Relative Anatomical Study Of Atlanto-occipital Joint For Posterior Transarticular Screw Fixation

Objective:To study the anatomy of the atlanto-occipital joint so as to explore the entry point,the anchorage point,the optimal angle and length of the screw trajectory for posterior atlanto-occipital transarticular screw fixation.Methods:Anatomic characteristics were observed and measured of dried occipital condyle and atlas specimens,then according to the above ralative parameters,operation of posterior atlanto-occipital transarticular screw fixation was performed on fresh cadaver occipitalcervical specimens.Roentgen rays were examined postoperatively and autopsy was done to study the positions of screws.Results:The ideal angle of the atlanto-occipital transarticular screw trajectory was 20.3°±2.0°medial to the sagittal plane and 49.4°±2.4°cephalad to the transverse plane.The optimal length of the screw was 13.94±0.96mm in the occipital condyle and 12.84±0.62mm in the atlas.Postoperative examinations demonstrated that fixation were made with transarticular screws placed through the atlanto-occipital articulation and just beneath the hypoglossal canal,68.75% of the anchorage point was at a best region of approximately triangle which located in anterior and superior aspect of the occipital condyle.Conclusions:Anatomically,there is a feasibility of using posterior atlanto-occipital transarticular screw fixation.The entry point is in the middle of the junction of the C₁ posterior arch and the midpoint of the posterior inferior part of the C₁ lateral mass,the strew can be implanted safely in the condition of selecting appropriate length and standard angle of the trajectory.