Objective To explore the clinical efficacy of secondary posterior internal fixation after transoral anterior atlantoaxial release under 3D operative microscopy for treatment of irreducible atlantoaxial dislocation.Methods From January 2014 to May 2016,12 patients with irreducible atlantoaxial dislocation were treated with secondary posterior internal fixation after transoral anterior atlantoaxial release under 3D operative microscopy in our hospital.They were 7 males and 5 females,with an average age of 37.1 years (from 25 to 54 years).The efficacy was analyzed in terms of their visual analogue scale (VAS),Japanese Orthopaedic Association (JOA) scoring,improvement rate of neurological function,American Spinal Injury Association (ASIA) grading,atlas-dens interval (ADI),space available for the cord (SAC) and cervicomedullary angle (CMA) before and one year after operation.Results The patients were followed up for more than one year.All the atlantoaxial joints obtained anatomic reduction.Their preoperative values of VAS (5.73 ± 1.36 points),JOA score (9.03 ± 2.12 points),ADI (8.34 ± 1.12 mm),SAC (9.53 ± 0.69 mm) and CMA (121.23°±4.32°) were significantly improved one year after operation (1.21 ±0.63 points,14.32±2.51 points,2.83 ± 0.36 mm,14.23 ± 1.22 mm and 153.53° ± 9.25°, respectively) (P ＜0.05).The improvement rate of neurological function increased gradually with the postoperative time,reaching 94.14％ ±5.11％ one year after operation.The postoperative ASIA grading was significantly improved too one year after operation (P ＜ 0.05).Conclusion Secondary posterior internal fixation after transoral anterior atlantoaxial release under 3D operative microscopy may lead to fine clinical efficacy one year after operation for patients with irreducible atlantoaxial dislocation.