TOMOGRAPHIC STUDY OF THE ATLAS CONCERNING SCREW FIXATION ON LATERAL MASS

Introduction: Harms's technique for atlanto-axial arthrodesis fusion makes possible the use of intraoperative reduction maneuvers of the atlanto-axial dislocation and facilitates the fixation of this joint, especially in hiperkyphotic patients and in situations where the fixation with transarticular screw on C1-C2 segment is impossible. Objective: to describe the morphometric parameters of atlases related to the Harms's technique for atlanto-axial arthrodesis, the optimal path and the safety aisle for screw insertion into atlas' lateral mass measured with multiplanar reconstruction imaging with helical computed tomography scan at the planes, Materials and Methods: 30 atlases of local origin have been submitted to computed tomography imaging and measured with digital cursor. Results: The posteroanterior length of the implant measured 16.5 mm to the right and 16.3 mm to the left. When using it as entry point at the central portion of the lateral mass just below the posterior arc, the path must have a congruence of 15 degrees in the coronal plan and an upper angle of 20 degrees at the sagittal plane. Conclusion: The safety aisle for screw fixation into the lateral mass would have a lower bending of 22 degrees and an upper bending of 33 degrees at sagittal plane and 36 degrees medially and 26 degrees laterally, at the axial plane.