Background: K-line is widely recognized as a useful index to evaluate alignment and size of the cervical ossification of the posterior longitudinal ligament (OPLL) in one parameter. The purpose of this study was to investigate that K-line could be a tool to predict the prognosis of LP for cervical spondylotic myelopathy (CSM) as well. Methods: Sixty-eight patients who underwent LP were enrolled. C2-7 angle, local kyphosis angle, and K-line which is the straight line connecting the midpoints of

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... g the midpoints of the spinal canal at C2 and C7 was evaluated on T2- weighted sagittal magnetic resonance imaging (MRI). The JOA score and the recovery rate of the JOA score were evaluated at pre-operation and at follow-up. C2/C7 angle, local kyphosis angle, the JOA score, and the recovery rate were compared between K-line (-) and K-line (+) groups. Results: The recovery rate of K-line (+) group (50.6%) was significantly better than that of K-line (-) (19.4%). In K-line (-), the disc type in which the protruded disc was absorbed during the follow-up showed statistically better recovery rate (27.6%) at follow-up compared to other K-line (-) in which anterior cord compression due to the osteophyte or the kyphotic beak was not absorbed (osseous type, 5.0%).Conclusion: The present study has indicated that K-line can be a factor to predict the clinical outcome of LP for CSM. In K-line (-), the disc type showed somewhat better outcomes compared to the osseous type. However, the results were not sufficient.