Response to the letter to the editor (Lumbar spine endplate sclerosis is a protective factor for cage subsidence in minimally invasive transforaminal lumbar interbody fusion)

This response acknowledges the readers’ insightful queries regarding our study on lumbar spine endplate Hounsfield units (HU) and cage subsidence in minimally invasive transforaminal lumbar interbody fusion (MI-TLIF), addressing optimism bias in threshold generation and patient-level clustering. For internal validation, bootstrap resampling (1,000 iterations) was performed to assess the variability of our 221 HU cutoff. The bootstrap-derived cutoff values ranged from 208 to 340 HU (95% CI) due to sample size limitations, while the area under the curve yielded a 95% CI of 0.532–0.785. Adjusted odds ratios (ORs) using the 221 HU threshold remained significantly above. To account for the clustered data structure (464 endplates nested within 169 patients), we re-analyzed the cohort using generalized estimating equations (GEE). GEE analysis confirmed that both obesity (BMI >25 kg/m ² : OR 2.522, 95% CI: 1.216–5.227, P = 0.0129) and endplate HU (OR 0.989, 95% CI: 0.983–0.995, P = 0.0005) remained robust independent predictors of subsidence. Furthermore, within the L1 HU <117 subgroup, an endplate HU <221 maintained a strong association with subsidence (OR 4.437, 95% CI: 1.683–11.718, P = 0.0026). These rigorous statistical adjustments successfully address optimism bias and clustering effects, confirming the validity of our findings. In conclusion, this work strongly reinforces our position as the very first study to demonstrate that preoperative lumbar spine endplate HU represents a key protective factor against cage subsidence, providing clinicians with a practical, cost-effective risk-stratification tool.