Objective: To evaluate the clinical significance of a negative K-line in the neck flexion position (FK-line [-]), which indicates that cervical ossification of the posterior longitudinal ligament (OPLL) crosses the K-line during flexion, and to compare surgical outcomes between laminoplasty (LP) and laminectomy with fusion (LF) for multilevel FK-line (-) cervical OPLL.Methods: A total of 349 patients with multiple cervical OPLL who underwent posterior decompression surgery (LP or LF) with a minimum of 2 years of follow-up were stratified by FK-line status. Clinical and radiological parameters were compared between the FK-line (+) and FK-line (-) groups. Subgroup analysis of FK-line (-) patients evaluated the efficacy of LP versus LF. Multivariate regression identified predictors of neurological recovery.Results: Patients with FK-line (-) OPLL exhibited a smaller FK-line distance, more kyphotic alignment, greater cervical flexion, and lower recovery ratios compared to those with FK-line (+). In the FK-line (-) subgroup, LF achieved a significantly greater increase in FK-line distance, better correction of the flexion angle, and more neurological recovery than LP. Multivariate analyses identified postoperative FK-line distance, C2–7 flexion angle, and preoperative dynamic extension reserve as independent predictors of neurological outcomes.Conclusion: FK-line status reflects the sagittal cord position and predicts surgical outcomes in cervical OPLL. In FK-line (-) patients, LF provides better neurological recovery and more effective posterior cord shift and kyphotic alignment correction than LP. Incorporating FK-line assessment to guide surgical planning could improve individualized treatment outcomes for multilevel OPLL.

Shin JJ, et al. Yonsei Med J. 2026 Jan;67:e2. https://doi.org/10.3349/ymj.2024.0094

Background/Objectives: Optimal trajectories for S2-alar-iliac (S2AI) screw placement have been widely studied; however, in fluoroscopy-assisted free-hand techniques, exact reproduction is rarely achievable. This study aimed to quantify direction-specific safety margins around patient-specific optimal trajectories and to determine their relationship with pelvic parameters. Methods: We retrospectively analyzed patients who underwent S2AI screw fixation with available preoperative and postoperative CT imaging. Pelvic parameters, including pelvic tilt (PT), sacral slope (SS), and pelvic incidence (PI), were measured. Optimal transverse and sagittal screw angles were determined using CT-based planning. Postoperative CT was used to assess actual screw trajectories and cortical violations. Direction-specific generalized estimating equation models were used to evaluate associations between trajectory deviation and screw malposition. Receiver operating characteristic (ROC) analysis was performed to determine cutoff values for safe deviation. Results: A total of 62 patients (105 screws) were included in axial analysis and 41 patients (76 screws) in sagittal analysis. PT and PI showed significant inverse correlations with both optimal transverse and sagittal angles (all p < 0.001). Greater lateral and medial deviations were significantly associated with corresponding cortical violations (OR 2.33, 95% CI 1.51–3.59; and OR 2.10, 95% CI 1.40–3.15 per degree, respectively; both p < 0.001). Inferior deviation was significantly associated with violation in the sagittal plane (OR 1.39, 95% CI 1.18–1.65 per degree; p < 0.001), whereas superior deviation was not significant. ROC analysis demonstrated asymmetric safety margins: 1.5° lateral (AUC = 0.972), 8.1° medial (AUC = 0.965), and 18.5° inferior (AUC = 0.897). Conclusions: S2AI screw placement may be conceptualized as a tolerance-based process centered on a patient-specific optimal trajectory. Safety margins are direction-dependent and asymmetric, with a narrow tolerance for lateral deviation. These findings provide practical guidance for intraoperative trajectory adjustment in free-hand techniques.

Background/Objectives: To evaluate the clinical and radiological outcomes of surgical interventions stratified by dynamic K-line status and to identify predictors of neurological recovery in multilevel cervical ossification of the posterior longitudinal ligament (OPLL). Methods: This study analyzed 535 patients with multilevel cervical OPLL who underwent anterior cervical discectomy and fusion (ACDF), laminoplasty (LP), or laminectomy with fusion (LF), with a minimum 24 months of follow-up. Patients were classified based on dynamic K-line status—neutral (NK-line) and flexion (FK-line)—into three groups: Group 1 (NK-line [+]/FK-line [+]), Group 2 (NK-line [+]/FK-line [−]), and Group 3 (NK-line [−]/FK-line [−]). Radiographic parameters, JOA scores, and VAS were compared, and multivariate regression identified predictors of recovery. A multinomial inverse probability of treatment weighting (IPTW) analysis was conducted to reduce treatment selection bias. Results: Progressive dynamic K-line negativity was associated with greater cervical kyphosis, a higher canal-occupying ratio, reduced FK-line distance, and poorer neurological recovery. After IPTW analysis, ACDF showed higher adjusted recovery across subgroups. In Group 1, younger age and fewer operative levels predicted better recovery. In Groups 2 and 3, LF demonstrated significantly greater neurological recovery than LP. A larger preoperative FK-line distance and a greater postoperative FK-line distance increase were independent predictors of favorable outcomes. Conclusions: Dynamic K-line stratification has good prognostic value in multilevel cervical OPLL. ACDF remains the most effective procedure across dynamic K-line status groups, and LF is preferred over LP in patients with latent or fixed FK-line (−). Incorporating dynamic K-line metrics into surgical planning could improve procedure selection and enhance neurological recovery.

Background/Objectives: The external auditory meatus (EAM) is widely used as a surrogate cranial reference point, based on its proximity to the midpoint of the nasion–inion line (MNI). However, its radiographic clarity has not been thoroughly validated. This study aimed to evaluate the radiographic clarity and interobserver reproducibility of the EAM compared with the sella turcica as a control landmark. Methods: A retrospective review was performed on patients who underwent surgical correction for sagittal spinal deformity between 2021 and 2024. Preoperative standing whole-spine radiographs were analyzed. Horizontal and vertical distances from the EAM and the posterior border of the sella turcica to the MNI were measured. Radiographic clarity was categorized into three groups. Five independent neurosurgeons conducted all measurements, and interobserver reliability was assessed using the intraclass correlation coefficient with a two-way random-effects model [ICC(2,1)]. Results: The EAM was horizontally closer to the MNI (1.1 mm vs. 13.8 mm) but exhibited poorer radiographic clarity, with only 14.1% classified as single point and clear compared with 84.5% for the sella turcica. Interobserver reproducibility was lower for the EAM (ICC: 0.84 horizontal, 0.89 vertical) than for the sella turcica (0.97, 0.95). Horizontal deviation among observers was significantly greater for the EAM (major deviation 6.3 mm vs. 2.2 mm, p < 0.001), whereas vertical deviation did not differ significantly. Conclusions: Although anatomically close to the MNI, the EAM demonstrated inferior radiographic clarity and reproducibility. These findings suggest that the EAM may have limitations as a cranial reference landmark.