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STUDY DESIGN A prospective multicenter study. OBJECTIVE To compare the surgical outcomes of anterior and posterior fusion surgeries in patients with K-line (-) cervical ossification of the posterior longitudinal ligament (OPLL). SUMMARY OF BACKGROUND DATA Although laminoplasty is effective for patients with K-line (+) OPLL, fusion surgery is recommended for those with K-line (-) OPLL. However, whether the anterior or posterior approach is preferable for this pathology has not been effectively determined. METHODS A total of 478 patients with myelopathy due to cervical OPLL from 28 institutions were prospectively registered from 2014 to 2017 and followed up for 2 years. Of the 478 patients, 45 and 46 with K-line (-) underwent anterior and posterior fusion surgeries, respectively. After adjusting for confounders in baseline characteristics using a propensity score-matched analysis, 54 patients in both the anterior and posterior groups (27 patients each) were evaluated. Clinical outcomes were assessed using the cervical Japanese Orthopaedic Association and the Japanese Orthopaedic Association Cervical Myelopathy Evaluation Questionnaire. RESULTS Both approaches showed comparable neurological and functional recovery. The cervical range of motion was significantly restricted in the posterior group because of the large number of fused vertebrae compared with the anterior group. The incidence of surgical complications was comparable between the cohorts, but the posterior group demonstrated a higher frequency of segmental motor paralysis, whereas the anterior group more frequently reported postoperative dysphagia. CONCLUSION Clinical improvement was comparable between anterior and posterior fusion surgeries for patients with K-line (-) OPLL. The ideal surgical approach should be informed based on the balance between the surgeon's technical preference and the risk of complications.