## 3 Results  
This study examined the preservation of the disc height in a total of 89 patients. The mean follow-up time was 159 months. Out of 146 patients, 89, along with an additional 58 who were not included in this study, demonstrated outcomes according to the Modified Odom's Criteria: 30.8% were rated as good, 67.1% as better, and 2.1% as fair, and there were no poor outcomes [23]. The upper adjacent MDH was maintained at 84.6 \(\pm\) 15.1% across all levels of surgery, without a significant difference being observed between the number of levels operated on (\(p\) = 0.12). The lower disc height was maintained at 89.4 \(\pm\) 8.4% overall, and the level of surgery was not related to the MDH (\(p\) = 0.95). The occurrence of spondylolisthesis at the last follow-up was 5.7% (five patients) in the total population. Dividing by the number of levels operated revealed that 7% (four patients) had single-level surgeries, 3.8% (one patient) had two-level surgeries, and none had three-level surgeries, which was not statistically significant (\(p\) = 1). Fifteen (16.9%) of the total cohort developed ASD. There was a notable increase in ASD incidence with the number of levels operated: 13.8% (eight patients) had single-level surgeries, 15.4% (four patients) had two-level surgeries, and 60.0% (three patients) had three-level surgeries (\(p\) = 0.03). Five cases of rod fracture and one case of cage subsidence greater than 2 mm occurred, although there were no correlated symptoms (Table 2). One patient in her seventh year after L4-5 surgery underwent revision surgery for clinical ASD. Aside from this case, no other patients required revision surgery (illustrative case, Figure 3). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Total (N = 89)** & \begin{tabular}{c} **1-Level Surgery** \\ **(N = 58)** \\ \end{tabular} & \begin{tabular}{c} **2-Level Surgery** \\ **(N = 26)** \\ \end{tabular} & \
\begin{tabular}{c} **3-Level Surgery** \\ **(N = 5)** \\ \end{tabular} &
\begin{tabular}{c} **\(p\)-Value** \\ \end{tabular} \\ \hline Upper disc height preservation (\%) & 84.6 \(\pm\) 15.2 & 87.3 \(\pm\) 9.1 & 82.8 \(\pm\) 20.0 & 63.4 \(\pm\) 25.7 & 0.12 \\ Lower disc height preservation (\%) & 89.2 \(\pm\) 8.4 & 89.4 \(\pm\) 8.6 & 89.1 \(\pm\) 8.6 & 92.0 & 0.95 \\ Spondylolisthesis & & & & & 1 \\ No & 84 (94.4\%) & 54 (93.1\%) & 25 (96.2\%) & 5 (100\%) & \\ Yes & 5 (5.6\%) & 4 (6.9\%) & 1 (3.8\%) & 0 (0.0\%) & \\ Adjacent segment disease & & & & & 0.03 * \\ No & 74 (83.1\%) & 50 (86.2\%) & 22 (84.6\%) & 2 (40.0\%) & \\ Yes & 15 (16.9\%) & 8 (13.8\%) & 4 (15.4\%) & 3 (60.0\%) & \\ \hline \hline \end{tabular}  
* Disc height maintenance (\%) refers to the percentage of disc height preserved at the final follow-up compared to the preoperative measurement. The descriptive data are presented as the mean \(\pm\) standard deviation or number (%).
* \(p\) < 0.05 indicates a statistically significant difference.  
\end{table} 
Table 2: Surgical outcomes based on level of surgery. Figure 3: Illustrative case. (**A**) A 48-year-old male patient underwent L4-5-S1 fusion surgery using a Nitinol spring rod due to severe lower back pain and bilateral radiculopathy stemming from L4–5 stenosis and L5-S1 spondylolisthesis. (**A**) A preoperative lateral radiograph. (**B**) A radiograph taken 1 year post-operation. (**C**) A radiograph taken 16 years post-operation. No mechanical failure is observed, and solid fusion is evident. (**D**) A sagittal view of the computer tomography 16 years post-operation showing solid fusion at both operated levels. (**E**) An axial MRI image of the adjacent level (L3–4) 16 years post-operation. The preoperative LL was documented at 37.8 \(\pm\) 10.9\({}^{\circ}\). Immediately post-operatively, LL showed remarkable consistency with an immediate post-operative measurement of 37.9 \(\pm\) 9.6\({}^{\circ}\).