Evaluating the effect of reference line on interpretation of cephalometric analysis

Abstract Background: Natural head position (NHP) is a standard and reproducible position and its short term reproducibility has been reported to be much lower than standard deviation of extra-cranial lines. Objectives: Due to racial and ethnical differences in cephalometric measurements this study was carried out to compare the variations of extra-cranial reference lines to true vertical line among Iranian people while registration was in NHP. Methods: This was a diagnostic study conducted on 60 dentistry students aged between 19-29 years (females: 22.50±2.76 males: 23.45±2.94 30 each). The exclusion criteria were heriditary and acquired dentofacial disorders, facial malformations, history of orthodontic treatment, orthognathic surgery, TMD and neck musculopathies. All types of occlusions were included. Lateral cephalograms were taken from each individual in NHP by means of a mirror in the same device and by one single operator. Also, a photograph by the same device was taken to register soft tissue Frankfurt. The angles between the SN, BaNa, and soft and hard tissue FH to true horizontal were measured. Mean SD and SE of each angle, their correlation, and also determination coefficient regarding each line were calculated. The differences between the paired angles were evaluated by paired t test. Findings: The lowest SD and SE was associated with SN (SE: 0.051, SD: 3.86) and the highest related to BaNa (SE: 0.51, SD: 4.53). The lowest correlation coefficient was associated with the soft and hard tissue Frankfort plane compared to horizontal line (r2=0.39, p<0.003, r=0.015). The highest correlation was observed between BaNa and SN compared to true horizontal line (r=0.086, p<0.000, r=0.079). Conclusion: Diagnosis of dentofacial disorders using extra-cranial reference lines will be more reliable if NHP is registered accurately.