Previous imaging assessment of degenerative lower cervical instability has mainly relied on intervertebral translation and intervertebral angulation [7]. In recent years, increasing evidence has shown that judging segmental instability by absolute translation or absolute angulation alone may be inaccurate. Jiang et al. [13] pointed out that degenerative cervical spondylolisthesis has long been underrecognized in clinical practice and that nearly half of cases show segmental instability on flexion-extension radiographs. Alizada et al. [6] further showed that reliance on neutral radiographs alone may miss some degenerative slipping segments and that MRI also has limited ability to identify dynamic slippage. Some recent studies have therefore focused on pedicle-facet angle, facet fluid signs, and T1 slope-cervical lordosis mismatch, all of which have been reported to be associated with degenerative cervical spondylolisthesis or instability risk [5, 14,15,16]. Multiple dynamic MRI studies have likewise suggested that additional compressed segments and related dynamic changes not visible on static MRI may be detected during flexion and extension [17, 18]. Goldschlager et al. [19] reported that dynamic four-dimensional CT can detect and confirm subtle cervical instability across various cervical conditions. In addition to conventional radiography, CT, and MRI, ultrasonography has also been applied to the assessment of cervical instability. Chen et al. [20] found that imbalance of the superficial posterior cervical muscles was closely related to cervical instability and suggested that ultrasonography of these muscles may serve as a novel auxiliary diagnostic method.

The present study focused on radiograph-based TCDR as a normalized quantitative marker for the clinically prespecified responsible segment. Traditional absolute horizontal translation and intervertebral angulation remain important in evaluating cervical instability [7], but in clinical practice they are influenced by individual anatomic differences and intersegmental variation [6, 21]. Standardization against radiographic sagittal canal diameter incorporates an individual anatomic background into the assessment. In the present study, TCDR demonstrated good segment-level discriminative ability and outperformed absolute intervertebral sagittal translation and isolated angulation in cluster-bootstrap AUC comparisons. However, TCDR should be interpreted as an adjunctive quantitative radiographic marker rather than as a definitive diagnostic standard.

The responsible-segment agreement analysis further supports the reliability of the prespecified clinical-radiological selection process. Among 197 initially screened clinically suspected cases, the two initial assessors showed excellent agreement (kappa = 0.824), and 17 cases were excluded because no clinically dominant responsible segment could be identified. This design helps reduce the possibility that investigators selectively chose the most radiographically abnormal level after viewing the quantitative parameters.

The interobserver reliability analysis demonstrated excellent ICC values for all key radiographic measurements, and Bland-Altman analysis showed minimal mean bias between observers. These findings reduce the likelihood that the observed group differences were primarily caused by systematic interobserver measurement bias. However, the absolute limits of agreement were not negligible, particularly for intervertebral angulation and TCDR. For TCDR, the MDC95 was 0.048, which was smaller than the observed between-group difference but still not negligible relative to the proposed cutoff value of 0.087. Therefore, TCDR values close to the cutoff should be interpreted cautiously and in combination with clinical symptoms, neurological findings, and cross-sectional imaging rather than as a stand-alone diagnostic criterion.

This study also found moderate positive correlations of TCDR with Pfirrmann grade and Weishaupt grade. Lower cervical instability often coexists with disc degeneration, facet degeneration, reduced disc-space height, and local osseous structural change [5, 22]. As anterior column support weakens and posterior stabilizing function declines, abnormal segmental shear and translation become more likely [23], so a moderate correlation between TCDR and degenerative burden is not unexpected. Segment-stratified analysis showed that the AUC of TCDR varied across segments, with relatively higher values at C4/5 and C6/7 but a lower value at C5/6. Given intersegmental differences in mobility, facet morphology, and the local mechanical environment [24], this result more likely reflects segmental heterogeneity than insufficient diagnostic ability of TCDR itself.

The patient-level sensitivity analysis provides an important caution for clinical application. When the maximum TCDR among four control segments was used, the AUC decreased to 0.713. This indicates that TCDR should not be used as an isolated patient-level screening test. Its main value lies in quantitative assessment of a clinically prespecified responsible segment rather than indiscriminate classification of patients based on the maximum value across multiple levels. TCDR should therefore be interpreted within the clinical context and alongside neurological examination and CT/MRI findings.

This study has several limitations. First, the study and control groups were asymmetric in segment composition and sampling structure, and the control group consisted of healthy examinees rather than symptomatic patients without instability, creating a risk of spectrum bias. Therefore, the findings reflect discrimination between clinically suspected instability segments and asymptomatic segments rather than definitive differentiation among all symptomatic cervical disorders. Second, although the prespecified 3 + 1 composite reference standard explicitly excluded translation, angulation, TCDR, and their directly derived indices, it still included degenerative imaging evidence. Accordingly, mechanistic interpretation of Pfirrmann grade, Weishaupt grade, and some disc-space-related indicators may be influenced by residual definition coupling. Third, radiographic sagittal canal diameter represents a two-dimensional lateral radiographic measurement rather than a direct cross-sectional measurement of the true osseous canal diameter on CT or MRI. Projection geometry, patient positioning, image quality, and radiographic magnification may influence this measurement. Although standardized imaging, calibrated PACS measurement, and interobserver reliability analysis supported its reproducibility, future studies should compare radiographic TCDR with CT- or MRI-derived canal measurements. Fourth, intraobserver reliability was not assessed because repeated measurements by the same observer were not included in the original retrospective measurement protocol. Fifth, this study lacks prospective external validation and did not track clinical outcomes. The biomechanical rationale of TCDR remains theoretical and was not validated by finite-element modeling, cadaveric testing, dynamic MRI, or four-dimensional CT. More rigorous prospective studies and external validation are still needed.