Magnitude and reproducibility of smiling in 12-year-old Caucasian children in the Republic of Ireland: a comparison of Class I and Class II malocclusions

Abstract

Aims:• To determine if malocclusion (Class I, Class II division 1 and Class II division 2) influences the magnitude, the immediate intra-session and the short-term inter-session reproducibility of the rest position to posed smile and the rest position to maximal smile. • To determine if gender influences the magnitude, the immediate intra-session and the short-term inter-session reproducibility of the rest position to posed smile and the rest position to maximal smile. Materials and Methods: One hundred and ten Caucasian volunteers (55 males; 55 females) aged 12 years, with no previous history of orthodontic treatment, identifiable syndrome or facial asymmetry, were recruited. Three malocclusion categories were assessed: Class I (20 males, 20 females), Class II division 1 (20 males; 20 females) and Class II division 2 (15 males; 15 females). Three-dimensional (3D) images of three facial expressions (rest position, posed smile and maximal smile) of each subject were captured using the Di3D system. These images were repeated 15 minutes later to assess immediate intra-session reproducibility and two weeks later to assess short-term inter-session reproducibility. Twenty-six landmarks were digitally placed on all the images. Landmark identification error was assessed by re-landmarking 10 percent of the images, one month after initial landmarking. The magnitude of movement from rest to posed smile and from rest to maximal smile averaged over all the landmarks was calculated for each session. Results: The magnitude of mean movement averaged over all the landmarks differed significantly between rest to posed smile and rest to maximal smile (p < 0.0001). This difference was found in both genders (p = 0.0012) but was greater in males than in females (p <0.0001). Immediate intra-session reproducibility (p=0.1677) was high for both rest to posed smile and rest to maximal smile. A statistically significant difference (p <0.0001) of 0.27mm in short-term inter-session reproducibility was found for both rest to posed smile and rest to maximal smile. This was, however, clinically insignificant. Malocclusion had no effect on magnitude of either smile (p = 0.8138) or immediate intra-session reproducibility (p = 0.3878) or short-term inter-session reproducibility (p=0.3396). Similar results were found when the 10 lower-face landmarks were assessed independently. Conclusion Rest to posed smile and rest to maximal smile differed in terms of magnitude of movement for both genders with males displaying a greater difference. The rest to posed smile and rest to maximal smile demonstrated immediate intra-session and short-term inter-session reproducibility in males and females. Malocclusion had no effect on the magnitude or reproducibility of smiling.