## 4 Discussion
While dynamic stabilization can be useful in certain indications, its inability to increase the disc height limits its applicability, and some studies have reported a mechanical failure risk exceeding 20% [16; 17; 27]. Convinced that dynamic stabilization lacking solid fusion could precipitate mechanical failure, our approach incorporated the use of an interbody cage. In our study, this strategy was shown to be effective through a 100% fusion rate over an average follow-up period of 13.0 years like that of a solid fusion surgery. Biomechanical studies suggest that semi-rigid fixation, particularly in long-level fusion surgery, enhances mobility at the UIV. This facilitates a more gradual transition of motion between the instrumented and the adjacent non-instrumented segments of the spine [17]. It also decreases screw loading at the UIV, which can reduce the occurrence of proximal junctional kyphosis. In the context of long-level fusion surgery, some surgeons have employed soft-landing techniques using hooks or bandages to prevent proximal junctional failure and similar issues [17; 28; 29; 30]. We suggest that these mechanisms may have contributed to our study's observation of a 16.9% incidence rate of radiologic ASD. In our study, the role of the cage in the anterior part of the spine was to reduce the load applied to the posterior spring rod, thus preventing fracture in the spring rod and maintaining the disc space interval, stabilizing the anterior column of the spine. Normal spinal load sharing has an anterior-to-posterior ratio of approximately 80:20. In biomechanical studies by [PERSON] et al., rigid fixation altered this ratio to 59:41, whereas semi-rigid fixation showed a closer approximation to physiological load sharing at 75:25 and resulted in more uniform pressure distribution on the interbody cage compared to rigid rods [31]. Several limitations in this study should be pointed out, including its retrospective nature, the presence of selection bias due to a low follow-up rate, the small patient cohort, and the absence of a control group. Ideally, clinical outcomes should be quantitatively analyzed using health-related quality of life questionnaires. However, in this study, clinical outcomes could only be determined based on whether the patients underwent revision surgery for ASD. Additionally, there were no specific indications for whether surgeons should use rigid rods or Bioflex rods; the choices were made based on the surgeons' preferences and discussions with the patients. Moreover, there was no consistent serial follow-up during the long-term follow-up period, leading to heterogeneous follow-up durations among patients. However, an ongoing discussion about alternatives to rigid fixation is crucial, and we believe that our study on long-term outcomes holds significant importance for future research directions in this field. Through this study, we confirmed that surgery using the Bioflex rod and cage could be a good alternative to lumbar fusion surgery with rigid rods by potentially lowering the ASD rate and demonstrating long-term stability. These findings suggest that further results could be developed through larger-scale and prospective studies.