3 Years photographic reproducibility of Natural Head Position (a longitudinal study)

Recently registration of lateral cephalometry in natural head position (NHP) has been recommended. But there are few longitudinal studies on its long-term reproducibility. The purpose of this study was photographic estimation of 3 year reproducibility of NHP in adult cases. This was a longitudinal descriptive study performed on 27 volunteer dental students with 19-29 years of age during 1997-1998. Excluding criteria consisted of congenital and TMJ disorders, history of previous orthodontic treatment, orthosurgical or reconstructive surgery. Three photographs in NHP were taken. The second one (T2) was taken 4 to 10 minutes after the first photograph (short term period) and the third one (T3) was taken with an average of 2 years, 6 months and 9 days after the first one (long term period). They were taken by one technician using a single cephalometric device in standing position by means of a mirror in order to decrease the effect of confounding variables as much as possible. After superimposition of photographs registered on the true vertical line, the angle between true horizontal line from N and N'-Pog' line was calculated. The mean, SD, confidence interval of the angles, short and long term ME, correlation of the angles by time and sex difference were analyzed. Short term reproducibility was 1.78 degrees and long term was 6.60 degrees. There was a significant correlation between changes of the angles during time, which decreased respectively [r(T1/T2)=0.918 r(T2/T3)=0.538 r(T1/T3)=0.478). There was a significant difference between male and female samples only in the second registration (T2) (p<0.045). NHP reproducibility decreases significantly after 3 years and it becomes one third of its short term value nevertheless there is a significant correlation between its variations during a 3 year period.