difference in the reproducibility of NHP between men and women.

The reproducibility of NHP for consecutive stereophotogrammetric captures is generally acceptable. The reproducibility of NHP using the mirror position was slightly better compared with NHP in the sitting self-balanced position.