## 4 Discussion  
In our study, the upper disc height and lower disc height were maintained at 84.6% and 89.2%, respectively, over the follow-up period. Radiological ASD was observed in 16.9% of the patients. According to a study by [PERSON] et al., there was an 18.9-22% reduction in the adjacent disc height at an average of 114 months post-posterolateral fusion (PLF), while our study showed a 15.4% reduction at an average of 159 months [24]. [PERSON] et al. reported that the annual risk of re-operation due to reported ASD was 2-14%, and [PERSON] et al. reported a necessity for re-operation in 9% of cases at 5 years and 31% at 10 years [5; 9]. A study by [PERSON] et al. reported radiologic ASD at 37% in solid fusion and 17% in Dynesys at 3 years [15]. Our study also showed a similar rate of 16.9 for radiologic ASD. However, considering that our average follow-up was 13 years, the risk of ASD might be lower. Furthermore, only a single patient in our study required re-operation due to ASD, suggesting that ensuring the stability of the surgical segments and potentially delaying \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Non-ASD** & **ASD** & \(p\)**-Value** \\  & **(\(n\) = 74)** & **(\(n\) = 15)** & \\ \hline Age (years) & 53.5 & 51.0 & 0.99 \\ Gender & & & 0.13 \\ Female & 47 (78.3\%) & 13 (21.7\%) & \\ Male & 27 (93.1\%) & 2 (6.9\%) & \\ BMI (kg/m\({}^{2}\)) & 24.9 \(\pm\) 2.9 & 25.4 \(\pm\) 2.8 & 0.54 \\ Surgery level & & & 0.03 \({}^{*}\) \\
1 & 50 (86.2\%) & 8 (13.8\%) & \\
2 & 22 (84.6\%) & 4 (26.7\%) & \\
3 & 2 (40\%) & 3 (60\%) & \\ Follow-up periods (years) & 13.1 & 12.7 & 0.08 \\ Lumbar lordosis & & & \\ Preoperative & 37.3 \(\pm\) 11.9\({}^{\circ}\) & 39.5 \(\pm\) 5.6\({}^{\circ}\) & 0.41 \\ Post-operative & 35.2 \(\pm\) 10.8\({}^{\circ}\) & 33.3 \(\pm\) 9.2\({}^{\circ}\) & 0.33 \\ \hline \hline \end{tabular} For the parametric distributions, the values are expressed as the mean \(\pm\) standard deviation. For the non-parametric distributions, the values are presented as the median. The percentages were calculated based on the total number of patients. ASD, adjacent segment disease. BMI, body mass index. \({}^{*}p<\) 0.05, statistically significant difference.  
\end{table} 
Table 4: Comparison of demographic and clinical characteristics between patients with and without ASD at final follow-up. \begin{table}
\begin{tabular}{l c} \hline \hline
**Measurement** & **Value (Mean \(\pm\) SD)** \\ \hline Lumbar lordosis & \\ Preoperatively & 37.8 \(\pm\) 10.9\({}^{\circ}\) \\ Immediately post-operatively & 37.9 \(\pm\) 9.6\({}^{\circ}\) \\ Final follow-up & 36.5 \(\pm\) 11.4\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The lumbar lordosis measurement of the angle. adjacent segment disease could present an advantage over solid fusion. We used the Bioflex system, which has been used by [PERSON] et al. for dynamic stabilization and by [PERSON] et al. in conjunction with PLIF, showing positive short-term outcomes [18; 25]. An optimal system would include rods that are resistant to fatigue failure and that do not exhibit loosening at the device-bone interface. Because of these concerns, biomechanical tests were conducted using the BioFlex system [26]. The fatigue strength assessment, in accordance with the American Society for Testing and Materials F1717, involved a 9 mm displacement subject to 20 million cycles at 5 Hz, which did not subject the device to significant strains. The device maintained its structural integrity during the compression, torsion, and tension tests, indicating minimal strain transfer. Additionally, the biocompatibility of the Bioflex rod was examined. Over a period of 9 days, a nickel release test employing inductively coupled plasma-atomic emission spectroscopy did not show detectable nickel release, and a galvanic corrosion examination was conducted to determine the device's performance in contact with other metals, which did not show any adverse interactions.