Three-dimensional evaluation of soft and hard tissue changes in skeletal class III patients following orthognathic surgery

Purpose: This study aimed to quantitatively evaluate soft tissue changes in resting and social smile positions after bimaxillary orthognathic surgery in patients with skeletal Class III deformity, by superimposing three-dimensional (3D) stereophotogrammetry and cone-beam computed tomography (CBCT) data. Materials and Methods: Resting analysis was performed in 20 patients who underwent Le Fort I osteotomy and bilateral sagittal split ramus osteotomy (BSSO), while social smile analysis was conducted in 21 patients treated with the same surgical protocol. Cone-beam computed tomography (CBCT) and facial scans obtained using a three-dimensional facial photography and analysis system (3dMD) taken preoperatively (T0) and at the earliest 6 months postoperatively (T1) were aligned using anatomical reference points. Hard and soft tissue landmarks were measured relative to the Frankfurt Horizontal (FH) and vertical planes (VP), and soft tissue–hard tissue follow ratios were calculated. Social smile analysis included lip morphology, smile width, interlabial gap, facial dimensions, and symmetry parameters. Statistical analysis involved the Shapiro–Wilk test, paired t-test or Wilcoxon test, Benjamini–Hochberg false discovery rate (FDR) correction, and Cohen's d effect size. Results: In the resting position, vertical movements relative to the FH plane showed that soft tissue closely followed hard tissue in the chin (Me, Gn, Pog), nazal (Prn, Sn), and lip (Ls, Li) regions, with follow ratios exceeding 98% at menton and gnathion, 74.4% at pogonion, 79.8% at point A, and 86.0% at point B. Horizontal movements relative to the VP plane revealed changes in mandibular and maxillary reference points as well as lip, nose, and midface parameters; lower facial segments followed skeletal changes more closely, whereas midface regions exhibited greater variability. In social smile analysis (n = 21), lip morphology (upper/lower lip thickness, lip length), smile width, smile index, interlabial gap, facial height, and symmetry parameters were evaluated. Postoperatively, upper lip thickness tended to increase, while lower lip thickness slightly decreased. Smile width and smile index increased, with a tendency toward reduced buccal corridor ratios. Smile symmetry improved, particularly through reduced vertical asymmetry between the oral commissures. Cohen's d values indicated small-to-moderate effect sizes for lips and lower facial segments. Regional variations showed that soft tissue response was more limited in the nasolabial and paranasal areas compared to skeletal movement. Conclusion: This study presents a comprehensive 3D analysis of soft tissue–hard tissue relationships after orthognathic surgery in both resting and social smile positions, incorporating vertical and sagittal movements. Findings revealed high soft tissue follow ratios in lower facial segments, but reduced correspondence in nasolabial and paranasal areas. Social smile analysis demonstrated postoperative improvements in smile width, lip morphology, and symmetry. Effect size analysis suggested that surgery yields clinically perceptible aesthetic benefits. These results support the use of 3D facial analysis and CBCT fusion techniques as reliable tools for surgical planning and patient expectation management. Keywords: Orthognathic surgery, Stereophotogrammetry, Cone-beam computed tomography (CBCT), Soft tissue–hard tissue ratio, Smile analysis