Reproducibility of Natural Head Position assessed with stereophotogrammetry

Objective: The aim of this study was to assess the stability of Natural Head Position (NHP) over time using the 3dMDface System. Setting and sample population: 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. Materials and Methods: 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. Results: 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. Conclusion: 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.