Objective:To evaluate the long-term outcomes of posterior release, reduction, fixation, and fusion for irreducible atlantoaxial dislocation (AAD).Methods:Between January 2005 and June 2016, a total of 31 patients with irreducible AAD who had received posterior approach surgery were included. Among them, there were 13 males and 18 females, the average age was 39.1±13.5 years (range 9-72 years). The clinical data of the eligible individuals were collected and analyzed. Neck disability index (NDI) and Japanese Orthopaedic Association (JOA) scores were recorded to evaluate the recovery of neck and neurological functions. The atlantodental interval (ADI), clivus-canal angle (CCA), and cervico-medullary angle (CMA) were measured to evaluate the reduction of AAD. C 0-C 2 angle and C 2-C 7 angle were measured to evaluate the recovery of cervical alignment. For individuals with basilar invagination, the distances from the tip of odontoid process to Chamberlain line and Wackenheim line were measured to assess the reduction in the vertical direction. The duration of bony fusion and complications were also analyzed. Results:The mean follow-up period was 82.7±26.4 months (range 61-170 months). In terms of functional scores, the NDI dropped from 43.41%±11.60% before surgery to 12.19%±6.97% at the six months follow-up, and 9.45%±7.51% at the last follow-up ( F=89.56, P<0.001). The JOA increased from 9.48±2.41 points before surgery to 14.71±1.42 points at the six months follow-up, and 14.97±1.47 points at the last follow-up ( F=52.89, P<0.001). Regarding the horizontal and vertical dislocations, the ADI decreased from 9.16±2.32 mm before surgery to 1.39±1.04 mm at the six months follow-up, and 1.29±1.08 mm at the last follow-up ( F=189.61, P<0.001). The distance from the tip of odontoid process to Chamberlain line decreased from 11.15±4.35 mm before surgery to 2.03±2.83 mm at the six months follow-up, and 2.15±3.02 mm at the last follow-up ( F=37.58, P<0.001). The distance from the tip of odontoid process to Wackenheim line reduced from 6.81±2.57 mm before surgery to -2.23±1.58 mm at the six months follow-up, and -2.27±1.58 mm at the last follow-up ( F=122.16, P<0.001). For the amelioration of the compression on medulla and spinal cord, the CCA increased from 113.68°±12.67° before surgery to 143.39°±7.38° at the six months follow-up, and 142.39°±7.13° at the last follow-up ( F=67.13, P<0.001). The CMA increased from 115.71°±13.69° before operation to 145.58°±10.78° at the last follow-up ( F=41.44, P<0.001). Regarding the curvature of the cervical spine, the C 0-C 2 angle recovered from 1.94°±15.82° before surgery to 14.84°±6.45° at the last follow-up ( F=11.97, P<0.001), and the C 2-C 7 angle ameliorated from 27.26°±8.49° before operation to 19.26°±5.44° at the last follow-up ( F=11.13, P<0.001). Bony fusion was achieved in all cases, the fusion time was 9.71±2.55 months (range 5-15 months). A total of five complications occurred in the cases (two cerebrospinal fluid leakages, one deep infection, one transient neurologic deficit, and one dysphagia). They were all cured with corresponding treatments. In the last follow-up, none of the cases developed failure of internal fixation or re-dislocation. Conclusion:Posterior approach release, reduction, fixation and fusion technique is a safe and efficient surgical strategy with favorable long-term follow-up outcomes for irreducible AAD.