Abstract: Objective: To investigate the effect of Spee's curve and crowding on the 2D and 3D measurement of orthodontic model. Methods: Study models of 30 patients with permanent dentition in clinical treatment were collected. They were measured by 2D (plaster models and photocopies) and 3D (didital models). The size of teeth, arch length and width, crowding and the depth of Spee curve were measured with grouping by crowding and the depth of Spee's curve. The results mentioned above were evaluated by one-way ANOVA and further compared by LSD method if difference was significant. Results: The differences of the teeth's sizes influenced by Spee's curve and crowding were statistically significant (P<0.05). Pairwise comparison results showed that the difference between digital and photocopies models was obvious. Maxillary mild crowding group was 1.16±1.34mm (P=0.000), mandibular mild crowding group was (1.14±1.40) mm (P=0.000), and mandibular severe crowding group was (1.35±2.19) mm (P=0.028). With the increasing of the depth of Spee's curve, there were statistically significant difference in the teeth's sizes of plaster models, digital models and photocopies. The difference of digital models and the photocopies in the mild Spee's curve group was (1.70±1.39) mm (P=0.001), and in the severe Spee's curve group was (1.66±1.46) mm (P=0 .002). Conclusion: Three measuring methods had good accuracy. Measuring differences were related to crowding and the depth of the Spee's curve.

Key words: 3D measurement, 2D measurement, Spee's curve, Dental crowding