## 4 Discussion  
According to previous studies, the etiology of ASD is considered to be excessive loading and abnormal sagittal alignment [3; 5; 8; 9; 22]. Among these causes, joint mechanics may act as an important factor affecting adjacent segment degeneration [8; 22]. However, the association between postoperative cervical spinal biomechanics and ASD remains uncertain. Hence, from a biomechanical perspective, we tried to understand the effects of spinal fusion surgery on the adjacent segments using a cervical spine MSK MBD model. For the inverse dynamics analysis, the input motion data after spinal fusion is a key factor for the accuracy and reliability of the MSK MBD simulation. As mentioned above, the segmental motion after cervical spinal fusion is different from the normal condition, according to the in vivo kinematics observation [7]. Limited ROM and the re-distributed cervical spine rhythm after spinal fusion were both considered in the fusion model. In this way, spinal loading and internal motion after cervical spinal fusion were predicted. In summary, the spinal loading pattern after fusion presents a distinctly level-dependent feature, similar to the NS. However, there are still some differences in spinal loading patterns between the FS and the NS. Firstly, regarding the maximum CF, the FS was about 25% greater than the NS in the head extension phase (Figure 3). A significant increase in adjacent IDPs after spinal fusion was also found in the in vitro investigations [12; 13]. [PERSON] et al. [13] reported that IDPs at adjacent segments after C5-C6 fusion increased by 20.7% in the extension phase \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Disc Level**} & \multirow{2}{*}{**Force (N)**} & \multicolumn{2}{c}{\(\mathbf{0^{\circ}\) **to \(\mathbf{1^{\circ}\) a**}}} & \multicolumn{2}{c}{\(\mathbf{1^{\circ}\) **to \(\mathbf{2^{\circ}}\)**}} & \multicolumn{2}{c}{\(\mathbf{2^{\circ}\) **to \(\mathbf{3^{\circ}}\)**}} & \multicolumn{2}{c}{\(\mathbf{3^{\circ}\) **to \(\mathbf{4^{\circ}}\)**}} & \multicolumn{2}{c}{\(\mathbf{4^{\circ}\) **to \(\mathbf{5^{\circ}}\)**}} & \multicolumn{2}{c}{\(\mathbf{5^{\circ}\) **to \(\mathbf{6^{\circ}}\)**}} \\ \cline{3-14}  & & **NS** & **FS** & **NS** & **FS** & **NS** & **FS** & **NS** & **FS** & **NS** & **FS** & **NS** & **FS** \\ \hline \multirow{3}{*}{C3–C4} & CF & 134.3 & 151.0 & 107.6 & 117.3 & 83.9 & 110.1 & 79.2 & 90.6 & 76.0 & 82.7 & 68.7 & 70.7 \\  & SF & 23.8 & 31.8 & 16.6 & 19.8 & 8.2 & 16.7 & 3.5 & 6.9 & 1.2 & 3.3 & \(-\)0.1 & 0.5 \\  & FJF & 30.0 & 33.4 & 25.3 & 24.4 & 16.6 & 21.0 & 8.1 & 8.4 & 2.3 & 2.6 & 0.5 & 0.5 \\ \hline \multirow{3}{*}{C4–C5} & CF & 124.4 & 82.5 & 98.8 & 77.6 & 89.7 & 78.5 & 87.4 & 81.9 & 80.8 & 79.2 & 71.2 & 71.3 \\  & SF & 18.1 & 20.5 & 11.2 & 12.7 & 5.4 & 5.8 & 0.8 & 1.4 & \(-\)0.6 & 0.1 & \(-\)0.6 & \(-\)0.4 \\  & FJF & 29.4 & 29.1 & 21.8 & 21.3 & 13.8 & 12.0 & 5.0 & 4.0 & 0.9 & 1.1 & 0.3 & 0.4 \\ \hline \multirow{3}{*}{C6–C7} & CF & — & \(161.0\) & 133.5 & 141.4 & 133.9 & 111.1 & 127.0 & 99.3 & 114.7 & 90.9 & 86.1 & 76.4 \\  & SF & — & \(-26.1\) & \(-10.4\) & \(-20.9\) & \(-11.9\) & \(-14.2\) & \(-11.2\) & \(-13.4\) & \(-8.2\) & \(-12.8\) & \(-2.6\) & \(-\)10.0 \\ \cline{1-1}  & FJF & — & 35.3 & 28.2 & 29.5 & 19.6 & 19.5 & 9.4 & 9.6 & 2.1 & 2.4 & 0.6 & 0.6 \\ \hline \hline \end{tabular}  
* NS: normal subject; FS: fusion subject; CF: average compressive force; SF: average anterior–posterior shear force, where a positive force means an anterior direction; FJF: average facet joint force,a the interval of the disc motion, where a positive angle means flexion. \(\mathbf{\cdot^{-}}\)– means no motion in this interval for the disc.  
\end{table} 
Table 1: Comparison of the average compressive force, shear forces, and facet joint forces against intervertebral angle between the fusion subject and the normal subject, during the head flexion movement.