Background: Posterior cervical decompression results in favorable outcomes for K-line (+) ossification of the posterior longitudinal ligament (OPLL) patients. However, even for patients with K-line (+) in the neck neutral position, K-line (−) in the neck-flexed position (flexion K-line (−)) may affect surgical outcomes. We investigated the influence of flexion K-line (−) on surgical outcomes after muscle-preserving selective laminectomy using multivariate analysis. Methods: This study involved 113 OPLL patients with K-line (+) in the neck neutral position who underwent muscle-preserving selective laminectomy. Patients were divided into flexion K-line (+) (n = 90) and flexion K-line (−) (n = 23) groups. We analyzed the influence of a flexion K-line (−) on radiological and surgical outcomes. We conducted a multivariate analysis to analyze the factors affecting surgical outcomes. Results: The patients with a flexion K-line (−) had a larger C2–C7 sagittal vertical axis (preoperatively, P = 0.042; postoperatively, P = 0.021), narrower postoperative clearance of the spinal cord (P = 0.003), a smaller proportion of segmental-type OPLL (P < 0.001), and a greater OPLL occupancy ratio (P < 0.001). The recovery rate measured by the Japanese Orthopedic Association (JOA) score was poorer in patients with a flexion K-line (−) (17.6 ± 32.2%) than in those with a flexion K-line (+) (35.3 ± 29.5%) (P = 0.013). Multiple linear regression analysis revealed that the flexion K-line (−) affected the recovery rate of the JOA score (β = −0.233, P = 0.013). Conclusions: Even for patients with K-line (+) OPLL, the flexion K-line (−) affects surgical outcomes. The flexion K-line (−) is a useful predictor of poor surgical outcomes after posterior decompression surgery.
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