Objective:To explore the clinical efficacy and imaging assessment of posterior arthrodesis with pedicle screw fixation in the treatment of atlantoaxial instability or dislocation. Methods:24 cases of atlantoaxial instability or dislocation were treated with posterior atlantoaxial pedicle screw fixation from February 2005 to June 2009

of these cases there were 7 cases of odontoid process old fracture

4 cases of congenital odontoid process freeing

9 cases of odontoid process type ⅡC fresh fracture and 4 cases of atlas transverse ligament injury. The parameters were assessed by preoperative CT and three-dimensional reconstruction such the entry point and angle of atlantoaxial pedicle screws. The entry point of C 2 pedicle screw was located in the mid-point of inferior articular process of axis

using the medial border of the C 2 lamina as a landmark

25° in a medial inclination and 25° in a cephalad direction.The entry point of C 1 was placed in longitudinal vertical outside the entry point of C 2 pedicle screw about 1mm and the point of intersection above 2 mm inferior border of posterior arch

5° in a medial inclination and 5° in a cephalad direction.The diameter of screws was 3.5/4.0 mm

and the screw length of C 1 and C 2 pedicle screws were 22～30 mm and 22～28 mm. Postoperatively

the placement of the inserted screws were evaluated by means of X-ray

CT-scan and three-dimensional reconstruction. Results:A total of 95 screws were successfully placed in atlas and axis. Laminar hook was implanted instead of one atlas pedicle screw because of implant failure intraoperative. Postoperative CT scan

one screw perforated the atlas spinal canal

one penetrated the axial transverse foramen

without any clinical response. There were no neural or vascular injury was observed. All patients were followed up from 6 to 29 months

averaged 16 months.The clinical symptom was improved in some extend. Bony fusion was observed in all 24 cases within 3 to 6 months without complications of internal fixation failure and loose. Conclusion:The transpedicular fixation is an effective method for the treatment of atlantoaxial instability or dislocation

and it is necessary to fully assess imaging characteristics of atlantoaxial pedicle and risks of atlantoaxial pedicle screw implantation before surgery.