[Reliability of direct probing isthmus of axial vertebrae to defining the screw path in atlas during Magerl technique].

Objective: To evaluate the accuracy and reliability of atlantoaxial transarticular screw insertion (Magerl's technique) in atlas under lateral fluoroscopic monitoring without anteroposterior view.
Method: Seventy-five consecutive patients with atlantoaxial instability, 48 males and 27 females, aged 35.1, were treated by Magerl's technique. The screw path in atlas was achieved by direct probing the isthmus of axial vertebrae then identifying its position through the atlantoaxial joint correlation. Postoperative transoral X ray and CT film were used for analysis of the position of the 150 screws in the mass of atlas. Three areas were delimitated in and around the lateral mass of atlas: area A (inside the joint face), area: B (outside the joint face but still in lateral mass), and area C area (outside the lateral mass) so as to analyze the location of screws.
Results: Among the 150 screws 139 (92.7%) were located in area A, 8 in area B(1) and 3 in area B(2) (7.3%), and none in area C. All cases got atlantoaxial union 3 months after operation.
Conclusion: The method of "direct probing" is reliable for identifying the path of screw in atlas.