How do different bimaxillary orthognathic surgeries affect facial soft tissues?

This study evaluated three-dimensional (3D) soft tissue changes and their correlations with hard tissue movements after combined bimaxillary orthognathic surgery. A retrospective analysis was performed using cone-beam computed tomography scans obtained from 22 adults before surgery (T1) and at least 6 months after surgery (T2). Participants were divided into two groups according to the surgical procedure performed: mandibular setback combined with maxillary advancement (MdSet + MxAdv; n = 10) or mandibular advancement combined with maxillary impaction (MdAdv + MxImp; n = 12). Three-dimensional displacements of hard and soft tissue landmarks were assessed using ITK-SNAP and 3D Slicer software. Pearson's and Spearman's correlation coefficients were used to identify associations between hard and soft tissue changes. Significant correlations between anteroposterior hard and soft tissue movements were observed in the subnasal region in the MdSet + MxAdv group (% = 75/70; r = 0.768/0.875) and in the upper lip (% = 83/87/71; r = 0.689/0.760/0.812) and lower lip (% = 54; r = 0.755) regions in the MdAdv + MxImp group. Soft tissue responses in the lower lip and chin regions were significantly correlated with underlying hard tissue movements in both groups; however, the MdAdv + MxImp group demonstrated greater soft tissue response magnitudes (% = 123/136; r = 0.975/0.982) than the MdSet + MxAdv group (% = 75/70; r = 0.810/0.727). Significant vertical correlations in the MdAdv + MxImp group were identified at point A' (% = 86; r = 0.851) and mandibular landmarks (% = 67/109/108; = 0.855/0.981/0.996), whereas the MdSet + MxAdv group showed significant correlations at the lower lip and Pog' point (% = 137/73; r = 0.785/0.921). Both groups demonstrated stronger correlations for mandible-related soft tissues in the anteroposterior and vertical directions. These quantified soft-to-hard tissue ratios may improve the prediction of facial soft tissue changes and contribute to more accurate planning of combined bimaxillary orthognathic procedures.