# Differential Relapse of Proximal and Distal Segments after Mandibular Setback Surgery
## 4 Discussion  
In the present study, the positional changes in the proximal and distal segments after BSSRO setback surgery were evaluated. During surgery, the condyles rotated medially with inferior and lateral movement, a result which is consistent with the previous study of [23]. During mandibular setback surgery, the proximal segments are routinely rotated medially to reduce the transverse gap between the proximal and setback distal segments, which inevitably causes the condyle's inferior and lateral movement out of the mandibular fossa. During postsurgical orthodontic treatment, condyles returned partially to the pretreatment position via lateral rotation with superior and medial movement [24]. In another study, it was explained that the medial rotation of the condyle during surgery was due to screw insertion during fixation [25]. The present study evaluated only the right condyle in patients with less than 5 mm of chin asymmetry. Another study showed that although the condylar heads moved downward after BSSRO surgery in both the asymmetry and symmetry groups, there were almost no changes in the condylar head between pretreatment and post-treatment in the axial and sagittal plane [26]. In the present study, although the condylar position changes during surgery were reduced during postsurgical orthodontic treatment, significant changes in the anteroposterior position of the medial and lateral poles remained. In the present study, the amount of mandibular setback, the developed VBS, and the resolved VBS did not correlate with condylar movements (Table S1), highlighting the difficulty in predicting these movements. This may be due to the complex proximal segment movements influenced by the bony interferences and gaps between the distal and two proximal segments that arise during mandibular setback. According to a previous study [27], gender does not significantly affect postsurgical mandibular relapse, so this study did not examine gender differences. Additionally, no significant difference in relapse was reported between single- and double-jaw surgeries [28], which is consistent with the present study results. Regarding relapse according to the surgical methods used for mandibular setback, counterclockwise rotation of the mandible has been reported with BSSRO, whereas clockwise rotation has been reported with intraoral vertical ramus osteotomy [4]. When the distal segment is set back along the presurgical occlusal plane, VBS occurs, which can \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Type of Surgery**} \\ \cline{2-6}  & \multicolumn{2}{c}{**1-Jaw Surgery**} & \multicolumn{2}{c}{**2-Jaw Surgery**} \\ \cline{2-6}
**Variables (T2–T1)** & **Mean** & **SD** & **Mean** & **SD** & \(p\) **Value** \\ \hline PS rotation (\({}^{\circ}\)) & \(-\)2.59 & 1.84 & \(-\)2.31 & 2.52 & 0.731 \\ DS rotation (\({}^{\circ}\)) & \(-\)2.15 & 2.10 & \(-\)1.92 & 2.61 & 0.792 \\ Pog-Z (mm) & 4.23 & 1.56 & 3.69 & 2.20 & 0.445 \\ Me-\(Y\) (mm) & 2.31 & 0.78 & 2.20 & 1.52 & 0.799 \\ VBS (mm) & 4.41 & 1.54 & 4.31 & 2.19 & 0.886 \\ \hline \hline \end{tabular} Independent \(t\)-test was performed. \({}^{\ast}\) Statistically significant, \(p\) < 0.05.  
\end{table} 
Table 6: Comparison of postsurgical changes between single-jaw and double-jaw surgeries \({}^{\ast}\). extend the pterygomasseteric sling and cause postsurgical counterclockwise rotation of the mandible [10; 17]. Also, during postsurgical orthodontic treatment, the proximal and distal segments of the mandible showed different movements, resulting in a significant resolution of the VBS [17]. It has been reported that the proximal segments rotate counterclockwise due to the contraction of the masticatory muscles, while the distal segment rotates clockwise during the postoperative period with monocortical fixation [29]. They explained that the reason for such movements was that the proximal segment was pulled upwards by the masseer muscle but fixed at the TMJ, while the distal segment's upward movement was restricted by the maxillary molars.