K-line (−) in the neck-flexed position affects surgical outcomes in patients with ossification of the posterior longitudinal ligament after muscle-preserving selective laminectomy

Abstract Background Posterior cervical decompression results in favorable outcomes for K-line(+) ossificationof 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 laminectomyusing 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 analysisto 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 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.