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Effect of chin position on natural head orientation reproducibility.

BACKGROUND AND AIMS
Establishment of natural head position (NHP) in radiography or photography by estimation, which is called natural head orientation (NHO), has been recently advocated by various studies. This study aimed to determine the effect of chin position on the accuracy and reproducibility of NHO in a more realistic way.


MATERIALS AND METHODS
NHP-based digital photographs of 33 adults with normal profile were modified using the computer software Supper-Goo and Photoshop technique to produce 99 profile images in three groups with normal, forward and backward chin positions. The reconstructed images were presented to three observers who rotated the images through a circular hole on a computer monitor. The horizontal angle was read on screen and recorded whenever they found the best horizontally-oriented face. This was performed at four intervals: T0: baseline; T1: after 5 minutes; T2: after one week; and T3: one month after baseline.


RESULTS
After orientation, the mean orientation angles were 0.19°±1.39, 0.20°±1.53 and -0.31°±1.38 for the normal, forward and backward chin groups, respectively. T-test did not show any significant differences in NHO and NHP among the three groups of chin position: P values of 0.17, 0.19, and 0.26 for normal, forward, and backward chin positions, respectively. However, one-way ANOVA demonstrated significant differences in the NHO between the normal and backward, and backward and forward chin groups. The obtained reproducibility rate of NHO calculated via Med-Calc 8.1 online software for T0, T1, T2 and T3 intervals was more than 60%, which is regarded significant in medical studies.


CONCLUSION
Natural head position could be created with great reproducibility through orientation of profile images in different chin positions. NHO is under the influence of chin position; however, this influence is not clinically significant.