Natural head posture--considerations of reproducibility.

This three-part study investigated the reproducibility of natural head posture (NHP) using radiographs and photographs. In part 1, reproducibility of cephalograms 1 year after the introduction of NHP was investigated and found to be less favourable (Dahlberg coefficient of 2.99 degrees) than most other previous investigations. In order to minimize radiation exposure of patients, reproducibility of photographs and method agreement between photographs and cephalograms were investigated in part 2. Reproducibility of the two photographs was poor (2.71 degrees). However, method agreement between cephalograms and the photographs taken at the same time was good (1.39 degrees). Replacement of the radiographic method with the photographic method for assessing NHP reproducibility appeared justified. Changing the protocol for achieving NHP in part 3 of the study improved reproducibility substantially (1.41 degrees). Various statistical methods were used to assess reproducibility and method agreement. Bland and Altman's graphical representation was found to be the most appropriate for method agreement. The Dahlberg coefficient, commonly used to assess NHP repeatability/reproducibility, does not provide an extreme enough interval to allow a sufficient clinical assessment of a method to be undertaken, compared with the reproducibility coefficient. That is, the latter provides a 95 per cent range, compared with 52 per cent with Dahlberg.