# Differential Relapse of Proximal and Distal Segments after Mandibular Setback Surgery
## 5 Conclusions  
* The mandibular condyles rotated medially with lateral and downward movement during mandibular setback surgery. They gradually recovered during postsurgical orthodontic treatment but never fully returned to the pretreatment position.
* During the postoperative healing period, the mesial and distal fragments of the mandible each have different centers of rotation (C\({}_{\text{Rot}}\)) and move differently.
* Increased VBS during surgery was correlated with greater relapse of the mandibular setback and a corresponding postrosuperior shift of the C\({}_{\text{Rot}}\) of the distal segment.
* Two regression equations were formulated to predict the coordinates of the C\({}_{\text{Rot}}\) of the distal segment for the postsurgical relapse. These can be used to simulate relapse during the surgery simulation, although they need external validation.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14209299/s1](https://www.mdpi.com/article/10.3390/app14209299/s1)]([https://www.mdpi.com/article/10.3390/app14209299/s1](https://www.mdpi.com/article/10.3390/app14209299/s1)), Figure S1: Landmarks; Figure S2: A miniplate and 4 screws for semi-rigid fixation; Table S1: Correlation between the VBS and condye movement.  
Conceptualization, S.-H.L. and J.-D.K.; methodology, S.-H.L.; software, J.-D.K.; validation, J.-D.K.; formal analysis, J.-D.K.; investigation, S.-H.L.; resources, S.-H.L.; data curation, J.-D.K.; writing--original draft preparation, S.-H.L. and J.-D.K.; writing--review and editing, J.-D.K. and J.H.P.; visualization, S.-H.L. and J.-D.K.; supervision, S.-H.L. and J.H.P.; project administration, S.-H.L.; funding acquisition, S.-H.L. All authors have read and agreed to the published version of the manuscript.  
Figure 6: Illustration of surgical simulation considering relapse. (**A**) Postmandibular osteotomy with the mandible’s proximal (green) and distal (blue) segments split. (**B**) To account for the expected relapse, the surgical occlusion was adjusted to create a Class II open bite. A 16.3 mm mandibular setback (red arrow) created 9 mm of VBS (yellow arrow). The overlap between the proximal and distal segments (dark green) indicates the area to be removed during surgery. (**C**) Simulation of the relapse during postsurgical orthodontic treatment. The center of rotation (C\({}_{\text{Rot}}\)) of the distal segment was calculated based on the VBS, located 43.2 mm above and 10.6 mm behind the medial pole of the right condye (red dot). The distal segment of the mandible (yellow) was then rotated counterclockwise until the VBS measurement reached 2 mm (yellow), a position that is typically maintained after treatment. The proximal segment (purple) was rotated around the medial pole of the condye. (**D**) Superimposition of the post-treatment skull (pink) with the relapse simulation model (yellow and purple) demonstrates that the relapse simulation was accurate in this case. Both maxillary and mandibular incisors were retracted during postsurgical orthodontic treatment.  
**Funding:** This research was funded by Chosun University, 2021 (Chosun-2021). The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Institutional Review Board (IRB) of Chosun University Dental Hospital (IRB approval number CUDH IRB 2301 003, 26 January 2023).  
**Informed Consent Statement:** Patient consent was waived due to the retrospective design of this study.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author due to privacy or ethical restrictions.  
**Conflicts of Interest:** The authors declare no conflict of interest.