Fluctuating asymmetry of the normal facial skeleton.

The purpose of this study was to produce reliable estimations of fluctuating facial asymmetry in a normal population. Fifty-four computed tomography (CT) facial models of average-looking and symmetrical Chinese subjects with a class I occlusion were used in this study. Eleven midline landmarks and 12 pairs of bilateral landmarks were digitized. The repeatability of the landmark digitization was first evaluated. A Procrustes analysis was then used to measure the fluctuating asymmetry of each CT model, after all of the models had been scaled to the average face size of the study sample. A principal component analysis was finally used to establish the direction of the fluctuating asymmetries. The results showed that there was excellent absolute agreement among the three repeated measurements. The mean fluctuating asymmetry of the average-size face varied at each anthropometric landmark site, ranging from 1.0mm to 2.8mm. At the 95% upper limit, the asymmetries ranged from 2.2mm to 5.7mm. Most of the asymmetry of the midline structures was mediolateral, while the asymmetry of the bilateral landmarks was more equally distributed. These values are for the average face. People with larger faces will have higher values, while subjects with smaller faces will have lower values.

Point-based superimposition of a digital dental model on to a three-dimensional computed tomographic skull: an accuracy study in vitro.

We investigated the accuracy of point-based superimposition of a digital dental model on to a 3-dimensional computed tomographic (CT) skull with intact dentition. The physical model was scanned by CT to give a virtual skull model, and a plaster dental model was taken and laser-scanned to give a digital dental model. Three different background investigators were recruited and calibrated to make the point-based superimposition, and afterwards were asked to repeat 5 superimpositions each. Five bone-to-tooth measurements for the maxilla and 6 for the mandible were selected to indicate the relation of teeth to skull. Repeated measures were made on the physical model to act as a control group, and on the virtual model to act as the test group. The absolute agreement intra-class correlation coefficient (ICC) was used to assess the intra/inter-investigator reliability; Bland-Altman analysis was used to calculate the general differences, limits of agreement, and precision ranges of the estimated limits. Inter/intra-investigator reliability was excellent with ICC varying from 0.986 to 1; Bland-Altman analysis indicated that general difference was 0.01 (0.25)mm, the upper limit of agreement was 0.50mm and the lower limit -0.47 mm, and the precision range for the upper limit was 0.43 mm to 0.57 mm and for the lower limit -0.54 mm to -0.40 mm. Clinically acceptable accuracy can be achieved using a direct point-based method to superimpose a digital dental model on to a 3-dimensional CT skull.