# Differential Relapse of Proximal and Distal Segments after Mandibular Setback Surgery
## 3 Results
### Relapse of Proximal and Distal Segments and Their Center of Rotation  
During T2-T1, the proximal segments rotated 2.4 \(\pm\) 2.1\({}^{\circ}\), and all proximal segments rotated counterclockwise. The C\({}_{\text{Rot}}\)s of the proximal segments were distributed around the condyle, and the mean C\({}_{\text{Rot}}\) of the proximal segments was 12.3 \(\pm\) 26.1 mm superior and 16.6 \(\pm\) 48.0 mm posterior to the medial pole (Figures 3A and 4A and Table 4). During T2-T1, the distal segments rotated 2.0 \(\pm\) 2.3\({}^{\circ}\) counterclockwise, and the mean C\({}_{\text{Rot}}\) was 3.0 \(\pm\) 92.3 mm superior and 13.5 \(\pm\) 59.3 mm anterior to the medial pole of the condyle (Figures 3B and 4B and Table 4). The C\({}_{\text{Rot}}\)s of the distal segments were distributed along the line passing the condyle posterosuperiorphy and anteroinferiroly (Figure 3B). Only five samples with the C\({}_{\text{Rot}}\) located inferior to the menton showed a clockwise rotation of the distal segment, and other samples showed a counterclockwise rotation. Both rotations resulted in an anterosuperior movement of the distal segments. This indicates that the relapse of the BSSRO mandibular setback always occurs in the anterosuperior direction, not in the anteroinferior direction. The difference was that a larger anterior movement of dentition, rather than the chin, occurs with clockwise rotation, and the opposite occurs with counterclockwise rotation. The regression equation between the \(z\)- and \(y\)-coordinates of the C\({}_{\text{Rot}}\) of the distal segment obtained through linear regression analysis was as follows:  
\[y=-1.45\times z+22.7\qquad(95\%\text{ CI}:\ (-1.668,-1.236)) \tag{1}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Variables**} & **T1–T0** & \(\boldsymbol{p}\)**Value** & **T2–T1** & \(\boldsymbol{p}\)**Value** \\ \hline Proximal & PS rotation & & & \\ Segment (PS) & (\({}^{\circ}\)) & 2.1 \(\pm\) 2.9 & \textless{}0.001 \({}^{\star}\) & \(-2.4\pm 2.1\) & \textless{}0.001 \({}^{\star,\ddagger}\) \\ \hline \multirow{4}{*}{Distal} & DS rotation & & & \(-2.0\pm 2.3\) & \textless{}0.001 \({}^{\star}\) \\  & (\({}^{\circ}\)) & & & \(-2.0\pm 2.3\) & \textless{}0.001 \({}^{\star}\) \\ \cline{1-1}  & VBS (mm) & \(-6.2\pm 2.9\) & \textless{}0.001 \({}^{\star}\) & \(4.4\pm 1.8\) & \textless{}0.001 \({}^{\star}\) \\ \cline{1-1}  & Pog-Z (mm) & 13.6 \(\pm\) 5.3 & \textless{}0.001 \({}^{\star}\) & \(4.0\pm 1.9\) & \textless{}0.001 \({}^{\star}\) \\ \cline{1-1}  & Me-Y (mm) & \(-0.8\pm 1.9\) & 0.015 \({}^{\star,\ddagger}\) & \(2.3\pm 1.2\) & \textless{}0.001 \({}^{\star}\) \\ \hline \hline \end{tabular} Values are mean \(\pm\) standard deviation. Significance was determined using paired \(t\)-test. The Wilcoxon signed-rank test results are presented when normality was not satisfied (\({}^{\ddagger}\)). \({}^{\star}\) Statistically significant, \(p<0.05\).  
\end{table} 
Table 4: Positional changes in proximal and distal segments of mandible \({}^{\ddagger,\star}\). Figure 2: Movements of the right condyle during treatment. (**A**) Surgical movement; (**B**) postsurgical movement. Gray shows pretreatment, yellow shows postsurgery, and pink shows post-treatment condyles. During surgery, the right condyle rotated medially with lateral-inferior movement (3-dimensional red arrows) and then partially returned to its original position during postsurgical orthodontic treatment (3-dimensional blue arrows). Statistically significant, \({}^{\star}p<0.05\); \({}^{\star\star}p<0.01\); \({}^{\star\star}p<0.001\). Figure 4: Illustration of the average positions of C\({}_{\text{Rot}}\) and the amount of rotational relapse of the proximal and distal segments of the mandible during postsurgical orthodontic treatment. (**A**) Proximal segment; (**B**) distal segment. The relapse of the distal segment is the same as the relapse of the chin, and this is the result of the relapse of the proximal segment and the change in the relationship between the proximal and distal segments.