To the editor,
We sincerely appreciate the correspondent’s thoughtful and well-referenced commentary on our recent article [
1]. The additional points about individual segment analysis, the impact of sagittal alignment, and the need for longitudinal data are not only valid but also demonstrate the evolving understanding of mechanobiological bone remodeling in spinal surgery [
2].
As the letter correctly pointed out, our study employed Hounsfield units (HU) to tackle the important issue of level-specific bone remodeling in multilevel lumbar fusion. We appreciate that the correspondent emphasized the strengths of our research, especially the validation of HU stability between preoperative and immediate postoperative scans. This finding underscores the reliability of HU-based assessments, even after instrumentation.
We acknowledge that a longitudinal study following the same individuals over multiple time points would yield the most definitive evidence of bone remodeling trajectories. However, in our clinical cohort, the number of patients who received regular and frequent postoperative computed tomography (CT) scans over an extended period was insufficient for a robust longitudinal analysis, which aligns with findings from other studies [
3]. Consequently, we adopted a subgroup analysis by follow-up period as a pragmatic alternative. This approach prioritized minimizing inter-group variability to ensure data integrity. We believe this provides a foundational framework for future longitudinal studies to build upon.
Regarding cohort homogeneity, we intentionally excluded patients with osteoporosis or those on bone-active medications to focus our analysis on disuse-driven bone loss. While this method was sound, we acknowledge that “real-world” populations often include such patients. Comparing our cohort with patients who have compromised bone quality will be essential for drawing more robust clinical conclusions for high-risk surgical planning.
We also addressed potential selection bias related to the reasons for follow-up CT scans. As shown in Table 3, patients with longer follow-up durations were more likely to be evaluated for adjacent segment disease. However, our multivariable linear regression analysis indicated that the reason for obtaining a CT scan did not significantly correlate with changes in HU. Future studies with larger cohorts and propensity score matching may help further reduce these biases.
We appreciate the suggestion to incorporate segment-by-segment analysis and sagittal alignment data. We concur that load distribution is greatly affected by these biomechanical factors. Additionally, biomechanical studies have examined longer fusion constructs, highlighting that both the properties of the rods and sagittal alignment significantly impact biomechanical stress [
4-
6]. We are currently planning further research to analyze longer constructs in deformity correction surgery, focusing on how global alignment correlates with localized bone quality.
Finally, we would like to highlight the primary clinical implication of our study, as illustrated in Fig. 4. Our data indicate that the HU of the fusion construct begin to show a negative change compared to preoperative values starting 4 years after surgery. Notably, in patients with follow-up periods of 6 years or longer, we observed a significant decline in bone quality exceeding 18%. This finding serves as a crucial warning for clinicians to consider the potential weakening of bone quality within the fusion construct when contemplating implant removal or planning revision surgery in long-standing cases [
7].
We once again thank the correspondent for their contribution, which significantly enhances our work and advances the scholarly discussion on optimizing long-term care for patients undergoing lumbar fusion.
NOTES
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Conflict of Interest
The authors have nothing to disclose.
REFERENCES
- 1. Jang HJ, Kim D, Moon BJ, et al. Disuse bone loss in fusion constructs after multilevel lumbar fusion: a computed tomography Hounsfield unit analysis. Neurospine 2026;23:176-86.
- 2. Cai W, Huo Y, Liu Y, et al. Biomechanics in bone regeneration and mechanobiology in osteoblasts: Fundamental concepts and recent progress. EngMedicine 2025;2:100057.
- 3. Patel VV, Andersson GB, Garfin SR, et al. Utilization of CT scanning associated with complex spine surgery. BMC Musculoskelet Disord 2017;18:52.
- 4. Son DM, Lee SB, Lee SJ, et al. Biomechanical comparison of multilevel lumbar instrumented fusions in adult spinal deformity according to the upper and lower fusion levels: a finite element analysis. Biomed Res Int 2022;2022:2534350.
- 5. Vieweg U, Keck J, Krüger S, et al. Biomechanical comparison of different rod-to-rod connectors to a conventional titanium-and cobalt chromium posterior spinal fixation system. Brain Spine 2023;3:101708.
- 6. Galbusera F, Wilke HJ, Brayda-Bruno M, et al. Influence of sagittal balance on spinal lumbar loads: a numerical approach. Clin Biomech (Bristol) 2013;28:370-7.
- 7. Teles AR, Yavin D, Zafeiris CP, et al. Fractures after removal of spinal instrumentation: revisiting the stress-shielding effect of instrumentation in spine fusion. World Neurosurg 2018;116:e1137-43.
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