## 4 Discussion
However, the fixation effects of the NIPD and the DIAM in torsion are less suitable than those in extension because when the NIPD and the DIAM are placed at the midline of the lumbar spine, the fixation effect is limited and an asymmetrical load is generated. If the lumbar spine exhibits spinal cord compression and spinal instability, rigid spinal instruments such as rods and screws, rather than the semirigid interspinous process devices discussed in this paper, are required to stabilize the lumbar spine. Studying the variations in the disc stress distribution in the in vitro test was difficult. FE simulation helps in observing the different stresses for different lumbar spine implants. In the validation of disc stress, the disc compressive stress pattern in FE simulation was consistent with the intradiscal pressure pattern in the in vitro test in our previous study [22]. Therefore, we analyzed the disc stress in this study because disc stress is associated with compression of the spinal cord. We found that the disc stress increased in the DEF model at the surgery level in flexion and extension. Laminotomy not only decreased the spinal stability but also increased the disc stress after spinal decompressive surgery. Therefore, laminotomy with the NIPD can help mitigate the increase in disc stress at the bridged level. [PERSON] et al. [23] found that the implantation of an interspinous process device has limited influence on the adjacent disc; our results also exhibited a similar trend. The NIPD increased the stress at the cephalic adjacent level by less than 5% compared with \begin{table}
\begin{tabular}{c c c c c} \hline  & **Flexion** & **Extension** & **Rotation** & **Lateral Bending** \\ \hline DIAM [11] & 110\% & 117\% & 109\% & 116\% \\ \hline NIPD & 105\% & 77\% & 116\% & 103\% \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison between the NIPD and the current device DIAM in the normalized data in the INT model ([DIAM or NIPD]/INT] \(\times\) 100%). the INT model. Thus, the NIPD can help avoid stress-related discogenic back pain at the adjacent level. In the NIPD model, the facet joint contact force in extension was low and changed negligibly in torsion and lateral bending. The aforementioned results are consistent with those presented by [PERSON] and [PERSON], who reported that implantation of an interspinous process device decreases the facet joint stress at the implanted level [24]. Since the NIPD is placed at the central position of the lumbar spine, it successfully resists the extension moment. The placement of the NIPD was similar to that of the DIAM. The decrease in the facet joint contact force achieved using the NIPD (58%) was higher than that achieved using the DIAM (29%) because the U-shaped NIPD provides higher stability than does the V-shaped DIAM in extension [11]. However, the change in torsion and lateral bending was negligible because the NIPD or the DIAM was placed at the central position and the resistance of the asymmetric twisting moment was limited. This study has some limitations. The FE analysis was similar to the traditional in vitro test in which an experimental load acts on the lumbar spine without muscle. However, in daily activities, the lumbar spine may bear greater physiological loads than the loading conditions tested in the FE model. Especially in the DEF model, activation of the erect spinae muscle would raise extension moment and increase spinal instability. The NIPD may offer more spinal stability because it can effectively resist extension moment. Minimally invasive decompression surgery is typically performed on the elderly population; however, degenerations such as osteophytes, osteoporosis, narrowing foramen, and decreasing disc height were not considered in the developed FE model. In practice, DIAMs are tied to the lumbar spine with cables. Accordingly, contact elements were used in this study to bond the lumbar spine and the NIPD.