The study of three-dimensional smile changes on orofacial morphology

Traditionally, two-dimensional (2D) frontal and lateral facial views have provided an incomplete understand true three-dimensional (3D) relationships. This study aimed to investigate the 3D changes in orofacial soft tissues during smiling and to examine the influence of craniofacial and dental morphology on these changes.

131 individuals aged 18-30 with attractive facial features were recruited. Rest and posed smile 3D facial photos captured by 3D camera and cone-beam computed tomography (CBCT) scans were taken. Data were recorded and analyzed in rest, smile images, and smiling change measurements, including 23 landmarks, 11 linear, and 6 angular measurements. Statistical methods were used to analyze the frequency distribution, the significant difference between rest and smile variables, and the relationship between craniofaciodental tissue and 3D smile changes.

The study involved 65 males (mean age 23.5 ± 3.2 years) and 66 females (mean age 22.5 ± 1.5 years). The soft tissue landmarks changed during smiling reveals that 21 landmarks in z-plane, 22 landmarks in y-plane, and 15 landmarks in x-plane landmarks showed significant differences. In addition, a significant correlation was noted between upper lip length (sn-uli) (r = 0.46, P = 0.057), lower lip length (lls-SB) (r = 0.44, P = 0.022), and the hard tissue measurement Id-Me.

The significant 3D change of soft tissue landmarks during smiling occurs mostly in z and y-plane, rather than x-plane. Clinically, all dimensions should be considered in orthodontic diagnosis and treatment planning, especially in the z plane the change cannot be seen in the traditional 2D analysis.