The minimal region required for accurate CBCT-3D facial registration in evaluating soft-hard tissue changes after orthodontic extraction

To determine the minimal registration area required for accurate best-fit integration of cone-beam computed tomography (CBCT) and three-dimensional (3D) facial images, and to apply the optimized multimodal model to quantify hard-soft tissue changes around the lips after orthodontic treatment. This retrospective study analyzed 20 patients (6 males and 14 females; mean age 17.05 ± 7.37 years) who underwent same-day CBCT and 3D facial scanning. Four registration regions were tested: A0 (zygomatic and nasal root), and A1-A3 (progressively larger areas). Alignment accuracy was assessed using mean surface distance. The minimal effective region was then used to construct integrated models for measuring sagittal, vertical, and horizontal distances between 15 hard and 10 soft tissue landmarks before and after treatment. Differences in mean surface distance among the various registration regions were analyzed using one-way ANOVA, a paired-sample t-test was conducted to evaluate changes in sagittal, vertical, and horizontal soft and hard tissue distances before and after orthodontic extraction treatment. The minimal region, A0 (zygomatic 2–3 cm; nasal root 1–2 cm), achieved alignment accuracy comparable to A1-A3, with no significant differences in mean surface distance (p = 0.921). Post-treatment measurements showed significant sagittal increases in lip soft tissue thickness (≈ 3–4 mm), most prominently at the philtrum and vermilion border. Philtrum retraction was approximately one-third that of the vermilion. Maxillary incisors exhibited 1–2 mm vertical intrusion and mild horizontal canine displacement, whereas most other hard-soft tissue relationships remained stable. The zygomatic (2–3 cm in diameter) and nasal root (1–2 cm in diameter) regions form a minimal yet reliable registration area for accurate CBCT-3D facial image integration. After orthodontic extraction, lip soft tissue thickened sagittally despite incisor retraction, while other soft-hard relationships were largely preserved. These findings support the clinical utility of localized registration and highlight lip soft tissue adaptation as a key factor in treatment evaluation. The protocol of this study (PKUSSIRB-202392145) was registered at the Chinese Clinical Trial Registry (Chictr.org.cn) with the identifier ChiCTR2000034288 (01/07/2020).