Aim: There are significant differences in morphological features of various racial and ethnic groups. The purpose of the present study was to identify cephalometric norms of Tweed diagnostic facial triangle in North East adolescents of Iran and to evaluate the effect of sexual dimorphism on cephalometric criteria of this population.
Materials and methods: The sample comprised lateral cephalograms taken in natural head position of 40 pupils with normal occlusion (20 girls, 20 boys). Tweed diagnostic triangle was traced on each cephalogram and its angles were measured to the nearest 0.5°. The data were analyzed by SPSS software using independent samples t-tests and liotelling's test .
Results: The lower incisors were significantly more proclined and mandibular plane was significantly steeper in north eastern Iranians compared to Caucasians (p<0.00 ). The average FMIA angle was significantly smaller than 65° as proposed by Tweed (p<0.001). There was no statistical difference in mean values of the three angles between boys and girls (p>0.05).
Conclusion: There are significant differences in cephalometric norms of Tweed triangle between Iranians and Caucasians, showing important implications in orthodontic treatment planning. With respect to the significant differences in cephalometric norms of Iranians and Caucasians, it seems necessary to use standards of each racial group for orthodontic diagnosis and treatment of malocclusions in that population.