## 4 Discussion  
Traditional laminectomy is a common lumbar decompression surgery and involves the removal of the lamina, ligament flavour, supraspinous ligament, and interspinous ligament. However, such spinal decompression can cause iatrogenic spinal instability because the supraspinous and interspinous ligaments help in stabilizing the lumbar spine  
Figure 8: Disc stress distribution in the three FE models in lateral bending: (**A**) implanted level L3–L4 and (**B**) adjacent level L2–L3. Note: the arrow indicates the region of high stress. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **INT** & **DEF** & **NIPD** \\  & **Moment: 1.2 Nm** & **Moment: 1.2 Nm** & **Moment: 1.2 Nm** \\ \hline \multicolumn{4}{c}{**ROM (degree)**} \\ \hline L1–L2 & 0.82 & 0.82 & 0.82 \\ L2–L3 & 0.82 (100\%) & 0.82 (100\%) & 0.82 (100\%) \\ L3–L4 & 1.01 (100\%) & 1.29 (128\%) & 1.17 (116\%) \\ L4–L5 & 1.22 & 1.23 & 1.28 \\ Total & 3.87 & 4.16 & 4.09 \\ \hline \multicolumn{4}{c}{**Stiffness (Nm/degree)**} \\ \hline L3–L4 & 1.19 (100\%) & 0.93 (78\%) & 1.03 (86\%) \\ \hline \multicolumn{4}{c}{**Disc max. stress (KPa)**} \\ \hline L2–L3 & 211 (100\%) & 210 (100\%) & 210 (100\%) \\ L3–L4 & 271 (100\%) & 281 (105\%) & 278 (104\%) \\ \hline \multicolumn{4}{c}{**Facet joint force (N)**} \\ \hline \multicolumn{4}{c}{Left} & Right & Left & Right \\ L2–L3 & 0 & 15 & 0 & 15 \\ L3–L4 & 0 & 13 & 0 & 12 & 0 \\ \hline \end{tabular} Note: values in parentheses indicate the normalized results ([[DEF or NIPD]/INT] \(\times\) 100%).  
\end{table} 
Table 5: Comparison of biomechanical characteristics among the three FE models in torsion. in flexion. Moreover, extensive wound areas occur in late recovery. Therefore, minimally invasive laminotomy is an effective treatment because it entails a small wound area and is reported to yield good outcomes [19]. The proposed laminotomy procedure aims to preserve the spinous process, interspinous ligaments, and facet capsules, removing only the bilateral half of the inferior part of the upper lamina and a limited amount of the superior part of the lower lamina, including the adjacent ligamentum flavum, in the treated segment. However, a part of the posterior lamina is destroyed, which decreases the stability of the lumbar spine. The results of this study indicate that the ROM of the laminotomies in the DEF model increased by 16% in flexion, 23% in extension, 4% in lateral bending, and 28% in rotation compared with the corresponding values in the INT model. The results of this study were consistent with those of previous studies that have reported that partial disecectomy increases the ROM at the decompressed lumbar segment [20; 21]. The increased ROM can induce the impingement of the nerve root and narrow the foramen. Especially in flexion and extension, both motions frequently act on the lumbar spine in daily activities. Therefore, an interspinous process device is essential for augmenting spinal stability after spinal decompression. Consequently, we developed an NIPD. The ROM in the NIPD model was 18% and 23% lower than that in the INT model in flexion and extension, respectively; thus, the NIPD model increased the spinal stability in flexion and extension. We investigated the use of a currently used interspinous process device, namely the DIAM, by using the FE model that was employed in the previous study in which laminotomy with the DIAM was analyzed [11]. This study reported that the DIAM increased the ROM by 10% in flexion and by 17% in extension, as listed in Table 6. Thus, the NIPD outperforms the DIAM. The superior performance of the NIPD can be attributed to its shape. The DIAM is V-shaped, whereas the NIPD is U-shaped; thus, the NIPD provides a larger contact area with the spine than the DIAM and mitigates an excessive increase in ROM. Therefore, the NIPD (\(-62\%\)) decreased the facet contact force to a greater extent than the DIAM (\(-29\%\)) at the surgery level in extension.