Letter to the Editor: Achieving the Minimum Clinically Important Difference in Japanese Orthopaedic Association Score After Surgery for Degenerative Cervical Myelopathy: Predictive Factors and Impact on Patient-Reported Outcome Measures

Dear Editor,

We commend the authors explored predictors of achieving the minimum clinically important difference (MCID) in JOA after surgery for degenerative cervical myelopathy (DCM). ¹ Several issues, however, warrant clarification to strengthen the interpretability and external validity of the findings.

First, the choice of MCID anchor and method is insufficiently detailed. MCID estimates for DCM vary widely depending on anchor-based vs distribution-based methods, patient baseline severity, and instrument responsiveness. ² Without an explicit anchor (eg, patient global impression of change) and sensitivity analyses using alternative thresholds or continuous change scores, the risk of misclassification remains. Reporting calibration and reclassification metrics when varying MCID cut-points would help demonstrate robustness.

Second, modeling choices could lead to over-optimism. The reported discrimination (AUC 0.775) is promising, yet there is no indication of internal validation (eg, bootstrap optimism correction) or assessment of model calibration (calibration slope, intercept, and calibration curves). Given potential collinearity among sagittal alignment parameters and baseline function, penalized regression with internal validation and decision-curve analysis would more reliably quantify clinical utility. ³

Third, missing data handling is unclear. In large prospective cohorts, missingness is common and rarely completely at random. Complete-case analysis can bias estimates and diminish power, especially when missingness correlates with severity or comorbidity. Multiple imputation with appropriate auxiliary variables and pooling of model estimates is preferable, accompanied by a missingness flow diagram and sensitivity analyses.

Fourth, potential treatment effect modification was not explored. Prior work suggests baseline myelopathy severity, symptom duration, and T2 cord hyperintensity may modify surgical benefit. ⁴ Testing interaction terms (eg, baseline JOA × alignment; MRI signal change × approach) could identify subgroups with differential probabilities of achieving MCID and inform personalized counseling.

Finally, the reliance on a dichotomous endpoint (MCID achieved vs not) may obscure clinically relevant gradations. Recent BMJ reports have underscored that categorizing inherently continuous measures can lead to information loss, reduced statistical power, arbitrary cut-points, and misclassification, ultimately distorting effect estimates and clinical interpretation. ⁵ Complementary analyses using continuous change in JOA and PROMs, responder curves, and cumulative distribution plots would provide a more nuanced depiction of benefit and aid shared decision-making.

In summary, while the study advances understanding of predictors of meaningful neurological improvement after DCM surgery, greater transparency in MCID derivation, rigorous validation and calibration of prediction models, and exploration of effect modification would enhance credibility and clinical applicability.