[Reproducibility and repeatability of the determination of occlusal plane on digital dental models]

To assess the repeatability(intraobserver comparison)and reproducibility(interobserver comparison)of two different methods for establishing the occlusal plane on digital dental models.

With Angle's classification as a stratification factor,48 cases were randomly extracted from 806 ones which had integrated clinical data and had their orthodontic treatment from July 2004 to August 2008 in Department of Orthodontics ,Peking University School and Hospital of Stomatology.Post-treatment plaster casts of 48 cases were scanned by Roland LPX-1200 3D laser scanner to generate geometry data as research subjects.In a locally developed software package,one observer repeated 5 times at intervals of at least one week to localize prescriptive landmarks on each digital model to establish a group of functional occlusal planes and a group of anatomic occlusal planes, while 6 observers established two other groups of functional and anatomic occlusal planes independently.Standard deviations of dihedral angles of each group on each model were calculated and compared between the related groups.The models with the five largest standard deviations of each group were studied to explore possible factors that might influence the identification of the landmarks on the digital models.

Significant difference of intraobserver variability was not detected between the functional occlusal plane and the anatomic occlusal plane (P>0.1), while that of interobserver variability was detected (P<0.001). The pooled experimental standard deviation the 6 observers established for the functional occlusal plane was 0.2° smaller than that of the anatomic occlusal plane.The functional occlusal plane's variability of intraobserver and interobsever did not differ significantly (P>0.1), while the anatomic occlusal plane's variability of the intraobserver was significantly smaller than that of the interobserver (P<0.001), with a 0.12° difference value of the pooled experimental standard deviation.

When one observer measures a number of digital models,both the functional and the anatomic occlusal planes are suitable as a conference plane with equal repeatability. When several observers measure a large number of digital models,the functional occlusal plane is more reproducible than the anatomic occlusal plane ,but the difference is small.