Improvement of smile characteristics in dentofacial deformity patients following orthognathic surgery

The purpose of this study was to investigate the effect of bimaxillary surgery on smile characteristics, including smile line, smile arc, smile index (SI), most posterior teeth displayed(MPTD), buccal corridor ratio (BCR), and smile symmetry. The study consisted of 87 patients with dentofacial deformity: Class II (30 patients), Class III (37 patients), and asymmetry (20 patients). Pearson's chi-squared test and ANOVA were used to analyze the differences in smile characteristics across the preoperative groups. Pearson's chi-square test and the paired-sample t-test were utilized to determine the differences in smile characteristics between the preoperative and postoperative periods. The results showed significant differences in lip thickness ratio (p = 0.021∗), smile line (p = 0.003∗∗), smile arc (p = 0.000∗∗), MPTD (p = 0.015∗), and BCR (p = 0.000∗∗) among the preoperative groups. After orthognathic surgery, in the Class II group there was an increase in the proportion of smile line that transformed from high to average and low (p = 0.001∗∗), an increase in the percentage of upper lip curvature that changed from downward to upward and straight (p = 0.011∗), and an increase in both postoperative SI and BCR (p = 0.007∗∗ and p = 0.002∗∗, respectively), while MPTD decreased (p = 0.031∗). For Class III, the postoperative change in upper lip curvature tended to be more upward and straight (p = 0.002∗∗), the smile arc became more parallel and straight (p = 0.000∗∗), postoperative SI and MPTD increased (p = 0.000∗∗), while BCR decreased (p = 0.000∗). Regarding asymmetry, both MPTD and BCS showed symmetry postoperatively (p = 0.038, p = 0.000∗∗). The findings of this study indicate that orthognathic surgery could enhance some smile characteristics.