K-line in patients with cervical ossification of the posterior longitudinal ligament: relationship with change in sagittal cervical curvature and laminoplasty outcomes

Introduction To analyze how K-line is related to change in sagittal cervical curvature and laminoplasty outcomes in patients with cervical ossification of the posterior longitudinal ligament (OPLL). Materials and methods The study retrospectively analyzed 81 patients with OPLL who had undergone posterior cervical single-door laminoplasty and arch plate fixation between June 2011 and June 2017. Fifty-five were K-line positive (K[+]) and 26 were K-line negative (K[−]). Clinical and radiological results were compared between the groups. Patients were followed up for at least 2 years. Results Before the operation, Japanese Orthopedic Association (JOA) score, visual analogue scale (VAS) score, neck disability index (NDI), and short-form-36 (SF-36) quality of life score did not differ significantly between the groups. Neurological function was improved in both groups after the procedure. At last follow-up, JOA score, VAS score, NDI, SF-36 score, and JOA score improvement rate differed significantly between the groups. Before the operation, at the 3-month and final follow-ups, C2–7 Cobb angle, T1 slope, and C2–7 SVA differed significantly between the groups. The changes were more marked in the K(−) group than in the K(+) group. The incidence of cervical kyphosis differed significantly between the groups (P < 0.05), as well as between patients with lordosis < 7° and those with lordosis ≥ 7°. Conclusions K-line negativity and lordosis < 7° may predict kyphosis after laminoplasty in patients with OPLL. The cervical curvature in patients with OPLL tends towards kyphosis and anteversion after laminoplasty, which contributes to the reduced clinical effect of the procedure.