| BackgroundThe dislocation of atlantoaxial joint is major injury in upper cervical vertebra, arising from craniocerebral trauma, basilar impression, Arnold-chiari deformity, rheumatoid arthritis, etal. It can be divided into reducible, hardly reducible, irreducible style in clinic. The methods of treatment depend on extent of injury of atlas transverse ligament. There are a large amount of methods including Gallie's, Brook's, Margerl's, Cervifix's, Harm's plate and Transoroparyngeal Atlantoaxial Reduction Plate system(TARP) in the field of irreducible atlantoaxial dislocation. The dislocation atlantoaxial joint could be classified into congenital, trauma, pathology style according to etiological factor. The congenital dislocation atlantoaxial joint include deformity of odontoid bone, basilar impression,, et, al. Among them, deformity of odontoid bone is the most common, for example, liberation of odontoid bone, agenesis of odontoid bone. The factors of trauma dislocation atlantoaxial joint contains fracture of odontoid bone and injury of atlas transverse ligament. In addition, alar ligament brings to rotation of atlantoaxial joint. Rheumatoid arthritis, infective inflammation, tumor, tubercle contribute to pathology dislocation of atlantoaxial joint.ObjectiveTo assess the feasibility of pedicle screw fixation transoral approach to axis, based on the anatomical observation and measurement of axis. To evaluate the clinical value of pedicle screw fixation transoral approach to axis according to CT imagings. To evaluate the clinical effect of transoral atlantoaxial pedicle screw technique in the treatment of irreducible atlantoaxial dislocationMethodSixty dry axis specimens were underwent to measure anatomic data, including distance from screw entrance point to superior articular process, to the sagittal midline of spine, to medial edge of transverse foramen, and the screw length in axial pedicle, screw insertion outside angle(α), screw angle to tail(β). Pedicle screws insert twenty dry axis specimen, measuring screw angles, the length in axial pedicle, apply to six patients with irreducible atlantoaxial dislocation. Six patients were treated with atlantoaxial pedicle screw technique to observe the clinic outcome according to follow-up.ResultThe mean distance from screw entrance localization to superior articular process was (5.0±1.0) mm, to the sagittal midline was (7.8±0.7) mm, the distance to medial wall of transverse foramen was averagely about (6.1±1.7) mm, the averaged length of pedicle was (26.4±1.5)mm, screw insertion outside angle (α) averaged 18°±4°, screw angle to side to tail (β) by pedicle of axis averaged 14°±4°. According to CT measurement, the average length of axial pedicle was 25 mm, distance from screw point to sagittal midline of spine was 6.6 mm, screw insertion outside angle was 21 degree, and screw angle to side to tail was 10 degree. The dislocation in 5 cases was reduced completely and that in one case reduced up largely. There was no infection of incision cerebral spinal fluid leakage or deterioration of spinal cord function. All cases were followed up for 6 months to 18 months. Radiographs shows successful fusion in the surface of atlantoaxial joint.Conclusion(1)The best screw entrance in the transoral approach was 0(distance from midline of axis was 7mm, inferior edge of superior articular process was 5.0mm). (2) The safety direction of axial pedicle screw was that screw insertion outside angle was 20°and screw angle to side to tail averaged 12°。It can be concluded that it is safe for C2 pedicle screw。(3)CT scan is necessary for irreducible atlantoaxial dislocation to guide effectively the screw internal fixation surgery. Transoral atlantoaxial pedicle screw technique with solid fixtion simple which is suitable for irreducible atlantoaxial dislocation.
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