## 4 Discussion
\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** \\ \hline \multirow{3}{*}{C3–C4} & CF & 60.2 & 58.7 & 52.9 & 52.5 & 51.9 & 57.9 & 55.2 & 64.5 & 59.3 & 71.1 & 63.6 & 78.5 \\  & SF & \(-1.3\) & \(-1.6\) & \(-2.5\) & \(-3.9\) & \(-4.1\) & \(-7.3\) & \(-5.9\) & \(-10.5\) & \(-8.3\) & \(-13.3\) & \(-10.8\) & \(-16.1\) \\ \hline \multirow{3}{*}{C4–C5} & CF & 62.5 & 58.8 & 55.2 & 56.8 & 54.8 & 64.7 & 58.8 & 73.2 & 62.4 & 82.8 & 66.1 & 91.1 \\  & SF & \(-0.8\) & \(-1.5\) & \(-1.1\) & \(-4.1\) & \(-2.0\) & \(-6.7\) & \(-3.0\) & \(-8.7\) & \(-3.8\) & \(-10.2\) & \(-4.7\) & \(-10.7\) \\ \hline \multirow{3}{*}{C6–C7} & CF & 63.2 & 63.5 & 72.8 & 58.9 & 80.3 & 64.4 & 85.3 & 70.3 & 89.5 & 76.7 & 93.2 & 84.5 \\  & SF & 0.2 & \(-8.2\) & 2.0 & \(-8.0\) & 4.2 & \(-8.8\) & 6.0 & \(-9.1\) & 7.9 & \(-8.8\) & 9.8 & \(-7.8\) \\ \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. \(\mathbf{a^{\circ}}\) the interval of the disc motion, where a positive angle means flexion.  
\end{table} 
Table 2: Comparison of the average compressive and shear forces against the intervertebral angle between the fusion subject and the normal subject, during the head flexion movement. according to the cadaveric experiment. [PERSON] et al. [12] found a significant increase (up to 46.5 \(\pm\) 18.8%) in IDP at the superior segment during the extension movement. Excessive spinal loading after fusion would be bound to affect the intervertebral disc-bearing capacity, even to damage its physiological structure [9]. By contrast, the same magnitudes and trends were predicted in the head flexion phase. Differently, the increments of the average CFs of the FS during the head flexion movement were mostly greater than those of the NS at the superior adjacent segments (Table 2). Moreover, according to the in vivo dynamic motion, the postoperative ROMs of the head flexion of fusion patients were decreased [20]. The predicted results indicated that adjacent segments after fusion would sustain more loading under the same intervertebral motion angle. These obvious postoperative changes of the spinal biomechanics might affect the structure and function of spinal soft tissues. [PERSON] et al. [8] stated that a weakened intervertebral disc structure is the main cause of the disc degeneration process. Similarly, [PERSON] et al. [23] indicated that an increased CF persistently applied to the disc would change the physiological structure over a period of time. Presumably, these postoperative variations might result in accelerated disc degeneration over the years. Hence, treated patients should be advised to avoid excessive motion in daily activities. Secondly, minor variations in the A-P SFs at the superior adjacent segments were found between the FS and the NS, except for the beginning and end stages of the head motion cycle. Particularly, an obvious increase in the head extension phase was found at C4-C5. Excessive shear stress may result in the circumferential tears of the disc, thus leading to structural failure, even disc degeneration [8; 24]. For the inferior adjacent segment, the A-P SF after spinal fusion trends towards the posterior direction in comparison to the NS. In the simulation process, almost the same ROMs of C6-C7 between the FS and the NS were adopted during the head flexion movement.