STRUCTURED ABSTRACT: Study Design. A retrospective single-center study.Objective. To clarify preoperative factors predicting unsatisfactory indirect decompression after laminoplasty in patients with cervical spondylotic myelopathy (CSM).Summary of Background Data. Many authors have shown that inadequate indirect decompression following laminoplasty can inhibit neural recovery and should be considered a complication. We previously demonstrated that residual anterior compression of the spinal cord (ACS) impaired recovery of upper extremity motor function. Although the K-line has been established as a predictive index indicating laminoplasty is required in patients with ossification of the posterior longitudinal ligament, it remains unclear what preoperative factors can predict insufficient posterior cord decompression in patients with CSM.Methods. Forty-six consecutive patients who underwent laminoplasty for the treatment of CSM at our hospital were reviewed. A modified K-line was defined as the line connecting the midpoints of the spinal cord at C2 and C7 on T1-weighted sagittal magnetic resonance imaging. We also determined the minimum interval between the preoperative modified K-line and the anterior impingement (INTmin) on the mid-sagittal image. Data analysis involved logistic regression and receiver operating characteristics curve (ROC) analysis to select the most valuable index for predicting postoperative ACS.Results. Ten patients had ACS immediately after laminoplasty. Logistic regression analysis showed that INTmin was a significant predictive factor for the occurrence of postoperative ACS (odds ratio = 0.485; 95% confidence interval = 0.29-0.81; p = 0.02). ROC analysis showed an area under the curve of 0.871. A cut-off of 4.0 mm had a sensitivity of 80% and a specificity of 80.6% for prediction of postoperative ACS.Conclusion. The parameter INTmin correlated with the occurrence of postoperative ACS. A cut-off point of 4.0 mm is most appropriate for alerting spine surgeons to a high likelihood of postoperative ACS.